- Email: [email protected]
Facile solid phase synthesis of an activated diazo linker
"IWrRAHEDRON LETTERS Tetrahedron Letters 39 (1998) 7803-7806
Pergamon
Facile solid phase synthesis of an activated diazo linker G u r d i p B h a l a y * and A n d r e w R. D u n s t a n Novartis Horsham Research Centre, Wimblehurst Road, Horshara, West Sussex, RHI2 5AB, UK.
Received 10 July 1998; accepted 11 August 1998 Abstract The synthesis of a resin bound diazo species has been achieved in a highly efficient manner starting from commercially available reagents. This has produced an activated linker, providing a chemoselective method for the attachment of functionalised carboxylic acids to the solid phase via the Wang linker, in a rapid and colourometric reaction. O 1998ElsevierScienceLtd. All rightsreserved. Keywords: Diazo compounds;Hydrazones;Racemisation;Solid phase synthesis
Combinatorial chemistry has become popular amongst medicinal chemists for both lead discovery and lead optimization [1]. This partially emanates from the ability to accelerate the chemical synthesis of target molecules by overcoming the need for labour intensive purification by conventional methods. Attachment of the template to a solid support allows the use of washing and filtration as the sole method of purification during a synthesis. A central component in solid phase synthesis is the linker [2]. The linker is used to attach the compound to be synthesized to the solid support. One of the most widely used linkers for the attachment of carboxylic acids to solid support is the Wang Linker 1 [3]. Originally designed for peptide synthesis using the Fmoc protecting group strategy, this linker has been adopted by solid phase medicinal chemists to produce non peptide based libraries. The versatile nature of this linker allows for the cleavage of the tethered compound by acidolysis with TFA, or by nucleophilic cleavage [4] as in our recent benzodiazepine synthesis [5].
00~~/OH Wang Linker
Activated D i m Linker
1
2
*[email protected]
0040-4039/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(98)01706-7
7804 In solid phase synthesis attachment of the first residue to the Wang linker I usually employs one of the plethora of methods conceived by peptide chemists [6]. These proceed with carboxyl activation, often introducing the possibility of racemisation of an o~-stereogenic centre. Usually a large excess of the carboxy residue is used to ensure high loading, thus making this wasteful. We now report the synthesis of our activated diazo linker 2 which counters each of these drawbacks. The linker is based upon phenyldiazomethane, the properties of which are described in [7]. We decided to investigate the use of this reactive reagent in a resin bound form. In contrast to phenyldiazomethane, the resin bound analogue appears to be stable even when exposed to the atmosphere and light at R.T., for weeks, but decomposes within seconds, with loss of colour and without incident (lg scale) when heated to 100°C. The solid phase synthesis (Scheme 1) was initiated by treating commercially available Wang aldehyde resin [8] 3 with 2,4,6-triisopropylbenzenesulfonyl hydrazide 6 (2 equiv, THF, 16h, R.T.) to give the resin bound sulfonyl hydrazone 4 as an orange coloured resin. The reaction was monitored using infra-red (recorded using Nicolet Avatar 360 FT-IR directly on the resin beads) spectroscopy by observing the disappearance of the aldehyde signal (1680cm1), the washings were retained to recover the unused hydrazide 6. Next the diazo species 2 was generated using the solution phase protocol of Reese [9]. The orange coloured hydrazone 4 was treated with potassium hydroxide (2 equiv) in methanol/THF (1:4 v/v) mixture at 90°C for 7 mins, during which time the resin turned deep red. Infra-red spectroscopy showed an intense stretch at 2050cm1 indicating the formation of the diazo species 2. The subsequent esterification reactions were carded out at room temperature by adding a solution of the carboxylic acid 7 (1.2 equiv in DCM, THF or DMF) to the activated resin. The reactions of 8, 9 and 12 were over in 5rains, 13 took 20mins and 10, 11 required 90rains; and all are accompanied by the evolution of nitrogen gas. In all cases the deep red colour of the resin 2 becomes yellow (except 10, 11 turns pink) during the reaction, providing a convenient visual end-point to esterification, giving the resin bound ester 5. The esterification reaction requires the protonation of the diazo alkane to give the corresponding benzyl diazonium salt which undergoes nucleophilic attack to give the benzyl ester with the evolution of nitrogen gas. The proposed mechanism explains the chemoselectivity observed for the attachment of hydroxy acids such as 10, 11 and 12 (Scheme 2)
7805
without the need for alcohol protection. Free amino acids such as phenylalanine 13 can also be attached to solid support without resorting to N-protection by treating the zwitterionic amino acid with p-toluenesulfonic acid to protonate the amine [7]. Each of the acids 8-13 were attached and cleaved from resin (TFA/H20 95:5 v/v, R.T., 20mins) in excellent yields (>85%, based upon the loading of 3 over three steps) and purities (>90% by IH-nmr). H2N-N-Trs 6 i
=
/Trs
~ N - N 0
°
H
H
3 Trs=
I" RCOOH
o
7
~---N
e-o
iii 5
Reagents: i) 6 (2 exluiv),THF, R.T. 16h, ii) KOH (2 equiv), MeOI-VrHF(1:4v/v),90°C, 7mins., iii) 7 (I .2 exluiv),DCM, R.T. 5mins.
