Expanding the limits of isonitrile-mediated amidations: On the remarkable stereosubtleties of macrolactam formation from synthetic seco -cyclosporins

Article abstract of DOI:10.1021/ja2103372

The scope of isonitrile-mediated amide bond-forming reactions is further explored in this second-generation synthetic approach to cyclosporine (cyclosporin A). Both type I and type II amidations are utilized in this effort, allowing access to epimeric cyclosporins A and H from a single precursor by variation of the coupling reagents. This work lends deeper insight into the relative acylating ability of the formimidate carboxylate mixed anhydride (FCMA) intermediate, while shedding light on the far-reaching impact of remote stereochemical changes on the effective preorganization of seco-cyclosporins.

Full text of DOI:10.1021/ja2103372

Article
pubs.acs.org/JACS
Expanding the Limits of Isonitrile-Mediated Amidations: On the
Remarkable Stereosubtleties of Macrolactam Formation from
Synthetic Seco-Cyclosporins
Xiangyang Wu,† Jennifer L. Stockdill,† Peter K. Park,† and Samuel J. Danishefsky*,†,‡
†Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, New York
10065, United States
‡Department of Chemistry, Columbia University, Havemeyer Hall, 3000 Broadway, New York, New York 10027, United States
S
* Supporting Information
ABSTRACT: The scope of isonitrile-mediated amide bond-
forming reactions is further explored in this second-generation
synthetic approach to cyclosporine (cyclosporin A). Both type
I and type II amidations are utilized in this effort, allowing
access to epimeric cyclosporins A and H from a single
precursor by variation of the coupling reagents. This work
lends deeper insight into the relative acylating ability of the
formimidate carboxylate mixed anhydride (FCMA) intermedi-
ate, while shedding light on the far-reaching impact of remote
stereochemical changes on the effective preorganization of
seco-cyclosporins.
Scheme 1. Isonitrile-Based Approaches to the Construction
of N-Methyl Amides
INTRODUCTION
■
Long-standing interest in the chemical synthesis of proteins1
has served to spur the development of numerous methods for
construction of the amide bond.2 These efforts have led to the
advent of highly reactive coupling agents3 for step-by-step
elongation as well as powerful strategies for segment
condensation such as native chemical ligation.4 Notwithstand-
ing the impressive progress that has unquestionably been
achieved, only a subset of these methodologies are suitable for
the formation of hindered peptide linkages involving amino
acids that are either N-alkylated or doubly alkylated at the α-
carbon.5 Modifications of bioactive polypeptide structures such
as N-methylation can confer improved pharmacoproperties,6
but from a synthetic perspective their incorporation may not be
simple. Hence, there remains a continuing impetus for
investigating novel modes of amide formation.
II).7k In this format, an isonitrile serves to activate the acid,
but the N-bound functionality of the isonitrile is not
incorporated into the product. Use of a sterically encumbered
“throwaway” isonitrile8 can be quite helpful in favoring the
required bimolecular acylation pathway over internal O (or S)
→ N rearrangement, which would lead to an N-formyl amide
carrying the nitrogen-bound functionality.7j
With these two general strategies in mind, we turned to the
synthesis of the historically important natural product cyclo-
sporine (1), a cyclic undecapeptide with seven N-methyl amide
functions (Figure 1). Discovered in the early 1970s in the
context of an extract from the fungus Tolypocladium inflatum
Gams,9 cyclosporin A was noted by scientists at the Sandoz
company to have a unique biological profile in that it possesses
significant immunosuppressive activity with manageable
cytotoxicity.10 Its structure was deduced on the basis of
Considerations such as these prompted us to revisit the
chemistry of isonitriles.7 Among other discoveries, two
conceptually different but complementary approaches were
developed to allow for ready access to N-methyl peptides
(Scheme 1). In the first, a carboxylic acid and an isonitrile are
combined under microwave irradiation to provide an N-formyl
amide (type Ia).7a The analogous transformation of thioacids
into N-thioformyl amides is even more facile, proceeding at
ambient temperature (type Ib).7h Under suitable reduction
protocols, these type I derived coupling products can be easily
converted to their N-methylated counterparts. In the second
method, the amide bond is fashioned when an exogenous
secondary amine intercepts the presumed formimidate
carboxylate mixed anhydride (FCMA) intermediate (type
Received: November 6, 2011
Published: January 17, 2012
© 2012 American Chemical Society
2378
dx.doi.org/10.1021/ja2103372 | J. Am. Chem.Soc. 2012, 134, 2378−2384

