- Email: [email protected]
Facile synthesis of α-hydroxy carboxylic acids from the corresponding α-amino acids
Tetrahedron Letters 55 (2014) 4149–4151
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Facile synthesis of a-hydroxy carboxylic acids from the corresponding a-amino acids Nicolai Stuhr-Hansen a,b,⇑, Shahrokh Padrah a, Kristian Strømgaard a a b
Department of Drug Design and Pharmacology, University of Copenhagen, Universitetsparken 2, DK-2100 Copenhagen, Denmark Department of Chemistry, Faculty of Sciences, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark
a r t i c l e
i n f o
Article history: Received 10 April 2014 Revised 13 May 2014 Accepted 22 May 2014 Available online 9 June 2014 Keywords: a-Hydroxy carboxylic acids Diazotization Hydroxylation Depsipeptides
a b s t r a c t An effective and improved procedure is developed for the synthesis of a-hydroxy carboxylic acids by treatment of the corresponding protonated a-amino acid with tert-butyl nitrite in 1,4-dioxane–water. The amino moiety must be protonated and located a to a carboxylic acid function in order to undergo initial diazotization and successive hydroxylation, since neither b-amino acids nor acid derivatives such as esters and amides undergo hydroxylations. The method is successfully applied for the synthesis of 18 proteinogenic amino acids. Ó 2014 Elsevier Ltd. All rights reserved.
Peptides containing ester bonds in their backbone, which are known as depsipeptides,1 are found in Nature, often from marine sources, as cyclic depsipeptides2 with many exciting biological properties. Most notably, romidepsin is a histone deacetylase (HDAC) inhibitor and an approved drug for the treatment of cutaneous T-cell lymphoma.3 Depsipeptides are potential drug targets displaying effects on drug resistance in tumor cells4 and are potent antitumor agents.5–7 Furthermore, they exhibit antitubercular activity8 and inhibit S1-serine protease.9 In addition, natural depsipeptides such as antillatoxin,10 somamide A,11 apratoxins,12 lyngbyabelins,13 spiruchostatin,14 and stigonemapeptin15 have been isolated from various species. Amide-to-ester mutations16 have been introduced in proteins as a method17 to evaluate, for example, the effects of specific protein backbone–backbone hydrogen bonds on protein folding. By replacing the backbone amide with an ester,18 the hydrogen bonding ability of the amide NH is removed, and the carbonyl becomes a much weaker hydrogen bond acceptor19 in accordance with theoretical investigations.20 Recently, we employed amideto-ester mutations both in peptide ligands and in proteins to evaluate the importance of backbone hydrogen bonds for protein– protein interactions, specifically the interaction of peptide ligands with PSD-95/Discs-large/ZO-1 (PDZ) domains.21,22 The synthesis of depsipeptides has typically been conducted by solid-phase peptide synthesis,23 introducing the appropriate ester moieties by incorporating a-hydroxy carboxylic acids either using ⇑ Corresponding author. Tel.: +45 22353153. E-mail address: [email protected] (N. Stuhr-Hansen). http://dx.doi.org/10.1016/j.tetlet.2014.05.090 0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
peptide coupling reagents24,25 or by application of the hexafluoroacetone bidentate protecting/activating technique.26,27 a-Hydroxy carboxylic acids have also been used as synthetic tools for enantiomeric additions to aldehydes facilitating the synthesis of optically active alcohols28 and as monomers for poly(a-hydroxy) acids29 with potential for production of biodegradable polymers.30 Only a few appropriately side-chain protected a-hydroxy carboxylic acids for use in Boc-based solid phase chemistry are commercially available in acceptable quantities and purities, these include: glycolic acid (HO-Gly-OH), (S)-lactic acid (HO-Ala-OH), (S)-2-hydroxyisocaproic acid (HO-Leu-OH), (2S,3R)-2-hydroxy-3methylpentanoic acid (HO-Ile-OH), (S)-2-hydroxy-3-methylbutyric acid (HO-Val-OH, valic acid), (S)-3-phenyllactic acid (HO-Phe-OH), and (S)-3-(benzyloxy)-2-hydroxypropanoic acid (HO-Ser(Bn)-OH). Furthermore, the synthetic challenges