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2-Bromoethyl glycosides for synthesis of glycoconjugates on solid support
Tetrahedron 61 (2005) 2421–2429
2-Bromoethyl glycosides for synthesis of glycoconjugates on solid support Ulf Ellervik,* Ma˚rten Jacobsson and Jo¨rgen Ohlsson Organic and Bioorganic Chemistry, Center for Chemistry and Chemical Engineering, Lund University, PO Box 124, SE-221 00 Lund, Sweden Received 24 September 2004; revised 26 November 2004; accepted 7 January 2005 Available online 28 January 2005
Abstract—2-Bromoethyl glycosides can easily and in high yields be transformed into sulfones by treatment with a suitable thiol followed by oxidation with mCPBA. The observation that the so formed sulfones were cleaved by treatment with NaOMe/MeOH was used to design a new safety catch linker for synthesis of glycoconjugates on solid support. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction Since the introduction of solid phase peptide synthesis by Merrifield in 1963,1 polymer-supported synthesis has become the method of choice for synthesis of peptides and oligonucleotides. The first investigations into solidphase oligosaccharide synthesis were conducted in the early 1970s and, whilst some groundbreaking work was done, progress was halted by the lack of powerful glycosylating agents, diverse protecting groups and on-bead analytical techniques.2 As advances were made in the solution phase synthesis of complex carbohydrates, new protecting groups, and selective glycosylating agents became available during the 1990s thus fuelling the interest in solid phase oligosaccharide synthesis. Also, on-bead analytical techniques such as high-resolution magic angle spinning NMR and FT-IR became widely available.
more advanced and automated synthesis is becoming a realistic goal2,7 further linker systems are needed. Especially attractive are safety-catch linkers, that is, linkers that are stable to most conditions until activated by, for example, oxidation or reduction.8,9 2-Bromoethyl was introduced as an anomeric protecting group that can be transformed into a spacer for glycoconjugates.10 The 2-bromoethyl group is easily introduced by normal glycosylation of 2-bromoethanol and is stable towards standard reagents used in carbohydrate synthesis.11 It shows unusual stability towards anomerization.12 The formed glycosides can then be alkylated using different thiols by treatment with, for example, Cs2CO3 in DMF.13
Presently, complex oligosaccharides are synthesized on solid phase using a wide variety of linker systems, glycosyl donors and acceptors, promoter systems and strategies.2 A number of different linker systems suitable for solid phase oligosaccharide synthesis are available with differences in stability, cleavage reagents and the group introduced upon cleavage. Notable examples are silyl ether linkers,3,4 acidand base-labile linkers such as amino-functionalized Rink resin5 and a benzylidene acetal-type linker to Wang aldehyde resin.6 Also, thioglycoside linkers have been used, as well as linkers cleaved by oxidation, hydrogenation, photocleavage and even olefin metathesis.2 Even though solid phase oligosaccharide synthesis is becoming Keywords: Solid support; Glycoconjugates; Safety catch linker. * Corresponding author. Tel.: C46 46 2228220; fax: C46 46 2228209; e-mail: [email protected] 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2005.01.015
Scheme 1. Deprotection of 1 under basic conditions (0.05 M NaOMe– MeOH) gave the desired product 2 as well as glucose (3).
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These thioethers are easily oxidized to sulfones using m-chloroperbenzoic acid (mCPBA). An attempt to deacetylate the sulfone 1 (Scheme 1) under basic conditions (0.05 M NaOMe–MeOH) gave the desired product 2 as well as a small amount of free glucose (3), that is, the aglycon was cleaved off. We realized that 2-bromoethyl glycosides can be coupled to solid supported thiols and thereby be used for solid phase glycoside synthesis.
2. Cleavage of bromoethyl glycosides in solution Before performing the reaction on solid support, an investigation of the system was performed in solution. As a model for the linker, compound 1 was chosen. The model compound was synthesized from 2-bromoethyl tetra-Oacetyl-b-D-glucopyranoside 414,15 in two steps (Scheme 2). Treatment of 4 with thiophenol and Cs2CO3 gave 5 in 95% yield. The sulfide 5 was then oxidized to the corresponding sulfone 1 in 98% yield using mCPBA.
cleavage was likewise obtained with lower concentrations of NaOMe, although requiring longer reaction times. As a lower limit a 0.05 M solution was used, resulting in a reaction time of 56 h. However, investigation of the cleavage products showed that compound 616 (Fig. 1) was formed instead of the expected alkene. Performing the reaction in NaOEt–EtOH (1.0 M) similarly gave D-glucose in quantitative yield, along with compound 716 (Fig. 1). An NMR-study was performed in order to elucidate the reaction mechanism. The model compound 1 was thus treated with a 0.7 M NaOCD3–CD3OD-solution and NMR-spectra were recorded over a period of 300 min. After 1 min complete deacetylation was observed along with about 50% exchange of the protons in a-position to the sulfone. The exchange of a-protons was complete within 5 min. The ratio of the integral of the anomeric NMRsignals for the formed glucose compared to the total integral of the anomeric protons of glucose (3) and deacetylated bromoethyl glucoside (2) was measured and is shown in Figure 2. About 10% glucose was formed within 5 min, whereas complete conversion required 300 min. During the experiment no signals corresponding to alkene protons were observed.
Scheme 2. (a) Thiophenol, Cs2CO3, DMF, 2 h. (b) mCPBA, EtOAc, 30 min. (c) NaOMe–MeOH, 4–56 h.
It was anticipated that basic treatment of compound 1 would result in abstraction of an a-proton (a relative to the sulfone), leading to an elimination reaction with concomitant expulsion of the glucoside. Thus, treatment of 1 with NaOMe–MeOH (1.0 M) smoothly furnished an a/b-mixture of D-glucose in quantitative yield within 4 h. Complete
Figure 2. Formation of glucose (3) by treating the sulfone 1 with 0.7 M NaOCD3–CD3OD. The graph shows the percentage of the integral for the NMR-signals of the anomeric protons of glucose (3) compared to the total integral for the anomeric protons of glucose (3) and deacetylated bromoethyl glycoside (2).
Figure 1. Cleavage products formed during alkoxide treatment of 1.
The fact that the deuterium–hydrogen exchange of the protons in a-position to the sulfone is much faster than formation of glucose indicates that the reaction follows a reversible carbanion elimination (E1cB)R, followed by a fast addition of the nucleophile (methoxide or ethoxide). This is supported by observations in similar systems, PhSO2CH2CH2Z, with leaving groups (Z) other than carbohydrates.16
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However, to disprove an SN2-mechanism, we performed a second experiment where a glucoside carrying a propyl linker (compound 10) was synthesized in two steps from the known 2-bromopropyl 2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside15 (8, Scheme 3), in a manner similar to that described for compound 1. Treatment of 10 with NaOMe– MeOH (1.0 M) gave an almost quantitative yield of compound 11; hence no cleavage product was observed.
Scheme 4. (a) NaOMe-MeOH, 45 min. (b) DMF, NaH, BnBr, 1 h. (c) mCPBA, EtOAc, 0.5 h. (d) NaOMe–MeOH, 22 h.
with anthraldehyde dimethyl acetal, and then acetylated using standard conditions to give compound 16 (Scheme 5).
Scheme 3. (a) Thiophenol, Cs2CO3, DMF, 1 h. (b) mCPBA, EtOAc, 0.5 h. (c) NaOMe–MeOH, CH2Cl2, 24 h.
As a final experiment compound 15 was synthesized from 4 in four steps, in order to verify the generality of the cleavage reaction. Compound 4 was deacetylated and subsequently benzylated under basic conditions to give 13. The sulfide 13 was then oxidized to the sulfone 14. Treatment of 14 with NaOMe–MeOH (1.0 M) gave compound 15 in 74% yield together with compound 6 (Scheme 4).
3. Use of 2-bromoethyl glycosides on solid support To evaluate the bromoethyl group as a linker for solid phase carbohydrate synthesis we performed a series of experiments in order to optimize the coupling of the glycosides to thiol resin. Cesium carbonate was used as base in solution phase but the poor solubility in DMF makes it less suitable for solid phase reactions. Instead the non-nucleophilic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was investigated and turned out to work well. To follow the reactions we used the newly developed anthraldehyde protective group which shows a strong UVabsorbance at around 365 nm and also a strong fluorescence.17 Compound 4 was deprotected using NaOMe– MeOH, transformed into the fluorescent acetal by treatment
Scheme 5. (i) NaOMe–MeOH–CH2Cl2, 20 min. (ii) Anthraldehyde dimethyl acetal, MeCN, pTSA, overnight. (iii) Pyridine, Ac2O, overnight.
We decided to use a relatively inexpensive scavenger resin, 3-(3-mercaptophenyl)propanamidomethylpolystyrene resin, as solid support. The resin was first treated with tributyl phosphine (0.7 M in THF–H2O 95:5) to reduce any disulfides and then swelled in DMF. DBU (1 equiv to resin substitution) was then added followed by different equivalents of compound 16. Samples were taken from the solution at various time intervals, diluted with MeCN and the absorbance measured at 364 nm. Treatment with 0.5 equiv (to resin substitution) of 16 resulted in an almost complete loss of absorbance in about 10 min, indicating that all material was coupled to the resin. Addition of 1.0 equiv of 16 resulted in about 70% reduction of the original
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absorbance, which gives an indication of the practically useful loading of the resin for this coupling (Fig. 3).
To verify that compound 16 had been coupled to the resin the two samples, treated with 0.5 equiv of 16 and 17, respectively, were washed with several volumes of DMF and CH2Cl2 and then swelled in a mixture of MeOH and CH2Cl2 and illuminated with long wave UV-radiation (365 nm). As expected, only the resin treated with 16 showed a strong fluorescence (Fig. 4). The stability of the thioethyl linker was investigated by subjecting compound 5 or 12 to standard conditions used in synthetic carbohydrate chemistry, such as basic, acidic or reductive protective group manipulations as well as typical glycosylation conditions (NIS–TfOH or BF3$OEt2). The results, summarized in Table 1, show that the thioethyl linker is stable to typical conditions. However, treatment with BF3$OEt2 caused anomerization (16%). Table 1. Stability of the thioethyl linker. Compound 5 or 12 was subjected to the conditions below. The samples were purified and analyzed using NMR
Figure 3. Coupling of 16 to thiophenyl resin using 0.5 equiv (C) and 1.0 equiv (&). As a control the resin was treated with 0.5 equiv of the unreactive 17 (,). Samples were taken from the solution at various time intervals and diluted with acetonitrile and the absorbance measured at 364 nm.
As a control, compound 1717 with an unreactive trimethylsilylethyl group was subjected to the same conditions and showed, as expected, only a minute drop in absorbance due to dilution but no coupling.
Figure 4. Visual comparison between resin treated with the unreactive compound 17 (left) and the reactive 16 (right) illuminated with long wave UV-radiation (365 nm). The resin treated with 16 shows a strong fluorescence.
