Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Catalytic Oligopeptide Synthesis Zijian Liu, Hidetoshi Noda, Masakatsu Shibasaki,* and Naoya Kumagai* Institute of Microbial Chemistry (BIKAKEN), Tokyo, 3-14-23 Kamiosaki, Shinagawa-ku, Tokyo 141-0021, Japan S Supporting Information *
ABSTRACT: Waste-free catalytic assembly of α-amino acids is fueled by a multiboron catalyst that features a characteristic B3NO2 heterocycle, providing a versatile catalytic protocol wherein functionalized natural α-amino acid units are accommodated and commonly used protecting groups are tolerated. The facile dehydrative conditions eliminate the use of engineered peptide coupling reagents, exemplifying a greener catalytic alternative for peptide coupling. The catalysis is sufficiently robust to enable pentapeptide synthesis, constructing all four amide bond linkages in a catalytic fashion.
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mide is an omnipresent functional group1 interconnecting carboxylic acid and amine fragments in diverse natural products and biomacromolecules, as well as artificial chemical entities, e.g., functional polymers and small molecule therapeutics. The highly ubiquitous nature of the amide bond linkage is underscored by the recent statistical analysis revealing that amidation is by far the most frequently used chemical transformation in medicinal chemistry.2 The sustained demand for amides has advanced synthetic means of accessing this class of compounds,3 producing a vast array of reagents that render carboxylic acids and amines coupled under various conditions.4 The state-of-the-art chemical toolbox for amide bond formation also includes a catalytic alternative to allow for dehydrative amidation to avoid coproduction of reagentderived waste,5 which is never fulfilled by reagent-driven amidation. While more than two decades have passed since the first catalytic amidation,6 neither boron-based catalysts7 nor group IV metal catalysts8 have been widely adopted in practical synthesis due to the limited catalytic activity and substrate generality. In particular, embracing α-amino acids as substrates to catalytically forge amide bonds of peptides, a representative amide-rich chemical entity of significant biological interest, remains challenging to synthetic chemists. Prior studies using boronic acid,7l,m,9 borinic acids,7n or borates7r as catalysts are plagued with low catalyst turnover, and the use of unconventional protecting groups limited their synthetic utility (Scheme 1A). Moreover, α-amino acids bearing heteroatom functional residues are little explored.10 The chemical synthesis of peptides is a research area of significant biological and practical impact with high demand,11 and engineered and expensive coupling reagents4b,c are primarily utilized to couple α-amino acid units for reliable conversion and stereochemical integrity. In this context, we envisaged a systematic study on the catalytic amidation of α-amino acids with 1,3-dioxa-5-aza-2,4,6triborinane (DATB) catalyst 1, a multiboron catalyst amenable to dehydrative coupling of a wide range of substrate types (Scheme 1B).12 We reasoned that the high catalytic activity of 1 © XXXX American Chemical Society
Scheme 1. Drawbacks in Catalytic Peptide Synthesis (A) and This Work (B)
dictated by the distinct B3NO2 core would exhibit competence for catalytic peptide coupling. In this letter, we demonstrate that DATB 1 could be used to interconnect both nonfunctionalized and functionalized natural α-amino acid units in a catalytic manner, with turnover numbers (TON) ranging from 16 to 180. Commonly employed protecting groups13 were tolerated, and the synthesis of a biologically active pentapeptide highlights the synthetic utility of the present catalytic protocol. Our investigation of the catalytic coupling of α-amino acids commenced with a feasibility study to furnish nonfunctionalized tripeptides via the coupling of dipeptidyl acid Boc-GlyReceived: December 1, 2017
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DOI: 10.1021/acs.orglett.7b03735 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Leu-OH 2 and the HCl salt of α-amino acid Bn esters 3 (Scheme 2). Epimerization during the coupling reaction
Scheme 3. Catalytic Dipeptide Synthesis Using Functionalized N-Fmoc α-Amino Acids 5 and H-Leu-OBn HCl Salt 6
Scheme 2. Catalytic Tripeptide Synthesis Using Boc-GlyLeu-OH 2 and Nonfunctionalized α-Amino Acid HCl Salts 3
a
AA denotes amino acid. Isolated yield is reported. bEr and dr were determined by HPLC analysis.