Journal of the American Chemical Society
Article
Scheme 2. Retrosynthetic Analysis of Cyclosporin A
Figure 1. Structure of cyclosporin A.
chemical degradation, NMR, and X-ray crystallographic
analysis.11 Subsequently, Wenger accomplished a total synthesis
of cyclosporine, which he disclosed in a series of landmark
papers published in 1983−1984.12 The remarkable ability of
certain cyclosporins to prevent rejection of grafted organs
transformed the field of organ transplantation.13 Moreover,
elucidation of its mechanism of action14 served to illuminate a
key signal transduction pathway in T cell biology.15
In light of these structurally realized translational features, we
undertook a new total synthesis of cyclosporin A and did
indeed succeed in a “first generation” synthesis in which
isonitriles served to mediate the construction of five of the
seven N-methyl amides as well as the final macrolactamiza-
tion.7l Herein, we sought to establish all of the amides
(secondary and tertiary) of cyclosporin A via isonitrile-
mediated chemistry, thereby confronting some significant new
issues in isonitrile-mediated amidation. In the course of this
work, we serendipitously unveiled some intriguing modes of
selectivity during the macrolactamization of diastereomeric
cyclosporin seco-acids.
Scheme 3. Synthesis of Tetrapeptide Carboxylic Acid 3a
RESULTS AND DISCUSSION
■
Our retrosynthetic strategy, in broad outline, followed the lead
of Wenger,12c who disconnected the macrocycle at the Ala(7)-
D-Ala(8) junction, the only site between consecutive non-N-
methylated amides (Scheme 2). We further anticipated that the
requisite linear undecapeptide 2 might be fashioned by the
joining of tetrapeptide 3 and heptapeptide 4, appreciating that
such a coupling might well be challenging due to steric
hindrance near the reacting functional groups and owing to the
potential for epimerization. In a similar vein, we hoped to
evaluate the proficiency of both carboxylic acid 3a and thioacid
3b in the reaction. Further disconnection of 4 revealed the
MeBmt ((4R)-4-((E)-2-butenyl)-4,N-dimethyl-L-threonine)-
derived residue 5, dipeptide acid 67l and tetrapeptide 7 as
milestone building blocks. The unnatural amino acid derivative
5 was to be prepared via the route described by Evans and co-
workers.16 The remaining individual peptide fragments would
be assembled via sequential type II couplings.
72% yield over two steps (Scheme 3). Our early efforts at
accomplishing this transformation had stalled at efficiencies of
40%, presumably due to the formation of side products,
including thioacid anhydride and thioformamide.7h Ultimately,
a simple experimental modification (slow addition of the
thioacid via syringe pump) was made to maintain a low
concentration of thioacid during the course of the reaction, thus
minimizing side product formation due to undesired reaction of
the thioacid with the FCMA intermediate.7j Attempts to extend
the resulting dipeptide 10 with leucine thioacid 8 via tert-butyl
isonitrile activation were complicated by formation of
significant amounts (ca. 30%) of diketopiperazine 12.12b This
undesired cyclization product had been encountered earlier in
the synthesis of multiply N-methylated peptides due to the
slower coupling rates and increased propensity for occupying
We began our synthetic efforts by targeting tetrapeptide
carboxylic acid 3a. Coupling of leucine-derived thioacid 87l with
valine isonitrile 97f followed by radical reduction (with n-
Bu3SnH and azobisisobutyronitrile (AIBN)) at 100 °C in
toluene afforded the corresponding N-methyl dipeptide 7 in
2379
dx.doi.org/10.1021/ja2103372 | J. Am. Chem.Soc. 2012, 134, 2378−2384

