Stereoselective Access to Azetidine-Based α-Amino Acids and Applications to Small Peptide Synthesis

Article abstract of DOI:10.1021/acs.orglett.0c03131

Non-natural azetidine-based amino acids (Aze) present interesting features in protein engineering. A simple organometallic route toward unsaturated carboxylic acid precursors is presented. Subsequent metal-catalyzed asymmetric reduction allowed for the sy

Full text of DOI:10.1021/acs.orglett.0c03131

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Letter
Stereoselective Access to Azetidine-Based α‑Amino Acids and
Applications to Small Peptide Synthesis
Felix Reiners, Emanuel Joseph, Benedikt Nißl, and Dorian Didier*
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ABSTRACT: Non-natural azetidine-based amino acids (Aze)
present interesting features in protein engineering. A simple
organometallic route toward unsaturated carboxylic acid precursors
is presented. Subsequent metal-catalyzed asymmetric reduction
allowed for the synthesis of a new library of 2-azetidinylcarboxylic
acids, which were finally employed in the formation of small
peptide chains.
Scheme 1. Synthesis of Aze Derivatives
onstituting the backbone of every protein, α-amino acids
are essential building blocks. The potential behind the
C
development of new non-natural amino acids is broad since it
unlocks new features in protein engineering. The introduction
of cyclic amino acid scaffolds causes substantial changes in the
secondary structure of peptide chains, justifying therefore the
interest in azetidine 2-carboxylic acid compounds (Aze) as
versatile foldamer elements.1
2
3
́
́
For instance, the groups of Martin-Martinez and Toniolo
reported that γ-turns can be induced by the presence of 2-
azetidinylcarboxylic acids within peptide chains.
2-Azetidinylcarboxylic acids are traditionally synthesized by
cyclization of an adequately protected α-amino acid possessing
a leaving group in the γ-position under basic conditions
(Scheme 1A).4 Recently, the groups of Schreiber and Baran
have showcased the use of directing groups at position 2 of
stereodefined derivatives to promote a cis-selective function-
alization at position 3 through C−H activation strategies
(Scheme 1B).5 In addition, efforts have recently been made
toward the synthesis of 3-azetidinylcarboxylic acids.6
Following our recent advances on the synthesis of four-
membered carbo- and heterocycles,7−9 we set out to
investigate the formation of non-natural substituted amino
acids through a simple stereocontrolled sequence involving the
intermediate formation of unsaturated prochiral azetinyl-
carboxylic acids. We envisioned that further asymmetric
hydrogenation would furnish the desired functionalized
azetidine carboxylic acid compounds diastereo- and enantio-
selectively (Scheme 1C).10 The addition of organometallic
nucleophiles to commercial sources of 3-azetidinones (Scheme
2) allows for an efficient access to tertiary alcohol 1, which is
then transformed into the corresponding methyl ether 2.11 As
previously established by our group,8a the formation of
lithiated species [C] is accomplished through an α-lithiation/
β-elimination/α-lithiation sequence via addition of s-BuLi.
Corresponding carboxylic acids 3 are simply obtained after
bubbling CO2 in the reaction mixture. A model substrate
possessing a phenyl group at position 3 led to the
corresponding carboxylic acid 3a in 67% yield. Functional
group tolerance was examined next by introducing various
substituents on the aryl moiety of the substrate. Electron-
donor groups such as methoxy- and polymethoxy-substituents
Received: September 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03131
© XXXX American Chemical Society
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Halogen-substituted structures were also tolerated, furnishing
3h and 3i in moderate yields up to 62%. It is important to note
that these strained unsaturated carboxylic acids proved to be
stable toward air and moisture. We were therefore able to
crystallize compound 3i (X-ray given in Scheme 2C), which
showed hydrogen bonding between the proton of the
carboxylic acid and the N-Boc protecting group, partially
accounting for the overall stability of these structures. Alkenyl
and alkynyl groups were also introduced (3j, 3k), although
lower yields were generally observed in comparison with alkyl
groups (3l−3q, 62−80%). A moderate yield was obtained for
the unsubstituted derivative 3r, isolated in 48%.