associated with obtaining side-chain protected a-hydroxy carboxylic acids for incorporation in depsipeptides have led researchers to mutate peptides and proteins, that is, not using the native residues, due to the unavailability of these building blocks. Even though side-chain protected a-hydroxy carboxylic acids are versatile building blocks with a broad range of applications, they have previously been synthesized exclusively by a diazotization procedure using HNO2 generated from sodium nitrite under protic conditions.31 In some cases these conditions are too harsh for side-chain protecting groups,32 and in our hands only a limited selection of a-hydroxy carboxylic acids could be isolated in satisfactory yields and purities using this diazotization procedure. It was therefore desirable to employ milder conditions for the synthesis of these important synthons and herein we describe the synthesis of a-hydroxy carboxylic acids
4150
N. Stuhr-Hansen et al. / Tetrahedron Letters 55 (2014) 4149–4151 R
R OH
BocHN
TFA
O
vs. commercially available HO-Ala-OH (approximately +2.5; c = 2.5 in water, 20 °C) and HO-Phe-OH (approximately +21; c = 2.0 in water, 20 °C) were recorded. By performing chiral HPLC on a Crownpack CR( ) column (Chiral Technologies Europe) to examine the optical purities of the synthesized HO-Ala-OH, HOLeu-OH, HO-Val-OH, and HO-Phe-OH versus the corresponding four commercially available hydroxy acids, no indication of inversion of the configurations was observed. The use of the synthesized R-HO-Phe-OH and R-HO-Val-OH as internal standards resulted in different retention times being demonstrated compared with the corresponding S-hydroxy acids, with clear base-line separations. Based on these chirality measurements it can be concluded that the enantiomeric excesses of the examined synthetic a-hydroxy carboxylic acids were above 99% with full retention of configuration in agreement with reported literature values.36 These observations allowed us to postulate a mechanism proceeding via intramolecular hydroxy group attack at the diazonium ion generated by diazotization of the amine, followed by solvolysis of the intermediate oxiranium ion (Scheme 2). Boc-glutamine (1) could not be converted into the corresponding a-hydroxy acid (HO-Gln-OH). Almost no reaction was observed utilizing the general conditions. However, upon concentration and prolonged stirring, glutamic acid (2) (61% yield) and an inseparable approximately 4:1 mixture of 5-oxo-proline (3) and 5-oxo-tetrahydrofuran-2-carboxylic acid (4) (22% overall yield, Scheme 3) were isolated by HPLC, but HO-Gln-OH was not formed. Compound 2 is presumably formed by initial diazotization of the amide function followed by hydrolysis.37 Diazotization of either of the two NH2 moieties and spontaneous ring closure affords 3 independent of which nitrogen was diazotized. Compound 4 may be formed by lactonization of HO-Glu-OH formed by diazotization and successive hydroxylation of both NH2 moieties (see Supporting information). In order to examine the importance of a free carboxylic acid in the a-amino acid for the diazotization/hydroxylation procedure, we subjected the ethyl ester of alanine to the diazotization conditions. No hydroxylated products were observed and the unreacted ester could be isolated in quantitative yield. This shows that a free carboxylic acid moiety is essential for formation of the diazonium ion and therefore for overall hydroxylation to occur. The importance of the position of the free carboxylic acid moiety for diazotization to occur was demonstrated by attempting the reaction with b-phenylalanine, which, in contrast to a-phenylalanine, did not undergo reaction and unreacted starting material was isolated in quantitative yield. The latter experiments demonstrate that an acarboxylic acid assisted formation of the diazonium ion was key in the formation of the corresponding hydroxylated compounds. Furthermore, when attempting reactions of zwitterionic amino acids in the absence of additional acid, diazotization was not
R t-BuONO 1,4-dioxane-H 2O rt, 1 h
OH
+ H 3N O
OH HO O
Scheme 1.