Reaction
Conditions
Product
Acylation Deacylation
Pyridine, Ac2O, 16 h NaOMe-MeOH (0.05 M), 1 h HCl-MeOH, 16 h PhCH(OMe)2, pTsOH, CH3CN, 3 h AcOH (80%), 90 8C, 3 h BnBr, NaH, DMF, 16 h Pd-C (10%), AcOH, H2 (500 psi), 2 h NIS, TfOH, MS-AW 300, CH2Cl2, 0 8C, 30 min BF3OEt2, CH2Cl2, 1 h
5 12 12 5 5 13 5 5
Acetal formation Acetal cleavage Benzylation Debenzylation Glycosylation
5 (a/b 1:5)
To estimate the possible overall yield using the bromoethyl linker we coupled compound 4 (0.5 equiv to resin substitution) to the resin using DBU as base. Methyl iodide was then added to cap the residual thiol groups and the resin was washed and the sulfide oxidized to sulfone using mCPBA. The resin was again washed and then cleaved by treatment with 0.2 M NaOMe–MeOH–CH2Cl2 overnight to give glucose in an excellent 55% yield after purification. With a working protocol for using bromoethyl glycosides on solid support we decided to illustrate the usefulness of this novel linker with a more advanced compound. Compound 22 was synthesized in four steps from the known compound 1818 (Scheme 6). The benzylidene acetal protecting group of compound 18 was cleaved off using 80% AcOH at 90 8C. In a one-pot procedure the primary hydroxyl group of the formed diol 19 was selectively benzoylated at K78 8C and the secondary hydroxyl group was then transformed into a trifluoromethanesulfonate and consecutively inverted using NaN3 in DMF. The total yield for this conversion was 86%. All attempts to glycosylate compound 20 using 2-bromoethanol resulted either in de-O-acetylation of position 2 (using NIS and TMSOTf) or formation of the chloro sugar (using ICl and AgOTf).19 Instead compound 20 was de-Oacetylated and then O-benzoylated. The benzoylated compound was easily glycosylated using NIS and TMSOTf to give 22 in 89% yield.20 Compound 22 was coupled to the thiol resin by the standard procedure. The benzoyl protecting groups were removed by
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propose that the reaction follows a reversible carbanion elimination (E1cB)R, followed by a fast addition of the nucleophile. We have also shown that 2-bromoethyl glycosides can be coupled to solid supported thiols and thereby be used for synthesis of glycoconjugates on solid support. 5. Experimental 5.1. General The structures of all new compounds were determined by careful NMR analysis, including COSY, NOESY, HETCOR, and long-range HETCOR. NMR-spectra were recorded with 300 and 400 MHz instruments. Chemical shifts are given in ppm downfield from the signal for Me4Si, with reference to internal CHCl3 or MeOH. All new compounds were determined to be O95% pure by 1H NMR spectroscopy. Molecular sieves were activated by heating under vacuum. CH2Cl2 and MeCN were dried by distillation from CaH2, DMF was distilled. Column chromatography was performed on SiO2 (Matrex LC-gel; 60A, 35-70 MY, Grace), and TLC on Merck SiO2 60 F256. 3-(3-Mercaptophenyl)propanamidomethylpolystyrene was purchased from Argonaut Technologies. Compounds 4,15 8,15 17,17 and 1818 were prepared as described in the literature. Compounds 3 and 15 are commercially available. Scheme 6. (a) AcOH (80%, aq), 90 8C. (b) CH2Cl2, pyridine, BzCl, Tf2O, then NaN3, DMF. (c) NaOMe-MeOH, 2 h, then BzCl, pyridine, DMAP overnight. (d) CH2Cl2, 2-bromoethanol, NIS, TMSOTf, MS AW-300.
treatment with NaOMe–MeOH (0.05 M) for 1 h. The azide was then reduced using DTT–DBU–DMF21 and condensed with 2-naphthoyl chloride. The sulfide was then oxidized to the sulfone and treated with NaOMe–MeOH–CH2Cl2 to give compound 23 as an anomeric mixture in 27% yield after purification, a recovery comparable to other methods for synthesis of glycoconjugates on solid support (Scheme 7).2
Scheme 7. (a) DMF, DBU, thiol resin. (b) DMF, DBU, MeI. (c) NaOMeMeOH-CH2Cl2 (0.05 M). (d) DTT, DMF, DBU. (e) DMF, pyridine, 2naphthoyl chloride. (f) mCPBA, CH2Cl2. (g) NaOMe-MeOH, CH2Cl2 (0.2 M).
5.1.1. 2-(Phenylsulfonyl)ethyl 2,3,4,6-tetra-O-acetyl-b-Dglucopyranoside (1). To a solution of 5 (490 mg, 0.93 mmol) in EtOAc (32 mL) was added mCPBA (60%, 590 mg) and the resulting mixture was stirred at room temperature. After 30 min, the mixture was filtered through a short column of alumina, concentrated, and chromatographed (SiO2, heptane–EtOAc, 1:2) to give 1 as a white 1 solid (512 mg, 98%). [a]20 D ZK9.4 (c 1.2, CHCl3). H NMR (CDCl3): d 7.55–8.00 (m, 5H, Ar), 5.17 (t, 1H, JZ9.5 Hz, H-3), 5.01 (t, 1H, JZ9.8 Hz, H-4), 4.83 (dd, 1H, JZ9.6, 8.0 Hz, H-2), 4.51 (d, 1H, JZ8.0 Hz, H-1), 4.23 (dd, 1H, JZ12.3, 5.0 Hz, H-6), 4.09–4.16 (m, 2H, H-6, CH2), 3.98– 4.06 (m, 1H, CH2), 3.68 (ddd, 1H, JZ10.0, 5.0, 2.4 Hz, H-5), 3.35–3.50 (m, 2H, CH2), 2.10, 2.04, 2.03, 1.99 (s, 3H each, OAc). 13C NMR (CDCl3): d 170.79, 170.37, 169.60, 169.58, 139.71, 134.15, 129.52, 128.24, 72.79, 72.12, 71.05, 68.39, 63.28, 62.01, 56.38, 20.94, 20.89, 20.78. HRMS calcd for C22H28O12S (MCNa) 539.1199, found 539.1206. 5.1.2. D -Glucose (3). To a solution of 1 (15 mg, 0.029 mmol) in MeOH (1.28 mL) was added NaOMe (0.067 mL, 1 M in MeOH) and the resulting mixture was stirred at room temperature. After 56 h AcOH (5% in MeOH) was added until neutral solution (moist pH-paper). The mixture was concentrated and chromatographed (SiO2, CH2Cl2–MeOH–H2O, 66:33:4/10:20:1) to give 3 as a white solid (4.9 mg, 94%) and 6 as a clear oil (5.8 mg, 99%).
4. Conclusion We have shown that 2-bromoethyl glycosides can easily be transformed into sulfones which can be cleaved by treatment with NaOMe–MeOH to yield hemiacetals. We
5.1.3. 2-(Phenylthio)ethyl 2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside (5). To a solution of 2-bromoethyl-2,3,4,6tetra-O-acetyl-b-D-glucopyranoside (4) (92 mg, 0.20 mmol) in DMF (4 mL) were added thiophenol (0.038 mL,
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0.36 mmol) and Cs2CO3 (120 mg, 0.36 mmol) and the resulting mixture was stirred at room temperature. After 2 h the reaction mixture was diluted with CH2Cl2, washed with water, dried (MgSO4), concentrated, co-concentrated with toluene and chromatographed (SiO2, toluene–acetone, 10:1) to give 5 as a white solid (93 mg, 95%). [a]20 D ZK9.1 (c 1.1, CHCl3). 1H NMR (CDCl3): d 7.10–7.30 (m, 5H, Ar), 5.13 (t, 1H, JZ9.5 Hz, H-3), 5.01 (t, 1H, JZ9.8 Hz, H-4), 4.93 (dd, 1H, JZ9.6, 8.0 Hz, H-2), 4.45 (d, 1H, JZ8.0 Hz, H-1), 4.17 (dd, 1H, JZ12.3, 4.8 Hz, H-6), 4.05 (dd, 1H, JZ12.3, 2.4 Hz, H-6), 3.94 (ddd, 1H, JZ10.4, 7.7, 5.5 Hz, CH2), 3.57–3.65 (m, 2H, H-5, CH2), 2.97–3.07 (m, 2H, CH2), 2.00, 1.98, 1.95, 1.94 (s, 3H each, OAc). 13C NMR (CDCl3): d 170.86, 170.49, 169.60, 135.67, 129.84, 129.25, 126.65, 101.22, 72.93, 72.06, 71.35, 68.87, 68.54, 62.09, 33.45, 20.94, 20.82, 20.80. HRMS calcd for C22H28O10S (MCNa) 507.1301, found 507.1300. 5.1.4. Deprotection study of 2-(phenylsulfonyl)ethyl 2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside (1). Compound 1 (10 mg, 0.019 mmol) was dissolved in CD3OD (1.0 mL) and the mixture was added to an NMR-tube and an NMR-spectrum was recorded. CD3ONa (0.075 mL, 1 M in CD3OD) was added and NMR-spectra were recorded after 1, 15, 30, 120, and 300 min, respectively. After 1 min, 1 was completely deacylated and the shift of the anomeric proton was 4.23 ppm. The amount cleaved product was determined as the percentage of the integral for the NMR-signals of the anomeric protons of 3 (5.18 and 4.62 ppm) compared to the total integral for the anomeric protons of glucose and deacetylated bromoethyl glycoside. 5.1.5. 3-(Phenylthio)propyl 2,3,4,6-tetra-O-acetyl-b-Dglucopyranoside (9). Thiophenol (0.055 mL, 0.38 mmol) and Cs2CO3 (124 mg, 0.38 mmol) were added to a solution of 8 (120 mg, 0.26 mmol) in DMF (5 mL) and the resulting mixture was stirred at room temperature. After 1 h the reaction mixture was diluted with CH2Cl2, washed with water, dried (MgSO4), concentrated, co-concentrated with toluene and chromatographed (SiO2, heptane–EtOAc, 3:1) to give 9 as a pale yellow oil (135 mg, 96%). [a]20 D ZC3.4 (c 1.0, CHCl3). 1H NMR (CDCl3): d 7.08–7.26 (m, 5H, Ar), 5.13 (t, 1H, JZ9.5 Hz, H-3), 5.02 (t, 1H, JZ9.8 Hz, H-4), 4.93 (dd, 1H, JZ9.6, 8.0 Hz, H-2), 4.42 (d, 1H, JZ8.0 Hz, H-1), 4.19 (dd, 1H, JZ12.3, 4.7 Hz, H-6), 4.06 (dd, 1H, JZ 12.3, 2.4 Hz, H-6), 3.88 (dt, 1H, JZ9.7, 5.5 Hz, CH2), 3.55– 3.65 (m, 2H, H-5, CH2), 2.85–3.95 (m, 2H, CH2), 2.01, 1.95 (s, 3H each, OAc), 1.93 (s, 6H, OAc), 1.75–1.90 (m, 2H, CH2). 13C NMR (CDCl3): d 171.11, 170.71, 169.83, 169.75, 136.57, 129.45, 129.37, 126.38, 101.34, 73.22, 72.22, 71.70, 68.82, 68.35, 30.17, 29.26, 21.18, 21.05, 21.03. HRMS calcd for C23H30O10S (MCNa) 521.1457, found 521.1455. 5.1.6. 3-(Phenylsulfonyl)propyl 2,3,4,6-tetra-O-acetyl-b(10). To a solution of 9 (45 mg, 0.083 mmol) in EtOAc (2.5 mL) was added mCPBA (53 mg, 0.18 mmol, 60%) and the resulting mixture was stirred at room temperature. After 30 min, the mixture was filtered through a short column of alumina, concentrated, and chromatographed (SiO2, heptane–EtOAc, 1:2) to give 10 as a clear oil (46 mg, 97%). [a]20 D ZK12.4 (c 1.3, CHCl3). 1H NMR (CDCl3): d 7.50–8.00 (m, 5H, Ar), 5.19 (t, 1H, JZ9.5 Hz, H-3), 5.06 (t, 1H, JZ9.8 Hz, H-4), 4.95 (dd,
D-glucopyranoside
1H, JZ9.6, 8.0 Hz, H-2), 4.47 (d, 1H, JZ8.0 Hz, H-1), 4.24 (dd, 1H, JZ12.3, 4.8 Hz, H-6), 4.12 (dd, 1H, JZ12.3, 2.3 Hz, H-6), 3.88–3.95 (m, 1H, CH2), 3.65–3.70 (m, 2H, H-5, CH2), 3.10–3.25 (m, 2H, CH2), 2.09, 2.03, 2.02, 2.00 (s, 3H each, OAc), 2.00–2.05 (m, 2H, CH2). 13C NMR (CDCl3): d 170.86, 170.43, 169.61, 169.54, 133.99, 129.57, 128.18, 100.90, 72.89, 72.08, 71.36, 68.48, 67.70, 62.03, 53.06, 23.34, 20.95, 20.87, 20.80. HRMS calcd for C23H30O12S (MCNa) 553.1356, found 553.1346. 5.1.7. 3-Phenylsulfonyl-propyl b-D-glucopyranoside (11). Compound 10 (23 mg, 0.040 mmol) was dissolved in NaOMe–MeOH (1 M, 1 mL) and the resulting mixture was stirred at room temperature. After 24 h, AcOH (5% in MeOH) was added until neutral solution (moist pH-paper). The mixture was concentrated and chromatographed (SiO2, CH2Cl2–MeOH–H2O, 66:33:4) to give 11 as a clear oil 1 (16 mg, 99%). [a]20 D ZK14.2 (c 0.5, MeOH). H NMR (CD3OD): d 7.60–7.95 (m, 5H, Ar), 4.19 (d, 1H, JZ7.8 Hz, H-1), 3.94 (dt, 1H, JZ10.1, 5.9 Hz, CH2), 3.84 (dd, 1H, JZ 12.0, 2.0 Hz, H-6), 3.60–3.67 (m, 2H, H-6, CH2), 3.20–3.45 (m, 5H, H-3, H-4, H-5, CH2), 3.14 (dd, 1H, JZ9.0, 7.9 Hz, H-2), 1.92–2.00 (m, 2H, CH2).13C NMR (CD3OD): d 140.65, 135.17, 130.69, 129.27, 101.45, 78.11, 75.14, 71.71, 68.48, 62.87, 54.10, 24.69. HRMS calcd for C15H22O8S (MCNa) 385.0933, found 385.0928. 5.1.8. 2-(Phenylthio)ethyl b-D-glucopyranoside (12). Compound 4 (120 mg, 0.25 mmol) was dissolved in NaOMe–MeOH (0.05 M, 5 mL) and stirred for 45 min at room temperature and then neutralized using Amberlite IR-120 HC. The mixture was concentrated and chromatographed (SiO2, CH2Cl2–MeOH 5:1) to give 12 as a white 1 solid (78 mg, 89%). [a]20 D ZK24.8 (c 0.9, MeOH). H NMR (CD3OD): d 7.15–7.41 (m, 5H, Ar), 4.28 (d, 1H, JZ7.8 Hz, H-1), 3.97 (ddd, 1H, JZ10.4, 8.2, 6.4 Hz, CH2), 3.85 (dd, 1H, JZ10.0, 2.1 Hz, H-6), 3.75 (ddd, 1H, JZ10.4, 8.2, 6.5 Hz, CH2), 3.66 (dd, 1H, JZ11.9, 5.3 Hz, H-6), 3.23– 3.37 (m, 3H, H-3, H-4, H-5), 3.15–3.21 (m, 3H, H-2, CH2). 13 C NMR (CD3OD): d 137.42, 130.46, 130.21, 127.34, 104.69, 78.13, 75.18, 71.67, 69.73, 62.80, 33.91. HRMS calcd for C14H20O6S (MCNa) 339.0878, found 339.0864. 5.1.9. 2-(Phenylthio)ethyl 2,3,4,6-tetra-O-benzyl-b-Dglucopyranoside (13). To a solution of 12 (69 mg, 0.22 mmol) in DMF (distilled, 3 mL) was added NaH (60%, 90 mg) and BnBr (0.15 mL). The reaction mixture was stirred for 1 h and then quenched by addition of MeOH (1 mL). The mixture was then diluted with CH2Cl2, washed with water, dried (Na2SO4), concentrated and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 13 as a white waxy solid (131 mg, 89%). [a]20 D ZC15.9 (c 1.7, CHCl3). 1 H NMR (CDCl3): d 7.10–7.35 (m, 25H, Ar), 4.95, 4.68 (ABq, 1H each, JZ11.0 Hz, OBn), 4.90, 4.76 (ABq, 1H each, JZ11.0 Hz, OBn), 4.79, 4.50 (ABq, 1H each, JZ 11.1 Hz, OBn), 4.57, 4.52 (ABq, 1H each, JZ12.1 Hz, OBn), 4.35 (d, 1H, JZ7.8 Hz, H-1), 4.07 (ddd, 1H, JZ10.2, 7.7, 5.7 Hz, CH2), 3.55–3.75 (m, 5H, H-3, H-5, H-6, CH2), 3.38–3.45 (m, 2H, H-2, H-4), 3.10–3.21 (m, 2H, CH2). 13C NMR (CDCl3): d 138.78, 138.68, 138.30, 138.27, 135.27, 129.59, 129.19, 128.58, 128.42, 128.18, 128.08, 127.99, 127.87, 127.84, 126.40, 104.01, 84.82, 82.39, 77.94, 75.91, 75.22 75.08, 75.06, 73.70, 72.32, 69.03, 68.71, 33.49.