is frequent concern in peptide synthesis and, more often, occurs at the α-position of an acid unit when the acid has an αN-acyl group due to the involvement of an intermediary oxazolone I by activation of the carboxyl group. Recently, we developed a catalytic amidation protocol using DATB 1 as a catalyst that was applicable to a wide range of carboxylic acids and amines.12 We anticipated that the primary driving force of DATB 1 catalysis is not the stringent electrophilic activation of carboxylic acid, and hence the epimerization via oxazolone I would be suppressed. Indeed, the amidation of Boc-Gly-LeuOH 2 and H-Gly-OBn·HCl 3a proceeded with 5 mol% of 1 at 80 °C in toluene, affording the desired tripeptide Boc-Gly-LeuGly-OBn 4a in 80% yield without any loss of enantiopurity.14 Sterically more demanding H-Phe-OBn·HCl 3b and H-LeuOBn·HCl 3c as amine units were also coupled under identical conditions to deliver the corresponding tripeptides 4b and 4c in decent yields without epimerization. The reliable catalytic access to tripeptides led us to incorporate α-amino acids with heteroatom functional groups in the DATB catalysis. For prospective application in solidphase peptide synthesis (SPPS),15 a series of N-Fmoc α-amino acids 5 bearing functionalized residues were submitted to catalytic dehydrative amidation with H-Leu-OBn·HCl 6 as an amine counterpart (Scheme 3). Functional groups at the residue were protected with commonly used representative protecting groups.13 For O-functionalized α-amino acids, Fmoc-Glu(Bn)-OH 5a, Fmoc-Gln(Tr)-OH 5b, Fmoc-Tyr(tBu)-OH 5c, and Fmoc-Thr(tBu)-OH 5d were coupled under standard conditions without notable stereochemical erosion and undesired deprotection with the exception of 7c, in which minor epimerization took place. While Fmoc-Ser(tBu)OH 5e exhibited partial epimerization at 80 °C, its high reactivity allowed for amide coupling at a lower temperature (50 °C) and dipeptide Fmoc-Ser(tBu)-Leu-OBn 7e was
a
AA denotes amino acid. Isolated yield is reported. bDr was determined by HPLC analysis. cPbf = 2,2,4,6,7-pentamethyl-2,3dihydrobenzofuran-5-sulfonyl.
obtained without notable loss of stereochemical integrity. NFunctionalized α-amino acids were generally compatible, and Fmoc-Lys(Boc)-OH 5i, Fmoc-Trp(Boc)-OH 5j, Fmoc-Arg(Pbf)-OH 5k, and Fmoc-His(NτTr)-OH 5l having standard acid-labile protecting groups were coupled with H-Leu-OBn· HCl 6 in moderate yields with a dr >20/1. 5k was slightly prone to epimerization, and the reaction was performed at 60 °C. As frequently observed for reagent-driven peptide couplings,16 Cys was less tractable and partial epimerization was observed for the S-trityl protected substrate.17 The electron-rich PMB group was found to be exquisitely effective in suppressing epimerization, and catalytic coupling at 50 °C delivered Fmoc-Cys(PMB)-Leu-OBn 7f in 80% yield and dr >20/1.16b Another S-functionalized α-amino acid Fmoc-MetOH 5g was coupled with no deleterious effects. Fmoc-Pro-OH 5h exemplified reactions of other functionalized α-amino acids. Dipeptide formation using functionalized amine fragments was then briefly examined (Scheme 4). The racemization-prone αamino acid Ser in Scheme 3 was successfully coupled when HSer(Bn)-OMe·HCl 9a was submitted to catalytic amidation with Fmoc-Leu-OH 8, affording the product 10a with a high yield and a dr >20/1. Other representative O-functionalized B
DOI: 10.1021/acs.orglett.7b03735 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
°C to give dipeptide Fmoc-Gly-Gly-OtBu 13. After removal of the Fmoc group with Et2NH, coupling with Fmoc-Phe-OH 14 was performed with 5 mol% of 1, affording tripeptide fragment Fmoc-Phe-Gly-Gly-OtBu·HCl 15. The dipeptide fragment at the N-terminus, Fmoc-Tyr(tBu)-Gly-OH 16, was accessed by DATB 1-catalyzed coupling of Fmoc-Tyr(tBu)-OH 5c and HGly-OBn·HCl 3a, followed by hydrogenolysis of the Bn ester in 84% in two steps. DATB 1 was competent for fragment coupling of the thus-obtained tripeptide 15 and dipeptide 16 after Fmoc deprotection, furnishing the desired pentapeptide Fmoc-Tyr(tBu)-Gly-Phe-Gly-Gly-O tBu that conforms to OGP(10−14).19 In summary, catalytic amidations were demonstrated using αamino acids as substrates. Both nonfunctionalized and functionalized natural α-amino acids were accommodated without racemization in most cases. The standard protecting groups were tolerated, and undesirable deprotection during the catalysis was not observed. The synthetic utility of this catalytic protocol is highlighted by the catalytic assembly of a pentapeptide for OGP(10−14), a potent osteogenic growth peptide. Because DATB 1 became miscible upon coordination by amines in solution phase to exert catalytic function, 1 is unlikely competent in SPPS where amine fragments are in the solid phase. The development of a soluble and more active derivative of 1 for catalytic SPPS is ongoing.