Journal of the American Chemical Society
Article
the cis amide conformation.5 Indeed, because of this issue,
Wenger was unable to synthesize tripeptide 11 bearing a C-
terminal benzyl ester by direct N-terminal extension (using the
mixed pivalic anhydride method).12b Fortunately, formation of
this side product could be limited by slow addition of the
dipeptide amine hydrochloride salt in chloroform to a premixed
solution of thioacid and isonitrile, resulting in a 65% yield of 11,
with only a 10% yield of 12. Further extension of 11 by a
similar carbamate cleavage/isonitrile-mediated coupling se-
quence furnished tetrapeptide 13 in 80% yield. Hydrogenolysis
of the benzyl ester afforded the corresponding tetrapeptide
carboxylic acid 3a in 95% yield.
Scheme 5. Synthesis of Tetrapeptide 7
In order to evaluate the relative merits of our type IIa and
type IIb coupling methods in the key segment coupling (3a + 4
and 3b + 4, respectively), we required access to tetrapeptide 3b
(Scheme 4). The thioacid was initially introduced in protected
Scheme 6. Construction of Heptapeptide 4
Scheme 4. Synthesis of Tetrapeptide Thioacid 3b
form by reaction of valine-derived carboxylic acid 14 with (9H-
fluoren-9-yl) methanethiol (FmSH, 15)17 in the presence of
N,N′-dicyclohexylcarbodiimide (DCC) and DMAP to form 16
in 76% yield.18 Cleavage of the carbamate linkage under acidic
conditions furnished valine-derived thioester 17. However,
further extension of 17 by sequential C→N isonitrile couplings
proved troublesome. The main issue was again diketopiperazine
formation, though more significant this time due to the
enhanced leaving group ability of the FmS. Ultimately, it was
found to be more practical to arrive at 19 by an alternate route
in which tripeptide acid 1812b was coupled to amino thioester
17, even though some degree of epimerization was observed.
Use of 3-(diethylphosphoryloxy)-1,2,3-benzotriazin-4(3H)-one
(DEPBT) in the coupling contained the epimerization to a
manageable ∼3:1 ratio, allowing the desired product 1919 to be
obtained in 65% yield from 16. Final removal of the Fm group
with piperidine in DMF afforded the desired tetrapeptide
thioacid 3b in 90% yield.
With tetrapeptide fragments 3a and 3b in hand, we turned
our attention to the synthesis of tetrapeptide 7, which was
readily accomplished using our type II chemistry in sequential
fashion (Scheme 5). First, thioacid 8 was treated with tert-butyl
isonitrile and amine salt 20 to produce dipeptide 21 in 85%
yield. Subsequent removal of the carbamate and coupling with
valine thioacid furnished tripeptide 22 in 65% yield over two
steps. Tetrapeptide 7 was obtained in 75% yield by a similar N-
terminal extension sequence.
found, however, that hexapeptide 24 could be obtained in
significantly higher yield (85% from 7) by merging acid 6 and
tetrapeptide 23 in the type IIa mode under microwave
irradiation at 80 °C (Scheme 6). The subsequent joining of
fragment 5 to hexapeptide 25 was also achieved by employing a
type IIa reaction. Thus, microwave irradiation of the protected
MeBmt residue 5 and hexapeptide amine salt 25 at 80 °C in the
presence of tert-butyl isonitrile provided the desired heptapep-
tide 4 in 68% yield over two steps. To our surprise, the
coupling was accompanied by concomitant loss of the Boc
group. Possibly, cleavage of the urethane function could have
been facilitated by intramolecular hydrogen bonding with the
neighboring free alcohol at the elevated temperatures induced
by microwave heating. Alternatively, once amino acid 5 is
activated by FCMA formation, it could undergo cyclization to
form the N-carboxyanhydride with formal loss of tert-butyl
cation. The observed product could have arisen by aminolysis
of the cyclic anhydride with 25. The precise mechanism
remains unclear.
The stage seemed well set for our proposed isonitrile-based
coupling of tetrapeptide 3 and heptapeptide 4 (Scheme 7). In
the event, carboxylic acid 3a and amine 4 were heated at 80 °C
under microwave irradiation in the presence of tert-butyl
isonitrile. One major product of correct mass was isolated in
71% yield (Scheme 7, entry 1). Surprisingly, this material did
not coelute when coinjected on the HPLC with the linear
undecapeptide that we synthesized via PyBOP coupling as
described in our previous report.7l Instead, it coeluted with the
minor product of the PyBOP coupling, which we tentatively
had assigned as the epimer of 26 at the α-carbon of MeVal(11)
(epi-26). Conducting the isonitrile coupling with thioacid 3b at
With the required fragments assembled, we now sought to
link them together to complete the synthesis of cyclosporin A.
In our previous synthesis, a segment coupling between the
thioacid analogue of dipeptide 6 and tetrapeptide 23 provided
hexapeptide 24 in 63% yield under type IIb conditions.7l We
2380
dx.doi.org/10.1021/ja2103372 | J. Am. Chem.Soc. 2012, 134, 2378−2384

