- Email: [email protected]
The Traceless Solid-Phase Synthesis of Organic Molecules
164
[9]
linkers and their applications
[9] The Traceless Solid-Phase Synthesis of Organic Molecules By David Tumelty, Yijun Pan, and Christopher P. Holmes Introduction
As the field of solid-phase organic synthesis continues to progress and expand, one aim of practitioners is to synthesize molecules that do not signal their solid-phase ‘‘origins’’ by the presence of extraneous remnants left after cleavage from the resin. These often appeared in earlier work as primary or secondary carboxamides or carboxyl groups appended to the target molecules and conveniently overlooked. These additional functionalities could hinder or obscure biological activity in an otherwise promising target compound or scaffold, as well as being chemically rather unaesthetic. As solid-phase routes and linkers have become increasingly sophisticated in recent years, many workers in the field have forwarded novel chemical methods to attempt to overcome some of these previously mentioned limitations.1 This chapter describes two such approaches, which illustrate different tactics used in the goal of synthesizing organic compounds in a traceless manner. In the first strategy, a linker and scaffold combine synergistically to achieve a traceless synthesis of diverse substituted benzimidazole compounds and libraries.2 Second, a novel linker is used in a more global fashion to synthesize target compounds by activation of chemically diverse phenols.3,4 Traceless Solid-Phase Synthesis of Benzimidazoles Background
Several solid-phase syntheses of benzimidazoles have been reported in recent years.5,6,7 Recently some have been described in which the final products could be regarded as traceless.2,8–10 Our initial goal was to design 1
V. Krchnak and M. W. Holladay, Chem. Rev. 102, 61 (2002). D. Tumelty, K. Cao, and C. P. Holmes, Org. Lett. 3, 83 (2001). 3 Y. Pan and C. P. Holmes, Org. Lett. 3, 2769 (2001). 4 Y. Pan, B. Ruhland, and C. P. Holmes, Angew. Chem. Int. Ed. Engl. 40, 4488 (2001). 5 D. Tumelty, M. K. Schwarz, K. Cao, and M. C. Needels, Tetrahedron Lett. 40, 6185 (1999). 6 D. Tumelty, M. K. Schwarz, and M. C. Needels, Tetrahedron Lett. 39, 7467 (1998). 7 J. P. Mayer, G. S. Lewis, C. McGee, and D. Bankaitis-Davis, Tetrahedron Lett. 39, 6655 (1998). 2
METHODS IN ENZYMOLOGY, VOL. 369
Copyright 2003, Elsevier Inc. All rights reserved. 0076-6879/03 $35.00
[9]
traceless solid-phase organic synthesis
165
a synthetic route that would permit the traceless release of benzimidazoles from single beads to support Affymax’s encoded combinatorial library screening technologies.11 We required that the final compounds would be released without the benefit of further solution-phase reactions to complete the synthesis and that purification steps would not be carried out (in this particular format). This necessitated the development of a novel synthetic strategy in which the required products were synthesized on the solid support in a quaternary salt form, and treatment with base released the products in a Hofmann elimination reaction. The basic concept has been previously reported for the synthesis of simple amines on the REM linker.12 Our modified plan to synthesize and release the benzimidazole compounds is shown in Fig. 1. Here, we envisage building the benzimidazole scaffold directly onto the linker and, by analogy with a regular tertiary amine synthesis on the REM linker, we can quaternize the resin-bound benzimidazole compounds by reaction with reactive bromides. The quaternary salt can then be liberated by a Hofmann elimination reaction upon treatment with base. Development of the Traceless Route
The uncertain part of the synthesis scheme (prior to testing it experimentally) was whether quaternization on the ring nitrogen that was not directly attached to the resin would provide a sufficiently strong electronwithdrawing force to permit the Hofmann elimination under mild conditions. This idea was initially tested in a double-linker scheme as shown in Fig. 2. This construct, although not the final chemical route, was extremely valuable in proving that the concept was valid and enabling various portions of the scheme to be optimized more effectively. This construct has also proven useful in the development of related chemistries using a similar Hofmann elimination strategy. In this scheme, Fmoc*--alanine is coupled to ArgoGel-Wang resin using standard methodology to give resin 1, and then the Fmoc group is 8
V. Krchnak, J. Smith, and J. Vagner, Tetrahedron Lett. 42, 1627 (2001). A. Mazurov, Bioorg. Med. Chem. Lett. 10, 67 (2000). 10 W. Huang and R. M. Scarborough, Tetrahedron Lett. 40, 2665 (1999). 11 Z. J. Ni, D. Maclean, C. P. Holmes, and M. A. Gallop, Methods Enzymol. 267, 261 (1996). 12 J. R. Morphy, Z. Rankovic, and D. C. Rees, Tetrahedron Lett. 37, 3209 (1996). * Abbreviations: AcOH, acetic acid; Alloc, allyloxycarbonyl; ArB(OH)2, a generalized aromatic boronic acid; BINAP, (R)-(þ)-2,20 -Bis(diphenylphosphino)-1,10 -binaphthyl, a chiral chelating ligand useful in palladium-mediated reactions; DCM, 1,2-dichloromethane, a common organic solvent (caution: suspected carcinogen); DIEA, N,N0 -diisopropylethylamine, a hindered organic base; DMF, N,N0 -dimethylformamide, a polar organic solvent; 9
166
[9]
linkers and their applications O
R2 + HN R1
O
O
NH2 O
R2 R2 N R1
O
O
O
N N
O
R1
R3 N
R2 R2 O
+
N
R3
R1
O
O
R2
N
+
O
R3 N R1
R1
R3 N
R2 N
R1
Fig. 1. Comparison of a regular tertiary amine synthesis on REM resin (left) with the planned traceless benzimidazole route.
DMSO, dimethylsulfoxide, a polar organic solvent; dppp, 1,3-bis(diphenyl-phosphino)propane; Fmoc, 9-fluorenylmethyloxycarbonyl, a protecting group of amines; GC-MS, combined gas chromatography and mass spectrometry instrumentation; HPLC, highpressure (performance) liquid chromatography; LC-MS, combined liquid chromatography and mass spectrometry instrumentation; MeOH, methanol; Na(CN)BH3, sodium cyanoborohydride, a reducing agent often used for the reduction of imines to amines; NMP, Nmethylpyrrolidinone, an organic solvent; Na2S2O4, sodium hyposulfite (sodium dithionite), a mild, water-soluble reducing agent; Oxone, potassium peroxymonosulfate, an oxidizing agent (Dupont); Pd(dppf)Cl2, [1,10 -bis(diphenylphosphino)ferrocene]dichloropalladium(II); Pd(OAc)2, palladium(II) acetate; PEG-PS, a resin composed of low-cross-linked polystyrene linked with polyethylene glycol of various lengths; PFS linker/resin, perfluoroalkylsulfonyl linker attached to resin; SnCl22H2O, tin(II) dichloride dihydrate, often used for reduction of aromatic nitro groups to corresponding anilines; tBoc, tertbutyloxycarbonyl, a common, acid-labile protecting group for amines; TEA, triethylamine, an organic base; TFA, 1,1,1-trifluoroacetic acid, a strong organic acid; THF, tetrahydrofuran, an organic solvent; TLC, thin-layer chromatography.
[9]
167
traceless solid-phase organic synthesis O H N
O O
O
O
a,b
O
H N
O
O
+
N
O
R1
O
1
2 c R2
O O
N
R1
e R2 HO
3
f
R2
O
4a
O
R1
O
N N
O
R1
N
g
R3 N
R2 O
+
R3 N Br +
5
O
O
NH2
H N
O
O
4
O
d
N
R1
e
R2 HO
N
R3 N CF3CO2+
N
O
O
5a
R1
R1 + TEA.HBr 6
Fig. 2. Double-linker route used in development: (a) 20% piperidine/NMP; (b) ortho nitrofluoro/-chloro-R1-arene, DIEA, NMP, 12 h, 60 ; (c) SnCl22H2O, NMP, 12 h; (d) R2 CHO, NMP, 12 h, 50 ; (e) TFA, DCM, 30 min; (f) R3-bromide, NMP, 18 h, 60 ; (g) TEA, DCM, 18 h, 25 .
removed with piperidine. The exposed resin-bound amine acts as the starting anchor for building diverse benzimidazole compounds by the sequential reaction of three components, namely ortho-fluoro- or ortho-chloronitroarenes, aldehydes, and alkyl- or benzylbromides. The fluoronitroarenes are added to give resin 2, with the expected steric and electronic considerations dictating the kinetics of the nucleophilic substitution reaction. We compensate for the differing kinetics to a large extent by using as high a concentration of the amine as possible in a polar organic solvent, such as NMP or DMSO. Several chloroarenes are useful in the scheme, although they tend to require heating and the reactions are often difficult to force to completion. The extent of the reaction is monitored by the ninhydrin test and in cases where an incomplete reaction occurs, the unreacted resin-bound
168
linkers and their applications
[9]
amine is capped by reaction with acetic anhydride/pyridine/DMF for 20 min. Fortunately, this capping step does not acetylate the resin-bound aniline under these conditions, presumably due to the low nucleophilicity of the resin-bound ortho-nitroaniline. The resin-bound phenylene diamine intermediates 3 are then generated by nitro group reduction with tin(II) chloride in NMP and cyclization/ aromatization with a wide variety of aldehydes gave the resin-bound benzimidazole intermediates 4. The treatment of this intermediate with 50% TFA/DCM liberates the substituted 3-(benzoimidazol-1-yl)-propionic acid derivative 4a. Analysis of this intermediate by HPLC and LC-MS gave a measure of the purity of the resin-bound product and enabled the optimization of conditions for the incorporation of the R1-nitroarenes and R2-aldehydes by an iterative process. Incorporation of the third and final diversity element to give resin 5 is achieved via quaternization of the resin-bound benzimidazole intermediates using a large excess of a primary alkyl or benzyl bromide. A high concentration solution of the desired bromide in NMP or DMSO (at least 1.5 M, with at least 30 equivalents over the estimated resin loading) proved effective in most cases, and heating this reaction to around a maximum of 55 helps to achieve a higher final yield in this model system. We observed some premature release of the product from the resin when heating at 60 and above. For the final scheme (see below), modifications to the linker enabled a higher temperature to be used for the reaction with bromides, which increases the yields of the final products by an average of about 20%. Despite the slight temperature sensitivity of this two-linker model system, it was valuable in determining the optimal conditions for quaternization using different classes of alkylating agent. Treatment of 5 with TFA releases the quaternary salt derivative 5a, which enables assessment of the success of the alkylation conditions. Again, evaluating different conditions enabled us to optimize this reaction for a range of alkylating agents. Despite the fact that the quaternization event occurs on a nitrogen atom that is not directly attached to the resin, we were gratified to observe that the products 6 are indeed released by base treatment. Release of the desired compounds from the resin was carried out in the development stages with various proportions of TEA or DIEA in DCM. We later observed that release could be carried out in several different solvents using one of a variety of organic and inorganic bases.13 For single compound 13
For example, ammonia in dioxane or methanol and inorganic bases, such as sodium hydroxide or ammonium hydroxide solutions. Such cleavages gave satisfactory results in a variety of solvents, such as N-methylpyrrolidinone, dimethyl sulfoxide, acetonitrile, methanol, dioxane, and acetone.