Table 1 Boc-AA(Pg)-OH
i. TFA, rt, 15 min ii. t-BuONO, 1,4-dioxane-H 2O (1:1) rt, 1 h
HO-AA(Pg)-OH
HO-AA(Pg)-OH
Yield (%)
HO-AA(Pg)-OH
Yield (%)
HO-Ala-OH HO-lle-OH HO-Phe-OH HO-Met-OH HO-Ser(Bn)-OH HO-Thr(Bn)-OH HO-Tyr(Bn)-OH HO-Cys(4-Me-Bn)-OH
58 21 53 15 41 36 27 8
HO-Lys(2-Cl-Z)-OH HO-Asp(cHx)-OH HO-Glu(cHx)-OH HO-Asn-OH HO-Gln-OH HO-His(Bom)-OH HO-Arg(Tos)-OH HO-Trp(CHO)-OH
49 35 62 51 — 72 15 18
Pg = side chain protecting group.
directly from appropriately side-chain protected Boc-solid-phase peptide chemistry compatible a-amino acids (Scheme 1). In order to develop a general method for the synthesis of a-hydroxy carboxylic acids from the possible 19 proteinogenic amino acids which have primary amino groups, we exploited the mildest possible diazotization conditions. Since these hydroxy acids are to be applied primarily for the construction of depsipeptides or proteins involving Boc-based solid-phase peptide synthesis,33 it was therefore necessary to utilize side-chain protecting groups that tolerate TFA, but on the other hand, that can be cleaved with HF34 in the final step. The standard properly side-chain protected Boc-amino acids used in Boc-based SPPS fulfilled these requirements. On treating these compounds with TFA the protonated amine salts were obtained. Because diazotization of these amino acids with aqueous sodium nitrite in the presence of a strong acid had not been applied for the synthesis of all nineteen a-hydroxy carboxylic acids,32 milder alternative conditions were sought in order to avoid the formation of undesired decomposition products. Treatment of the TFA-salts of the proteinogenic amino acids with one equivalent of tert-butyl nitrite in a 1:1 mixture of water–1,4-dioxane (Scheme 1) gave 18 of the corresponding a-hydroxy carboxylic acids (Table 1).35 All the a-hydroxy carboxylic acids were isolated directly by normal phase chromatography, except HO-Arg(Tos)-OH and HO-His(Bom)-OH, which were purified by HPLC. Identical optical rotations of the synthesized
R H
R H H2N O
+
NO - H2O
.. OH
+
N N
R H -N 2
H2O
O
R H +
..
OH
O
H
R H OH
+
H 2O
+
-H
O
O
O
Scheme 2.
O
Boc-Gln-OH 1
i. TFA, rt, 15 min ii. t-BuONO, 1,4-dioxane-H2O (1:1) rt, 100 h
OH
OH
H 2N
+ O
2 (61%)
Scheme 3.
O
N H 3 (18%)
CO2H +
OH
HO
O
O 4 (4%)
CO2H
N. Stuhr-Hansen et al. / Tetrahedron Letters 55 (2014) 4149–4151
observed. This suggests that for diazotization to occur and subsequent hydroxylation, the amine had to be protonated and needed a carboxylic acid function present on the a-carbon. One of the advances utilizing the presented diazotization procedure is the mild conditions employed, since it proved possible to prepare the a-hydroxy carboxylic acids: HO-Met-OH, HO-Cys(4Me-Bn)-OH, and HO-His(Bom)-OH by direct diazotization using these conditions, which is not possible utilizing sodium nitrite based procedures from Boc-compatible amino acids. In these cases it was necessary32 to prepare the hydroxyl acids using ‘non-diazo conditions’ via two-step routes due to by-product formation. Direct HPLC purification was possible for all the a-hydroxy carboxylic acids, but traces of nitrites and 1,4-dioxane proved to shorten the life-time of reverse phase columns. Therefore, normal