Catalytic Oligopeptide Synthesis

Article abstract of DOI:10.1021/acs.orglett.7b03735

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 accomm

Full text of DOI:10.1021/acs.orglett.7b03735

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
B NO heterocycle, providing a versatile catalytic protocol
3
2
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.
1
mide is an omnipresent functional group interconnecting
Scheme 1. Drawbacks in Catalytic Peptide Synthesis (A) and
This Work (B)
A
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
2
transformation in medicinal chemistry. The sustained demand
for amides has advanced synthetic means of accessing this class
3
of compounds, producing a vast array of reagents that render
4
carboxylic acids and amines coupled under various conditions.
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 reagent-
5
derived waste, which is never fulfilled by reagent-driven
amidation. While more than two decades have passed since the
6
7
first catalytic amidation, neither boron-based catalysts nor
8
group IV metal catalysts 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
7
l,m,9
7n
7r
boronic acid,
borinic acids, or borates as catalysts are
dictated by the distinct B NO core would exhibit competence
3 2
plagued with low catalyst turnover, and the use of unconven-
tional protecting groups limited their synthetic utility (Scheme
for catalytic peptide coupling. In this letter, we demonstrate
that DATB 1 could be used to interconnect both non-
functionalized and functionalized natural α-amino acid units in
a catalytic manner, with turnover numbers (TON) ranging
from 16 to 180. Commonly employed protecting groups 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 nonfunctional-
ized tripeptides via the coupling of dipeptidyl acid Boc-Gly-
1
A). Moreover, α-amino acids bearing heteroatom functional
10
residues are little explored. The chemical synthesis of
13
peptides is a research area of significant biological and practical
11
impact with high demand, and engineered and expensive
4b,c
coupling reagents
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-2,4,6-
triborinane (DATB) catalyst 1, a multiboron catalyst amenable
to dehydrative coupling of a wide range of substrate types
Received: December 1, 2017
1
2
(
Scheme 1B). We reasoned that the high catalytic activity of 1
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03735
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Leu-OH 2 and the HCl salt of α-amino acid Bn esters 3
Scheme 3. Catalytic Dipeptide Synthesis Using
Functionalized N-Fmoc α-Amino Acids 5 and H-Leu-OBn
(
Scheme 2). Epimerization during the coupling reaction
HCl Salt 6
Scheme 2. Catalytic Tripeptide Synthesis Using Boc-Gly-
Leu-OH 2 and Nonfunctionalized α-Amino Acid HCl Salts 3
a
b
AA denotes amino acid. Isolated yield is reported. Er 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 α-
N-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
1
2
and amines. 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-Leu-
OH 2 and H-Gly-OBn·HCl 3a proceeded with 5 mol% of 1 at
a
b
AA denotes amino acid. Isolated yield is reported. Dr was
c
determined by HPLC analysis. Pbf = 2,2,4,6,7-pentamethyl-2,3-
dihydrobenzofuran-5-sulfonyl.
80 °C in toluene, affording the desired tripeptide Boc-Gly-Leu-
14
Gly-OBn 4a in 80% yield without any loss of enantiopurity.
obtained without notable loss of stereochemical integrity. N-
Functionalized α-amino acids were generally compatible, and
Fmoc-Lys(Boc)-OH 5i, Fmoc-Trp(Boc)-OH 5j, Fmoc-Arg-
Sterically more demanding H-Phe-OBn·HCl 3b and H-Leu-
OBn·HCl 3c as amine units were also coupled under identical
conditions to deliver the corresponding tripeptides 4b and 4c
in decent yields without epimerization.
τ
(Pbf)-OH 5k, and Fmoc-His(N Tr)-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
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 solid-
15
16
phase peptide synthesis (SPPS), 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
couplings, Cys was less tractable and partial epimerization
1
7
was observed for the S-trityl protected substrate. 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.