[9]
traceless solid-phase organic synthesis
169
work at the development stage, a single treatment with 5% TEA in DCM for 16 h gives the highest yields of compound recovery. The excess TEA salts can be removed by extraction, prior to purification of the desired compound (if necessary), to recover the product 6 as a white solid in most cases. The double-linker construct again proved valuable in determining the end point and yield of the cleavage reaction under different reaction conditions. After treatment with base to release the product, the resin is thoroughly washed with DCM then subjected to TFA treatment. This allows us to examine the products remaining on the resin. If there was incomplete Hofmann elimination of the desired product, compound 5a is observed. By optimization of the basic conditions, the presence of 5a can be reduced to low levels or often completely eliminated. This procedure is allied with standard gravimetric analysis of the expected compound to determine the cleavage yields for a range of compounds. In our hands, the cleavage yields (i.e., target compound removed from the resin) is typically >80%, with generally less than 10% of the desired material remaining bound to the resin.14 Library Rehearsal
Despite the variable yields of the products, we observe that they are very pure, almost without exception, directly after cleavage from the resin (usually >90% of target compound by HPLC trace integral). Interestingly, combinations of the R1, R2, and R3 monomers that interact unfavorably to produce a low yield of target compound (as judged by both the resin and gravimetric analyses as outlined above) always give pure products upon cleavage. It became obvious that optimization of the R1 and R2 monomer combinations (that lead to high-purity resin-bound intermediate 4) would give the best chance for eventual product formation. Most commercially available aromatic and some aliphatic aldehydes work well in the synthetic scheme. Some examples of the R1 monomers used for subsequent library formation are shown in Fig. 3. Having determined the most successful R1/R2 combinations, we screened a wide variety of alkylating agents for their ability to quaternize resin intermediate 4. For several types of resin intermediate 4 (chosen 14
To further elucidate this point, we were able to remove from the resin about four-fifths of the quaternized target compound that had been synthesized (this was judged using the double-linker approach mentioned above). The variability in the actual amount of target material synthesized on (and competent to be released from) the resin was dictated by the reaction (in)compatibility of the three monomers comprising each individual benzimidazole compound.
170
[9]
linkers and their applications
F O2N
O2N
O2N
F
F
Cl
O 2N
S
O2N
F
F
Cl
O
F
Cl
O
O2N F
F
O
F
Cl
O2N F
F Br
O2N
F
O 2N
F
O2N
O2N
O
O O
O
O 2N O
Cl
F
O2N F
O O2N
F
O
O2N
Br N
Cl
O
Fig. 3. A selection of nitroarenes used in library production.
to have differing steric and electronic properties), a certain number of alkylating species give acceptable formation of resin 5 under common condi tions (2 M in NMP, 18 h, 60 ). The yield of this reaction largely determines the final yield of the reaction, almost independently of the conditions used for the final base-promoted elimination. Several of the alkylating species used in subsequent library production are shown in Fig. 4. Final Improved Reaction Route
Several modifications were made to the scheme to improve the stability of some of the resin intermediates and provide improved overall yields of the cleaved products. The final scheme used to synthesize tagged libraries is shown in Fig. 5. Two major changes to the double-linker resin used in development were introduced: a modification to the REM-type linker itself and the inclusion of tags for chemical encoding. The new resin/linker is easily made from commercially available reagents. In our hands, the use of PEG-PSbased resins works best for the scheme, which does call for both organic and aqueous reaction conditions. We have previously reported the use of an unencoded version of this route.2 A halogenated PEG-PS resin (TentaGel-Br or ArgoGel-Cl) is reacted with tert-butyl-N-(2-mercaptoethyl)carbamate to give resin 7. Coupling of this reagent to the resin introduces both the amine function that serves as an anchor for benzimidazole synthesis, as well as a sulfur group. The sulfur is later oxidized to a sulfoxide providing the driving force for the elimination reaction that ultimately releases the final products. For library synthesis,
[9]
171
traceless solid-phase organic synthesis Br
Br
Br
Br
Br
Br
F
Br
F
F
F
F Br
Br
NC
Br O2 N
Cl
F Br
Br
CF3 O
Br
Br
CF3
F CF3 Br
Br
Cl
Br
O O2 N
N
S
CF3
O
N
Br
CF3
Br
O
Br
O
Br
Br
Br
NH2 Br
O
Fig. 4. A selection of alkyl and benzyl bromides used in library production.
H N
S HN
Alloc
h, i, b
R2 O S O HN
Tag1 Tag2
S
tBoc
7
HN
R1
R1
10
O S O
f HN
S
R1 HN
R2
N
Tag 1 Tag 2
N
g
9
Tag1 Tag2
R3 N Br + R1
11
N N
j, d, h
8
Tag1 Alloc
N
R2
NO2
H N
O S O HN
Tag1 Tag2
12
R3 N
R2 +
k
N R1 + TEA.HBr
6
Fig. 5. Final library scheme. Conditions as in Fig. 2 and (h) Alloc removal and tag coupling; (i) TFA, dimethylsulfide, DCM, 2 30 min, then DIEA, DCM, 2 30 min, 25 ; (j) Na2S2O4, water, MeOH, 16 h, 25 ; (k) aqueous Oxone, 12 h, 25 .
we have devised a method for differentiating the resin, such that approximately one-tenth of the available functionality was reserved for encoding procedures. Prior to acidolysis to remove the tBoc protecting group, the resin is divided and each resin pool is encoded using the tagging strategy. Each tagged resin pool is then treated with the nitroarenes as before to give resin 8 and the resins are pooled prior to nitro group reduction.
172
linkers and their applications
[9]
The nitro group reduction is carried out on the pooled resin using sodium hydrosulfite in water to form the resin-bound substituted phenylene diamine precursor.15 The resin is again split into smaller pools and the second diversity elements (various aliphatic and aromatic aldehydes) are added. As previously observed, no exogenous oxidants are necessary to form the required resin-bound benzimidazoles. After the final round of tagging, the resin 9 is once again pooled and treated with a chemical oxidant to convert the sulfur group to a sulfoxide. Aqueous Oxone proved most effective, with complete conversion observed after overnight treatment. Prewashing of the resin with methanol aids in its subsequent solvation by the Oxone solution. The resin is then split into pools for addition of the final diversity elements, the alkyl and benzyl bromides. This proved to be the step in the synthesis scheme that largely determines the final yield of product, as noted before. Quaternization of the resin-bound intermediates 10 is carried out with a high concentration solution of the desired alkylating agent in NMP or DMSO to give resin 11. Heating this reaction to around a maximum of 70 helps to achieve higher final yield. We had previously determined that this sulfur-based linker has increased temperature stability compared to that used in the development stage. In model studies, we observe some premature release of the product from this resin only when heating at 90 and above. These final diversity elements were ‘‘spatially encoded,’’ i.e., the resin pools are kept separated and assayed separately, to allow for identification of the final monomers without the need for a further chemical encoding step. As a result no further pooling was necessary after the quaternization reaction and we were able to tailor the most favorable reaction conditions for incorporation of the required alkylating agents in each reaction. The release of the final products from the beads can be achieved using several different procedures, as previously determined at the development stage. For library production, we are able to release sufficient compound by solvating the beads with DMSO and subjecting them to treatment with ammonia gas.16 After release of the desired compound 6, we recover the encoding bead 12 and carry out the decoding procedures to determine the identity of the R1 and R2 monomers from beads of interest (after various assays have been carried out on the released compounds).
15 16
R. A. Scheuerman and D. Tumelty, Tetrahedron Lett. 41, 6531 (2000). R. Brown, J. Comb. Chem. 1, 283 (1999).
[9]
traceless solid-phase organic synthesis
173
Conclusion
This section described the successful development and implementation of a traceless synthetic route to create libraries of chemically diverse benzimidazole compounds. The chemical route delivers compounds in moderate yields but in high purity directly after cleavage from the solid support. The basic concept of this traceless approach has been applied to several other related heterocyclic systems that will be reported in due course.17 Experimental
Reagents and General Methods We have previously described the basic resin handling and washing procedures, as well as nitro group reduction, cyclization with aldehydes to form the benzimidazole ring, and chemical encoding procedures for a related benzimidazole system.18 Reagents and solvents used are available from Aldrich (Milwaukee, WI) and Calbiochem-Novabiochem (San Diego, CA). General Procedure for Coupling of o-Fluoro/Chloro-Nitroarenes The same procedure is used for the formation of resins 2 and 8 by reaction of the nitroarenes with the resin-bound -alanine or mercaptan/ amine linker, respectively. The o-fluoronitroarenes (Fig. 3) are dissolved in DMSO or NMP (at a concentration between 1.5 and 2 M) and added to the resin, followed by diisopropylethylamine (10 equivalents) and additional DMSO or NMP (if required) to ensure resin solvation. Although many nitroarenes react rapidly at room temperature, in a library format the resin/nitroarene mixture is heated overnight at 50 to help achieve equivalent reaction kinetics between different monomers. Under the same conditions, some o-chloronitroarenes are synthetically useful. The extent of the reaction can be assessed qualitatively (or quantitatively, if desired) by carrying out a ninhydrin test to check for the presence of free amine. In any case, the resins are acetylated with five equivalents of acetic anhydride/pyridine/DMF (1:1:10) for 20 min to cap any unreacted amine.