phase purification is recommended in general cases. In conclusion, we have synthesized 18 a-hydroxy carboxylic acids from the corresponding proteinogenic a-amino acids using mild diazotization conditions. All the products were synthesized directly from the appropriately protected Boc-chemistry compatible a-amino acids. The only anomaly apart from the logical exception proline was glutamine, which afforded glutamic acid, 5-oxo-proline, and 5-oxo-tetrahydrofuran-2-carboxylic acid instead of the corresponding a-hydroxy carboxylic acid (HO-Glu-OH). Furthermore, a short mechanistic survey revealed that diazotization and successive hydroxylation of an amine using tert-butyl nitrite only took place when a free carboxylic acid was present at the a-carbon. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.05. 090. References and notes 1. Shemyakin, M. M.; Ovchinnikov, Y. A. Recent Dev. Chem. Nat. Carbon Comp. 1967, 2, 1–46. 2. Hamada, Y.; Shioiri, T. Chem. Rev. 2005, 105, 4441–4482. 3. Prince, H. M.; Dickinson, M.; Khot, A. Future Oncol. 2013, 9, 1819–1827. 4. Ogino, J.; Moore, R. E.; Patterson, G. M. L.; Smith, C. D. J. Nat. Prod. 1996, 59, 581–586. 5. Gamble, W. R.; Durso, N. A.; Fuller, R. W.; Westergaard, C. K.; Johnson, T. R.; Sackett, D. L.; Hamel, E.; Cardellina Ii, J. H.; Boyd, M. R. Bioorg. Med. Chem. 1999, 7, 1611–1615. 6. Hamel, E. Biopolymers 2002, 66, 142–160. 7. Meickle, T.; Gunasekera, S. P.; Liu, Y.; Luesch, H.; Paul, V. J. Bioorg. Med. Chem. 2011, 19, 6576–6580.
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8. Narayanaswamy, V. K.; Albericio, F.; Coovadia, Y. M.; Kruger, H. G.; Maguire, G. E. M.; Pillay, M.; Govender, T. J. Pept. Sci. 2011, 17, 683–689. 9. Stolze, S. C.; Meltzer, M.; Ehrmann, M.; Kaiser, M. Eur. J. Org. Chem. 2012, 1616– 1625. 10. Orjala, J.; Nagle, D. G.; Hsu, V.; Gerwick, W. H. J. Am. Chem. Soc. 1995, 117, 8281–8282. 11. Nogle, L. M.; Williamson, R. T.; Gerwick, W. H. J. Nat. Prod. 2001, 64, 716–719. 12. Luesch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J.; Corbett, T. H. J. Am. Chem. Soc. 2001, 123, 5418–5423. 13. Luesch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J. J. Nat. Prod. 2000, 63, 1437– 1439. 14. Ford, P. W.; Gustafson, K. R.; McKee, T. C.; Shigematsu, N.; Maurizi, L. K.; Pannell, L. K.; Williams, D. E.; Dilip de Silva, E.; Lassota, P.; Allen, T. M. J. Am. Chem. Soc. 1999, 121, 5899–5909. 15. Kang, H.-S.; Krunic, A.; Orjala, J. J. Nat. Prod. 2012, 75, 807–811. 16. Deechongkit, S.; Nguyen, H.; Powers, E. T.; Dawson, P. E.; Gruebele, M.; Kelly, J. W. Nature 2004, 430, 101–105. 17. Powers, E. T.; Deechongkit, S.; Kelly, J. W. In Advances in Protein Chem; Robert, L. B., David, B., Eds.; Academic Press, 2005; 72, pp 39–78. 18. Valiyaveetil, F. I.; Sekedat, M.; MacKinnon, R.; Muir, T. W. J. Am. Chem. Soc. 2006, 128, 11591–11599. 19. Jaffe, H. H.; Freedman, L. D.; Doak, G. O. J. Am. Chem. Soc. 1953, 75, 2209–2211. 20. Kang, Y. K.; Byun, B. J. J. Phys. Chem. B 2008, 112, 9126–9134. 21. Eildal, J. N.; Hultqvist, G.; Balle, T.; Stuhr-Hansen, N.; Padrah, S.; Gianni, S.; Strømgaard, K.; Jemth, P. J. Am. Chem. Soc. 2013, 135, 12998–13007. 22. Pedersen, S. W.; Pedersen, S. B.; Anker, L.; Hultqvist, G.; Kristensen, A. S.; Jemth, P.; Strømgaard, K. Nat. Commun. 