Scheme 1.
Synthesis of the activated diazo linker 2.
O
O
OH
O ~.~ o
B(OH)2
NHFmoc
HO
; OH
8(92%, 91%)
10 (90%, 95%)
12 (88%, 94%)
OH
O
HO HO
9 (95%, 98%) Scheme 2.
HO : NHFmoc
11 (86%, 95%)
! NH; TsO"
13 (91%, 94%)
Range of acids attached to resin, yields and purities shown respectively in parenthesis.
7806
Typical Experimental Procedure To the Wang aldehyde resin 3 (4.0g, 12.96mmol) was added 2,4,6-triisopropylbenzenesufonyl hydrazide 6 (7.7g, 25.92mmol) as a solution in dry THF (40nil). The suspension was stirred for 16h at R.T., filtered and washed in the usual way with MeOH and DCM, to give the orange coloured resin bound hydrazone 4. Dry THF (40ml) was added and the resin allowed to preswell for 1Groins before the addition of a solution of potassium hydroxide (1.5g, 25.92mmol) in dry methanol (10ml). The flask was immersed into a hot oil bath at 90°C for 7rains, during which time the resin became deep red in colour. The resin was filtered and washed with dry DCM [IR (2050 cml)]. To the activated resin 2 (300mg, 0.97retool) in DMF (5ml) at R.T., was added a solution of the carboxylic acid 8 (200mg, 1.20mmol) in DMF (10ml). The resin instantly turned yellow and effervesence was observed. After 5rains the resin was filtered and washed in the usual way with MeOH and DCM, to give the resin bound ester [IR (1730cm1)]. In conclusion, we have described the first reported synthesis of an activated diazo linker in two steps starting from readily available reagents. This highly coloured resin provides a chemoselective method for the rapid attachment of carboxylic acids to resin via the Wang linker with a visible end-point. The reaction proceeds by a mechanism which should not compromise the integrity of an ¢~-stereogenic centre. In addition, the activated resin could also be used as a scavenger for carboxylic acids.
References [1]
(a) Terrett NK, Gardener M, Gordon DW, Kobylecki RJ, Steele J. Tetrahedron 1995;30:8135-8173. (b) Balkenhohi F, Bussche-Hunnefeld C, Lansky A, Zechel C. Angew. Chem. Int. Ed. Engl. 1996;35:2288-2337. (c) Nefzi A, Ostresh JM, Houghten RA. Chem. Rev. 1997: 449-472.
[2]
Terrett NK. Combinatorial Chemistry. Oxford: Oxford University Press, 1998.
[3]
Wang S-S. J. Am. Chem. Soc. 1973;95:1328-1333.
[4]
(a) DeWitt SH, Kiely JS, Stankovic CJ, Schroeder MC, Cody DMR, Pavia MR. Proc. Natl. Acad. Sci. USA. 1993;32:69096913. (b) Gordon DW, Steele J. Bioorg. Med. Chem. Lett. 1995;5:47-50.
[5]
Bhalay G, Blaney P, Palmer VH, Baxter AD. Tetrahedron Lett. 1997;38:8375-8378.
[6]
Jones JH. The Chemical Synthesis of Peptides. Oxford: Oxford University Press, 199 !.
[7]
Sammakia T. In: Paquette LA, editor. Reagents for Organic Synthesis. New York: John Wiley & Sons, 1995:3969-3971.
[8]
Available from Novabiochem.