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HRMS calcd for C42H44O6S (MCNa) 699.2756, found 699.2723.
30.10, 21.02, 20.97. HRMS calcd for C27H27BrO8 (MCNa) 581.0787, found 581.0781.
5.1.10. 2-(Phenylsulfonyl)ethyl 2,3,4,6-tetra-O-benzyl-b(14). To a solution of 13 (73 mg, 0.11 mmol) in EtOAc (4 mL) was added mCPBA (60%, 70 mg) and the resulting mixture was stirred at room temperature. After 30 min, the mixture was filtered through a short column of alumina to give 14 as a white solid 1 (75 mg, 99%). [a]20 D ZC14.5 (c 1.6, CHCl3). H NMR (CDCl3): d 7.82 (dd, 2H, JZ8.6, 1.3 Hz, Ar), 7.53 (dt, 1H, JZ7.4 Hz, Ar), 7.41 (t, 1H, JZ7.7 Hz, Ar), 7.00–7.30 (m, 20H, Ar), 4.80, 4.67 (ABq, 1H each, JZ10.9 Hz, OBn), 4.73, 4.45 (ABq, 1H each, JZ11.2 Hz, OBn), 4.56, 4.47 (ABq, 1H each, JZ11.3 Hz, OBn), 4.52, 4.44 (ABq, 1H each, JZ12.1 Hz, OBn), 4.21 (d, 1H, JZ7.8 Hz, H-1), 4.12 (dt, 1H, JZ10.9, 6.6 Hz, CH2), 3.85 (dt, 1H, JZ10.9, 6.6 Hz, CH2), 3.61, 3.56 (dABq, 1H each, JZ10.7, 4.4, 2.1 Hz, H-6), 3.28–3.52 (m, 5H, H-3, H-4, H-5, CH2), 3.10 (t, 1H, JZ8.0 Hz, H-2). 13C NMR (CDCl3): d 138.62, 138.56, 138.20, 133.92, 129.36, 128.65, 128.62, 128.59, 128.57, 128.47, 128.15, 128.11, 128.09, 128.00, 127.94, 127.88, 103.84, 84.62, 82.31, 77.74, 76.91, 75.93, 75.22, 75.00, 73.72, 72.31, 68.87, 63.26, 56.69. HRMS calcd for C42H44O8S (MCNa) 731.2655, found 731.2606.
5.1.13. Coupling of 2-bromoethyl 2,3-di-O-acetyl-4,6-O(9-anthracenyl)methylene-b-D-glucopyranoside (4) or 2-(trimethylsilyl)ethyl 2,3-di-O-acetyl-4,6-O-(9-anthracenyl)methylene-b- D -glucopyranoside (17) to thiol scavenger resin. 3-(3-Mercaptophenyl)propanamidomethylpolystyrene resin (1.54 mmol/g, 50 mg) was swelled in DMF (0.480 mL) for 5 min and then DBU (0.020 mL) was added and the resin was shaken for another 5 min. Compound 16 (20 mg or 40 mg) or 17 (20 mg) was dissolved in DMF (0.700 mL) and added to the resin. Samples (0.010 mL) were taken from the solution at different times and added to MeCN (0.990 mL). The samples (0.500 mL) were diluted with MeCN (2.000 mL) and the UV-absorbance measured at 364 nm.
D-glucopyranoside
5.1.11. 2,3,4,6-Tetra-O-benzyl-D-glucose (15). To a solution of 14 (58 mg, 0.082 mmol) in CH2Cl2 (2 mL) was added NaOMe–MeOH (1 M, 2 mL) and the mixture was stirred for 22 h and then neutralized using Amberlite IR-120 HC. The mixture was concentrated and purified on Sephadex LH-20 (CH2Cl2–MeOH 1:1) to give 15 as a white solid (33 mg, 74%) and 6 as a clear oil (7 mg, 43%). 5.1.12. 2-Bromoethyl 2,3-di-O-acetyl-4,6-O-(9-anthracenyl)methylene-b-D-glucopyranoside (16). To a solution of 4 (600 mg, 1.32 mmol) in MeOH (10 mL) and CH2Cl2 (5 mL) was added NaOMe–MeOH (1 M, 0.45 mL). The mixture was stirred for 20 min and then neutralized by addition of AcOH (1 mL). The mixture was concentrated and filtered through a column of SiO2 (CH2Cl2–MeOH 5:1). The residue was then dissolved in MeCN (10 mL) and anthraldehyde dimethyl acetal (323 mg, 1.65 mmol) and pTSA (5 mg) were added and the mixture was stirred at room temperature overnight protected from light and then neutralized by addition of Et3N (1 mL) and co-concentrated with toluene 3 times. The residue was chromatographed (SiO2, toluene/6:1 toluene–acetone) and the crude product was then dissolved in pyridine (5 mL) and acetic anhydride (4 mL) was added. The mixture was stirred overnight and then co-concentrated with toluene 3 times and the residue was chromatographed (SiO2, heptane–EtOAc 1:1) to give 16 as a white solid (354 mg, 48%). [a]20 DZ K70.7 (c 1.2, CHCl3). 1H NMR (CDCl3): d 8.60 (d, 2H, JZ 9.2 Hz, Ar), 8.51 (s, 1H, Ar), 8.01 (d, 2H, JZ8.3 Hz, Ar), 7.45–7.55 (m, 4H, Ar), 6.90 (s, 1H, ArCH), 5.45 (t, 1H, JZ 9.3 Hz, H-3), 5.10 (dd, 1H, JZ9.1, 7.9 Hz, H-2), 4.83 (d, 1H, JZ7.8 Hz, H-1), 4.55–4.60 (m, 1H, H-6), 4.23 (p, 1H, JZ5.6 Hz, CH2), 3.85–4.15 (m, 4H, H-4, H-5, H-6, CH2), 3.50 (t, 2H, JZ5.6 Hz, CH2), 2.10, 1.90 (s, 3H each, OAc*2). 13C NMR (CDCl3): d 170.31, 169.96, 131.65, 130.25, 129.89, 129.24, 126.51, 126.46, 125.13, 124.97, 101.84, 100.99, 79.52, 72.66, 71.90, 70.19, 69.80, 66.97,
5.1.14. Stability of 2-(phenylthio)ethyl 2,3,4,6-tetra-Oacetyl-b-D-glucopyranoside (5). Compound 5 (24 mg, 0.05 mmol) was subjected to the following conditions: (a) 5 was dissolved in pyridine (1.0 mL) and acetic anhydride (0.8 mL) was added. The mixture was stirred for 22 h and then co-concentrated with toluene and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 5 (24 mg, quant). (b) 5 was dissolved in NaOMe–MeOH (0.05 M, 1 mL) and stirred for 1 h. The mixture was neutralized by addition of Amberlite IR-120 HC and then concentrated to give 12 (16 mg, quant). (c) 5 was dissolved in MeOH saturated with HCl and stirred for 17 h. The mixture was concentrated and chromatographed (SiO2, CH2Cl2–MeOH, 5:1) to give 12 (15 mg, 94%). (d) 5 was dissolved in MeCN (1 mL, filtered through Al2O3) and a,a-dimethoxytoluene (0.10 mL) and pTSA (cat) were added. The mixture was stirred for 17 h, Et3N (0.050 mL) was added and the mixture was coconcentrated with toluene and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 5 (24 mg, quant). (e) 5 was dissolved in AcOH (80%, 3 mL) and stirred at 90 8C for 3 h. The mixture was co-concentrated with toluene to give 5 (23 mg, 95%). (f) 5 was dissolved in AcOH (3 mL) and Pd– C (10%, 25 mg) was added. The mixture was pressurized with H2 (500 psi) for 2 h and then filtered through Celite and concentrated to give 5 (20 mg, 84%). (g) 5 was dissolved in CH2Cl2 (2 mL) and BF3$OEt2 (0.019 mL, 3 equiv) was added. The mixture was stirred for 1 h, then quenched by addition of Et3N (0.040 mL). The mixture was then coconcentrated with toluene and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 5 (24 mg, quant, a–b 1:5). (h) 5 was dissolved in CH2Cl2 (2 mL) and MS AW-300 (50 mg) was added and the mixture was cooled down to 0 8C, under Ar. NIS (16 mg) was added, the mixture was stirred for 10 min and then TfOH (cat) was added. The mixture was stirred for another 10 min at 0 8C and then quenched by addition of Et3N (0.10 mL), concentrated and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 5 (24 mg, quant). 5.1.15. Coupling and deprotection of 2-bromoethyl 2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside (4) to thiol scavenger resin. 3-(3-Mercaptophenyl)propanamidomethylpolystyrene resin (1.54 mmol/g, 100 mg) was swelled in DMF (1.0 mL) for 5 min and then DBU (0.040 mL) was added and the resin was shaken for another 5 min. Compound 4 (35 mg, 0.5 equiv to resin substitution)
