Disulfide-Bridged Peptides That Mediate Enantioselective Cycloadditions through Thiyl Radical Catalysis

Article abstract of DOI:10.1021/acs.orglett.8b00364

An enantioselective vinylcyclopropane ring-opening/cycloaddition cascade is described. The active thiyl radical catalysts are generated in situ via UV light-promoted homolysis of cystine-based dimers. Amide-functionalization of the peptide at the 4-proline position is essential for effective asymmetric induction. Stereochemical communication is dependent on steric interactions with this substituent that are enforced by H-bonding to the peptide backbone.

Full text of DOI:10.1021/acs.orglett.8b00364

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Disulfide-Bridged Peptides That Mediate Enantioselective
Cycloadditions through Thiyl Radical Catalysis
Jonathan M. Ryss,† Amanda K. Turek,† and Scott J. Miller*
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States
S
* Supporting Information
ABSTRACT: An enantioselective vinylcyclopropane ring-opening/cycloaddition cascade is described. The active thiyl radical
catalysts are generated in situ via UV light-promoted homolysis of cystine-based dimers. Amide-functionalization of the peptide
at the 4-proline position is essential for effective asymmetric induction. Stereochemical communication is dependent on steric
interactions with this substituent that are enforced by H-bonding to the peptide backbone.
cycloaddition cascade recently disclosed by Maruoka and co-
s a catalytic residue in enzymatic systems, cysteine
Amediates myriad transformations proceeding through workers.6 This report, which constitutes the singular example of
asymmetric catalysis of these processes, achieves asymmetric
induction through a steric blocking strategy. In contrast, we
speculated that enantiocontrol could be achieved through
attractive noncovalent interactions provided by a peptidic
scaffold. Thus, homolysis of a disulfide-bridged peptide
precatalyst was projected to yield the derived alkyl thiyl radical;
addition to the VCP substrate delivers the stabilized radical
intermediate (I) (Scheme 1e). Olefin addition to I provides
intermediate II, which can cyclize to the product and
regenerate the active thiyl radical catalyst.
Our first challenge in implementing this proposal involved
the in situ generation of cysteine thiyl radicals, which are less
accessible than aryl thiyl radicals due to differences in bond
dissociation energies.7 Alkyl thiyl radicals can be generated via
hydrogen atom transfer from thiols, but such an approach
requires external initiators, oxidants, and/or bases, which can
give rise to undesired side reactions. Alternatively, UV light-
promoted homolysis of disulfide bonds can directly produce 2
equivalents of the corresponding thiyl radical.8 We chose to
implement this strategy using cystine-based dimers of the active
peptide catalysts with continuous exposure of the reaction
mixture to unfiltered UV light.9
both polar and radical mechanisms. Ubiquitous cysteine
proteases and other enzymes crucial to DNA biosynthesis
operate via two-electron nucleophilic mechanisms (Scheme
1a).1 Equally essential are cysteine-mediated biochemical
processes in which a sulfur-based radical is generated within
the active site and directly participates in a chemical reaction, as
in ribonucleotide reductase and pyruvate formate lyase
(Scheme 1b).2
Our group has previously reported a cysteine catalyst for the
polar Rauhut−Currier reaction,3 which has been suggested to
have a biosynthetic counterpart (Scheme 1c).4 In addition,
mimics of protein disulfide isomerase have been developed to
catalyze disulfide bridge interchanges.5 However, unlike the
polar chemistry of cysteine-based systems, single-electron
chemistry with cysteine thiyl radicals has yet to be employed
in organic synthesis. Inspired by the mechanistic versatility of
cysteine in biological processes, and in line with our interest in
the convergence of biological motifs with stereoselective
catalysis, we explored the potential of cysteine-based thiyl
radicals to catalyze asymmetric transformations. These studies
culminate in the design of a cysteine thiyl radical catalyst
embedded within a peptide framework. We demonstrate the
effectiveness of these catalysts in an enantioselective vinyl-
cyclopropane (VCP) ring-opening/cycloaddition cascade
(Scheme 1d).