17 18
D. Tumelty, unpublished results (2000). D. Tumelty, L.-C. Dong, K. Cao, L. Le, and M. C. Needels, in ‘‘High Throughput Synthesis’’ (I. Sucholeiki, ed.), p. 93. Marcel Dekker, New York, 2001.
174
linkers and their applications
[9]
Procedures for Reduction of the Aromatic Nitro Group For formation of resin 3, the resin is washed in NMP (20 ml/g of resin), filtered, and left solvated. Separately, tin(II) chloride dihydrate (approximately 40 equivalents with respect to resin-bound nitro groups) is dissolved in NMP with vigorous stirring, then the solution is added to the resin and mixed by nitrogen bubbling for 12 h at room temperature. The resin is filtered, washed, and left solvated prior to the next synthetic step. For reduction of resin 8, concerns about the possibility of traces of tin by-products contaminating subsequent assays led to the adoption of a different reduction procedure for library production.15 The tagged resins 8 are combined into one pool in a large peptide synthesis vessel, washed with methanol, filtered, and the resin left solvated. Separately, an aqueous solution of 0.5 M aqueous sodium hydrosulfite/0.5 M potassium carbonate is prepared and added to the resin (40 ml/g of resin) and then the resin/solution is bubbled with nitrogen at room temperature for 16 h. The resulting resin is washed with water, water/MeOH (1:1), MeOH, MeOH/NMP (1:1), NMP, DCM, MeOH, and ether, then filtered and dried overnight in vacuo prior to the next step. General Procedure for Quaternization with Alkyl/benzyl Bromides Resin 4 or 10 is solvated by washing in NMP. For quaternization, a benzyl or alkyl bromide (50 equivalents) is dissolved in NMP to give a final solution with a concentration of 2 M. This solution is added to the resin in a glass vial and stirred at 50–70 for 18 h. After this time, the dark brown resin is transferred to a polypropylene tube and washed with NMP, DCM, MeOH, then finally diethyl ether and dried overnight in vacuo. Preparation of Resin 7 A PEG-PS resin is subjected to a proprietary procedure where about 90% of the initial amine functionality is loaded with a moiety bearing a reactive bromide, while the remainder has an amine function protected by an allyloxycarbonyl group. The resin (40 g, approximately 15 mmol with respect to the bromine group) is solvated with NMP (200 ml) in a 1-liter pear-shaped flask, fitted with a nitrogen-bubbler. t-Butyl-N-(2-mercaptoethyl)carbamate (Aldrich, 20 ml, 7 equivalents; caution: Stench!) is added, followed by solid potassium carbonate (9 g, 4 equivalents), and the resulting resin/solution is stirred with an overhead paddle-stirrer at 60 for 12 h in a thermostatically controlled oil bath. After this time, the slurry is transferred to a 2-liter peptide synthesis vessel and the resin is subsequently washed under vigorous nitrogen bubbling using NMP, NMP/water, MeOH/ water, water, MeOH/water, NMP/water, NMP, DCM, MeOH, and ether
[9]
traceless solid-phase organic synthesis
175
(3 250 ml each), and finally dried overnight in vacuo. A pale yellow resin is obtained (43 g). A small resin sample is taken and, after removal of the tBoc group, the loading of the resin is assessed using two complementary methods: either a quantitative ninhydrin test19 or coupling an Fmoc group to the exposed amine, deprotecting with piperidine in DMF (1:4), and quantitatively assessing the concentration of the dibenzofulvene adduct formed at 302 nm.20 Either method usually gives loading values between 0.30 and 0.33 mmol/g for the amine linker. Preparation of Resin 10 The tagged resins are pooled into one large batch in a 2-liter peptide vessel, washed with methanol, and then left solvated. Separately, solid Oxone is dissolved in water (to a final concentration of 0.4 M), sonicating for 5 min to aid in solvation. The aqueous Oxone solution (10 equivalents with respect to the nitro group loading of the resin) is added to the methanolsolvated resin and stirred/bubbled for 16 h at room temperature. The resulting resin 10 was then washed with water, MeOH/water (1:1), MeOH, MeOH/NMP, NMP, DCM, and ether, then dried in vacuo overnight prior to the next step. Traceless Syntheses Using a Novel Triflate-Type Linker Background
Our goal for this work was somewhat different from the preceding traceless benzimidazole syntheses. Here we aimed to develop a novel solid-phase linker that would serve as an activating group for a wide variety of phenols, permitting subsequent transformations to occur between the resin-bound phenol and a variety of different classes of input molecules. The strategy is based on the well-known activation properties of triflates, which are widely used as precursors for aryl and vinyl cations due to their excellent leaving group properties.21 Once an oxygen atom on the phenol moiety is activated by the triflate (trifluoromethanesulfonyl) group, it becomes possible to carry out a reductive cleavage (essentially deoxygenating the phenol) or cross-coupling reactions, e.g., through palladium-catalyzed Suzuki, Stille, and Heck reactions.22 This gives rise to a variety of 19
V. K. Sarin, S. B. H. Kent, J. P. Tam, and R. B. Merrifield, Anal. Biochem. 117, 147 (1981). M. K. Schwarz, D. Tumelty, and M. A. Gallop, J. Org. Chem. 64, 2219 (1999). 21 J. F. Hartwig, Angew. Chem. Int. Ed. Engl. 37, 2046 (1998). 22 B. A. Lorsbach and M. J. Kurth, Chem. Rev. 99, 1549 (1999). 20
176
[9]
linkers and their applications
substituted aromatics or olefins at the ‘‘inert’’ phenolic or vinyl oxygen position. Our goal therefore was to design a triflate-like linker upon which we can conduct such triflate-directed transformations on solid-phase resin and this section describes the successful implementation of this strategy. Perfluoroalkylsulfonyl (PFS) Linker/Resin
We recently reported the synthesis of a perfluoroalkylsulfonyl linker attached to TentaGel resin 13 (Fig. 6), which proves to act in a fashion similar to triflates as we had hoped, and demonstrated its application for the traceless cleavage of phenols using palladium-catalyzed reduction and Suzuki cross-coupling reactions.3,4 The new polymer-supported linker allows the attachment of phenolic rings to the solid phase through the formation of aryl nonflates and subsequent traceless cleavage of the hydroxyl groups on aryl rings. A variety of phenols can be attached to this resin through the formation of the polymersupported perfluoroalkylsulfonates 14 in DMF at room temperature using potassium carbonate as base (Fig. 7). The attachment of phenols to this linker is especially attractive since the mild reaction conditions allow many useful functional groups (such as aldehydes, nitro, carboxylic acids, ketones, and alcohols) to be incorporated without additional protection, and these
H N O
O O S F F F
F F FF O O S O F F F FF
13 Fig. 6. Structure of the perfluoroalkylsulfonyl (PFS) linker/resin.
O O S F F F 13
l R1 HO
O O S O F F 14
m
R1
R1
o
R1 R2 R2
n
R1 N R3
Fig. 7. Divergent syntheses from a PFS-derivatized linker. (l) Substituted phenol, potassium carbonate, DMF; (m) Suzuki reaction; (n) reduction; (o) amine displacement.
[9]
traceless solid-phase organic synthesis
177
groups can themselves serve as combinatorial sites for the synthesis of large libraries. We have found that the resin-bound perfluoroalkylsulfonate species have similar reactivities to aryl triflates, such that most of the known palladium-catalyzed reactions involving aryl triflates were possible on the support (Fig. 7). Thus, a cleavage/cross-coupling strategy of simultaneously introducing diversity while liberating the desired molecule from the support provides a powerful technique for the traceless synthesis of molecules. We have initially targeted the Suzuki and Buchwald amination reactions as methods for generating biaryls and anilines, respectively. Cleavage of the Resin-Bound Phenols Using the Suzuki Coupling Reaction
The Suzuki coupling reaction is a powerful tool for carbon–carbon bond formation in combinatorial library production.23 Many different reaction conditions and catalyst systems have been reported for the cross-coupling of aryl triflates and aromatic halides with boronic acids in solution. After some experimentation, we found that the ‘‘Suzuki cleavage’’ of the resinbound perfluoroalkylsulfonates proceeded smoothly by using [1,10 -bis (diphenylphosphino)ferrocene]dichloropalladium(II), triethylamine, and boronic acids in dimethylformamide at 80 within 8 h afforded the desired biaryl compounds in good yields.24 The desired products are easily isolated by a simple two-phase extraction process and purified by preparative TLC to give the biaryl compounds in high purity, as determined by HPLC, GC-MS, and LC-MS analysis. A small library of biaryl compounds was synthesized in order to examine the scope and generality of the resin-bound PFS linker and the traceless ‘‘Suzuki cleavage’’ strategy, as shown in Fig. 8. The aryl perfluoroalkylsulfonate resin 15 is prepared by attaching 4-hydroxybenzaldehyde to resin. Resin 16 is prepared by a reductive amination of 15 with primary amines using sodium cyanoborohydride as the reducing agent. The presence of some acetic acid is also important in this step to promote the reductive amination reaction. The secondary amines generated in this step are used as another diversity site through functionalization of this amine. Biaryls 18 are produced in a traceless fashion from resin 17 in yields ranging from 65 to 90% upon exposure to the Suzuki conditions. We have observed that most boronic acids are suitable for this cleavage/cross-coupling procedure to generate a wide variety of molecules.
23 24
J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, and M. Lemaire, Chem. Rev. 102, 1359 (2002). B. Ruhland, A. Bombrun, and M. A. Gallop, J. Org. Chem. 62, 7820 (1997).