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was dissolved in DMF (1.0 mL) and added to the resin. After 1 h MeI (0.010 mL) was added and the resin shaken for another 10 min and then washed with DMF, CH2Cl2, MeOH and diethyl ether and dried under vacuum. To the dried resin was added CH2Cl2 (2 mL) and mCPBA (70%, 70 mg) and the mixture was shaken for 30 min and then washed with CH2Cl2, MeOH and diethyl ether and dried under vacuum. To the dried resin were added CH2Cl2 (1 mL), MeOH (1 mL) and NaOMe–MeOH (1 M, 0.40 mL) and the mixture was shaken overnight and then neutralized with Amberlite IR-120 HC, filtered and washed with MeOH. The residue was purified on Sephadex (LH-20, CH2Cl2–MeOH, 1:1) to give 3 as a white solid (7.6 mg, a–b 1:1, 55%). 5.1.16. 4-Methylphenyl 2,3-di-O-acetyl-1-thio-b-D-glucopyranoside (19). Compound 18 (2.63 g, 5.74 mmol) was dissolved in AcOH (80%, aq 70 mL) and stirred at 90 8C for 2.5 h and then co-concentrated with toluene. The residue was chromatographed (SiO2, toluene–acetone, 2:1) to give 19 as a white solid (1.98 g, 93%). [a]20 D ZK33.5 (c 1.2, CHCl3). 1H NMR (CDCl3): d 7.40 (bd, 2H, JZ8.1 Hz, Ar), 7.17 (bd, 2H, JZ7.9 Hz, Ar), 5.06 (t, 1H, JZ9.3 Hz, H-3), 4.95 (dd, 1H, JZ9.9, 9.3 Hz, H-2), 4.72 (d, 1H, JZ10.0 Hz, H-1), 3.97, 3.85 (dddABq, 1H each, JZ12.4, 6.0, 3.4 Hz, H-6), 3.75 (dt, 1H, JZ9.5, 4.9 Hz, H-4), 3.47 (ddd, 1H, JZ 9.6, 4.7, 3.4 Hz, H-5), 2.75 (d, 1H, JZ4.9 Hz, HO-4), 2.38 (s, 3H, ArCH3), 2.15, 2.12 (s, 3H each, OAc), 2.05 (dd, 1H, JZ7.3, 6.1 Hz, HO-6). 13C NMR (CDCl3): d 172.10, 169.95, 139.17, 133.82, 130.26, 128.32, 86.33, 79.99, 70.34, 69.73, 62.73, 21.61, 21.30, 21.27. HRMS calcd for C17H22O7S (MCNa) 393.0984, found 393.0984. 5.1.17. 4-Methylphenyl 2,3-di-O-acetyl-4-azido-6-O-benzoyl-4-deoxy-1-thio-b-D-galactopyranoside (20). Compound 19 (1.00 g, 2.70 mmol) was dissolved in CH2Cl2 (5 mL) and pyridine (0.87 mL) and cooled to K78 8C under Ar. Benzoyl chloride (0.329 mL, 2.84 mmol) was added dropwise and the mixture was stirred at K78 8C for 30 min and then left to retain room temperature during 60 min. The mixture was then again cooled to K78 8C and trifluoromethanesulphonic anhydride (0.89 mL, 5.40 mmol) was added dropwise. After 30 min at K78 8C the mixture was allowed to retain room temperature during 60 min and then diluted with CH2Cl2 and washed with satd aq NaHCO3, dried and concentrated. The residue was dissolved in DMF (18 mL) and NaN3 (3.60 g) was added and the mixture was stirred overnight and then filtered (SiO2, heptane–EtOAc, 2:1) and chromatographed (SiO2, heptane–EtOAc 2:1) to give 20 as a white solid (1.16 g, 86%). [a]20 D ZK50.2 (c 1.3, CHCl3). 1H NMR (CDCl3): d 6.97–8.02 (m, 9H, Ar), 5.26 (t, 1H, JZ9.8 Hz, H-2), 5.12 (dd, 1H, JZ9.7, 3.7 Hz, H-3), 4.58 (d, 1H, JZ10.0 Hz, H-1), 4.54, 4.45 (ABq, 1H each, JZ11.5, 5.4 Hz, H-6), 4.05–4.12 (m, 1H, H-4), 3.93 (dt, 1H, JZ5.7, 1.3 Hz, H-5), 2.27 (s, 3H, ArCH3), 2.09, 2.08 (s, 3H each, OAc). 13C NMR (CDCl3): d 170.57, 169.80, 166.47, 138.81, 133.86, 133.43, 130.22, 130.12, 129.81, 128.94, 87.48, 74.85, 74.51, 67.79, 63.74, 60.71, 21.60, 21.28, 20.43. HRMS calcd for C24H25N3O7S (MCNa) 522.1311, found 522.1308. 5.1.18. 4-Methylphenyl 4-azido-2,3,6-tri-O-benzoyl-4deoxy-1-thio-b-D-galactopyranoside (21). To a mixture
of 20 (530 mg, 1.06 mmol) in MeOH (25 mL) was added NaOMe–MeOH (0.25 mL, 1 M) and the resulting mixture was stirred for 5 h and then neutralized using Amberlite IR-120 HC. The residue was dissolved in pyridine (30 mL) and benzoyl chloride (0.45 mL, 3.88 mmol) and a catalytic amount of DMAP was added and the solution was stirred overnight. MeOH was added and the mixture was concentrated and flash chromatographed (SiO2, heptane– EtOAc, 2:1) to give 21 as a white solid (615 mg, 93%). 1 [a]21 D ZK28.3 (c 1.2, CHCl3). H NMR (CDCl3): d 8.10– 8.15 (m, 6H, Ar), 7.40–7.70 (m, 11H, Ar), 7.04 (d, 2H, JZ 8.0 Hz, Ar), (5.77 (t, 1H, JZ9.9 Hz, H-2), 5.61 (dd, 1H, JZ 9.8, 3.6 Hz, H-3), 4.91 (d, 1H, JZ10.0 Hz, H-1), 4.70, 4.68 (ABq, 1H each, JZ11.5, 7.0 Hz, H-6), 4.38 (dd, 1H, JZ3.6, 1.2 Hz, H-4), 4.17 (dt, 1H, JZ7.0, 1.2 Hz, H-5), 2.34 (s, 3H, ArCH3). 13C NMR (CDCl3): d 166.51, 166.12, 165.57, 138.87, 134.22, 133.85, 133.76, 133.68, 130.50, 130.27, 130.25, 130.11, 129.86, 129.71, 128.98, 128.96, 128.91, 128.84, 128.64, 87.72, 75.13, 75.08, 68.32, 63.79, 61.02, 21.61. HRMS calcd for C34H29N3O7S (MCNa) 646.1624, found 646.1617. 5.1.19. 2-Bromoethyl 2,3,6-tri-O-benzoyl-4-azido-4deoxy-b- D -galactopyranoside (22). Compound 21 (388 mg, 0.62 mmol) was dissolved in CH2Cl2 (20 mL) and MS AW-300 (600 mg) was added and the resulting mixture was stirred for 30 min and then cooled to 0 8C. 2-Bromoethanol (0.070 mL, 1.00 mmol), N-iodosuccinimide (196 mg, 0.87 mmol) and a catalytic amount of trimethylsilyl trifluoromethanesulfonate were added and the mixture was stirred for 10 min, then Et3N (1.0 mL) was added. The mixture was filtered through a short SiO2column and then chromatographed (SiO2, heptane–EtOAc, 4:1) to give 22 as a white solid (345 mg, 89%). [a]21 DZ K53.0 (c 1.3, CHCl3). 1H NMR (CDCl3): d 8.11–7.96 m, 6H, Ar), 7.65–7.37 (m, 9H, Ar), 5.79 (dd, 1H, JZ10.3, 7.8 Hz, H-2), 5.59 (dd, 1H, JZ10.3, 3.7 Hz, H-3), 4.82 (d, 1H, JZ7.9 Hz, H-1), 4.67, 4.58 (dABq, 1H each, JZ11.4, 6.4 Hz, H-6), 4.35 (dd, 1H, JZ3.7, 1.2 Hz, H-4), 4.16 (m, 2H, H-5, CH2), 3.90 (m, 1H, CH2), 3.45 (m, 2H, CH2). 13C NMR (CDCl3): d 166.05, 165.70, 165.17, 133.81, 133.49, 133.24, 130.06, 129.77, 129.72, 129.26, 128.58, 128.56, 128.33, 128.13, 101.60, 73.31, 70.96, 69.59, 69.14, 62.82, 60.28, 29.57. HRMS calcd for C29H26BrN3O8 (MCNa) 646.0801, found 646.0775. 5.1.20. 4-Deoxy-4-(2-naphthoyl)-amido-D-galactose (23). 3-(3-Mercaptophenyl)propanamidomethylpolystyrene resin (1.54 mmol/g, 100 mg) was swelled in DMF (1.0 mL) for 5 min and then DBU (0.040 mL) was added and the resin was shaken for another 5 min. Compound 22 (48 mg, 0.5 equiv to resin substitution) was dissolved in DMF (1.0 mL) and added to the resin. After 1 h MeI (0.050 mL) was added and the resin shaken for another 10 min and then washed with DMF, CH2Cl2, MeOH and diethyl ether and dried under vacuum. The dried resin was swelled in CH2Cl2 (1 mL) and MeOH (1 mL) and NaOMe–MeOH (1 M, 0.10 mL) were added. The resin was shaken for 1 h and then washed with MeOH and CH2Cl2. DTT (76 mg) in DMF (1 mL) was added followed by DBU (0.010 mL) and the resin was shaken overnight and then washed with DMF, CH2Cl2, MeOH and diethyl ether and dried under vacuum. The dried resin was swelled in DMF (2 mL) and pyridine
U. Ellervik et al. / Tetrahedron 61 (2005) 2421–2429
(0.20 mL) and 2-naphthoyl chloride (50 mg) were added. The resin was shaken for 4 h and then washed with DMF, CH2Cl2, MeOH and diethyl ether and dried under vacuum. To the dried resin were added CH2Cl2 (4 mL) and mCPBA (70%, 70 mg) and the mixture was shaken for 30 min and then washed with CH2Cl2, MeOH and diethyl ether and dried under vacuum. To the dried resin was added CH2Cl2 (2 mL), MeOH (2 mL) and NaOMe–MeOH (1 M, 0.80 mL) and the mixture was shaken overnight and then neutralized with acetic acid and washed with MeOH. The residue was chromatographed (SiO2, CH2Cl2–MeOH 5:1) to give 23 as a complex anomeric and rotameric mixture (6.8 mg, 27%). HRMS calcd for C17H19NO6 (MCNa) 356.1110, found 356.1123. 1H NMR can be found as Supplementary Material. 13C NMR (CD3OD): d 170.80, 136.43, 134.22, 130.17, 192.72, 129.35, 129.10, 128.96, 128.92, 128.76, 128.12, 127.94, 125.30, 124.60, 99.71, 98.49, 71.22, 70.04, 66.41, 66.22, 65.94, 64.89, 63.43, 62.11, 55.54, 55.37. Acknowledgements This work was supported by the Swedish Research Council.
Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tet.2005.01. 015
References and notes 1. Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149–2154.