[9]
Reese CB, Dudman CC. Synthesis 1982;5:419-421.
Pergamon
Facile solid phase synthesis of an activated diazo linker G u r d i p B h a l a y * and A n d r e w R. D u n s t a n Novartis Horsham Research Centre, Wimblehurst Road, Horshara, West Sussex, RHI2 5AB, UK.
Received 10 July 1998; accepted 11 August 1998 Abstract The synthesis of a resin bound diazo species has been achieved in a highly efficient manner starting from commercially available reagents. This has produced an activated linker, providing a chemoselective method for the attachment of functionalised carboxylic acids to the solid phase via the Wang linker, in a rapid and colourometric reaction. O 1998ElsevierScienceLtd. All rightsreserved. Keywords: Diazo compounds;Hydrazones;Racemisation;Solid phase synthesis
Combinatorial chemistry has become popular amongst medicinal chemists for both lead discovery and lead optimization [1]. This partially emanates from the ability to accelerate the chemical synthesis of target molecules by overcoming the need for labour intensive purification by conventional methods. Attachment of the template to a solid support allows the use of washing and filtration as the sole method of purification during a synthesis. A central component in solid phase synthesis is the linker [2]. The linker is used to attach the compound to be synthesized to the solid support. One of the most widely used linkers for the attachment of carboxylic acids to solid support is the Wang Linker 1 [3]. Originally designed for peptide synthesis using the Fmoc protecting group strategy, this linker has been adopted by solid phase medicinal chemists to produce non peptide based libraries. The versatile nature of this linker allows for the cleavage of the tethered compound by acidolysis with TFA, or by nucleophilic cleavage [4] as in our recent benzodiazepine synthesis [5].
00~~/OH Wang Linker
Activated D i m Linker
1
2
*[email protected]
0040-4039/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(98)01706-7
7804 In solid phase synthesis attachment of the first residue to the Wang linker I usually employs one of the plethora of methods conceived by peptide chemists [6]. These proceed with carboxyl activation, often introducing the possibility of racemisation of an o~-stereogenic centre. Usually a large excess of the carboxy residue is used to ensure high loading, thus making this wasteful. We now report the synthesis of our activated diazo linker 2 which counters each of these drawbacks. The linker is based upon phenyldiazomethane, the properties of which are described in [7]. We decided to investigate the use of this reactive reagent in a resin bound form. In contrast to phenyldiazomethane, the resin bound analogue appears to be stable even when exposed to the atmosphere and light at R.T., for weeks, but decomposes within seconds, with loss of colour and without incident (lg scale) when heated to 100°C. The solid phase synthesis (Scheme 1) was initiated by treating commercially available Wang aldehyde resin [8] 3 with 2,4,6-triisopropylbenzenesulfonyl hydrazide 6 (2 equiv, THF, 16h, R.T.) to give the resin bound sulfonyl hydrazone 4 as an orange coloured resin. The reaction was monitored using infra-red (recorded using Nicolet Avatar 360 FT-IR directly on the resin beads) spectroscopy by observing the disappearance of the aldehyde signal (1680cm1), the washings were retained to recover the unused hydrazide 6. Next the diazo species 2 was generated using the solution phase protocol of Reese [9]. The orange coloured hydrazone 4 was treated with potassium hydroxide (2 equiv) in methanol/THF (1:4 v/v) mixture at 90°C for 7 mins, during which time the resin turned deep red. Infra-red spectroscopy showed an intense stretch at 2050cm1 indicating the formation of the diazo species 2. The subsequent esterification reactions were carded out at room temperature by adding a solution of the carboxylic acid 7 (1.2 equiv in DCM, THF or DMF) to the activated resin. The reactions of 8, 9 and 12 were over in 5rains, 13 took 20mins and 10, 11 required 90rains; and all are accompanied by the evolution of nitrogen gas. In all cases the deep red colour of the resin 2 becomes yellow (except 10, 11 turns pink) during the reaction, providing a convenient visual end-point to esterification, giving the resin bound ester 5. The esterification reaction requires the protonation of the diazo alkane to give the corresponding benzyl diazonium salt which undergoes nucleophilic attack to give the benzyl ester with the evolution of nitrogen gas. The proposed mechanism explains the chemoselectivity observed for the attachment of hydroxy acids such as 10, 11 and 12 (Scheme 2)
7805
without the need for alcohol protection. Free amino acids such as phenylalanine 13 can also be attached to solid support without resorting to N-protection by treating the zwitterionic amino acid with p-toluenesulfonic acid to protonate the amine [7]. Each of the acids 8-13 were attached and cleaved from resin (TFA/H20 95:5 v/v, R.T., 20mins) in excellent yields (>85%, based upon the loading of 3 over three steps) and purities (>90% by IH-nmr). H2N-N-Trs 6 i
=
/Trs
~ N - N 0
°
H
H
3 Trs=
I" RCOOH
o
7
~---N
e-o
iii 5
Reagents: i) 6 (2 exluiv),THF, R.T. 16h, ii) KOH (2 equiv), MeOI-VrHF(1:4v/v),90°C, 7mins., iii) 7 (I .2 exluiv),DCM, R.T. 5mins.