OH 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 α-
amino acid Ser in Scheme 3 was successfully coupled when H-
Ser(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
1
3
16b
protecting groups. For O-functionalized α-amino acids,
Fmoc-Glu(Bn)-OH 5a, Fmoc-Gln(Tr)-OH 5b, Fmoc-Tyr-
Another S-functionalized α-amino acid Fmoc-Met-
t
t
(
Bu)-OH 5c, and Fmoc-Thr( Bu)-OH 5d were coupled
under standard conditions without notable stereochemical
erosion and undesired deprotection with the exception of 7c, in
t
which minor epimerization took place. While Fmoc-Ser( Bu)-
OH 5e exhibited partial epimerization at 80 °C, its high
reactivity allowed for amide coupling at a lower temperature
t
(
50 °C) and dipeptide Fmoc-Ser( Bu)-Leu-OBn 7e was
B
DOI: 10.1021/acs.orglett.7b03735
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
t
Scheme 4. Catalytic Dipeptide Synthesis Using Fmoc-Leu-
OH 8 and Functionalized α-Amino Acid HCl Salts 9
°C to give dipeptide Fmoc-Gly-Gly-O Bu 13. After removal of
the Fmoc group with Et NH, coupling with Fmoc-Phe-OH 14
was performed with 5 mol% of 1, affording tripeptide fragment
Fmoc-Phe-Gly-Gly-O Bu·HCl 15. The dipeptide fragment at
the N-terminus, Fmoc-Tyr( Bu)-Gly-OH 16, was accessed by
2
t
t
t
DATB 1-catalyzed coupling of Fmoc-Tyr( Bu)-OH 5c and H-
Gly-OBn·HCl 3a, followed by hydrogenolysis of the Bn ester in
8
4% 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
t
t
Fmoc-Tyr( Bu)-Gly-Phe-Gly-Gly-O Bu that conforms to
19
OGP(10−14).
In summary, catalytic amidations were demonstrated using α-
amino 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.
a
b
AA denotes amino acid. Isolated yield is reported. Dr was
determined by HPLC analysis.
t
t
(
(
H-Glu( Bu)-O Bu·HCl 9b) and N-functionalized (H-Trp-
t t
Boc)-O Bu·HCl 9c, H-Lys(Boc)-O Bu·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-Gly-
Phe-Gly-Gly-OH, is a biologically active pentapeptide that was
truncated from the C-terminus of osteogenic growth peptide
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03735.
Experimental details, spectroscopic and HPLC data of
products (PDF)
AUTHOR INFORMATION
Corresponding Authors
(
OGP). OGP(10−14) is a naturally occurring osteoblastic
■
mitogen that is equipotent to full-length tetradecapeptide
1
8
OGP. Based on the absence of undesired oxazolone
formation in the reaction of α-N-acyl carboxylic acid (Scheme
*
*
E-mail: mshibasa@bikaken.or.jp.
E-mail: nkumagai@bikaken.or.jp.
2
), catalytic assembly of five α-amino acid components of
ORCID
OGP(10−14) was attempted in a convergent fashion (Scheme
5
t
). Coupling of Fmoc-Gly-OH 11 and H-Gly-O Bu·HCl 12
Hidetoshi Noda: 0000-0001-6529-8870
Masakatsu Shibasaki: 0000-0001-8862-582X
reached completion with as little as 0.5 mol% of DATB 1 at 90
t
t
Scheme 5. Catalytic Assembly of Five α-Amino Acids To Afford Pentapeptide Fmoc-Tyr( Bu)-Gly-Phe-Gly-Gly-O Bu
C
DOI: 10.1021/acs.orglett.7b03735
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
11) (a) Kent, S. B. H. Annu. Rev. Biochem. 1988, 57, 957.
Naoya Kumagai: 0000-0003-1843-2592
(
(
(
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.
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 amide-
forming reactions.
L.; Soellner, M. B.; Raines, R. T. Annu. Rev. Biophys. Biomol. Struct.
005, 34, 91. (f) Kent, S. B. Chem. Soc. Rev. 2009, 38, 338.
12) (a) Noda, H.; Furutachi, M.; Asada, Y.; Shibasaki, M.; Kumagai,
2
(
N. Nat. Chem. 2017, 9, 571. (b) Noda, H.; Asada, Y.; Shibasaki, M.;
Kumagai, N. Chem. Commun. 2017, 53, 7447.
́
ACKNOWLEDGMENTS
(13) Isidro-Llobet, A.; Alvarez, M.; Albericio, F. Chem. Rev. 2009,
■
1
1
09, 2455.
This work was financially supported by KAKENHI (25713002,
7H03025, and JP16H01043 in Precisely Designed Catalysts
(
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,
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 spectro-
scopic analysis.
(
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.;
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Namdar-Attar, M.; Gavish, H.; Vidson, M.; Chorev, M. J. Pept. Res.
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DOI: 10.1021/acs.orglett.7b03735
Org. Lett. XXXX, XXX, XXX−XXX