Synthesis of cyclic proline-containing peptides via ring-closing metathesis.

Article abstract of DOI:10.1021/ol034370e

[reaction: see text] Several dienes embedded in di- and tripeptides which incorporate proline have been prepared and subjected to ring-closing metathesis. Bicyclic peptides of well-defined amide geometry and of varying ring sizes were prepared. Several li

Full text of DOI:10.1021/ol034370e

ORGANIC
LETTERS
2003
Vol. 5, No. 11
1847-1850
Synthesis of Cyclic Proline-Containing
Peptides via Ring-Closing Metathesis
Paul W. R. Harris, Margaret A. Brimble,* and Peter D. Gluckman
NeuronZ Medicinal Chemistry Group, Department of Chemistry,
UniVersity of Auckland, 23 Symonds St., Auckland, New Zealand
m.brimble@auckland.ac.nz
Received March 2, 2003
ABSTRACT
Several dienes embedded in di- and tripeptides which incorporate proline have been prepared and subjected to ring-closing metathesis.
Bicyclic peptides of well-defined amide geometry and of varying ring sizes were prepared. Several limitations of the cyclization step were
revealed.
The use of cyclic peptides provides an elegant approach for
the synthesis of peptides of rigid geometry that can be used
to probe the bioactive conformation of a given peptide. This
strategy can be used to establish the structure of a more
potent analogue.1
A common method for the synthesis of cyclic peptides2
relies on construction of an acyclic precursor, typically a
diene, that then undergoes cyclization with use of Grubbs
ring-closing metathesis (RCM).3 The work reported herein
involves the synthesis of dienes from suitable amino acid
precursors bearing an allyl group at C(2) or C(5) of proline.
RCM of these dienes affords cyclic proline-containing
peptides of defined rotamer geometry about the proline amide
bond. Several cyclic peptides of varying ring sizes have been
prepared and the effect of both stereochemistry and heteroa-
tom location on the metathesis reaction has been investigated.
Synthetic Strategy. We were interested in synthesizing a
range of conformationally restricted macrocyclic di- and
tripeptides containing proline, which adopt either cis or trans
stereochemistry about the peptide bond, depending on the
substitution on the proline ring.4 Given that cis/trans
isomerase enzymes often deliver the bioactive conformation
of a peptide,5 peptidomimetics of defined conformation are
useful for incorporation into a peptide scaffold. Etzkorn et
al.6 have recently synthesized amide bond isosteres of cis-
and trans-prolines for incorporation into peptide synthesis.
To prepare a series of cis and trans bicyclic dipeptides
incorporating proline, it was decided to introduce an allyl
group at either the C(2) or C(5) position. We chose a
glycylprolylglutamic acid scaffold onto which were also
(1) Marshall, G. R. Tetrahedron 1993, 49, 3547-3558.
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10.1021/ol034370e CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/30/2003