Under these conditions, excellent reactivity is observed using
diester substrates of the type studied by Maruoka, with (Boc-
As a context to explore these ideas, we were inspired by the
enantioselective, aryl thiyl radical-catalyzed VCP ring-opening/
Received: January 31, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00364
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a
Scheme 1. Polar and Radical Cysteine Catalysts in Biology
and Synthesis
Scheme 2. Peptide Optimization Studies
Cys-OMe)2 as the catalyst (eq A). High levels of reactivity are
maintained with the use of more complex peptides that were
anticipated to provide some level of selectivity (eq B).
However, nearly racemic product is obtained with these
catalysts, indicating that the determinants of selectivity in
Maruoka’s system are quite dissimilar from ours. To further
take advantage of peptide−substrate H-bonding interactions,
we converted to amide-functionalized VCP substrates (eq C).
With this different substrate class, promising levels of
enantioselectivity are obtained, indicating that the H-bond-
donating ability of the substrate is important for achieving
selectivity. To further take advantage of this presumed
interaction, subsequent screens were performed on the more
electron-deficient bis(amide) substrate 1b.
a
Reactions were performed on a 0.05 mmol scale and were quenched
by exposure to air. Conversions, diastereomeric ratios, and
enantiomeric excesses were determined by CSP-HPLC (254 nm,
uncorrected). Ar = 4-CF3Ph.
likely distant from any chiral information in the catalyst and
hypothesized that additional functionality orthogonal to the
peptide backbone would provide an expanded chiral pocket and
enhance stereochemical communication (Scheme 2b).11
Incorporation of a substituent at the 4-position of the proline
can achieve this extension of chiral information with minimal
disruption to the secondary structure of the peptide catalyst
(Scheme 2c). Indeed, this strategy was successful as phenyl-
acetamide-bearing catalyst P3 yields both diastereomers with
significantly improved enantioselectivity. A survey of other
functional groups at this position reveals that amide
substituents are the most promising for achieving high
enantioselectivity (Scheme 2d). Extensive investigation to
identify a suitable amide at the 4-proline position led us to
catalyst P10, which bears a diphenylmethylacetamide sub-
stituent. Modifications of nonprolyl residues reveals that no
other position impacts enantioselectivity as significantly as
functionalization at the 4-proline position. Such changes,
including the C-terminal capping group and the N-terminal
protecting group, do not provide any increase in selectivity.12
Under our standard reaction conditions13 using catalyst P10,
the diester substrates examined by Maruoka and co-workers
still afford nearly racemic product (Table 1, entry 1). However,
amide-substituted VCPs, which can presumably interact with
the peptide through H-bonding interactions, are more
successful substrates for this transformation. Specifically, several
electron-deficient amide substrates deliver the cycloaddition
products with high enantioselectivity and yield (entries 3, 8−
11). Further support for the proposed VCP−peptide H-bond is
provided by the result obtained in methanol (entry 5). This
A screen of catalyst motifs that exhibit high enantioselectivity
in other peptide-catalyzed reactions10 did not lead to a highly
selective catalyst for this cascade reaction; representative
peptides are shown in Scheme 2a. Modifications of these
sequences at all positions, using natural or commercially
available synthetic amino acids, affords no significant improve-
ment in enantioselectivity. These observations made clear that
our previously developed catalyst scaffolds are not able to
engage effectively with the amide substrate. We recognized that
the reactive centers of the putative radical intermediates are
B
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Table 1. Exploration of VCP Substrate Scope
Scheme 3. Breadth of Olefin-Coupling Partners
a
Reactions were performed on a 0.1 mmol scale and were quenched
by exposure to air. Isolated yield of a mixture of trans and cis
diastereomers. Diastereomeric ratios determined by 1H NMR;
enantiomeric ratios determined by CSP-HPLC (254 nm, uncor-
b
rected). Average of two separate experiments. Ar = 4-CF3Ph. 10 mol
c
% catalyst. Diastereomeric ratio reported after purification, crude dr
80:20 (trans/cis).