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2. Seeberger, P. H.; Haase, W. Chem. Rev. 2000, 100, 4349–4393 and references therein. 3. Wa˚llberg, A.; Weigelt, D.; Falk, N.; Magnusson, G. Tetrahedron Lett. 1997, 38, 4285–4286. 4. Doi, T.; Sugiki, M.; Yamada, H.; Takahashi, T. Tetrahedron Lett. 1999, 40, 2141–2144. 5. Silva, D. J.; Wang, H.; Allanson, N. M.; Jain, R. K.; Sofia, M. J. J. Org. Chem. 1999, 64, 5926–5929. 6. Hanessian, S.; Huynh, H. K. Synlett 1999, 1, 102–104. 7. Hewitt, M. C.; Snyder, D. A.; Seeberger, P. H. J. Am. Chem. Soc. 2002, 124, 13434–13436. 8. Gayo, M. L.; Suto, M. J. Tetrahedron Lett. 1997, 38, 211–214. 9. Fru¨chtel, J. S.; Jung, G. Angew. Chem. Int. Ed. Engl. 1996, 35, 17–42. 10. Dahme´n, J.; Frejd, T.; Magnusson, G.; Noori, G. Carbohydr. Res. 1982, 111, C1–C4. 11. Dahme´n, J.; Frejd, T.; Gro¨nberg, G.; Lave, T.; Magnusson, G.; Noori, G. Carbohydr. Res. 1983, 116, 303–307. 12. Ellervik, U.; Jansson, K.; Magnusson, G. J. Carbohydr. Chem. 1998, 17, 777–784. 13. Dahme´n, J.; Frejd, T.; Gro¨nberg, G.; Lave, T.; Magnusson, G.; Noori, G. Carbohydr. Res. 1983, 118, 292–301. 14. Helferich, B.; Lutzmann, H. Justus Liebigs Ann. Chem. 1939, 541, 1–16. 15. Ma´rquez, F.; Hernando, J. L. Anales 1966, 62, 721–727. 16. Marshall, D. R.; Thomas, P. J.; Stirling, C. J. M. J. Chem. Soc., Perkin Trans. 2 1977, 14, 1898–1908. 17. Ellervik, U. Tetrahedron Lett. 2003, 44, 2279–2281. 18. Ellervik, U.; Grundberg, H.; Magnusson, G. J. Org. Chem. 1998, 63, 9323–9338. 19. Ercegovic, T.; Meijer, A.; Magnusson, G.; Ellervik, U. Org. Lett. 2001, 3, 913–915. 20. The benzoylated analog of 19 showed severe disrupture of the chair conformation (proven by NMR) and no attempts to transform this compound into the azide were made. 21. Komba, S.; Werdelin, O.; Jensen, T.; Meldal, M. J. Pept. Sci. 2000, 6, 585–593.
2-Bromoethyl glycosides for synthesis of glycoconjugates on solid support Ulf Ellervik,* Ma˚rten Jacobsson and Jo¨rgen Ohlsson Organic and Bioorganic Chemistry, Center for Chemistry and Chemical Engineering, Lund University, PO Box 124, SE-221 00 Lund, Sweden Received 24 September 2004; revised 26 November 2004; accepted 7 January 2005 Available online 28 January 2005
Abstract—2-Bromoethyl glycosides can easily and in high yields be transformed into sulfones by treatment with a suitable thiol followed by oxidation with mCPBA. The observation that the so formed sulfones were cleaved by treatment with NaOMe/MeOH was used to design a new safety catch linker for synthesis of glycoconjugates on solid support. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction Since the introduction of solid phase peptide synthesis by Merrifield in 1963,1 polymer-supported synthesis has become the method of choice for synthesis of peptides and oligonucleotides. The first investigations into solidphase oligosaccharide synthesis were conducted in the early 1970s and, whilst some groundbreaking work was done, progress was halted by the lack of powerful glycosylating agents, diverse protecting groups and on-bead analytical techniques.2 As advances were made in the solution phase synthesis of complex carbohydrates, new protecting groups, and selective glycosylating agents became available during the 1990s thus fuelling the interest in solid phase oligosaccharide synthesis. Also, on-bead analytical techniques such as high-resolution magic angle spinning NMR and FT-IR became widely available.
more advanced and automated synthesis is becoming a realistic goal2,7 further linker systems are needed. Especially attractive are safety-catch linkers, that is, linkers that are stable to most conditions until activated by, for example, oxidation or reduction.8,9 2-Bromoethyl was introduced as an anomeric protecting group that can be transformed into a spacer for glycoconjugates.10 The 2-bromoethyl group is easily introduced by normal glycosylation of 2-bromoethanol and is stable towards standard reagents used in carbohydrate synthesis.11 It shows unusual stability towards anomerization.12 The formed glycosides can then be alkylated using different thiols by treatment with, for example, Cs2CO3 in DMF.13
Presently, complex oligosaccharides are synthesized on solid phase using a wide variety of linker systems, glycosyl donors and acceptors, promoter systems and strategies.2 A number of different linker systems suitable for solid phase oligosaccharide synthesis are available with differences in stability, cleavage reagents and the group introduced upon cleavage. Notable examples are silyl ether linkers,3,4 acidand base-labile linkers such as amino-functionalized Rink resin5 and a benzylidene acetal-type linker to Wang aldehyde resin.6 Also, thioglycoside linkers have been used, as well as linkers cleaved by oxidation, hydrogenation, photocleavage and even olefin metathesis.2 Even though solid phase oligosaccharide synthesis is becoming Keywords: Solid support; Glycoconjugates; Safety catch linker. * Corresponding author. Tel.: C46 46 2228220; fax: C46 46 2228209; e-mail: [email protected] 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2005.01.015
Scheme 1. Deprotection of 1 under basic conditions (0.05 M NaOMe– MeOH) gave the desired product 2 as well as glucose (3).
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These thioethers are easily oxidized to sulfones using m-chloroperbenzoic acid (mCPBA). An attempt to deacetylate the sulfone 1 (Scheme 1) under basic conditions (0.05 M NaOMe–MeOH) gave the desired product 2 as well as a small amount of free glucose (3), that is, the aglycon was cleaved off. We realized that 2-bromoethyl glycosides can be coupled to solid supported thiols and thereby be used for solid phase glycoside synthesis.
2. Cleavage of bromoethyl glycosides in solution Before performing the reaction on solid support, an investigation of the system was performed in solution. As a model for the linker, compound 1 was chosen. The model compound was synthesized from 2-bromoethyl tetra-Oacetyl-b-D-glucopyranoside 414,15 in two steps (Scheme 2). Treatment of 4 with thiophenol and Cs2CO3 gave 5 in 95% yield. The sulfide 5 was then oxidized to the corresponding sulfone 1 in 98% yield using mCPBA.
cleavage was likewise obtained with lower concentrations of NaOMe, although requiring longer reaction times. As a lower limit a 0.05 M solution was used, resulting in a reaction time of 56 h. However, investigation of the cleavage products showed that compound 616 (Fig. 1) was formed instead of the expected alkene. Performing the reaction in NaOEt–EtOH (1.0 M) similarly gave D-glucose in quantitative yield, along with compound 716 (Fig. 1). An NMR-study was performed in order to elucidate the reaction mechanism. The model compound 1 was thus treated with a 0.7 M NaOCD3–CD3OD-solution and NMR-spectra were recorded over a period of 300 min. After 1 min complete deacetylation was observed along with about 50% exchange of the protons in a-position to the sulfone. The exchange of a-protons was complete within 5 min. The ratio of the integral of the anomeric NMRsignals for the formed glucose compared to the total integral of the anomeric protons of glucose (3) and deacetylated bromoethyl glucoside (2) was measured and is shown in Figure 2. About 10% glucose was formed within 5 min, whereas complete conversion required 300 min. During the experiment no signals corresponding to alkene protons were observed.
Scheme 2. (a) Thiophenol, Cs2CO3, DMF, 2 h. (b) mCPBA, EtOAc, 30 min. (c) NaOMe–MeOH, 4–56 h.
It was anticipated that basic treatment of compound 1 would result in abstraction of an a-proton (a relative to the sulfone), leading to an elimination reaction with concomitant expulsion of the glucoside. Thus, treatment of 1 with NaOMe–MeOH (1.0 M) smoothly furnished an a/b-mixture of D-glucose in quantitative yield within 4 h. Complete
Figure 2. Formation of glucose (3) by treating the sulfone 1 with 0.7 M NaOCD3–CD3OD. The graph shows the percentage of the integral for the NMR-signals of the anomeric protons of glucose (3) compared to the total integral for the anomeric protons of glucose (3) and deacetylated bromoethyl glycoside (2).
Figure 1. Cleavage products formed during alkoxide treatment of 1.
The fact that the deuterium–hydrogen exchange of the protons in a-position to the sulfone is much faster than formation of glucose indicates that the reaction follows a reversible carbanion elimination (E1cB)R, followed by a fast addition of the nucleophile (methoxide or ethoxide). This is supported by observations in similar systems, PhSO2CH2CH2Z, with leaving groups (Z) other than carbohydrates.16
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However, to disprove an SN2-mechanism, we performed a second experiment where a glucoside carrying a propyl linker (compound 10) was synthesized in two steps from the known 2-bromopropyl 2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside15 (8, Scheme 3), in a manner similar to that described for compound 1. Treatment of 10 with NaOMe– MeOH (1.0 M) gave an almost quantitative yield of compound 11; hence no cleavage product was observed.
Scheme 4. (a) NaOMe-MeOH, 45 min. (b) DMF, NaH, BnBr, 1 h. (c) mCPBA, EtOAc, 0.5 h. (d) NaOMe–MeOH, 22 h.
with anthraldehyde dimethyl acetal, and then acetylated using standard conditions to give compound 16 (Scheme 5).
Scheme 3. (a) Thiophenol, Cs2CO3, DMF, 1 h. (b) mCPBA, EtOAc, 0.5 h. (c) NaOMe–MeOH, CH2Cl2, 24 h.
As a final experiment compound 15 was synthesized from 4 in four steps, in order to verify the generality of the cleavage reaction. Compound 4 was deacetylated and subsequently benzylated under basic conditions to give 13. The sulfide 13 was then oxidized to the sulfone 14. Treatment of 14 with NaOMe–MeOH (1.0 M) gave compound 15 in 74% yield together with compound 6 (Scheme 4).
3. Use of 2-bromoethyl glycosides on solid support To evaluate the bromoethyl group as a linker for solid phase carbohydrate synthesis we performed a series of experiments in order to optimize the coupling of the glycosides to thiol resin. Cesium carbonate was used as base in solution phase but the poor solubility in DMF makes it less suitable for solid phase reactions. Instead the non-nucleophilic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was investigated and turned out to work well. To follow the reactions we used the newly developed anthraldehyde protective group which shows a strong UVabsorbance at around 365 nm and also a strong fluorescence.17 Compound 4 was deprotected using NaOMe– MeOH, transformed into the fluorescent acetal by treatment
Scheme 5. (i) NaOMe–MeOH–CH2Cl2, 20 min. (ii) Anthraldehyde dimethyl acetal, MeCN, pTSA, overnight. (iii) Pyridine, Ac2O, overnight.
We decided to use a relatively inexpensive scavenger resin, 3-(3-mercaptophenyl)propanamidomethylpolystyrene resin, as solid support. The resin was first treated with tributyl phosphine (0.7 M in THF–H2O 95:5) to reduce any disulfides and then swelled in DMF. DBU (1 equiv to resin substitution) was then added followed by different equivalents of compound 16. Samples were taken from the solution at various time intervals, diluted with MeCN and the absorbance measured at 364 nm. Treatment with 0.5 equiv (to resin substitution) of 16 resulted in an almost complete loss of absorbance in about 10 min, indicating that all material was coupled to the resin. Addition of 1.0 equiv of 16 resulted in about 70% reduction of the original
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absorbance, which gives an indication of the practically useful loading of the resin for this coupling (Fig. 3).
To verify that compound 16 had been coupled to the resin the two samples, treated with 0.5 equiv of 16 and 17, respectively, were washed with several volumes of DMF and CH2Cl2 and then swelled in a mixture of MeOH and CH2Cl2 and illuminated with long wave UV-radiation (365 nm). As expected, only the resin treated with 16 showed a strong fluorescence (Fig. 4). The stability of the thioethyl linker was investigated by subjecting compound 5 or 12 to standard conditions used in synthetic carbohydrate chemistry, such as basic, acidic or reductive protective group manipulations as well as typical glycosylation conditions (NIS–TfOH or BF3$OEt2). The results, summarized in Table 1, show that the thioethyl linker is stable to typical conditions. However, treatment with BF3$OEt2 caused anomerization (16%). Table 1. Stability of the thioethyl linker. Compound 5 or 12 was subjected to the conditions below. The samples were purified and analyzed using NMR
Figure 3. Coupling of 16 to thiophenyl resin using 0.5 equiv (C) and 1.0 equiv (&). As a control the resin was treated with 0.5 equiv of the unreactive 17 (,). Samples were taken from the solution at various time intervals and diluted with acetonitrile and the absorbance measured at 364 nm.
As a control, compound 1717 with an unreactive trimethylsilylethyl group was subjected to the same conditions and showed, as expected, only a minute drop in absorbance due to dilution but no coupling.
Figure 4. Visual comparison between resin treated with the unreactive compound 17 (left) and the reactive 16 (right) illuminated with long wave UV-radiation (365 nm). The resin treated with 16 shows a strong fluorescence.