2014, 5, 3215. 23. Coin, I. J. Pept. Sci. 2010, 16, 223–230. 24. Avan, I.; Tala, S. R.; Steel, P. J.; Katritzky, A. R. J. Org. Chem. 2011, 76, 4884–4893. 25. Lüttenberg, S.; Sondermann, F.; Scherkenbeck, J. Tetrahedron 2012, 68, 2068– 2073. 26. Albericio, F.; Burger, K.; Ruíz-Rodríguez, J.; Spengler, J. Org. Lett. 2005, 7, 597– 600. 27. Isidro-Llobet, A.; Alvarez, M.; Burger, K.; Spengler, J.; Albericio, F. Org. Lett. 2007, 9, 1429–1432. 28. Bauer, T.; Gajewiak, J. Tetrahedron 2004, 60, 9163–9170. 29. Leemhuis, M.; van Steenis, Jan H.; van Uxem, Michelle J.; van Nostrum, Cornelus F.; Hennink, Wim E. Eur. J. Org. Chem. 2003, 3344–3349. 30. Cohen-Arazi, N.; Katzhendler, J.; Kolitz, M.; Domb, A. J. Macromolecules 2008, 41, 7259–7263. 31. Winitz, M.; Bloch-Frankenthal, L.; Izumiya, N.; Birnbaum, S. M.; Baker, C. G.; Greenstein, J. P. J. Am. Chem. Soc. 1956, 78, 2423–2430. 32. Deechongkit, S.; You, S.-L.; Kelly, J. W. Org. Lett. 2004, 6, 497–500. 33. Hojo, H.; Aimoto, S. Bull. Chem. Soc. Jpn. 1991, 64, 111–117. 34. Hackeng, T. M.; Griffin, J. H.; Dawson, P. E. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 10068–10073. 35. Typical procedure: The Boc-protected amino acid (1 mmol) was treated with TFA (3 mL) for 15 min. After evaporation, the residue was dissolved in 1,4dioxane–water (1:1, 4 mL) and the flask was placed in an ice bath. tertButylnitrite (0.13 mL, 1.1 mmol) was added and stirring was maintained under nitrogen at room temperature for 1 h. After pouring the reaction mixture onto Celite and evaporation (50 Pa, 30 °C) into a dry free-floating powder, separation was performed on silica with AcOH–MeOH–EtOAc (1:9:90) as the eluent affording the pure a-hydroxy carboxylic acid in the yield specified in Table 1. 36. Brewster, P.; Hughes, E. D.; Ingold, C. K.; Rao, P. A. D. S. Nature 1950, 166, 178– 179. 37. Yong, H. K.; Kweon, K.; Young, J. P. Tetrahedron Lett. 1990, 31, 3893–3894.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Facile synthesis of a-hydroxy carboxylic acids from the corresponding a-amino acids Nicolai Stuhr-Hansen a,b,⇑, Shahrokh Padrah a, Kristian Strømgaard a a b
Department of Drug Design and Pharmacology, University of Copenhagen, Universitetsparken 2, DK-2100 Copenhagen, Denmark Department of Chemistry, Faculty of Sciences, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark
a r t i c l e
i n f o
Article history: Received 10 April 2014 Revised 13 May 2014 Accepted 22 May 2014 Available online 9 June 2014 Keywords: a-Hydroxy carboxylic acids Diazotization Hydroxylation Depsipeptides
a b s t r a c t An effective and improved procedure is developed for the synthesis of a-hydroxy carboxylic acids by treatment of the corresponding protonated a-amino acid with tert-butyl nitrite in 1,4-dioxane–water. The amino moiety must be protonated and located a to a carboxylic acid function in order to undergo initial diazotization and successive hydroxylation, since neither b-amino acids nor acid derivatives such as esters and amides undergo hydroxylations. The method is successfully applied for the synthesis of 18 proteinogenic amino acids. Ó 2014 Elsevier Ltd. All rights reserved.