furnish the corresponding products with higher enantioselec-
tivity, suggesting that the free N−H bond may interfere with
enantioinduction. Of these products, trans-7 can be enantioen-
riched through filtration to remove an insoluble product, which
is assumed to be an aggregate of minor stereoisomers [enriched
to 92:8 dr (trans/cis), > 99:1 er (trans)].
a
Reactions were performed on a 0.1 mmol scale and were quenched
1,1-Disubstituted olefins are considerably less reactive but
afford similar enantiomeric excess as other olefins. The internal
olefin of a cyclic vinyl carbamate also undergoes the
transformation, delivering 5,5-bicyclic pyrrolidine (10). How-
ever, our optimal catalyst is evidently mismatched to the
intrinsic diastereoselectivity of this olefin (>95:5 cis/trans with
phenyl disulfide), and the product is instead obtained in lower
dr, and the minor trans diastereomer is formed with higher
enantioselectivity. This is the only case in which diastereose-
lectivity is perturbed in the peptide-catalyzed reaction relative
to the phenyl disulfide-catalyzed reaction.
Given the importance of the 4-proline substituent in
achieving enantioinduction in this system and its uniqueness
relative to our previously reported catalysts, we wanted to
better understand its function within this peptide catalyst and
its effect on enantioinduction. A brief study of electronically
varied benzamide catalysts revealed that the 4-proline
substituent appears to act neither as an H-bond donor nor as
an H-bond acceptor (Table 2a). Electron-deficient amides
provide the least enantioselective outcomes; however, other
Lewis basic substituents, such as a urea (P6), are also poorly
selective. However, the steric profile of this substituent is clearly
critical to enantioinduction; a small acetamide catalyst (P15) is
less selective than a bulkier pivalamide (P4) (Table 2b).
Enantioselectivity generally improves with increased steric bulk,
although excessive bulk is detrimental; after increasing the
substituent size to an adamantyl group, enantioselectivity
decreases (P17). Our attempts to find the optimal degree of
bulk on this substituent led us to catalyst P10. Catalyst P18,
which is as bulky as P4 but ostensibly a better H-bond donor,
gives approximately the same results as catalyst P4, reinforcing
by exposure to air. Reported data is the average of two separate
b
experiments, unless noted. Standard conditions: 2:1 hexanes/THF.
c
d
Isolated yield of a mixture of trans and cis diastereomers. Determined
e
1
by H NMR. Determined by CSP-HPLC (254 nm, uncorrected).
f
g
Single experiment. dr is crude before purification to remove cis; yield
is pure trans.
reaction proceeds with significantly diminished enantioselectiv-
ity, potentially due to disruption of the H-bonding interaction.
Consistent with our H-bonding hypothesis, electron-rich amide
substrates furnish the products with marginally diminished
enantioselectivity (entries 6 and 7). With this class of
bis(amide) substrates, diastereoselectivity is unperturbed by
the peptide and is intrinsic to each substrate. The trans
diastereomer predominates and is consistently formed with
higher enantioselectivity than the cis diastereomer.
Multiple olefins are successful olefin coupling partners, with
an evident requirement for an electron-rich olefin bearing an α-
heteroatom (Scheme 3). Other vinylogous functional groups,
including styrenes, acrylamides, and unactivated olefins, are
unreactive, likely due to polarity-matching requirements
between the electron-deficient α-dicarbonyl radical and the
olefin.14 The system developed by Maruoka and co-workers
also suffers from the same restriction. Diastereoselectivity
remains intrinsic to each VCP/olefin combination, again
favoring the trans diastereomer. The diminished enantioselec-
tivity observed when linear n-propyl vinyl ether (3) is used
suggests that enhanced bulk on the olefin partner is beneficial
for stereochemical communication. This is also consistent with
the results obtained from N-vinylacetamide (5), in contrast to
the bulkier vinyl carbamate (4). Olefins lacking an N−H bond
C
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a
relevant. However, given the extant roles for thiyl radicals in
biological systems, perhaps biochemical discovery of related
carbon−carbon bond-forming processes may yet emerge.