Reaction
Conditions
Product
Acylation Deacylation
Pyridine, Ac2O, 16 h NaOMe-MeOH (0.05 M), 1 h HCl-MeOH, 16 h PhCH(OMe)2, pTsOH, CH3CN, 3 h AcOH (80%), 90 8C, 3 h BnBr, NaH, DMF, 16 h Pd-C (10%), AcOH, H2 (500 psi), 2 h NIS, TfOH, MS-AW 300, CH2Cl2, 0 8C, 30 min BF3OEt2, CH2Cl2, 1 h
5 12 12 5 5 13 5 5
Acetal formation Acetal cleavage Benzylation Debenzylation Glycosylation
5 (a/b 1:5)
To estimate the possible overall yield using the bromoethyl linker we coupled compound 4 (0.5 equiv to resin substitution) to the resin using DBU as base. Methyl iodide was then added to cap the residual thiol groups and the resin was washed and the sulfide oxidized to sulfone using mCPBA. The resin was again washed and then cleaved by treatment with 0.2 M NaOMe–MeOH–CH2Cl2 overnight to give glucose in an excellent 55% yield after purification. With a working protocol for using bromoethyl glycosides on solid support we decided to illustrate the usefulness of this novel linker with a more advanced compound. Compound 22 was synthesized in four steps from the known compound 1818 (Scheme 6). The benzylidene acetal protecting group of compound 18 was cleaved off using 80% AcOH at 90 8C. In a one-pot procedure the primary hydroxyl group of the formed diol 19 was selectively benzoylated at K78 8C and the secondary hydroxyl group was then transformed into a trifluoromethanesulfonate and consecutively inverted using NaN3 in DMF. The total yield for this conversion was 86%. All attempts to glycosylate compound 20 using 2-bromoethanol resulted either in de-O-acetylation of position 2 (using NIS and TMSOTf) or formation of the chloro sugar (using ICl and AgOTf).19 Instead compound 20 was de-Oacetylated and then O-benzoylated. The benzoylated compound was easily glycosylated using NIS and TMSOTf to give 22 in 89% yield.20 Compound 22 was coupled to the thiol resin by the standard procedure. The benzoyl protecting groups were removed by
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propose that the reaction follows a reversible carbanion elimination (E1cB)R, followed by a fast addition of the nucleophile. We have also shown that 2-bromoethyl glycosides can be coupled to solid supported thiols and thereby be used for synthesis of glycoconjugates on solid support. 5. Experimental 5.1. General The structures of all new compounds were determined by careful NMR analysis, including COSY, NOESY, HETCOR, and long-range HETCOR. NMR-spectra were recorded with 300 and 400 MHz instruments. Chemical shifts are given in ppm downfield from the signal for Me4Si, with reference to internal CHCl3 or MeOH. All new compounds were determined to be O95% pure by 1H NMR spectroscopy. Molecular sieves were activated by heating under vacuum. CH2Cl2 and MeCN were dried by distillation from CaH2, DMF was distilled. Column chromatography was performed on SiO2 (Matrex LC-gel; 60A, 35-70 MY, Grace), and TLC on Merck SiO2 60 F256. 3-(3-Mercaptophenyl)propanamidomethylpolystyrene was purchased from Argonaut Technologies. Compounds 4,15 8,15 17,17 and 1818 were prepared as described in the literature. Compounds 3 and 15 are commercially available. Scheme 6. (a) AcOH (80%, aq), 90 8C. (b) CH2Cl2, pyridine, BzCl, Tf2O, then NaN3, DMF. (c) NaOMe-MeOH, 2 h, then BzCl, pyridine, DMAP overnight. (d) CH2Cl2, 2-bromoethanol, NIS, TMSOTf, MS AW-300.
treatment with NaOMe–MeOH (0.05 M) for 1 h. The azide was then reduced using DTT–DBU–DMF21 and condensed with 2-naphthoyl chloride. The sulfide was then oxidized to the sulfone and treated with NaOMe–MeOH–CH2Cl2 to give compound 23 as an anomeric mixture in 27% yield after purification, a recovery comparable to other methods for synthesis of glycoconjugates on solid support (Scheme 7).2
Scheme 7. (a) DMF, DBU, thiol resin. (b) DMF, DBU, MeI. (c) NaOMeMeOH-CH2Cl2 (0.05 M). (d) DTT, DMF, DBU. (e) DMF, pyridine, 2naphthoyl chloride. (f) mCPBA, CH2Cl2. (g) NaOMe-MeOH, CH2Cl2 (0.2 M).
5.1.1. 2-(Phenylsulfonyl)ethyl 2,3,4,6-tetra-O-acetyl-b-Dglucopyranoside (1). To a solution of 5 (490 mg, 0.93 mmol) in EtOAc (32 mL) was added mCPBA (60%, 590 mg) and the resulting mixture was stirred at room temperature. After 30 min, the mixture was filtered through a short column of alumina, concentrated, and chromatographed (SiO2, heptane–EtOAc, 1:2) to give 1 as a white 1 solid (512 mg, 98%). [a]20 D ZK9.4 (c 1.2, CHCl3). H NMR (CDCl3): d 7.55–8.00 (m, 5H, Ar), 5.17 (t, 1H, JZ9.5 Hz, H-3), 5.01 (t, 1H, JZ9.8 Hz, H-4), 4.83 (dd, 1H, JZ9.6, 8.0 Hz, H-2), 4.51 (d, 1H, JZ8.0 Hz, H-1), 4.23 (dd, 1H, JZ12.3, 5.0 Hz, H-6), 4.09–4.16 (m, 2H, H-6, CH2), 3.98– 4.06 (m, 1H, CH2), 3.68 (ddd, 1H, JZ10.0, 5.0, 2.4 Hz, H-5), 3.35–3.50 (m, 2H, CH2), 2.10, 2.04, 2.03, 1.99 (s, 3H each, OAc). 13C NMR (CDCl3): d 170.79, 170.37, 169.60, 169.58, 139.71, 134.15, 129.52, 128.24, 72.79, 72.12, 71.05, 68.39, 63.28, 62.01, 56.38, 20.94, 20.89, 20.78. HRMS calcd for C22H28O12S (MCNa) 539.1199, found 539.1206. 5.1.2. D -Glucose (3). To a solution of 1 (15 mg, 0.029 mmol) in MeOH (1.28 mL) was added NaOMe (0.067 mL, 1 M in MeOH) and the resulting mixture was stirred at room temperature. After 56 h AcOH (5% in MeOH) was added until neutral solution (moist pH-paper). The mixture was concentrated and chromatographed (SiO2, CH2Cl2–MeOH–H2O, 66:33:4/10:20:1) to give 3 as a white solid (4.9 mg, 94%) and 6 as a clear oil (5.8 mg, 99%).
4. Conclusion We have shown that 2-bromoethyl glycosides can easily be transformed into sulfones which can be cleaved by treatment with NaOMe–MeOH to yield hemiacetals. We
5.1.3. 2-(Phenylthio)ethyl 2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside (5). To a solution of 2-bromoethyl-2,3,4,6tetra-O-acetyl-b-D-glucopyranoside (4) (92 mg, 0.20 mmol) in DMF (4 mL) were added thiophenol (0.038 mL,
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0.36 mmol) and Cs2CO3 (120 mg, 0.36 mmol) and the resulting mixture was stirred at room temperature. After 2 h the reaction mixture was diluted with CH2Cl2, washed with water, dried (MgSO4), concentrated, co-concentrated with toluene and chromatographed (SiO2, toluene–acetone, 10:1) to give 5 as a white solid (93 mg, 95%). [a]20 D ZK9.1 (c 1.1, CHCl3). 1H NMR (CDCl3): d 7.10–7.30 (m, 5H, Ar), 5.13 (t, 1H, JZ9.5 Hz, H-3), 5.01 (t, 1H, JZ9.8 Hz, H-4), 4.93 (dd, 1H, JZ9.6, 8.0 Hz, H-2), 4.45 (d, 1H, JZ8.0 Hz, H-1), 4.17 (dd, 1H, JZ12.3, 4.8 Hz, H-6), 4.05 (dd, 1H, JZ12.3, 2.4 Hz, H-6), 3.94 (ddd, 1H, JZ10.4, 7.7, 5.5 Hz, CH2), 3.57–3.65 (m, 2H, H-5, CH2), 2.97–3.07 (m, 2H, CH2), 2.00, 1.98, 1.95, 1.94 (s, 3H each, OAc). 13C NMR (CDCl3): d 170.86, 170.49, 169.60, 135.67, 129.84, 129.25, 126.65, 101.22, 72.93, 72.06, 71.35, 68.87, 68.54, 62.09, 33.45, 20.94, 20.82, 20.80. HRMS calcd for C22H28O10S (MCNa) 507.1301, found 507.1300. 5.1.4. Deprotection study of 2-(phenylsulfonyl)ethyl 2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside (1). Compound 1 (10 mg, 0.019 mmol) was dissolved in CD3OD (1.0 mL) and the mixture was added to an NMR-tube and an NMR-spectrum was recorded. CD3ONa (0.075 mL, 1 M in CD3OD) was added and NMR-spectra were recorded after 1, 15, 30, 120, and 300 min, respectively. After 1 min, 1 was completely deacylated and the shift of the anomeric proton was 4.23 ppm. The amount cleaved product was determined as the percentage of the integral for the NMR-signals of the anomeric protons of 3 (5.18 and 4.62 ppm) compared to the total integral for the anomeric protons of glucose and deacetylated bromoethyl glycoside. 5.1.5. 3-(Phenylthio)propyl 2,3,4,6-tetra-O-acetyl-b-Dglucopyranoside (9). Thiophenol (0.055 mL, 0.38 mmol) and Cs2CO3 (124 mg, 0.38 mmol) were added to a solution of 8 (120 mg, 0.26 mmol) in DMF (5 mL) and the resulting mixture was stirred at room temperature. After 1 h the reaction mixture was diluted with CH2Cl2, washed with water, dried (MgSO4), concentrated, co-concentrated with toluene and chromatographed (SiO2, heptane–EtOAc, 3:1) to give 9 as a pale yellow oil (135 mg, 96%). [a]20 D ZC3.4 (c 1.0, CHCl3). 1H NMR (CDCl3): d 7.08–7.26 (m, 5H, Ar), 5.13 (t, 1H, JZ9.5 Hz, H-3), 5.02 (t, 1H, JZ9.8 Hz, H-4), 4.93 (dd, 1H, JZ9.6, 8.0 Hz, H-2), 4.42 (d, 1H, JZ8.0 Hz, H-1), 4.19 (dd, 1H, JZ12.3, 4.7 Hz, H-6), 4.06 (dd, 1H, JZ 12.3, 2.4 Hz, H-6), 3.88 (dt, 1H, JZ9.7, 5.5 Hz, CH2), 3.55– 3.65 (m, 2H, H-5, CH2), 2.85–3.95 (m, 2H, CH2), 2.01, 1.95 (s, 3H each, OAc), 1.93 (s, 6H, OAc), 1.75–1.90 (m, 2H, CH2). 13C NMR (CDCl3): d 171.11, 170.71, 169.83, 169.75, 136.57, 129.45, 129.37, 126.38, 101.34, 73.22, 72.22, 71.70, 68.82, 68.35, 30.17, 29.26, 21.18, 21.05, 21.03. HRMS calcd for C23H30O10S (MCNa) 521.1457, found 521.1455. 5.1.6. 3-(Phenylsulfonyl)propyl 2,3,4,6-tetra-O-acetyl-b(10). To a solution of 9 (45 mg, 0.083 mmol) in EtOAc (2.5 mL) was added mCPBA (53 mg, 0.18 mmol, 60%) and the resulting mixture was stirred at room temperature. After 30 min, the mixture was filtered through a short column of alumina, concentrated, and chromatographed (SiO2, heptane–EtOAc, 1:2) to give 10 as a clear oil (46 mg, 97%). [a]20 D ZK12.4 (c 1.3, CHCl3). 1H NMR (CDCl3): d 7.50–8.00 (m, 5H, Ar), 5.19 (t, 1H, JZ9.5 Hz, H-3), 5.06 (t, 1H, JZ9.8 Hz, H-4), 4.95 (dd,