Scheme 4. Catalytic Dipeptide Synthesis Using Fmoc-LeuOH 8 and Functionalized α-Amino Acid HCl Salts 9
a AA denotes amino acid. Isolated yield is reported. determined by HPLC analysis.
b
Dr was
(H-Glu(tBu)-OtBu·HCl 9b) and N-functionalized (H-Trp(Boc)-OtBu·HCl 9c, H-Lys(Boc)-OtBu·HCl 9d) amine units were coupled with Fmoc-Leu-OH 8 without compromising the stereochemical integrity. Of particular note is that amine·HCl salts, a readily available form of α-amino acids from commercial sources, were engaged in the catalytic amide coupling shown above without any pretreatment or additional use of bases, providing an operationally simple catalytic protocol of dehydrative peptide coupling. Successful implementation of α-amino acids in a catalytic amidation protocol provides the opportunity to access an oligopeptide via catalytic assembly. OGP(10−14), H-Tyr-GlyPhe-Gly-Gly-OH, is a biologically active pentapeptide that was truncated from the C-terminus of osteogenic growth peptide (OGP). OGP(10−14) is a naturally occurring osteoblastic mitogen that is equipotent to full-length tetradecapeptide OGP.18 Based on the absence of undesired oxazolone formation in the reaction of α-N-acyl carboxylic acid (Scheme 2), catalytic assembly of five α-amino acid components of OGP(10−14) was attempted in a convergent fashion (Scheme 5). Coupling of Fmoc-Gly-OH 11 and H-Gly-OtBu·HCl 12 reached completion with as little as 0.5 mol% of DATB 1 at 90
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03735. Experimental details, spectroscopic and HPLC data of products (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Hidetoshi Noda: 0000-0001-6529-8870 Masakatsu Shibasaki: 0000-0001-8862-582X
Scheme 5. Catalytic Assembly of Five α-Amino Acids To Afford Pentapeptide Fmoc-Tyr(tBu)-Gly-Phe-Gly-Gly-OtBu
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DOI: 10.1021/acs.orglett.7b03735 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
(11) (a) Kent, S. B. H. Annu. Rev. Biochem. 1988, 57, 957. (b) Humphrey, J. M.; Chamberlin, A. R. Chem. Rev. 1997, 97, 2243. (c) Dawson, P. E.; Kent, S. B. H. Annu. Rev. Biochem. 2000, 69, 923. (d) Bray, B. L. Nat. Rev. Drug Discovery 2003, 2, 587. (e) Nilsson, B. L.; Soellner, M. B.; Raines, R. T. Annu. Rev. Biophys. Biomol. Struct. 2005, 34, 91. (f) Kent, S. B. Chem. Soc. Rev. 2009, 38, 338. (12) (a) Noda, H.; Furutachi, M.; Asada, Y.; Shibasaki, M.; Kumagai, N. Nat. Chem. 2017, 9, 571. (b) Noda, H.; Asada, Y.; Shibasaki, M.; Kumagai, N. Chem. Commun. 2017, 53, 7447. (13) Isidro-Llobet, A.; Á lvarez, M.; Albericio, F. Chem. Rev. 2009, 109, 2455. (14) Stereochemical integrity was determined by HPLC analysis. See Supporting Infoirmation. (15) (a) Kent, S. B. Annu. Rev. Biochem. 1988, 57, 957. (b) Behrendt, R.; White, P.; Offer, J. J. Pept. Sci. 2016, 22, 4. (16) (a) Han, Y.; Albericio, F.; Barany, G. J. Org. Chem. 1997, 62, 4307. (b) Hibino, H.; Miki, Y.; Nishiuchi, Y. J. Pept. Sci. 2014, 20, 30. (17) The use of the conventional Tr protecting group for Cys led to partial racemization. See Supporting Information. (18) (a) Chen, Y.-C.; Bab, I.; Mansur, N.; Muhlrad, A.; Shteyer, A.; Namdar-Attar, M.; Gavish, H.; Vidson, M.; Chorev, M. J. Pept. Res. 2000, 56, 147. (b) Spreafico, A.; Frediani, B.; Capperucci, C.; Leonini, A.; Gambera, D.; Ferrata, P.; Rosini, S.; Di Stefano, A.; Galeazzi, M.; Marcolongo, R. J. Cell. Biochem. 2006, 98, 1007. (19) Débieux, J.-L.; Bochet, C. G. Chem. Sci. 2012, 3, 405.
Naoya Kumagai: 0000-0003-1843-2592 Notes
The authors declare the following competing financial interest(s): The Institute of Microbial Chemistry (BIKAKEN) has filed a patent on the DATB catalyst for direct amideforming reactions.
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ACKNOWLEDGMENTS This work was financially supported by KAKENHI (25713002, 17H03025, and JP16H01043 in Precisely Designed Catalysts with Customized Scaffolding) from JSPS and MEXT. N.K. thanks The Shorai Foundation For Science and Technology for financial support. Dr. Ryuichi Sawa, Ms. Yumiko Kubota, and Dr. Kiyoko Iijima at the Institute of Microbial Chemistry are gratefully acknowledged for their assistance with the spectroscopic analysis.
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DOI: 10.1021/acs.orglett.7b03735 Org. Lett. XXXX, XXX, XXX−XXX