Scheme 2. Synthetic Sequence Towards Azetinyl-Carboxylic
Acids 3
Having established a new library of unsaturated derivatives,
we started investigating the diastereospecific synthesis of
functionalized 2-azetidinecarboxylic acids 4 through palladium-
catalyzed cis-hydrogenation (Scheme 3).12
Scheme 3. Diastereospecific Hydrogenation Towards
Functionalized cis-Aze Compounds 4
a
The reaction was also performed on 3.5 mmol of 3b, giving 952 mg
(89%) of compound rac-4b.
Selected examples were hydrogenated using Pd/C (5 mol
%) under H2 atmosphere (20 bar) in methanol, furnishing cis-
isomers rac-4a−4j in excellent yields (up to 98%),
independently from the nature of the substituent at position
3. The relative stereochemistry of these derivatives was
assessed by analogy with X-ray measurements performed on
rac-4a. Although substrates possessing primary alkyl groups
gave the desired compounds rac-4h−4j in high yields, the
gave satisfactory yields (3b and 3c, 68%), and the N,N-
dimethylanilin derivative furnished 3d in 55% yield.
A decrease in efficiency was noted with dibenzofurans and
thiophenes (3e, 3f), which were only isolated in up to 40%
yield. The presence of a nitrile group resulted in the formation
of the corresponding ketone (3g) via 1,2-addition of s-BuLi.
B
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presence of a cyclopropyl only yielded 42% of compound rac-
4k. It is worth noting that the classical lithiation of saturated
cyclic systems usually leads to trans-isomers due to the sterical
hindrance engendered by surrounding substituents. Our
method allows for the selective formation of cis-isomers,5
offering therefore an efficient stereodivergent complementary
alternative to existing strategies.13
Optimizations of the asymmetric hydrogenation were
logically carried out next on model compound 3a in order to
identify the best ligand system to be used in the formation of
enantioenriched 2-azetidinylcarboxylic acid 4a.
results. Inspired by the pioneering work of Noyori on
asymmetric hydrogenation,16 BINAP-based ligands L1−L3
were evaluated first and gave both best yields and
enantioselectivities (up to 98% and er = 95:5 for L3). Other
usually efficient ligand systems such as SEGPHOS (L4−L6),17
JOSIPHOS (L7)18 and DIOP (L8)19 proved less efficient,
with enantiomeric ratios ranging from 75:25 to 90:10.
Using methanol as a solvent only gave 20% of the desired
product 4a. This study also showed that the nature of the
tertiary amine (Et3N or DIPEA) did not influence the reaction,
while pyridine only led to traces of 4a. Decreasing the pressure
of hydrogen to 10 bar gave similar results, although the
reaction had to be stirred for an extended time.
Results employing [Ru(p-cymene)Cl2]2 (2.5 mol %) are
given in Table 1, as transition metal complexes of rhodium14 or
iridium (Crabtree’s precatalyst)15 did not give satisfactory
With optimized conditions in hand, the scope of asymmetric
hydrogenation was evaluated on selected aryl, heteroaryl, and
alkyl derivatives (Scheme 4). Electron-donating and electron-
Table 1. Optimizations on Enantioselective Hydrogenation
Scheme 4. Ru-Catalyzed Enantioselective Reduction of
Azetinylcarboxylic Acids
a
The reaction was performed on 1.5 mmol of 3h, giving 447 mg of
compound (−)-4e.
deficient substituents in the para position of phenyl groups
furnished (−)-4b and (−)-4e−4f in high yields (91−96%) and
good enantiomeric ratios up to 94:6. With a dibenzothiophenyl
moiety, product 4h was isolated in 74% yield and 93:7 er.
However, the presence of an alkyl group (ethyl, (+)-4i)
diminished the enantiomeric ratio to 87:13.
With a novel library of racemic and enantioenriched amino
acid building blocks at our disposal, we last aimed at their
incorporation into small peptidic chains. Rac-4a was chosen to
undergo amidification with stereodefined L-phenylalanine
isopropyl ester (L-Phe-O-i-Pr) in the presence of HBTU as a
peptide coupling agent.20 The two enantiopure diaster-
eoisomers 5a and 5b could be separated via chromatography
and isolated with high yields (42% and 43%, respectively, 85%
overall). Similarly, p-methoxyphenyl substituted substrate rac-
4b gave two separable isomers 6a and 6b in 92% overall yield.
The absolute configuration of both isomers was ascertained by
X-ray crystallography of 6a and 6b, as shown in Scheme 5A.