D-glucopyranoside
1H, JZ9.6, 8.0 Hz, H-2), 4.47 (d, 1H, JZ8.0 Hz, H-1), 4.24 (dd, 1H, JZ12.3, 4.8 Hz, H-6), 4.12 (dd, 1H, JZ12.3, 2.3 Hz, H-6), 3.88–3.95 (m, 1H, CH2), 3.65–3.70 (m, 2H, H-5, CH2), 3.10–3.25 (m, 2H, CH2), 2.09, 2.03, 2.02, 2.00 (s, 3H each, OAc), 2.00–2.05 (m, 2H, CH2). 13C NMR (CDCl3): d 170.86, 170.43, 169.61, 169.54, 133.99, 129.57, 128.18, 100.90, 72.89, 72.08, 71.36, 68.48, 67.70, 62.03, 53.06, 23.34, 20.95, 20.87, 20.80. HRMS calcd for C23H30O12S (MCNa) 553.1356, found 553.1346. 5.1.7. 3-Phenylsulfonyl-propyl b-D-glucopyranoside (11). Compound 10 (23 mg, 0.040 mmol) was dissolved in NaOMe–MeOH (1 M, 1 mL) and the resulting mixture was stirred at room temperature. After 24 h, AcOH (5% in MeOH) was added until neutral solution (moist pH-paper). The mixture was concentrated and chromatographed (SiO2, CH2Cl2–MeOH–H2O, 66:33:4) to give 11 as a clear oil 1 (16 mg, 99%). [a]20 D ZK14.2 (c 0.5, MeOH). H NMR (CD3OD): d 7.60–7.95 (m, 5H, Ar), 4.19 (d, 1H, JZ7.8 Hz, H-1), 3.94 (dt, 1H, JZ10.1, 5.9 Hz, CH2), 3.84 (dd, 1H, JZ 12.0, 2.0 Hz, H-6), 3.60–3.67 (m, 2H, H-6, CH2), 3.20–3.45 (m, 5H, H-3, H-4, H-5, CH2), 3.14 (dd, 1H, JZ9.0, 7.9 Hz, H-2), 1.92–2.00 (m, 2H, CH2).13C NMR (CD3OD): d 140.65, 135.17, 130.69, 129.27, 101.45, 78.11, 75.14, 71.71, 68.48, 62.87, 54.10, 24.69. HRMS calcd for C15H22O8S (MCNa) 385.0933, found 385.0928. 5.1.8. 2-(Phenylthio)ethyl b-D-glucopyranoside (12). Compound 4 (120 mg, 0.25 mmol) was dissolved in NaOMe–MeOH (0.05 M, 5 mL) and stirred for 45 min at room temperature and then neutralized using Amberlite IR-120 HC. The mixture was concentrated and chromatographed (SiO2, CH2Cl2–MeOH 5:1) to give 12 as a white 1 solid (78 mg, 89%). [a]20 D ZK24.8 (c 0.9, MeOH). H NMR (CD3OD): d 7.15–7.41 (m, 5H, Ar), 4.28 (d, 1H, JZ7.8 Hz, H-1), 3.97 (ddd, 1H, JZ10.4, 8.2, 6.4 Hz, CH2), 3.85 (dd, 1H, JZ10.0, 2.1 Hz, H-6), 3.75 (ddd, 1H, JZ10.4, 8.2, 6.5 Hz, CH2), 3.66 (dd, 1H, JZ11.9, 5.3 Hz, H-6), 3.23– 3.37 (m, 3H, H-3, H-4, H-5), 3.15–3.21 (m, 3H, H-2, CH2). 13 C NMR (CD3OD): d 137.42, 130.46, 130.21, 127.34, 104.69, 78.13, 75.18, 71.67, 69.73, 62.80, 33.91. HRMS calcd for C14H20O6S (MCNa) 339.0878, found 339.0864. 5.1.9. 2-(Phenylthio)ethyl 2,3,4,6-tetra-O-benzyl-b-Dglucopyranoside (13). To a solution of 12 (69 mg, 0.22 mmol) in DMF (distilled, 3 mL) was added NaH (60%, 90 mg) and BnBr (0.15 mL). The reaction mixture was stirred for 1 h and then quenched by addition of MeOH (1 mL). The mixture was then diluted with CH2Cl2, washed with water, dried (Na2SO4), concentrated and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 13 as a white waxy solid (131 mg, 89%). [a]20 D ZC15.9 (c 1.7, CHCl3). 1 H NMR (CDCl3): d 7.10–7.35 (m, 25H, Ar), 4.95, 4.68 (ABq, 1H each, JZ11.0 Hz, OBn), 4.90, 4.76 (ABq, 1H each, JZ11.0 Hz, OBn), 4.79, 4.50 (ABq, 1H each, JZ 11.1 Hz, OBn), 4.57, 4.52 (ABq, 1H each, JZ12.1 Hz, OBn), 4.35 (d, 1H, JZ7.8 Hz, H-1), 4.07 (ddd, 1H, JZ10.2, 7.7, 5.7 Hz, CH2), 3.55–3.75 (m, 5H, H-3, H-5, H-6, CH2), 3.38–3.45 (m, 2H, H-2, H-4), 3.10–3.21 (m, 2H, CH2). 13C NMR (CDCl3): d 138.78, 138.68, 138.30, 138.27, 135.27, 129.59, 129.19, 128.58, 128.42, 128.18, 128.08, 127.99, 127.87, 127.84, 126.40, 104.01, 84.82, 82.39, 77.94, 75.91, 75.22 75.08, 75.06, 73.70, 72.32, 69.03, 68.71, 33.49.
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HRMS calcd for C42H44O6S (MCNa) 699.2756, found 699.2723.
30.10, 21.02, 20.97. HRMS calcd for C27H27BrO8 (MCNa) 581.0787, found 581.0781.
5.1.10. 2-(Phenylsulfonyl)ethyl 2,3,4,6-tetra-O-benzyl-b(14). To a solution of 13 (73 mg, 0.11 mmol) in EtOAc (4 mL) was added mCPBA (60%, 70 mg) and the resulting mixture was stirred at room temperature. After 30 min, the mixture was filtered through a short column of alumina to give 14 as a white solid 1 (75 mg, 99%). [a]20 D ZC14.5 (c 1.6, CHCl3). H NMR (CDCl3): d 7.82 (dd, 2H, JZ8.6, 1.3 Hz, Ar), 7.53 (dt, 1H, JZ7.4 Hz, Ar), 7.41 (t, 1H, JZ7.7 Hz, Ar), 7.00–7.30 (m, 20H, Ar), 4.80, 4.67 (ABq, 1H each, JZ10.9 Hz, OBn), 4.73, 4.45 (ABq, 1H each, JZ11.2 Hz, OBn), 4.56, 4.47 (ABq, 1H each, JZ11.3 Hz, OBn), 4.52, 4.44 (ABq, 1H each, JZ12.1 Hz, OBn), 4.21 (d, 1H, JZ7.8 Hz, H-1), 4.12 (dt, 1H, JZ10.9, 6.6 Hz, CH2), 3.85 (dt, 1H, JZ10.9, 6.6 Hz, CH2), 3.61, 3.56 (dABq, 1H each, JZ10.7, 4.4, 2.1 Hz, H-6), 3.28–3.52 (m, 5H, H-3, H-4, H-5, CH2), 3.10 (t, 1H, JZ8.0 Hz, H-2). 13C NMR (CDCl3): d 138.62, 138.56, 138.20, 133.92, 129.36, 128.65, 128.62, 128.59, 128.57, 128.47, 128.15, 128.11, 128.09, 128.00, 127.94, 127.88, 103.84, 84.62, 82.31, 77.74, 76.91, 75.93, 75.22, 75.00, 73.72, 72.31, 68.87, 63.26, 56.69. HRMS calcd for C42H44O8S (MCNa) 731.2655, found 731.2606.
5.1.13. Coupling of 2-bromoethyl 2,3-di-O-acetyl-4,6-O(9-anthracenyl)methylene-b-D-glucopyranoside (4) or 2-(trimethylsilyl)ethyl 2,3-di-O-acetyl-4,6-O-(9-anthracenyl)methylene-b- D -glucopyranoside (17) to thiol scavenger resin. 3-(3-Mercaptophenyl)propanamidomethylpolystyrene resin (1.54 mmol/g, 50 mg) was swelled in DMF (0.480 mL) for 5 min and then DBU (0.020 mL) was added and the resin was shaken for another 5 min. Compound 16 (20 mg or 40 mg) or 17 (20 mg) was dissolved in DMF (0.700 mL) and added to the resin. Samples (0.010 mL) were taken from the solution at different times and added to MeCN (0.990 mL). The samples (0.500 mL) were diluted with MeCN (2.000 mL) and the UV-absorbance measured at 364 nm.
D-glucopyranoside
5.1.11. 2,3,4,6-Tetra-O-benzyl-D-glucose (15). To a solution of 14 (58 mg, 0.082 mmol) in CH2Cl2 (2 mL) was added NaOMe–MeOH (1 M, 2 mL) and the mixture was stirred for 22 h and then neutralized using Amberlite IR-120 HC. The mixture was concentrated and purified on Sephadex LH-20 (CH2Cl2–MeOH 1:1) to give 15 as a white solid (33 mg, 74%) and 6 as a clear oil (7 mg, 43%). 5.1.12. 2-Bromoethyl 2,3-di-O-acetyl-4,6-O-(9-anthracenyl)methylene-b-D-glucopyranoside (16). To a solution of 4 (600 mg, 1.32 mmol) in MeOH (10 mL) and CH2Cl2 (5 mL) was added NaOMe–MeOH (1 M, 0.45 mL). The mixture was stirred for 20 min and then neutralized by addition of AcOH (1 mL). The mixture was concentrated and filtered through a column of SiO2 (CH2Cl2–MeOH 5:1). The residue was then dissolved in MeCN (10 mL) and anthraldehyde dimethyl acetal (323 mg, 1.65 mmol) and pTSA (5 mg) were added and the mixture was stirred at room temperature overnight protected from light and then neutralized by addition of Et3N (1 mL) and co-concentrated with toluene 3 times. The residue was chromatographed (SiO2, toluene/6:1 toluene–acetone) and the crude product was then dissolved in pyridine (5 mL) and acetic anhydride (4 mL) was added. The mixture was stirred overnight and then co-concentrated with toluene 3 times and the residue was chromatographed (SiO2, heptane–EtOAc 1:1) to give 16 as a white solid (354 mg, 48%). [a]20 DZ K70.7 (c 1.2, CHCl3). 1H NMR (CDCl3): d 8.60 (d, 2H, JZ 9.2 Hz, Ar), 8.51 (s, 1H, Ar), 8.01 (d, 2H, JZ8.3 Hz, Ar), 7.45–7.55 (m, 4H, Ar), 6.90 (s, 1H, ArCH), 5.45 (t, 1H, JZ 9.3 Hz, H-3), 5.10 (dd, 1H, JZ9.1, 7.9 Hz, H-2), 4.83 (d, 1H, JZ7.8 Hz, H-1), 4.55–4.60 (m, 1H, H-6), 4.23 (p, 1H, JZ5.6 Hz, CH2), 3.85–4.15 (m, 4H, H-4, H-5, H-6, CH2), 3.50 (t, 2H, JZ5.6 Hz, CH2), 2.10, 1.90 (s, 3H each, OAc*2). 13C NMR (CDCl3): d 170.31, 169.96, 131.65, 130.25, 129.89, 129.24, 126.51, 126.46, 125.13, 124.97, 101.84, 100.99, 79.52, 72.66, 71.90, 70.19, 69.80, 66.97,
5.1.14. Stability of 2-(phenylthio)ethyl 2,3,4,6-tetra-Oacetyl-b-D-glucopyranoside (5). Compound 5 (24 mg, 0.05 mmol) was subjected to the following conditions: (a) 5 was dissolved in pyridine (1.0 mL) and acetic anhydride (0.8 mL) was added. The mixture was stirred for 22 h and then co-concentrated with toluene and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 5 (24 mg, quant). (b) 5 was dissolved in NaOMe–MeOH (0.05 M, 1 mL) and stirred for 1 h. The mixture was neutralized by addition of Amberlite IR-120 HC and then concentrated to give 12 (16 mg, quant). (c) 5 was dissolved in MeOH saturated with HCl and stirred for 17 h. The mixture was concentrated and chromatographed (SiO2, CH2Cl2–MeOH, 5:1) to give 12 (15 mg, 94%). (d) 5 was dissolved in MeCN (1 mL, filtered through Al2O3) and a,a-dimethoxytoluene (0.10 mL) and pTSA (cat) were added. The mixture was stirred for 17 h, Et3N (0.050 mL) was added and the mixture was coconcentrated with toluene and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 5 (24 mg, quant). (e) 5 was dissolved in AcOH (80%, 3 mL) and stirred at 90 8C for 3 h. The mixture was co-concentrated with toluene to give 5 (23 mg, 95%). (f) 5 was dissolved in AcOH (3 mL) and Pd– C (10%, 25 mg) was added. The mixture was pressurized with H2 (500 psi) for 2 h and then filtered through Celite and concentrated to give 5 (20 mg, 84%). (g) 5 was dissolved in CH2Cl2 (2 mL) and BF3$OEt2 (0.019 mL, 3 equiv) was added. The mixture was stirred for 1 h, then quenched by addition of Et3N (0.040 mL). The mixture was then coconcentrated with toluene and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 5 (24 mg, quant, a–b 1:5). (h) 5 was dissolved in CH2Cl2 (2 mL) and MS AW-300 (50 mg) was added and the mixture was cooled down to 0 8C, under Ar. NIS (16 mg) was added, the mixture was stirred for 10 min and then TfOH (cat) was added. The mixture was stirred for another 10 min at 0 8C and then quenched by addition of Et3N (0.10 mL), concentrated and chromatographed (SiO2, heptane–EtOAc, 1:1) to give 5 (24 mg, quant). 5.1.15. Coupling and deprotection of 2-bromoethyl 2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside (4) to thiol scavenger resin. 3-(3-Mercaptophenyl)propanamidomethylpolystyrene resin (1.54 mmol/g, 100 mg) was swelled in DMF (1.0 mL) for 5 min and then DBU (0.040 mL) was added and the resin was shaken for another 5 min. Compound 4 (35 mg, 0.5 equiv to resin substitution)