Table 2. Catalyst Structure-Selectivity Investigation
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.8b00364.
Experimental methods, characterization of all com-
pounds, and copies of spectra (PDF)
Accession Codes
CCDC 1577970 and 1586383 contain the supplementary
crystallographic 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.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: scott.miller@yale.edu.
ORCID
Scott J. Miller: 0000-0001-7817-1318
Author Contributions
a
Reactions were performed on a 0.05 mmol scale and were quenched
by exposure to air. Conversions, diastereomeric ratios, and
enantiomeric ratios were determined by CSP-HPLC (254 nm,
uncorrected). Ar = 4-CF3Ph.
†J.M.R. and A.K.T. contributed equally to the project.
Notes
The authors declare no competing financial interest.
our hypothesis that the H-bonding capability of the amide is
secondary to its steric profile in terms of governing
enantioinduction.
ACKNOWLEDGMENTS
■
This work is supported by the National Institute of General
Medical Sciences of the United States National Institutes of
Health (R01-GM096403). J.M.R. acknowledges the Depart-
ment of Defense (DoD) for funding through the National
Defense Science & Engineering Graduate Fellowship
(NDSEG) Program. We also thank Dr. Brandon Q. Mercado
for X-ray crystallographic analyses and Dr. Eric Paulson for
NMR assistance.
In our catalyst optimization studies, we observed that
changes to other parts of the catalyst have minimal impact
on enantioselectivity. This prompted us to examine whether the
catalyst could be abbreviated at the proline residue and still
perform effectively (Table 2c). Enantioselectivity is largely
retained with these catalysts when the proline residue is capped
as an N,N-disubstituted amide (P19); a larger decrease is
observed when the amide bears a free N−H (P20). The
observation that enhanced Lewis basicity at this position is
beneficial, combined with the results obtained with steric
perturbations, leads us to advance a hypothetical stereochemical
model as shown in Table 2d. We propose that the VCP
substrate engages with the peptide backbone through an H-
bonding interaction with the proline amide, with a chiral pocket
defined by the steric profile of the 4-proline substituent. The
absolute configuration of 2g-trans was determined by X-ray
crystallography15 and is consistent with orientation of the tert-
butyl moiety away from the 4-proline substituent, further
supporting a steric role. Studies toward a mechanistic
understanding of the effect of this substituent on the
enantioselectivity-determining step are underway.
REFERENCES
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(9) Activation with a 365 nm filtered UV lamp provided appreciable
but incomplete conversion, possibly due to poor overlap of the alkyl
disulfide bond absorbance. UV−Vis spectra of (Boc-Cys-OMe)2 and
catalyst P10 are provided in the Supporting Information.
(10) For a detailed study of peptide structure, see: Metrano, A. J.;
Abascal, N. C.; Mercado, B. Q.; Paulson, E. K.; Hurtley, A. E.; Miller,
S. J. J. Am. Chem. Soc. 2017, 139, 492−516.
(11) Peptide model is based on the three-dimensional structure
elucidated in: Jakobsche, C. E.; Peris, G.; Miller, S. Angew. Chem., Int.
Ed. 2008, 47, 6707−6711.
(12) Additional optimization data can be found in the Supporting
Information.
(13) While the reaction proceeds in a variety of solvents (EtOAc,
PhMe, MeOH, MEK), nonpolar, ethereal solvent mixtures provided
the highest reactivity and enantioselectivity.
(14) Roberts, B. P. Chem. Soc. Rev. 1999, 28, 25−35.
(15) The X-ray crystallographic data are presented in the Supporting
Information.
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