These results were also used to determine the absolute
C
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Scheme 5. Di- and Tripeptide Synthesis
a
HBTU (1.1 equiv), DIPEA (2.2 equiv), CH2Cl2, rt, 6 h.
configurations of molecules presented in Scheme 4B, by
analogy.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03131.
Enantioenriched amino acid (−)-4b (92:8 er) was easily
resolved through peptide coupling with L-Phe-O-i-Pr under
previous conditions, yielding the enantiopure dipeptide (−)-6a
in 72%. The azetidinyl moiety was further deprotected with
TFA and engaged with an L-serine derivative (L-Ser-N-Boc)
toward the formation of tripeptide (+)-7 which was isolated in
its enantiopure form in 92% yield (Scheme 5B).
Representative example for the asymmetric reduction of
azetines (PDF)
Accession Codes
CCDC 2031215−2031218 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
In summary, the synthesis of enantioenriched four-
membered amino acids was achieved using a simple and
practical two-step procedure through intermediate formation
of isolable 2-azetinylcarboxylic acids. Their reduction was
performed with either palladium or chiral ruthenium
complexes, and the resulting saturated amino acids were
efficiently resolved after peptide coupling. Di- and tripeptides
were obtained in good yields and excellent enantiopurity. We
believe that such unusual architectures will be of high interest
in future protein engineering and in the study of azetidine-
containing secondary structures.
AUTHOR INFORMATION
■
Corresponding Author
Dorian Didier − Department of Chemistry and Pharmacy,
̈
̈
Ludwig-Maximilians-Universitat Munchen, 81377 Munich,
D
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Germany; orcid.org/0000-0002-6358-1485;
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Letter
(7) (a) Eisold, M.; Didier, D. Angew. Chem., Int. Ed. 2015, 54,
15884−15887. (b) Eisold, M.; Kiefl, G. M.; Didier, D. Org. Lett. 2016,
18, 3022−3025. (c) Eisold, M.; Baumann, A. N.; Kiefl, G. M.;
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Email: dorian.didier@cup.uni-muenchen.de
Authors
Felix Reiners − Department of Chemistry and Pharmacy,
̈
̈
Ludwig-Maximilians-Universitat Munchen, 81377 Munich,
Germany
Emanuel Joseph − Department of Chemistry and Pharmacy,
̈
̈
Ludwig-Maximilians-Universitat Munchen, 81377 Munich,
Germany
(9) (a) Eisold, M.; Reiners, F.; Muller-Deku, A.; Didier, D. Org. Lett.
̈
Benedikt Nißl − Department of Chemistry and Pharmacy,
2018, 20, 4654−4658. (b) Baumann, A. N.; Reiners, F.; Juli, T.;
Didier, D. Org. Lett. 2018, 20, 6736−6740. (c) Baumann, A. N.;
Reiners, F.; Siegle, A. F.; Mayer, P.; Trapp, O.; Didier, D. Chem. - Eur.
J. 2020, 26, 6029−6035.
̈
̈
Ludwig-Maximilians-Universitat Munchen, 81377 Munich,
Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03131
(10) Reiners, F.; Joseph, E.; Nißl, B.; Didier, D. ChemRxiv, 2020.
https://doi.org/10.26434/chemrxiv.12966803.v1
(11) See the Supporting Information.
Notes
(12) Nishimura, S. Handbook of Heterogeneous Catalytic Hydro-
genation for Organic Synthesis, 1st ed.; Wiley-Interscience: New York,
2001.
The authors declare no competing financial interest.
(13) Wykypiel, W.; Lohmann, J.-J.; Seebach, D. Helv. Chim. Acta
1981, 64, 1337−1346.
ACKNOWLEDGMENTS
■
D.D. and F.R. are grateful to the Fonds der Chemischen
Industrie, the Deutsche Forschungsgemeinschaft (DFG grant:
DI 2227/2-1 and Heisenberg fellowship: DI 2227/4-1), the
SFB749, and the Ludwig-Maximilians University for PhD
funding and financial support. Dr. Peter Mayer, Prof.
Konstantin Karaghiosoff, and Dr. Andreas N. Baumann
(LMU, Munich) are kindly acknowledged for X-ray and
NMR measurements, and preliminary experiments.
(14) Imamoto, T. Rhodium(I)-Catalyzed Asymmetric Hydro-
genation. In Rhodium Catalysis in Organic Synthesis: Methods and
Reactions; Tanaka, K., Ed.; Wiley-VCH, 2019; Chapter 1.
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