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was dissolved in DMF (1.0 mL) and added to the resin. After 1 h MeI (0.010 mL) was added and the resin shaken for another 10 min and then washed with DMF, CH2Cl2, MeOH and diethyl ether and dried under vacuum. To the dried resin was added CH2Cl2 (2 mL) and mCPBA (70%, 70 mg) and the mixture was shaken for 30 min and then washed with CH2Cl2, MeOH and diethyl ether and dried under vacuum. To the dried resin were added CH2Cl2 (1 mL), MeOH (1 mL) and NaOMe–MeOH (1 M, 0.40 mL) and the mixture was shaken overnight and then neutralized with Amberlite IR-120 HC, filtered and washed with MeOH. The residue was purified on Sephadex (LH-20, CH2Cl2–MeOH, 1:1) to give 3 as a white solid (7.6 mg, a–b 1:1, 55%). 5.1.16. 4-Methylphenyl 2,3-di-O-acetyl-1-thio-b-D-glucopyranoside (19). Compound 18 (2.63 g, 5.74 mmol) was dissolved in AcOH (80%, aq 70 mL) and stirred at 90 8C for 2.5 h and then co-concentrated with toluene. The residue was chromatographed (SiO2, toluene–acetone, 2:1) to give 19 as a white solid (1.98 g, 93%). [a]20 D ZK33.5 (c 1.2, CHCl3). 1H NMR (CDCl3): d 7.40 (bd, 2H, JZ8.1 Hz, Ar), 7.17 (bd, 2H, JZ7.9 Hz, Ar), 5.06 (t, 1H, JZ9.3 Hz, H-3), 4.95 (dd, 1H, JZ9.9, 9.3 Hz, H-2), 4.72 (d, 1H, JZ10.0 Hz, H-1), 3.97, 3.85 (dddABq, 1H each, JZ12.4, 6.0, 3.4 Hz, H-6), 3.75 (dt, 1H, JZ9.5, 4.9 Hz, H-4), 3.47 (ddd, 1H, JZ 9.6, 4.7, 3.4 Hz, H-5), 2.75 (d, 1H, JZ4.9 Hz, HO-4), 2.38 (s, 3H, ArCH3), 2.15, 2.12 (s, 3H each, OAc), 2.05 (dd, 1H, JZ7.3, 6.1 Hz, HO-6). 13C NMR (CDCl3): d 172.10, 169.95, 139.17, 133.82, 130.26, 128.32, 86.33, 79.99, 70.34, 69.73, 62.73, 21.61, 21.30, 21.27. HRMS calcd for C17H22O7S (MCNa) 393.0984, found 393.0984. 5.1.17. 4-Methylphenyl 2,3-di-O-acetyl-4-azido-6-O-benzoyl-4-deoxy-1-thio-b-D-galactopyranoside (20). Compound 19 (1.00 g, 2.70 mmol) was dissolved in CH2Cl2 (5 mL) and pyridine (0.87 mL) and cooled to K78 8C under Ar. Benzoyl chloride (0.329 mL, 2.84 mmol) was added dropwise and the mixture was stirred at K78 8C for 30 min and then left to retain room temperature during 60 min. The mixture was then again cooled to K78 8C and trifluoromethanesulphonic anhydride (0.89 mL, 5.40 mmol) was added dropwise. After 30 min at K78 8C the mixture was allowed to retain room temperature during 60 min and then diluted with CH2Cl2 and washed with satd aq NaHCO3, dried and concentrated. The residue was dissolved in DMF (18 mL) and NaN3 (3.60 g) was added and the mixture was stirred overnight and then filtered (SiO2, heptane–EtOAc, 2:1) and chromatographed (SiO2, heptane–EtOAc 2:1) to give 20 as a white solid (1.16 g, 86%). [a]20 D ZK50.2 (c 1.3, CHCl3). 1H NMR (CDCl3): d 6.97–8.02 (m, 9H, Ar), 5.26 (t, 1H, JZ9.8 Hz, H-2), 5.12 (dd, 1H, JZ9.7, 3.7 Hz, H-3), 4.58 (d, 1H, JZ10.0 Hz, H-1), 4.54, 4.45 (ABq, 1H each, JZ11.5, 5.4 Hz, H-6), 4.05–4.12 (m, 1H, H-4), 3.93 (dt, 1H, JZ5.7, 1.3 Hz, H-5), 2.27 (s, 3H, ArCH3), 2.09, 2.08 (s, 3H each, OAc). 13C NMR (CDCl3): d 170.57, 169.80, 166.47, 138.81, 133.86, 133.43, 130.22, 130.12, 129.81, 128.94, 87.48, 74.85, 74.51, 67.79, 63.74, 60.71, 21.60, 21.28, 20.43. HRMS calcd for C24H25N3O7S (MCNa) 522.1311, found 522.1308. 5.1.18. 4-Methylphenyl 4-azido-2,3,6-tri-O-benzoyl-4deoxy-1-thio-b-D-galactopyranoside (21). To a mixture
of 20 (530 mg, 1.06 mmol) in MeOH (25 mL) was added NaOMe–MeOH (0.25 mL, 1 M) and the resulting mixture was stirred for 5 h and then neutralized using Amberlite IR-120 HC. The residue was dissolved in pyridine (30 mL) and benzoyl chloride (0.45 mL, 3.88 mmol) and a catalytic amount of DMAP was added and the solution was stirred overnight. MeOH was added and the mixture was concentrated and flash chromatographed (SiO2, heptane– EtOAc, 2:1) to give 21 as a white solid (615 mg, 93%). 1 [a]21 D ZK28.3 (c 1.2, CHCl3). H NMR (CDCl3): d 8.10– 8.15 (m, 6H, Ar), 7.40–7.70 (m, 11H, Ar), 7.04 (d, 2H, JZ 8.0 Hz, Ar), (5.77 (t, 1H, JZ9.9 Hz, H-2), 5.61 (dd, 1H, JZ 9.8, 3.6 Hz, H-3), 4.91 (d, 1H, JZ10.0 Hz, H-1), 4.70, 4.68 (ABq, 1H each, JZ11.5, 7.0 Hz, H-6), 4.38 (dd, 1H, JZ3.6, 1.2 Hz, H-4), 4.17 (dt, 1H, JZ7.0, 1.2 Hz, H-5), 2.34 (s, 3H, ArCH3). 13C NMR (CDCl3): d 166.51, 166.12, 165.57, 138.87, 134.22, 133.85, 133.76, 133.68, 130.50, 130.27, 130.25, 130.11, 129.86, 129.71, 128.98, 128.96, 128.91, 128.84, 128.64, 87.72, 75.13, 75.08, 68.32, 63.79, 61.02, 21.61. HRMS calcd for C34H29N3O7S (MCNa) 646.1624, found 646.1617. 5.1.19. 2-Bromoethyl 2,3,6-tri-O-benzoyl-4-azido-4deoxy-b- D -galactopyranoside (22). Compound 21 (388 mg, 0.62 mmol) was dissolved in CH2Cl2 (20 mL) and MS AW-300 (600 mg) was added and the resulting mixture was stirred for 30 min and then cooled to 0 8C. 2-Bromoethanol (0.070 mL, 1.00 mmol), N-iodosuccinimide (196 mg, 0.87 mmol) and a catalytic amount of trimethylsilyl trifluoromethanesulfonate were added and the mixture was stirred for 10 min, then Et3N (1.0 mL) was added. The mixture was filtered through a short SiO2column and then chromatographed (SiO2, heptane–EtOAc, 4:1) to give 22 as a white solid (345 mg, 89%). [a]21 DZ K53.0 (c 1.3, CHCl3). 1H NMR (CDCl3): d 8.11–7.96 m, 6H, Ar), 7.65–7.37 (m, 9H, Ar), 5.79 (dd, 1H, JZ10.3, 7.8 Hz, H-2), 5.59 (dd, 1H, JZ10.3, 3.7 Hz, H-3), 4.82 (d, 1H, JZ7.9 Hz, H-1), 4.67, 4.58 (dABq, 1H each, JZ11.4, 6.4 Hz, H-6), 4.35 (dd, 1H, JZ3.7, 1.2 Hz, H-4), 4.16 (m, 2H, H-5, CH2), 3.90 (m, 1H, CH2), 3.45 (m, 2H, CH2). 13C NMR (CDCl3): d 166.05, 165.70, 165.17, 133.81, 133.49, 133.24, 130.06, 129.77, 129.72, 129.26, 128.58, 128.56, 128.33, 128.13, 101.60, 73.31, 70.96, 69.59, 69.14, 62.82, 60.28, 29.57. HRMS calcd for C29H26BrN3O8 (MCNa) 646.0801, found 646.0775. 5.1.20. 4-Deoxy-4-(2-naphthoyl)-amido-D-galactose (23). 3-(3-Mercaptophenyl)propanamidomethylpolystyrene resin (1.54 mmol/g, 100 mg) was swelled in DMF (1.0 mL) for 5 min and then DBU (0.040 mL) was added and the resin was shaken for another 5 min. Compound 22 (48 mg, 0.5 equiv to resin substitution) was dissolved in DMF (1.0 mL) and added to the resin. After 1 h MeI (0.050 mL) was added and the resin shaken for another 10 min and then washed with DMF, CH2Cl2, MeOH and diethyl ether and dried under vacuum. The dried resin was swelled in CH2Cl2 (1 mL) and MeOH (1 mL) and NaOMe–MeOH (1 M, 0.10 mL) were added. The resin was shaken for 1 h and then washed with MeOH and CH2Cl2. DTT (76 mg) in DMF (1 mL) was added followed by DBU (0.010 mL) and the resin was shaken overnight and then washed with DMF, CH2Cl2, MeOH and diethyl ether and dried under vacuum. The dried resin was swelled in DMF (2 mL) and pyridine
U. Ellervik et al. / Tetrahedron 61 (2005) 2421–2429
(0.20 mL) and 2-naphthoyl chloride (50 mg) were added. The resin was shaken for 4 h and then washed with DMF, CH2Cl2, MeOH and diethyl ether and dried under vacuum. To the dried resin were added CH2Cl2 (4 mL) and mCPBA (70%, 70 mg) and the mixture was shaken for 30 min and then washed with CH2Cl2, MeOH and diethyl ether and dried under vacuum. To the dried resin was added CH2Cl2 (2 mL), MeOH (2 mL) and NaOMe–MeOH (1 M, 0.80 mL) and the mixture was shaken overnight and then neutralized with acetic acid and washed with MeOH. The residue was chromatographed (SiO2, CH2Cl2–MeOH 5:1) to give 23 as a complex anomeric and rotameric mixture (6.8 mg, 27%). HRMS calcd for C17H19NO6 (MCNa) 356.1110, found 356.1123. 1H NMR can be found as Supplementary Material. 13C NMR (CD3OD): d 170.80, 136.43, 134.22, 130.17, 192.72, 129.35, 129.10, 128.96, 128.92, 128.76, 128.12, 127.94, 125.30, 124.60, 99.71, 98.49, 71.22, 70.04, 66.41, 66.22, 65.94, 64.89, 63.43, 62.11, 55.54, 55.37. Acknowledgements This work was supported by the Swedish Research Council.
Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tet.2005.01. 015
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