178
[9]
linkers and their applications O O S O F F 15
O
O O S O F F 17
N R1
NH R1
O O S O F F 16
p
O
q
O R2
r
N R1
Ar
R2
18
Fig. 8. A four-step synthesis on the PFS linker. (p) R1-NH2, Na(CN)BH3, AcOH, THF; (q) R2-NH2, TEA, CH2Cl2; (r) ArB(OH)2, Pd(dppf)Cl2, TEA, DMF.
Cleavage of the Resin-Bound Phenols Using Catalytic Reductive Elimination
The deoxygenation of phenols by a palladium-mediated reduction in solution phase is well established.25 Reductive cleavage of the polymersupported aryl triflate-type species allows phenols to be cleaved from the resin without any trace of the phenolic hydroxyl group. This strategy is valuable for combinatorial syntheses since the structure of the final product cleaved from the resin is independent of the position of the hydroxyl group on the starting phenol and enables greater flexibility in choosing the building blocks for library syntheses. The deoxygenation of the polymersupported aryl nonaflate species was studied through a palladium-mediated reduction reaction. We discovered that the polymer-bound aryl nonaflates were efficiently cleaved with a mixture of triethylamine and formic acid in the presence of a catalytic amount of palladium(II) acetate and 1,3-bis(diphenylphosphino)propane to afford high yields of the reduced arenes under mild conditions. The desired products are again isolated by a twophase extraction and the trace metal catalyst is removed by eluting the organic solution through a thin pad of silica gel. The resulting products are obtained in good yields as determined by HPLC, GC-MS, and LC-MS. Figure 9 shows two examples of the traceless cleavage of the resin-bound phenols using this palladium-catalyzed reductive elimination. It is notable that the cleaved product 20 does not have any remnant of the anchoring hydroxyl group and also that the polymer-bound perfluoroalkylsulfonate served here as a linker, a protecting group, and an activating group for the phenols. The aryl perfluoroalkylsulfonyl resin also permits monoattachment of a symmetric bisphenol to form resin 21 and only the attached
25
S. Cacchi, P. G. Ciattini, E. Morera, and G. Ortar, Tetrahedron Lett. 27, 5541 (1986).
[9]
179
traceless solid-phase organic synthesis O O S O F F 19
O O S O F F 21
N
s N 20
s OH
OH 22
Fig. 9. Reductive cleavage from the PFS linker. (s) Pd(OAc)2, dppp, TEA-HCO2H, DMF.
phenol group is deoxygenated, leading to a nonsymmetrical phenol 22 upon reductive cleavage. The application of resin 13 to the solid-phase synthesis of other useful target compounds was also explored and an example of this is the multistep synthesis of Meclizine (Fig. 10).26 The starting material, 3-methyl-4-hydroxybenzaldehyde, is attached to the PFS linker, and a polymer-bound amine intermediate is prepared by a reductive amination of resin 23 with amine 24. The resulting resin 25 is subjected to a palladium-mediated reductive cleavage to give Meclizine 26 in 80% yield, based on the original resin loading. Cleavage of the Resin-Bound Phenols Using Catalytic Amination
Substituted anilines often appear as a key element in biologically active compounds. The palladium-catalyzed amination of aryl triflates has drawn increasing interest as a synthetic route to a wide variety of aryl amines.27 The diversity of phenols and amines that is available, along with the simple attachment of phenols to the PFS linker, suggested to us that the catalytic amination of resin-bound aryl triflate species would provide another useful synthetic route to aryl amines. Figure 11 shows a general solid-phase protocol for the traceless cleavage of phenols from the PFS linker using catalytic amination. The reaction is carried out with phenolloaded resins 19 or 28, palladium(II) acetate, BINAP, cesium carbonate, and the corresponding amines in THF at 80 for 16 h. The desired products 27 and 29 are obtained in 70–80% yields based on the actual loading of the 26
Meclizine is an oral antiemetic used to treat nausea, vomiting, and dizziness associated with motion sickness. Compound 26, synthesized by our route, was spectroscopically identical with a commercially obtained sample (Sigma). 27 ˚ hman and S. L. Buchwald, Tetrahedron Lett. 38, 6363 (1997). J. A
180 O O S O F F 23
[9]
linkers and their applications
O
Cl
HN N
Cl
N
O O S O F F 25
p
N
24 Cl
N
s
N
26 Meclizine
Fig. 10. Synthesis of Meclizine on the PFS linker.
O O S O F F 19
N
t HN
O O S O F F 28
N
N N 27
N Et
N
t
N
O
N HN
O
29
Fig. 11. Cleavage from the PFS linker with amines. (t) Pd(OAc)2, BINAP, Cs2CO3, THF.
phenols. The traceless ‘‘amination cleavage’’ approach permits the introduction of a new aromatic amine functionality at the phenolic oxygen position during cleavage and provides a powerful method to synthesize libraries with rich synthetic diversity. Conclusion
The resin-bound perfluoroalkylsulfonyl linker is compatible with many common solid-phase reactions, such as tin dichloride-mediated aromatic nitro group reduction, trifluoroacetic acid-mediated tBoc deprotection, reductive amination reactions, acylation, and sulfonation. It is possible to perform several sequential synthetic reactions on the nonflate resin so that multistep syntheses can be carried out. The solid-phase approach provides an operationally simple, inexpensive, and general protocol for the cleavage
[9]
traceless solid-phase organic synthesis
181
of the aryl-oxygen bond. We anticipate that the ease of preparation, excellent stability, and synthetic versatility of the polymer-supported linker will prove useful in solid-phase and combinatorial chemistry. Since a large number of phenols with a variety of functional groups are commercially available, and a variety of palladium-mediated reactions can be used for the resin-bound aryl triflates, our novel resin-bound perfluoroalkylsulfonyl linker will provide a powerful method to synthesize structurally diverse libraries. Experimental
Reagents and General Methods All starting materials were obtained from Aldrich (Milwaukee, WI). TentaGel resin was obtained from Rapp Polymere (Tubingen, Germany). Procedures for the synthesis of the PFS linker and its attachment to resin to form 13 have been previous described.3 General Procedure for Attachment of Phenols to Resin 13 (to form 14, 15, 19, 21, 23, or 28) The phenol (20 equivalents), potassium carbonate (22 equivalents), and resin 13 (1 equivalent with respect to the sulfonyl fluoride group) are mixed with DMF (10 ml/g of resin) and shaken overnight at room temperature. The resin is filtered and washed with water, DMF, and DCM, and then is dried under vacuum overnight to give the required resin-bound phenol. General Procedure for Cleavage of Phenols Using the Suzuki Coupling Reaction: Preparation of Resins 15–17 and Compounds 18 A mixture of 4-hydroxybenzadehyde (5.0 mmol), potassium carbonate (6.6 mmol), and resin-bound linker 13 (3.0 g, 1.0 mmol) is added to DMF (8.0 ml) and the mixture was shaken at room temperature overnight. The resin was filtered and washed with water, DMF, and DCM, and then dried under vacuum overnight to give resin 15. A portion of the dried resin (0.50 g, 0.16 mmol) is then mixed with a primary amine (R1-NH2, 2.0 mmol), THF (2.0 ml), Na(CN)BH3 (1 N solution in THF, 2.0 ml, 2.0 mmol), mmol), and acetic acid (0.11 ml, 1.95 mmol) and the mixture is shaken overnight at room temperature. The beads are filtered and washed with water, DMF, and DCM, and dried under vacuum overnight to give resin 16. To a portion of the dried amine resin 15 (0.20 g, 0.066 mmol) is added TEA (2.3 mmol), DCM (4.0 ml), and an acid chloride (R2-COCl, 1.6 mmol) mmol) at 0 . The mixture was allowed to warm-up and then shaken at room
182
[10]
linkers and their applications
temperature overnight. The resin is filtered, washed with DMF and DCM, and dried under vacuum overnight, as before, to give resin 17. For the Suzuki cleavage reaction, a portion of the dry resin 17 (0.20 g, 0.07 mmol) is mixed with Pd(dppf)Cl2 (7.2 mg), an arylboronic acid (0.25 mmol), TEA (0.1 ml, 0.60 mmol), and DMF (2.0 ml) in a glass vial under nitrogen. The mixture is then shaken at 90 overnight. The polymer beads are next filtered and washed several times with Et2O and the combined organic phase is washed with aqueous 2% sodium carbonate and water and then evaporated to dryness. The crude products are purified by preparative TLC (or other suitable methods) to give the desired products 18 in 65–90% yields, with >98% purity as determined by HPLC. General Procedure for Cleavage of Phenols by a Reductive Elimination Reaction: Preparation of Compounds 20 and 22 To dried resins 19 and 21 (0.1 g resin, approximately 0.04 mmol with respect to the loading of the phenol) are added Pd(OAc)2 (8.0 mg), 1,3-bis (diphenyl-phosphino)propane (dppp, 17.0 mg), DMF (1.4 ml), and a mix ture of HCO2H (0.2 ml) and TEA (0.8 ml). The mixture is shaken at 85 for 2 h, and then the resin is filtered and washed several times with diethyl ether. The combined organic phase is washed with aqueous sodium carbonate solution then water and evaporated to dryness. The residue obtained is dissolved in diethyl ether and eluted through a short column of alumina to remove any remaining inorganic residues. The crude products are purified by preparative TLC (or other suitable methods) to give the desired products 20 and 22 in >95% purity.