Peptides containing ester bonds in their backbone, which are known as depsipeptides,1 are found in Nature, often from marine sources, as cyclic depsipeptides2 with many exciting biological properties. Most notably, romidepsin is a histone deacetylase (HDAC) inhibitor and an approved drug for the treatment of cutaneous T-cell lymphoma.3 Depsipeptides are potential drug targets displaying effects on drug resistance in tumor cells4 and are potent antitumor agents.5–7 Furthermore, they exhibit antitubercular activity8 and inhibit S1-serine protease.9 In addition, natural depsipeptides such as antillatoxin,10 somamide A,11 apratoxins,12 lyngbyabelins,13 spiruchostatin,14 and stigonemapeptin15 have been isolated from various species. Amide-to-ester mutations16 have been introduced in proteins as a method17 to evaluate, for example, the effects of specific protein backbone–backbone hydrogen bonds on protein folding. By replacing the backbone amide with an ester,18 the hydrogen bonding ability of the amide NH is removed, and the carbonyl becomes a much weaker hydrogen bond acceptor19 in accordance with theoretical investigations.20 Recently, we employed amideto-ester mutations both in peptide ligands and in proteins to evaluate the importance of backbone hydrogen bonds for protein– protein interactions, specifically the interaction of peptide ligands with PSD-95/Discs-large/ZO-1 (PDZ) domains.21,22 The synthesis of depsipeptides has typically been conducted by solid-phase peptide synthesis,23 introducing the appropriate ester moieties by incorporating a-hydroxy carboxylic acids either using ⇑ Corresponding author. Tel.: +45 22353153. E-mail address: [email protected] (N. Stuhr-Hansen). http://dx.doi.org/10.1016/j.tetlet.2014.05.090 0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
peptide coupling reagents24,25 or by application of the hexafluoroacetone bidentate protecting/activating technique.26,27 a-Hydroxy carboxylic acids have also been used as synthetic tools for enantiomeric additions to aldehydes facilitating the synthesis of optically active alcohols28 and as monomers for poly(a-hydroxy) acids29 with potential for production of biodegradable polymers.30 Only a few appropriately side-chain protected a-hydroxy carboxylic acids for use in Boc-based solid phase chemistry are commercially available in acceptable quantities and purities, these include: glycolic acid (HO-Gly-OH), (S)-lactic acid (HO-Ala-OH), (S)-2-hydroxyisocaproic acid (HO-Leu-OH), (2S,3R)-2-hydroxy-3methylpentanoic acid (HO-Ile-OH), (S)-2-hydroxy-3-methylbutyric acid (HO-Val-OH, valic acid), (S)-3-phenyllactic acid (HO-Phe-OH), and (S)-3-(benzyloxy)-2-hydroxypropanoic acid (HO-Ser(Bn)-OH). Furthermore, the synthetic challenges associated with obtaining side-chain protected a-hydroxy carboxylic acids for incorporation in depsipeptides have led researchers to mutate peptides and proteins, that is, not using the native residues, due to the unavailability of these building blocks. Even though side-chain protected a-hydroxy carboxylic acids are versatile building blocks with a broad range of applications, they have previously been synthesized exclusively by a diazotization procedure using HNO2 generated from sodium nitrite under protic conditions.31 In some cases these conditions are too harsh for side-chain protecting groups,32 and in our hands only a limited selection of a-hydroxy carboxylic acids could be isolated in satisfactory yields and purities using this diazotization procedure. It was therefore desirable to employ milder conditions for the synthesis of these important synthons and herein we describe the synthesis of a-hydroxy carboxylic acids