Scheme 1.
Synthesis of the activated diazo linker 2.
O
O
OH
O ~.~ o
B(OH)2
NHFmoc
HO
; OH
8(92%, 91%)
10 (90%, 95%)
12 (88%, 94%)
OH
O
HO HO
9 (95%, 98%) Scheme 2.
HO : NHFmoc
11 (86%, 95%)
! NH; TsO"
13 (91%, 94%)
Range of acids attached to resin, yields and purities shown respectively in parenthesis.
7806
Typical Experimental Procedure To the Wang aldehyde resin 3 (4.0g, 12.96mmol) was added 2,4,6-triisopropylbenzenesufonyl hydrazide 6 (7.7g, 25.92mmol) as a solution in dry THF (40nil). The suspension was stirred for 16h at R.T., filtered and washed in the usual way with MeOH and DCM, to give the orange coloured resin bound hydrazone 4. Dry THF (40ml) was added and the resin allowed to preswell for 1Groins before the addition of a solution of potassium hydroxide (1.5g, 25.92mmol) in dry methanol (10ml). The flask was immersed into a hot oil bath at 90°C for 7rains, during which time the resin became deep red in colour. The resin was filtered and washed with dry DCM [IR (2050 cml)]. To the activated resin 2 (300mg, 0.97retool) in DMF (5ml) at R.T., was added a solution of the carboxylic acid 8 (200mg, 1.20mmol) in DMF (10ml). The resin instantly turned yellow and effervesence was observed. After 5rains the resin was filtered and washed in the usual way with MeOH and DCM, to give the resin bound ester [IR (1730cm1)]. In conclusion, we have described the first reported synthesis of an activated diazo linker in two steps starting from readily available reagents. This highly coloured resin provides a chemoselective method for the rapid attachment of carboxylic acids to resin via the Wang linker with a visible end-point. The reaction proceeds by a mechanism which should not compromise the integrity of an ¢~-stereogenic centre. In addition, the activated resin could also be used as a scavenger for carboxylic acids.
References [1]
(a) Terrett NK, Gardener M, Gordon DW, Kobylecki RJ, Steele J. Tetrahedron 1995;30:8135-8173. (b) Balkenhohi F, Bussche-Hunnefeld C, Lansky A, Zechel C. Angew. Chem. Int. Ed. Engl. 1996;35:2288-2337. (c) Nefzi A, Ostresh JM, Houghten RA. Chem. Rev. 1997: 449-472.
[2]
Terrett NK. Combinatorial Chemistry. Oxford: Oxford University Press, 1998.
[3]
Wang S-S. J. Am. Chem. Soc. 1973;95:1328-1333.
[4]
(a) DeWitt SH, Kiely JS, Stankovic CJ, Schroeder MC, Cody DMR, Pavia MR. Proc. Natl. Acad. Sci. USA. 1993;32:69096913. (b) Gordon DW, Steele J. Bioorg. Med. Chem. Lett. 1995;5:47-50.
[5]
Bhalay G, Blaney P, Palmer VH, Baxter AD. Tetrahedron Lett. 1997;38:8375-8378.
[6]
Jones JH. The Chemical Synthesis of Peptides. Oxford: Oxford University Press, 199 !.
[7]
Sammakia T. In: Paquette LA, editor. Reagents for Organic Synthesis. New York: John Wiley & Sons, 1995:3969-3971.
[8]
Available from Novabiochem.
[9]
Reese CB, Dudman CC. Synthesis 1982;5:419-421.