[10] Unnatural Diamino Acid Derivatives as Scaffolds for Creating Diversity and as Linkers for Simplifying Screening in Chemical Libraries By Robert Pascal, Re´gine Sola, and Patrick Jouin Introduction
The introduction of conformational restrictions into flexible active molecules is a well-known strategy for trying to increase their potency and/or selectivity toward their biological targets.1 Several methods have been used for constraining flexible molecules. Cyclic derivatives of linear peptides or peptidomimetics can thus be prepared by reactions involving side-chain
METHODS IN ENZYMOLOGY, VOL. 369
Copyright 2003, Elsevier Inc. All rights reserved. 0076-6879/03 $35.00
[9]
linkers and their applications
[9] The Traceless Solid-Phase Synthesis of Organic Molecules By David Tumelty, Yijun Pan, and Christopher P. Holmes Introduction
As the field of solid-phase organic synthesis continues to progress and expand, one aim of practitioners is to synthesize molecules that do not signal their solid-phase ‘‘origins’’ by the presence of extraneous remnants left after cleavage from the resin. These often appeared in earlier work as primary or secondary carboxamides or carboxyl groups appended to the target molecules and conveniently overlooked. These additional functionalities could hinder or obscure biological activity in an otherwise promising target compound or scaffold, as well as being chemically rather unaesthetic. As solid-phase routes and linkers have become increasingly sophisticated in recent years, many workers in the field have forwarded novel chemical methods to attempt to overcome some of these previously mentioned limitations.1 This chapter describes two such approaches, which illustrate different tactics used in the goal of synthesizing organic compounds in a traceless manner. In the first strategy, a linker and scaffold combine synergistically to achieve a traceless synthesis of diverse substituted benzimidazole compounds and libraries.2 Second, a novel linker is used in a more global fashion to synthesize target compounds by activation of chemically diverse phenols.3,4 Traceless Solid-Phase Synthesis of Benzimidazoles Background
Several solid-phase syntheses of benzimidazoles have been reported in recent years.5,6,7 Recently some have been described in which the final products could be regarded as traceless.2,8–10 Our initial goal was to design 1
V. Krchnak and M. W. Holladay, Chem. Rev. 102, 61 (2002). D. Tumelty, K. Cao, and C. P. Holmes, Org. Lett. 3, 83 (2001). 3 Y. Pan and C. P. Holmes, Org. Lett. 3, 2769 (2001). 4 Y. Pan, B. Ruhland, and C. P. Holmes, Angew. Chem. Int. Ed. Engl. 40, 4488 (2001). 5 D. Tumelty, M. K. Schwarz, K. Cao, and M. C. Needels, Tetrahedron Lett. 40, 6185 (1999). 6 D. Tumelty, M. K. Schwarz, and M. C. Needels, Tetrahedron Lett. 39, 7467 (1998). 7 J. P. Mayer, G. S. Lewis, C. McGee, and D. Bankaitis-Davis, Tetrahedron Lett. 39, 6655 (1998). 2
METHODS IN ENZYMOLOGY, VOL. 369
Copyright 2003, Elsevier Inc. All rights reserved. 0076-6879/03 $35.00
[9]
traceless solid-phase organic synthesis
165
a synthetic route that would permit the traceless release of benzimidazoles from single beads to support Affymax’s encoded combinatorial library screening technologies.11 We required that the final compounds would be released without the benefit of further solution-phase reactions to complete the synthesis and that purification steps would not be carried out (in this particular format). This necessitated the development of a novel synthetic strategy in which the required products were synthesized on the solid support in a quaternary salt form, and treatment with base released the products in a Hofmann elimination reaction. The basic concept has been previously reported for the synthesis of simple amines on the REM linker.12 Our modified plan to synthesize and release the benzimidazole compounds is shown in Fig. 1. Here, we envisage building the benzimidazole scaffold directly onto the linker and, by analogy with a regular tertiary amine synthesis on the REM linker, we can quaternize the resin-bound benzimidazole compounds by reaction with reactive bromides. The quaternary salt can then be liberated by a Hofmann elimination reaction upon treatment with base. Development of the Traceless Route
The uncertain part of the synthesis scheme (prior to testing it experimentally) was whether quaternization on the ring nitrogen that was not directly attached to the resin would provide a sufficiently strong electronwithdrawing force to permit the Hofmann elimination under mild conditions. This idea was initially tested in a double-linker scheme as shown in Fig. 2. This construct, although not the final chemical route, was extremely valuable in proving that the concept was valid and enabling various portions of the scheme to be optimized more effectively. This construct has also proven useful in the development of related chemistries using a similar Hofmann elimination strategy. In this scheme, Fmoc*--alanine is coupled to ArgoGel-Wang resin using standard methodology to give resin 1, and then the Fmoc group is 8
V. Krchnak, J. Smith, and J. Vagner, Tetrahedron Lett. 42, 1627 (2001). A. Mazurov, Bioorg. Med. Chem. Lett. 10, 67 (2000). 10 W. Huang and R. M. Scarborough, Tetrahedron Lett. 40, 2665 (1999). 11 Z. J. Ni, D. Maclean, C. P. Holmes, and M. A. Gallop, Methods Enzymol. 267, 261 (1996). 12 J. R. Morphy, Z. Rankovic, and D. C. Rees, Tetrahedron Lett. 37, 3209 (1996). * Abbreviations: AcOH, acetic acid; Alloc, allyloxycarbonyl; ArB(OH)2, a generalized aromatic boronic acid; BINAP, (R)-(þ)-2,20 -Bis(diphenylphosphino)-1,10 -binaphthyl, a chiral chelating ligand useful in palladium-mediated reactions; DCM, 1,2-dichloromethane, a common organic solvent (caution: suspected carcinogen); DIEA, N,N0 -diisopropylethylamine, a hindered organic base; DMF, N,N0 -dimethylformamide, a polar organic solvent; 9
166
[9]
linkers and their applications O
R2 + HN R1
O
O
NH2 O
R2 R2 N R1
O
O
O
N N
O
R1
R3 N
R2 R2 O
+
N
R3
R1
O
O
R2
N
+
O
R3 N R1
R1
R3 N
R2 N
R1
Fig. 1. Comparison of a regular tertiary amine synthesis on REM resin (left) with the planned traceless benzimidazole route.
DMSO, dimethylsulfoxide, a polar organic solvent; dppp, 1,3-bis(diphenyl-phosphino)propane; Fmoc, 9-fluorenylmethyloxycarbonyl, a protecting group of amines; GC-MS, combined gas chromatography and mass spectrometry instrumentation; HPLC, highpressure (performance) liquid chromatography; LC-MS, combined liquid chromatography and mass spectrometry instrumentation; MeOH, methanol; Na(CN)BH3, sodium cyanoborohydride, a reducing agent often used for the reduction of imines to amines; NMP, Nmethylpyrrolidinone, an organic solvent; Na2S2O4, sodium hyposulfite (sodium dithionite), a mild, water-soluble reducing agent; Oxone, potassium peroxymonosulfate, an oxidizing agent (Dupont); Pd(dppf)Cl2, [1,10 -bis(diphenylphosphino)ferrocene]dichloropalladium(II); Pd(OAc)2, palladium(II) acetate; PEG-PS, a resin composed of low-cross-linked polystyrene linked with polyethylene glycol of various lengths; PFS linker/resin, perfluoroalkylsulfonyl linker attached to resin; SnCl22H2O, tin(II) dichloride dihydrate, often used for reduction of aromatic nitro groups to corresponding anilines; tBoc, tertbutyloxycarbonyl, a common, acid-labile protecting group for amines; TEA, triethylamine, an organic base; TFA, 1,1,1-trifluoroacetic acid, a strong organic acid; THF, tetrahydrofuran, an organic solvent; TLC, thin-layer chromatography.
[9]
167
traceless solid-phase organic synthesis O H N
O O
O
O
a,b
O
H N
O
O
+
N
O
R1
O
1
2 c R2
O O
N
R1
e R2 HO
3
f
R2
O
4a
O
R1
O
N N
O
R1
N
g
R3 N
R2 O
+
R3 N Br +
5
O
O
NH2
H N
O
O
4
O
d
N
R1
e
R2 HO
N
R3 N CF3CO2+
N
O
O
5a
R1
R1 + TEA.HBr 6
Fig. 2. Double-linker route used in development: (a) 20% piperidine/NMP; (b) ortho nitrofluoro/-chloro-R1-arene, DIEA, NMP, 12 h, 60 ; (c) SnCl22H2O, NMP, 12 h; (d) R2 CHO, NMP, 12 h, 50 ; (e) TFA, DCM, 30 min; (f) R3-bromide, NMP, 18 h, 60 ; (g) TEA, DCM, 18 h, 25 .
removed with piperidine. The exposed resin-bound amine acts as the starting anchor for building diverse benzimidazole compounds by the sequential reaction of three components, namely ortho-fluoro- or ortho-chloronitroarenes, aldehydes, and alkyl- or benzylbromides. The fluoronitroarenes are added to give resin 2, with the expected steric and electronic considerations dictating the kinetics of the nucleophilic substitution reaction. We compensate for the differing kinetics to a large extent by using as high a concentration of the amine as possible in a polar organic solvent, such as NMP or DMSO. Several chloroarenes are useful in the scheme, although they tend to require heating and the reactions are often difficult to force to completion. The extent of the reaction is monitored by the ninhydrin test and in cases where an incomplete reaction occurs, the unreacted resin-bound
168
linkers and their applications
[9]
amine is capped by reaction with acetic anhydride/pyridine/DMF for 20 min. Fortunately, this capping step does not acetylate the resin-bound aniline under these conditions, presumably due to the low nucleophilicity of the resin-bound ortho-nitroaniline. The resin-bound phenylene diamine intermediates 3 are then generated by nitro group reduction with tin(II) chloride in NMP and cyclization/ aromatization with a wide variety of aldehydes gave the resin-bound benzimidazole intermediates 4. The treatment of this intermediate with 50% TFA/DCM liberates the substituted 3-(benzoimidazol-1-yl)-propionic acid derivative 4a. Analysis of this intermediate by HPLC and LC-MS gave a measure of the purity of the resin-bound product and enabled the optimization of conditions for the incorporation of the R1-nitroarenes and R2-aldehydes by an iterative process. Incorporation of the third and final diversity element to give resin 5 is achieved via quaternization of the resin-bound benzimidazole intermediates using a large excess of a primary alkyl or benzyl bromide. A high concentration solution of the desired bromide in NMP or DMSO (at least 1.5 M, with at least 30 equivalents over the estimated resin loading) proved effective in most cases, and heating this reaction to around a maximum of 55 helps to achieve a higher final yield in this model system. We observed some premature release of the product from the resin when heating at 60 and above. For the final scheme (see below), modifications to the linker enabled a higher temperature to be used for the reaction with bromides, which increases the yields of the final products by an average of about 20%. Despite the slight temperature sensitivity of this two-linker model system, it was valuable in determining the optimal conditions for quaternization using different classes of alkylating agent. Treatment of 5 with TFA releases the quaternary salt derivative 5a, which enables assessment of the success of the alkylation conditions. Again, evaluating different conditions enabled us to optimize this reaction for a range of alkylating agents. Despite the fact that the quaternization event occurs on a nitrogen atom that is not directly attached to the resin, we were gratified to observe that the products 6 are indeed released by base treatment. Release of the desired compounds from the resin was carried out in the development stages with various proportions of TEA or DIEA in DCM. We later observed that release could be carried out in several different solvents using one of a variety of organic and inorganic bases.13 For single compound 13
For example, ammonia in dioxane or methanol and inorganic bases, such as sodium hydroxide or ammonium hydroxide solutions. Such cleavages gave satisfactory results in a variety of solvents, such as N-methylpyrrolidinone, dimethyl sulfoxide, acetonitrile, methanol, dioxane, and acetone.