4150
N. Stuhr-Hansen et al. / Tetrahedron Letters 55 (2014) 4149–4151 R
R OH
BocHN
TFA
O
vs. commercially available HO-Ala-OH (approximately +2.5; c = 2.5 in water, 20 °C) and HO-Phe-OH (approximately +21; c = 2.0 in water, 20 °C) were recorded. By performing chiral HPLC on a Crownpack CR( ) column (Chiral Technologies Europe) to examine the optical purities of the synthesized HO-Ala-OH, HOLeu-OH, HO-Val-OH, and HO-Phe-OH versus the corresponding four commercially available hydroxy acids, no indication of inversion of the configurations was observed. The use of the synthesized R-HO-Phe-OH and R-HO-Val-OH as internal standards resulted in different retention times being demonstrated compared with the corresponding S-hydroxy acids, with clear base-line separations. Based on these chirality measurements it can be concluded that the enantiomeric excesses of the examined synthetic a-hydroxy carboxylic acids were above 99% with full retention of configuration in agreement with reported literature values.36 These observations allowed us to postulate a mechanism proceeding via intramolecular hydroxy group attack at the diazonium ion generated by diazotization of the amine, followed by solvolysis of the intermediate oxiranium ion (Scheme 2). Boc-glutamine (1) could not be converted into the corresponding a-hydroxy acid (HO-Gln-OH). Almost no reaction was observed utilizing the general conditions. However, upon concentration and prolonged stirring, glutamic acid (2) (61% yield) and an inseparable approximately 4:1 mixture of 5-oxo-proline (3) and 5-oxo-tetrahydrofuran-2-carboxylic acid (4) (22% overall yield, Scheme 3) were isolated by HPLC, but HO-Gln-OH was not formed. Compound 2 is presumably formed by initial diazotization of the amide function followed by hydrolysis.37 Diazotization of either of the two NH2 moieties and spontaneous ring closure affords 3 independent of which nitrogen was diazotized. Compound 4 may be formed by lactonization of HO-Glu-OH formed by diazotization and successive hydroxylation of both NH2 moieties (see Supporting information). In order to examine the importance of a free carboxylic acid in the a-amino acid for the diazotization/hydroxylation procedure, we subjected the ethyl ester of alanine to the diazotization conditions. No hydroxylated products were observed and the unreacted ester could be isolated in quantitative yield. This shows that a free carboxylic acid moiety is essential for formation of the diazonium ion and therefore for overall hydroxylation to occur. The importance of the position of the free carboxylic acid moiety for diazotization to occur was demonstrated by attempting the reaction with b-phenylalanine, which, in contrast to a-phenylalanine, did not undergo reaction and unreacted starting material was isolated in quantitative yield. The latter experiments demonstrate that an acarboxylic acid assisted formation of the diazonium ion was key in the formation of the corresponding hydroxylated compounds. Furthermore, when attempting reactions of zwitterionic amino acids in the absence of additional acid, diazotization was not
R t-BuONO 1,4-dioxane-H 2O rt, 1 h
OH
+ H 3N O
OH HO O
Scheme 1.
Table 1 Boc-AA(Pg)-OH
i. TFA, rt, 15 min ii. t-BuONO, 1,4-dioxane-H 2O (1:1) rt, 1 h
HO-AA(Pg)-OH
HO-AA(Pg)-OH
Yield (%)
HO-AA(Pg)-OH
Yield (%)
HO-Ala-OH HO-lle-OH HO-Phe-OH HO-Met-OH HO-Ser(Bn)-OH HO-Thr(Bn)-OH HO-Tyr(Bn)-OH HO-Cys(4-Me-Bn)-OH
58 21 53 15 41 36 27 8
HO-Lys(2-Cl-Z)-OH HO-Asp(cHx)-OH HO-Glu(cHx)-OH HO-Asn-OH HO-Gln-OH HO-His(Bom)-OH HO-Arg(Tos)-OH HO-Trp(CHO)-OH
49 35 62 51 — 72 15 18
Pg = side chain protecting group.