placed an allyl group on either the glycine or glutamate
residue (Scheme 1). Introduction of an allyl group onto the
1211 and 13.12 Selective reduction of the lactam carbonyl with
LiEt3BH yielded aminols, which underwent C-allylation with
allyltributylstannane to afford 5-allylprolines 213 (cis/trans,
66:33) and 18 (cis/trans, 57:43) in good yield. Neither set of
diastereomers could be separated; however, removal of the
Boc group with HCl in dioxane allowed small-scale separa-
tion of the trans and cis tert-butyl esters 14 and 15. The
stereochemistry of 14 and 15 was determined with use of
nOe experiments and was supported by literature ana-
logues8,10 which suggested that the cis isomer 15 would be
the major product.
Scheme 1. General Synthesis of Diene Precursors for RCM
Based on a Glyclyprolylglutamate Scaffold
The preparation of (S)-2-allylproline methyl ester 3,14
N-allylglycine carbamates 4,15 7,17b and 8,16 of C-allylgly-
cines17 518 and 6,19 and of glutamate 107 followed literature
procedures or variations thereof.
Coupling of N-allylglycine derivative 4 with the mixture
of diastereoisomeric 5-allylprolines 14 and 15 was effected
with EDCI to yield the separable dienes 18 and 19 in
moderate yield (60%). The cis/trans ratio of the newly
created amide (Pro) bond in both these flexible acyclic
dipeptides was 1:1. Ring-closing metathesis of the individual
1,10-dienes 18 and 19 was accomplished with use of Grubbs
catalyst A to give the expected cyclononenes 20 and 21 in
moderate yield (Scheme 3). The ring closure was not affected
R-carbon and the nitrogen atom of glycine was deemed
straightforward. Glutamic acid was chosen in view of the
well-established methodology for the stereoselective intro-
duction of an allyl group at C(4).7
Scheme 3
The synthesis of proline derivatives bearing an allyl group
at C(5) has been reported8 (Scheme 2). Thus, the ethyl ester9
Scheme 2. Synthesis of 5-Allylprolinesa
by the stereochemistry of the allyl group at C(5) on the
proline moiety, both reactions giving similar yields with
a Reagents and conditions: (i) for 12, EtOH/SOCl2, for 13, 70%
HClO4, t-BuOAc (56%); (ii) Boc2O, DMAP (85% for 2, 95% for
1); (iii) LiEt3BH; (iv) for 1, allyltributylstannane, Me3SiOTf (70%),
for 2, allyltributylstannane, BF3‚Et2O (54%); (v) for 1, 4 M HCl,
dioxane 72% (14 31%, 15 41%), for 2, CF3CO2H, ca. 80%.
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33-36.
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2335-2344.
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S.; Bauchat, P.; Danion-Bougot, R.; Danion, D. Tetrahedron: Asymmetry
1998, 9, 2121-2131.
and tert-butyl ester10 of readily available (S)-pyroglutamic
acid 11 were protected as their respective Boc carbamates
(7) Hanessian, S.; Margarita, R. Tetrahedron Lett. 1998, 39, 5887-5890.
(8) Chiesa, M. V.; Manzoni, L.; Scolastico, C. Synlett 1996, 441-443.
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1996, 52, 3195-3222.
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Eur. J. 2000, 6, 949-958.
1848
Org. Lett., Vol. 5, No. 11, 2003

comparable reaction times and catalyst loadings. Very little
deallylation/isomerization occurred (cf. N- or O-allyl sys-
tems).20 In contrast to their respective precursors, both
macrocycles 20 and 21 adopted only the trans conformation
about the peptide bond.
Scheme 5a
Subsequent transformations of 20 and 21 proved to be
problematic due to the presence of the tert-butyl ester and a
benzylcarbamate, hence our attention was turned to the use
of an ethyl ester with a Boc as a protecting group. Due to
difficulties in separating the ethyl esters 16 and 17, the
mixture of diastereomers 2 was coupled with the C(2)-
allylglycine 6 with use of DCC to afford an inseparable
mixture of dipeptides 22 [C(2)/C(5), cis/trans, 77:23, cis/
trans (Pro) ratio undetermined due to complex NMR spectra]
in 55% yield (Scheme 4). Subjecting the diene mixture to
a Reagents and conditions: (i) DCC; (ii) cat. A (30 mol %),
CH2Cl2, reflux, 116 h.
forcing conditions and in lower yield than the corresponding
trans isomers. In the present work it was observed that
placing the allyl moieties closer together (C-allyl vs N-allyl)
and in certain orientations (cis vs trans) resulted in significant
differences in reactivity.
Scheme 4a
The above metathesis reactions had involved an allyl group
at C(5) on the proline and an allyl unit on the glycine,
forming macrocycles containing a trans (Pro) amide bond.
To obtain macrocycles with a cis (Pro) amide bond,
metathesis between an allyl group at C(2) on proline and an
allyl group on the glycine was required. Thus metathesis of
26 with the second generation Grubbs’s catalyst (B)22 yielded
bicyclic cyclooctene 272h as a single (cis) rotamer in good
yield (Scheme 6).
a Reagents and conditions: (i) CF3CO2H; (ii) DCC; (iii) cat. A
(10 mol %), CH2Cl2, reflux, 24 h.
Scheme 6a
the standard RCM conditions yielded only one macrocycle
23 in excellent yield (75%). On the basis of the fact that the
cis diastereomer was the major component in the precursor,
the cyclic metathesis product was assigned as cis C(2)/C(5)
stereochemistry. Clearly the minor trans isomer cyclizes
extremely slowly relative to the cis isomer, a trend that was
not observed earlier in the metathesis of the N-allyl dienes
18 and 19 (Scheme 3). Gratifyingly cyclooctene 23 adopted
only trans stereochemistry about the peptide bond.
To access the C2/C5 trans diastereomer 25 of 23 it was
necessary to use amine 14; coupling with C(2)-allylglycine
6 with use of DCC yielded the diene 24 in 80% yield
[Scheme 5, cis:trans (Pro) ratio undetermined due to complex
NMR spectra]. As anticipated, ring closure of 24 was
extremely slow, required high catalyst loading, and afforded
a lower yield of the cyclooctene 25. However, the peptide
bond within macrocycle 25 adopted only a trans conforma-
tion.
a Reagents and conditions: (i) DCC, HOBt; (ii) cat. B (10 mol
%), C6H6, 45 °C, 48 h.
All attempts to effect metathesis of the corresponding C(2)
allyl/N-allyl derivatives 28, 29, or 30 (Scheme 7) gave
complex mixtures probably due to competing deallylation,
which is accelerated by the electron-withdrawing Boc, CO2-
Bn, or Fmoc carbamates. N,N′-Diallylglycine23 derivative 31
was prepared in an attempt to avoid this effect; however,
only starting material was recovered after attempted RCM,
possibly due to quenching of the catalyst by the reaction
Grubbs et al.21 have reported that certain cis-substituted
cyclohexane derivatives undergo metathesis under more
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Org. Lett., Vol. 5, No. 11, 2003
1849