[9]
traceless solid-phase organic synthesis
169
work at the development stage, a single treatment with 5% TEA in DCM for 16 h gives the highest yields of compound recovery. The excess TEA salts can be removed by extraction, prior to purification of the desired compound (if necessary), to recover the product 6 as a white solid in most cases. The double-linker construct again proved valuable in determining the end point and yield of the cleavage reaction under different reaction conditions. After treatment with base to release the product, the resin is thoroughly washed with DCM then subjected to TFA treatment. This allows us to examine the products remaining on the resin. If there was incomplete Hofmann elimination of the desired product, compound 5a is observed. By optimization of the basic conditions, the presence of 5a can be reduced to low levels or often completely eliminated. This procedure is allied with standard gravimetric analysis of the expected compound to determine the cleavage yields for a range of compounds. In our hands, the cleavage yields (i.e., target compound removed from the resin) is typically >80%, with generally less than 10% of the desired material remaining bound to the resin.14 Library Rehearsal
Despite the variable yields of the products, we observe that they are very pure, almost without exception, directly after cleavage from the resin (usually >90% of target compound by HPLC trace integral). Interestingly, combinations of the R1, R2, and R3 monomers that interact unfavorably to produce a low yield of target compound (as judged by both the resin and gravimetric analyses as outlined above) always give pure products upon cleavage. It became obvious that optimization of the R1 and R2 monomer combinations (that lead to high-purity resin-bound intermediate 4) would give the best chance for eventual product formation. Most commercially available aromatic and some aliphatic aldehydes work well in the synthetic scheme. Some examples of the R1 monomers used for subsequent library formation are shown in Fig. 3. Having determined the most successful R1/R2 combinations, we screened a wide variety of alkylating agents for their ability to quaternize resin intermediate 4. For several types of resin intermediate 4 (chosen 14
To further elucidate this point, we were able to remove from the resin about four-fifths of the quaternized target compound that had been synthesized (this was judged using the double-linker approach mentioned above). The variability in the actual amount of target material synthesized on (and competent to be released from) the resin was dictated by the reaction (in)compatibility of the three monomers comprising each individual benzimidazole compound.
170
[9]
linkers and their applications
F O2N
O2N
O2N
F
F
Cl
O 2N
S
O2N
F
F
Cl
O
F
Cl
O
O2N F
F
O
F
Cl
O2N F
F Br
O2N
F
O 2N
F
O2N
O2N
O
O O
O
O 2N O
Cl
F
O2N F
O O2N
F
O
O2N
Br N
Cl
O
Fig. 3. A selection of nitroarenes used in library production.
to have differing steric and electronic properties), a certain number of alkylating species give acceptable formation of resin 5 under common condi tions (2 M in NMP, 18 h, 60 ). The yield of this reaction largely determines the final yield of the reaction, almost independently of the conditions used for the final base-promoted elimination. Several of the alkylating species used in subsequent library production are shown in Fig. 4. Final Improved Reaction Route
Several modifications were made to the scheme to improve the stability of some of the resin intermediates and provide improved overall yields of the cleaved products. The final scheme used to synthesize tagged libraries is shown in Fig. 5. Two major changes to the double-linker resin used in development were introduced: a modification to the REM-type linker itself and the inclusion of tags for chemical encoding. The new resin/linker is easily made from commercially available reagents. In our hands, the use of PEG-PSbased resins works best for the scheme, which does call for both organic and aqueous reaction conditions. We have previously reported the use of an unencoded version of this route.2 A halogenated PEG-PS resin (TentaGel-Br or ArgoGel-Cl) is reacted with tert-butyl-N-(2-mercaptoethyl)carbamate to give resin 7. Coupling of this reagent to the resin introduces both the amine function that serves as an anchor for benzimidazole synthesis, as well as a sulfur group. The sulfur is later oxidized to a sulfoxide providing the driving force for the elimination reaction that ultimately releases the final products. For library synthesis,
[9]
171
traceless solid-phase organic synthesis Br
Br
Br
Br
Br
Br
F
Br
F
F
F
F Br
Br
NC
Br O2 N
Cl
F Br
Br
CF3 O
Br
Br
CF3
F CF3 Br
Br
Cl
Br
O O2 N
N
S
CF3
O
N
Br
CF3
Br
O
Br
O
Br
Br
Br
NH2 Br
O
Fig. 4. A selection of alkyl and benzyl bromides used in library production.
H N
S HN
Alloc
h, i, b
R2 O S O HN
Tag1 Tag2
S
tBoc
7
HN
R1
R1
10
O S O
f HN
S
R1 HN
R2
N
Tag 1 Tag 2
N
g
9
Tag1 Tag2
R3 N Br + R1
11
N N
j, d, h
8
Tag1 Alloc
N
R2
NO2
H N
O S O HN
Tag1 Tag2
12
R3 N
R2 +
k
N R1 + TEA.HBr
6
Fig. 5. Final library scheme. Conditions as in Fig. 2 and (h) Alloc removal and tag coupling; (i) TFA, dimethylsulfide, DCM, 2 30 min, then DIEA, DCM, 2 30 min, 25 ; (j) Na2S2O4, water, MeOH, 16 h, 25 ; (k) aqueous Oxone, 12 h, 25 .
we have devised a method for differentiating the resin, such that approximately one-tenth of the available functionality was reserved for encoding procedures. Prior to acidolysis to remove the tBoc protecting group, the resin is divided and each resin pool is encoded using the tagging strategy. Each tagged resin pool is then treated with the nitroarenes as before to give resin 8 and the resins are pooled prior to nitro group reduction.
172
linkers and their applications
[9]
The nitro group reduction is carried out on the pooled resin using sodium hydrosulfite in water to form the resin-bound substituted phenylene diamine precursor.15 The resin is again split into smaller pools and the second diversity elements (various aliphatic and aromatic aldehydes) are added. As previously observed, no exogenous oxidants are necessary to form the required resin-bound benzimidazoles. After the final round of tagging, the resin 9 is once again pooled and treated with a chemical oxidant to convert the sulfur group to a sulfoxide. Aqueous Oxone proved most effective, with complete conversion observed after overnight treatment. Prewashing of the resin with methanol aids in its subsequent solvation by the Oxone solution. The resin is then split into pools for addition of the final diversity elements, the alkyl and benzyl bromides. This proved to be the step in the synthesis scheme that largely determines the final yield of product, as noted before. Quaternization of the resin-bound intermediates 10 is carried out with a high concentration solution of the desired alkylating agent in NMP or DMSO to give resin 11. Heating this reaction to around a maximum of 70 helps to achieve higher final yield. We had previously determined that this sulfur-based linker has increased temperature stability compared to that used in the development stage. In model studies, we observe some premature release of the product from this resin only when heating at 90 and above. These final diversity elements were ‘‘spatially encoded,’’ i.e., the resin pools are kept separated and assayed separately, to allow for identification of the final monomers without the need for a further chemical encoding step. As a result no further pooling was necessary after the quaternization reaction and we were able to tailor the most favorable reaction conditions for incorporation of the required alkylating agents in each reaction. The release of the final products from the beads can be achieved using several different procedures, as previously determined at the development stage. For library production, we are able to release sufficient compound by solvating the beads with DMSO and subjecting them to treatment with ammonia gas.16 After release of the desired compound 6, we recover the encoding bead 12 and carry out the decoding procedures to determine the identity of the R1 and R2 monomers from beads of interest (after various assays have been carried out on the released compounds).
15 16
R. A. Scheuerman and D. Tumelty, Tetrahedron Lett. 41, 6531 (2000). R. Brown, J. Comb. Chem. 1, 283 (1999).
[9]
traceless solid-phase organic synthesis
173
Conclusion
This section described the successful development and implementation of a traceless synthetic route to create libraries of chemically diverse benzimidazole compounds. The chemical route delivers compounds in moderate yields but in high purity directly after cleavage from the solid support. The basic concept of this traceless approach has been applied to several other related heterocyclic systems that will be reported in due course.17 Experimental
Reagents and General Methods We have previously described the basic resin handling and washing procedures, as well as nitro group reduction, cyclization with aldehydes to form the benzimidazole ring, and chemical encoding procedures for a related benzimidazole system.18 Reagents and solvents used are available from Aldrich (Milwaukee, WI) and Calbiochem-Novabiochem (San Diego, CA). General Procedure for Coupling of o-Fluoro/Chloro-Nitroarenes The same procedure is used for the formation of resins 2 and 8 by reaction of the nitroarenes with the resin-bound -alanine or mercaptan/ amine linker, respectively. The o-fluoronitroarenes (Fig. 3) are dissolved in DMSO or NMP (at a concentration between 1.5 and 2 M) and added to the resin, followed by diisopropylethylamine (10 equivalents) and additional DMSO or NMP (if required) to ensure resin solvation. Although many nitroarenes react rapidly at room temperature, in a library format the resin/nitroarene mixture is heated overnight at 50 to help achieve equivalent reaction kinetics between different monomers. Under the same conditions, some o-chloronitroarenes are synthetically useful. The extent of the reaction can be assessed qualitatively (or quantitatively, if desired) by carrying out a ninhydrin test to check for the presence of free amine. In any case, the resins are acetylated with five equivalents of acetic anhydride/pyridine/DMF (1:1:10) for 20 min to cap any unreacted amine.