directly from appropriately side-chain protected Boc-solid-phase peptide chemistry compatible a-amino acids (Scheme 1). In order to develop a general method for the synthesis of a-hydroxy carboxylic acids from the possible 19 proteinogenic amino acids which have primary amino groups, we exploited the mildest possible diazotization conditions. Since these hydroxy acids are to be applied primarily for the construction of depsipeptides or proteins involving Boc-based solid-phase peptide synthesis,33 it was therefore necessary to utilize side-chain protecting groups that tolerate TFA, but on the other hand, that can be cleaved with HF34 in the final step. The standard properly side-chain protected Boc-amino acids used in Boc-based SPPS fulfilled these requirements. On treating these compounds with TFA the protonated amine salts were obtained. Because diazotization of these amino acids with aqueous sodium nitrite in the presence of a strong acid had not been applied for the synthesis of all nineteen a-hydroxy carboxylic acids,32 milder alternative conditions were sought in order to avoid the formation of undesired decomposition products. Treatment of the TFA-salts of the proteinogenic amino acids with one equivalent of tert-butyl nitrite in a 1:1 mixture of water–1,4-dioxane (Scheme 1) gave 18 of the corresponding a-hydroxy carboxylic acids (Table 1).35 All the a-hydroxy carboxylic acids were isolated directly by normal phase chromatography, except HO-Arg(Tos)-OH and HO-His(Bom)-OH, which were purified by HPLC. Identical optical rotations of the synthesized
R H
R H H2N O
+
NO - H2O
.. OH
+
N N
R H -N 2
H2O
O
R H +
..
OH
O
H
R H OH
+
H 2O
+
-H
O
O
O
Scheme 2.
O
Boc-Gln-OH 1
i. TFA, rt, 15 min ii. t-BuONO, 1,4-dioxane-H2O (1:1) rt, 100 h
OH
OH
H 2N
+ O
2 (61%)
Scheme 3.
O
N H 3 (18%)
CO2H +
OH
HO
O
O 4 (4%)
CO2H
N. Stuhr-Hansen et al. / Tetrahedron Letters 55 (2014) 4149–4151
observed. This suggests that for diazotization to occur and subsequent hydroxylation, the amine had to be protonated and needed a carboxylic acid function present on the a-carbon. One of the advances utilizing the presented diazotization procedure is the mild conditions employed, since it proved possible to prepare the a-hydroxy carboxylic acids: HO-Met-OH, HO-Cys(4Me-Bn)-OH, and HO-His(Bom)-OH by direct diazotization using these conditions, which is not possible utilizing sodium nitrite based procedures from Boc-compatible amino acids. In these cases it was necessary32 to prepare the hydroxyl acids using ‘non-diazo conditions’ via two-step routes due to by-product formation. Direct HPLC purification was possible for all the a-hydroxy carboxylic acids, but traces of nitrites and 1,4-dioxane proved to shorten the life-time of reverse phase columns. Therefore, normal phase purification is recommended in general cases. In conclusion, we have synthesized 18 a-hydroxy carboxylic acids from the corresponding proteinogenic a-amino acids using mild diazotization conditions. All the products were synthesized directly from the appropriately protected Boc-chemistry compatible a-amino acids. The only anomaly apart from the logical exception proline was glutamine, which afforded glutamic acid, 5-oxo-proline, and 5-oxo-tetrahydrofuran-2-carboxylic acid instead of the corresponding a-hydroxy carboxylic acid (HO-Glu-OH). Furthermore, a short mechanistic survey revealed that diazotization and successive hydroxylation of an amine using tert-butyl nitrite only took place when a free carboxylic acid was present at the a-carbon. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.05. 090. References and notes 1. Shemyakin, M. M.; Ovchinnikov, Y. A. Recent Dev. Chem. Nat. Carbon Comp. 1967, 2, 1–46. 2. Hamada, Y.; Shioiri, T. Chem. Rev. 2005, 105, 4441–4482. 3. Prince, H. M.; Dickinson, M.; Khot, A. Future Oncol. 2013, 9, 1819–1827. 4. Ogino, J.; Moore, R. E.; Patterson, G. M. L.; Smith, C. D. J. Nat. Prod. 1996, 59, 581–586. 5. Gamble, W. R.; Durso, N. A.; Fuller, R. W.; Westergaard, C. K.; Johnson, T. R.; Sackett, D. L.; Hamel, E.; Cardellina Ii, J. H.; Boyd, M. R. Bioorg. Med. Chem. 1999, 7, 1611–1615. 6. Hamel, E. Biopolymers 2002, 66, 142–160. 7. Meickle, T.; Gunasekera, S. P.; Liu, Y.; Luesch, H.; Paul, V. J. Bioorg. Med. Chem. 2011, 19, 6576–6580.
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