Scheme 7
Scheme 9a
with the more basic nitrogen.24 Conversion of diallylamine
31 into its hydrochloride salt25 followed by attempted RCM
was also unsuccessful.
The ring-closing metathesis of dienes contained within a
Gly.Pro.Glu scaffold was also examined. The substrate was
synthesized (Scheme 8) from coupling 32 with 10 to yield
a Reagents and conditions: (i) EtOCOCl; (ii) cat. A (20 mol %),
CH2Cl2, reflux, 48 h; (iii) H2, 10% Pd/C.
Scheme 8a
stabilization of the structure by the formation of a γ-turn. A
peptide γ-turn is a structural motif present in many biologi-
cally active cyclic peptides that occurs to reverse the
orientation of the peptide chain. γ-Turns contain three
residues held together in a seven-membered cyclic confor-
mation by an intramolecular hydrogen bond.26
The synthesis and ring closure of a number of di- and
tripeptides incorporating proline has been accomplished via
construction of appropriate diallylated peptide precursors and
subsequent olefin metathesis. The proline-containing peptides
cyclized to afford cyclic products that adopted a single
conformation about the Gly-Pro amide bond (with the
exception of 34). N-Allyl dienes 18 and 19 afforded
cyclononenes 20 and 21 with similar yields under identical
reaction conditions. The cis isomer [C(2)/C(5)] of C-allyl
diene 22 underwent smooth cyclization to cyclooctene 23
in good yield in contrast to the trans isomer 24 that required
high catalyst loading to achieve a moderate yield of cy-
clooctene 25. In the two cases involving cyclization of dienes
in which one allyl group was located at the more hindered
C(2) position on proline, C-allyl diene 26 readily formed
the RCM product 27 whereas N-allyl diene 28 failed to
undergo cyclization to the analogous diazabicyclic product
(Scheme 7).
a Reagents and conditions: (i) EtOCOCl; (ii) cat. B (30 mol %),
C6H6, 45 °C, 65 h; (iii) H2, 10% Pd/C.
N-allylglycyl diene 33, predominantly as the trans (Pro)
rotamer. Exposure of 33 to Grubbs catalyst B followed by
hydrogenation gave macrocycle 34 in 58% yield after
purification by HPLC. Cyclotetradecene 34 existed as a 65:
35 mixture of trans:cis (Pro) rotamers. The increased
proportion of the cis rotamer may reflect increased flexibility
of the Pro amide bond when it is embedded in a larger 14-
membered ring.
Acknowledgment. The authors thank NeuronZ Ltd for
financial support.
Metathesis of the corresponding C-allylglycyl dipeptide
36 and subsequent global deprotection yielded the 13-
membered macrocycle 37 (58%) as a single trans rotamer
(Scheme 9). The observation of only the trans amide rotamer
may be a consequence of the smaller ring size and/or
Supporting Information Available: Experimental pro-
cedures and spectral data for compounds 20, 23, and 37. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL034370E
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Org. Lett., Vol. 5, No. 11, 2003