17 18
D. Tumelty, unpublished results (2000). D. Tumelty, L.-C. Dong, K. Cao, L. Le, and M. C. Needels, in ‘‘High Throughput Synthesis’’ (I. Sucholeiki, ed.), p. 93. Marcel Dekker, New York, 2001.
174
linkers and their applications
[9]
Procedures for Reduction of the Aromatic Nitro Group For formation of resin 3, the resin is washed in NMP (20 ml/g of resin), filtered, and left solvated. Separately, tin(II) chloride dihydrate (approximately 40 equivalents with respect to resin-bound nitro groups) is dissolved in NMP with vigorous stirring, then the solution is added to the resin and mixed by nitrogen bubbling for 12 h at room temperature. The resin is filtered, washed, and left solvated prior to the next synthetic step. For reduction of resin 8, concerns about the possibility of traces of tin by-products contaminating subsequent assays led to the adoption of a different reduction procedure for library production.15 The tagged resins 8 are combined into one pool in a large peptide synthesis vessel, washed with methanol, filtered, and the resin left solvated. Separately, an aqueous solution of 0.5 M aqueous sodium hydrosulfite/0.5 M potassium carbonate is prepared and added to the resin (40 ml/g of resin) and then the resin/solution is bubbled with nitrogen at room temperature for 16 h. The resulting resin is washed with water, water/MeOH (1:1), MeOH, MeOH/NMP (1:1), NMP, DCM, MeOH, and ether, then filtered and dried overnight in vacuo prior to the next step. General Procedure for Quaternization with Alkyl/benzyl Bromides Resin 4 or 10 is solvated by washing in NMP. For quaternization, a benzyl or alkyl bromide (50 equivalents) is dissolved in NMP to give a final solution with a concentration of 2 M. This solution is added to the resin in a glass vial and stirred at 50–70 for 18 h. After this time, the dark brown resin is transferred to a polypropylene tube and washed with NMP, DCM, MeOH, then finally diethyl ether and dried overnight in vacuo. Preparation of Resin 7 A PEG-PS resin is subjected to a proprietary procedure where about 90% of the initial amine functionality is loaded with a moiety bearing a reactive bromide, while the remainder has an amine function protected by an allyloxycarbonyl group. The resin (40 g, approximately 15 mmol with respect to the bromine group) is solvated with NMP (200 ml) in a 1-liter pear-shaped flask, fitted with a nitrogen-bubbler. t-Butyl-N-(2-mercaptoethyl)carbamate (Aldrich, 20 ml, 7 equivalents; caution: Stench!) is added, followed by solid potassium carbonate (9 g, 4 equivalents), and the resulting resin/solution is stirred with an overhead paddle-stirrer at 60 for 12 h in a thermostatically controlled oil bath. After this time, the slurry is transferred to a 2-liter peptide synthesis vessel and the resin is subsequently washed under vigorous nitrogen bubbling using NMP, NMP/water, MeOH/ water, water, MeOH/water, NMP/water, NMP, DCM, MeOH, and ether
[9]
traceless solid-phase organic synthesis
175
(3 250 ml each), and finally dried overnight in vacuo. A pale yellow resin is obtained (43 g). A small resin sample is taken and, after removal of the tBoc group, the loading of the resin is assessed using two complementary methods: either a quantitative ninhydrin test19 or coupling an Fmoc group to the exposed amine, deprotecting with piperidine in DMF (1:4), and quantitatively assessing the concentration of the dibenzofulvene adduct formed at 302 nm.20 Either method usually gives loading values between 0.30 and 0.33 mmol/g for the amine linker. Preparation of Resin 10 The tagged resins are pooled into one large batch in a 2-liter peptide vessel, washed with methanol, and then left solvated. Separately, solid Oxone is dissolved in water (to a final concentration of 0.4 M), sonicating for 5 min to aid in solvation. The aqueous Oxone solution (10 equivalents with respect to the nitro group loading of the resin) is added to the methanolsolvated resin and stirred/bubbled for 16 h at room temperature. The resulting resin 10 was then washed with water, MeOH/water (1:1), MeOH, MeOH/NMP, NMP, DCM, and ether, then dried in vacuo overnight prior to the next step. Traceless Syntheses Using a Novel Triflate-Type Linker Background
Our goal for this work was somewhat different from the preceding traceless benzimidazole syntheses. Here we aimed to develop a novel solid-phase linker that would serve as an activating group for a wide variety of phenols, permitting subsequent transformations to occur between the resin-bound phenol and a variety of different classes of input molecules. The strategy is based on the well-known activation properties of triflates, which are widely used as precursors for aryl and vinyl cations due to their excellent leaving group properties.21 Once an oxygen atom on the phenol moiety is activated by the triflate (trifluoromethanesulfonyl) group, it becomes possible to carry out a reductive cleavage (essentially deoxygenating the phenol) or cross-coupling reactions, e.g., through palladium-catalyzed Suzuki, Stille, and Heck reactions.22 This gives rise to a variety of 19
V. K. Sarin, S. B. H. Kent, J. P. Tam, and R. B. Merrifield, Anal. Biochem. 117, 147 (1981). M. K. Schwarz, D. Tumelty, and M. A. Gallop, J. Org. Chem. 64, 2219 (1999). 21 J. F. Hartwig, Angew. Chem. Int. Ed. Engl. 37, 2046 (1998). 22 B. A. Lorsbach and M. J. Kurth, Chem. Rev. 99, 1549 (1999). 20
176
[9]
linkers and their applications
substituted aromatics or olefins at the ‘‘inert’’ phenolic or vinyl oxygen position. Our goal therefore was to design a triflate-like linker upon which we can conduct such triflate-directed transformations on solid-phase resin and this section describes the successful implementation of this strategy. Perfluoroalkylsulfonyl (PFS) Linker/Resin
We recently reported the synthesis of a perfluoroalkylsulfonyl linker attached to TentaGel resin 13 (Fig. 6), which proves to act in a fashion similar to triflates as we had hoped, and demonstrated its application for the traceless cleavage of phenols using palladium-catalyzed reduction and Suzuki cross-coupling reactions.3,4 The new polymer-supported linker allows the attachment of phenolic rings to the solid phase through the formation of aryl nonflates and subsequent traceless cleavage of the hydroxyl groups on aryl rings. A variety of phenols can be attached to this resin through the formation of the polymersupported perfluoroalkylsulfonates 14 in DMF at room temperature using potassium carbonate as base (Fig. 7). The attachment of phenols to this linker is especially attractive since the mild reaction conditions allow many useful functional groups (such as aldehydes, nitro, carboxylic acids, ketones, and alcohols) to be incorporated without additional protection, and these
H N O
O O S F F F
F F FF O O S O F F F FF
13 Fig. 6. Structure of the perfluoroalkylsulfonyl (PFS) linker/resin.
O O S F F F 13
l R1 HO
O O S O F F 14
m
R1
R1
o
R1 R2 R2
n
R1 N R3
Fig. 7. Divergent syntheses from a PFS-derivatized linker. (l) Substituted phenol, potassium carbonate, DMF; (m) Suzuki reaction; (n) reduction; (o) amine displacement.
[9]
traceless solid-phase organic synthesis
177
groups can themselves serve as combinatorial sites for the synthesis of large libraries. We have found that the resin-bound perfluoroalkylsulfonate species have similar reactivities to aryl triflates, such that most of the known palladium-catalyzed reactions involving aryl triflates were possible on the support (Fig. 7). Thus, a cleavage/cross-coupling strategy of simultaneously introducing diversity while liberating the desired molecule from the support provides a powerful technique for the traceless synthesis of molecules. We have initially targeted the Suzuki and Buchwald amination reactions as methods for generating biaryls and anilines, respectively. Cleavage of the Resin-Bound Phenols Using the Suzuki Coupling Reaction
The Suzuki coupling reaction is a powerful tool for carbon–carbon bond formation in combinatorial library production.23 Many different reaction conditions and catalyst systems have been reported for the cross-coupling of aryl triflates and aromatic halides with boronic acids in solution. After some experimentation, we found that the ‘‘Suzuki cleavage’’ of the resinbound perfluoroalkylsulfonates proceeded smoothly by using [1,10 -bis (diphenylphosphino)ferrocene]dichloropalladium(II), triethylamine, and boronic acids in dimethylformamide at 80 within 8 h afforded the desired biaryl compounds in good yields.24 The desired products are easily isolated by a simple two-phase extraction process and purified by preparative TLC to give the biaryl compounds in high purity, as determined by HPLC, GC-MS, and LC-MS analysis. A small library of biaryl compounds was synthesized in order to examine the scope and generality of the resin-bound PFS linker and the traceless ‘‘Suzuki cleavage’’ strategy, as shown in Fig. 8. The aryl perfluoroalkylsulfonate resin 15 is prepared by attaching 4-hydroxybenzaldehyde to resin. Resin 16 is prepared by a reductive amination of 15 with primary amines using sodium cyanoborohydride as the reducing agent. The presence of some acetic acid is also important in this step to promote the reductive amination reaction. The secondary amines generated in this step are used as another diversity site through functionalization of this amine. Biaryls 18 are produced in a traceless fashion from resin 17 in yields ranging from 65 to 90% upon exposure to the Suzuki conditions. We have observed that most boronic acids are suitable for this cleavage/cross-coupling procedure to generate a wide variety of molecules.
23 24
J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, and M. Lemaire, Chem. Rev. 102, 1359 (2002). B. Ruhland, A. Bombrun, and M. A. Gallop, J. Org. Chem. 62, 7820 (1997).
178
[9]
linkers and their applications O O S O F F 15
O
O O S O F F 17
N R1
NH R1
O O S O F F 16
p
O
q
O R2
r
N R1
Ar
R2
18
Fig. 8. A four-step synthesis on the PFS linker. (p) R1-NH2, Na(CN)BH3, AcOH, THF; (q) R2-NH2, TEA, CH2Cl2; (r) ArB(OH)2, Pd(dppf)Cl2, TEA, DMF.
Cleavage of the Resin-Bound Phenols Using Catalytic Reductive Elimination
The deoxygenation of phenols by a palladium-mediated reduction in solution phase is well established.25 Reductive cleavage of the polymersupported aryl triflate-type species allows phenols to be cleaved from the resin without any trace of the phenolic hydroxyl group. This strategy is valuable for combinatorial syntheses since the structure of the final product cleaved from the resin is independent of the position of the hydroxyl group on the starting phenol and enables greater flexibility in choosing the building blocks for library syntheses. The deoxygenation of the polymersupported aryl nonaflate species was studied through a palladium-mediated reduction reaction. We discovered that the polymer-bound aryl nonaflates were efficiently cleaved with a mixture of triethylamine and formic acid in the presence of a catalytic amount of palladium(II) acetate and 1,3-bis(diphenylphosphino)propane to afford high yields of the reduced arenes under mild conditions. The desired products are again isolated by a twophase extraction and the trace metal catalyst is removed by eluting the organic solution through a thin pad of silica gel. The resulting products are obtained in good yields as determined by HPLC, GC-MS, and LC-MS. Figure 9 shows two examples of the traceless cleavage of the resin-bound phenols using this palladium-catalyzed reductive elimination. It is notable that the cleaved product 20 does not have any remnant of the anchoring hydroxyl group and also that the polymer-bound perfluoroalkylsulfonate served here as a linker, a protecting group, and an activating group for the phenols. The aryl perfluoroalkylsulfonyl resin also permits monoattachment of a symmetric bisphenol to form resin 21 and only the attached
25
S. Cacchi, P. G. Ciattini, E. Morera, and G. Ortar, Tetrahedron Lett. 27, 5541 (1986).
[9]
179
traceless solid-phase organic synthesis O O S O F F 19
O O S O F F 21
N
s N 20
s OH
OH 22
Fig. 9. Reductive cleavage from the PFS linker. (s) Pd(OAc)2, dppp, TEA-HCO2H, DMF.
phenol group is deoxygenated, leading to a nonsymmetrical phenol 22 upon reductive cleavage. The application of resin 13 to the solid-phase synthesis of other useful target compounds was also explored and an example of this is the multistep synthesis of Meclizine (Fig. 10).26 The starting material, 3-methyl-4-hydroxybenzaldehyde, is attached to the PFS linker, and a polymer-bound amine intermediate is prepared by a reductive amination of resin 23 with amine 24. The resulting resin 25 is subjected to a palladium-mediated reductive cleavage to give Meclizine 26 in 80% yield, based on the original resin loading. Cleavage of the Resin-Bound Phenols Using Catalytic Amination
Substituted anilines often appear as a key element in biologically active compounds. The palladium-catalyzed amination of aryl triflates has drawn increasing interest as a synthetic route to a wide variety of aryl amines.27 The diversity of phenols and amines that is available, along with the simple attachment of phenols to the PFS linker, suggested to us that the catalytic amination of resin-bound aryl triflate species would provide another useful synthetic route to aryl amines. Figure 11 shows a general solid-phase protocol for the traceless cleavage of phenols from the PFS linker using catalytic amination. The reaction is carried out with phenolloaded resins 19 or 28, palladium(II) acetate, BINAP, cesium carbonate, and the corresponding amines in THF at 80 for 16 h. The desired products 27 and 29 are obtained in 70–80% yields based on the actual loading of the 26
Meclizine is an oral antiemetic used to treat nausea, vomiting, and dizziness associated with motion sickness. Compound 26, synthesized by our route, was spectroscopically identical with a commercially obtained sample (Sigma). 27 ˚ hman and S. L. Buchwald, Tetrahedron Lett. 38, 6363 (1997). J. A
180 O O S O F F 23
[9]
linkers and their applications
O
Cl
HN N
Cl
N
O O S O F F 25
p
N
24 Cl
N
s
N
26 Meclizine
Fig. 10. Synthesis of Meclizine on the PFS linker.
O O S O F F 19
N
t HN
O O S O F F 28
N
N N 27
N Et
N
t
N
O
N HN
O
29
Fig. 11. Cleavage from the PFS linker with amines. (t) Pd(OAc)2, BINAP, Cs2CO3, THF.
phenols. The traceless ‘‘amination cleavage’’ approach permits the introduction of a new aromatic amine functionality at the phenolic oxygen position during cleavage and provides a powerful method to synthesize libraries with rich synthetic diversity. Conclusion
The resin-bound perfluoroalkylsulfonyl linker is compatible with many common solid-phase reactions, such as tin dichloride-mediated aromatic nitro group reduction, trifluoroacetic acid-mediated tBoc deprotection, reductive amination reactions, acylation, and sulfonation. It is possible to perform several sequential synthetic reactions on the nonflate resin so that multistep syntheses can be carried out. The solid-phase approach provides an operationally simple, inexpensive, and general protocol for the cleavage
[9]
traceless solid-phase organic synthesis
181
of the aryl-oxygen bond. We anticipate that the ease of preparation, excellent stability, and synthetic versatility of the polymer-supported linker will prove useful in solid-phase and combinatorial chemistry. Since a large number of phenols with a variety of functional groups are commercially available, and a variety of palladium-mediated reactions can be used for the resin-bound aryl triflates, our novel resin-bound perfluoroalkylsulfonyl linker will provide a powerful method to synthesize structurally diverse libraries. Experimental
Reagents and General Methods All starting materials were obtained from Aldrich (Milwaukee, WI). TentaGel resin was obtained from Rapp Polymere (Tubingen, Germany). Procedures for the synthesis of the PFS linker and its attachment to resin to form 13 have been previous described.3 General Procedure for Attachment of Phenols to Resin 13 (to form 14, 15, 19, 21, 23, or 28) The phenol (20 equivalents), potassium carbonate (22 equivalents), and resin 13 (1 equivalent with respect to the sulfonyl fluoride group) are mixed with DMF (10 ml/g of resin) and shaken overnight at room temperature. The resin is filtered and washed with water, DMF, and DCM, and then is dried under vacuum overnight to give the required resin-bound phenol. General Procedure for Cleavage of Phenols Using the Suzuki Coupling Reaction: Preparation of Resins 15–17 and Compounds 18 A mixture of 4-hydroxybenzadehyde (5.0 mmol), potassium carbonate (6.6 mmol), and resin-bound linker 13 (3.0 g, 1.0 mmol) is added to DMF (8.0 ml) and the mixture was shaken at room temperature overnight. The resin was filtered and washed with water, DMF, and DCM, and then dried under vacuum overnight to give resin 15. A portion of the dried resin (0.50 g, 0.16 mmol) is then mixed with a primary amine (R1-NH2, 2.0 mmol), THF (2.0 ml), Na(CN)BH3 (1 N solution in THF, 2.0 ml, 2.0 mmol), mmol), and acetic acid (0.11 ml, 1.95 mmol) and the mixture is shaken overnight at room temperature. The beads are filtered and washed with water, DMF, and DCM, and dried under vacuum overnight to give resin 16. To a portion of the dried amine resin 15 (0.20 g, 0.066 mmol) is added TEA (2.3 mmol), DCM (4.0 ml), and an acid chloride (R2-COCl, 1.6 mmol) mmol) at 0 . The mixture was allowed to warm-up and then shaken at room
182
[10]
linkers and their applications
temperature overnight. The resin is filtered, washed with DMF and DCM, and dried under vacuum overnight, as before, to give resin 17. For the Suzuki cleavage reaction, a portion of the dry resin 17 (0.20 g, 0.07 mmol) is mixed with Pd(dppf)Cl2 (7.2 mg), an arylboronic acid (0.25 mmol), TEA (0.1 ml, 0.60 mmol), and DMF (2.0 ml) in a glass vial under nitrogen. The mixture is then shaken at 90 overnight. The polymer beads are next filtered and washed several times with Et2O and the combined organic phase is washed with aqueous 2% sodium carbonate and water and then evaporated to dryness. The crude products are purified by preparative TLC (or other suitable methods) to give the desired products 18 in 65–90% yields, with >98% purity as determined by HPLC. General Procedure for Cleavage of Phenols by a Reductive Elimination Reaction: Preparation of Compounds 20 and 22 To dried resins 19 and 21 (0.1 g resin, approximately 0.04 mmol with respect to the loading of the phenol) are added Pd(OAc)2 (8.0 mg), 1,3-bis (diphenyl-phosphino)propane (dppp, 17.0 mg), DMF (1.4 ml), and a mix ture of HCO2H (0.2 ml) and TEA (0.8 ml). The mixture is shaken at 85 for 2 h, and then the resin is filtered and washed several times with diethyl ether. The combined organic phase is washed with aqueous sodium carbonate solution then water and evaporated to dryness. The residue obtained is dissolved in diethyl ether and eluted through a short column of alumina to remove any remaining inorganic residues. The crude products are purified by preparative TLC (or other suitable methods) to give the desired products 20 and 22 in >95% purity.
[10] Unnatural Diamino Acid Derivatives as Scaffolds for Creating Diversity and as Linkers for Simplifying Screening in Chemical Libraries By Robert Pascal, Re´gine Sola, and Patrick Jouin Introduction
The introduction of conformational restrictions into flexible active molecules is a well-known strategy for trying to increase their potency and/or selectivity toward their biological targets.1 Several methods have been used for constraining flexible molecules. Cyclic derivatives of linear peptides or peptidomimetics can thus be prepared by reactions involving side-chain
METHODS IN ENZYMOLOGY, VOL. 369
Copyright 2003, Elsevier Inc. All rights reserved. 0076-6879/03 $35.00