Journal of the American Chemical Society
Article
obtained (28) exhibited chromatographic and spectroscopic
properties matching an authentic natural sample of [D-
MeVal(11)]-cyclosporin A, better known as cyclosporin H.21,22
Although we anticipated that we might suffer some loss of
diastereomeric integrity at this stage due to oxazolonium ion
formation23 during the coupling, the nearly complete
epimerization that actually occurred had not been expected.
In retrospect, this behavior is in line with what Wenger
observed in his attempts to synthesize 26.12c While Wenger’s
coupling of 3a and 4 using BOP gave the correct
undecapeptide, he noted that, when pivaloyl chloride was
used as the activating agent, the reaction afforded only epi-26,
as we found here. Moreover, when Wenger conducted the BOP
coupling with epi-3a12b (i.e., with D-MeVal at the C-terminus),
he still obtained mainly the desired undecapeptide 26 and
virtually no D-MeVal-containing isomer. Taken together, these
data indicate that a kind of dynamic kinetic resolution is
operative, and that there is a different selectivity for formation
of 26 versus epi-26 depending on the choice of coupling
agent.24 Wenger concluded that this outcome must reflect
differences in the activated species that arise by each reagent.
Our results suggest that isonitrile-based activation falls within
the more “anhydride-like” manifold, not surprisingly, given the
highly active acyl donor character of the presumed FCMA
intermediate. Indeed, when we attempted the microwave-
mediated coupling of epi-3a with 4 using tert-butyl isonitrile,
the only product formed was again epi-26. Subsequent efforts to
overturn the selectivity by adding HOBt and converging on an
intermediate in common with PyBOP proved unfruitful.
Therefore, the condensation between fragment 3a and
heptapeptide 4 was carried out in the presence of PyBOP
and NMM in CH2Cl2 to afford the desired (precyclosporin A)
undecapeptide 26 in 52% yield (Scheme 7, entry 3).7l,22
Previously, our group disclosed a method by which the final
macrocyclization could be accomplished at 70 °C in 30% yield
via isonitrile-based technology under microwave mediation.7l
The yield was improved to 54% by addition of HOBt, possibly
due to interception of the FCMA intermediate by HOBt,
rendering side-product formation by [1,3]-O→N acyl transfer
or FCMA hydrolysis less likely. While this was a substantial
improvement, we surmised that the elevated temperatures
could also be responsible for eroding the yield via
decomposition of the starting material and/or product. Inspired
by our earlier findings in this area, we sought to perform this
macrolactamization at room temperature by employing a C-
terminal thioacid rather than a C-terminal carboxylic acid.
Saponification of 26 followed by coupling with FmSH in the
presence of DCC and DMAP furnished the desired
undecapeptide thioester 2 in 68% yield over two steps without
apparent epimerization (Scheme 9).22 Unmasking of the N-
terminus under acidic conditions was followed by treatment
with piperidine in DMF to afford the corresponding thioacid 29
in 62% yield over two steps. Happily, exposure of the
unprotected undecapeptide thioacid 29 to 10 equiv of
cyclohexyl isonitrile in the presence of HOBt at room
temperature resulted in nearly complete conversion to
cyclosporin A (1) after 10 h, with minimal formation of
byproducts. Efforts to purify the final compound were
somewhat complicated by significant chromatographic tailing,
which led to poor recovery from the stationary phase.
Nevertheless, the desired macrocycle was isolated by normal
phase HPLC in 55% yield.
Scheme 7. Synthesis of Undecapeptide 26
room temperature did not ameliorate the problem, as again the
major product was still epi-26 (Scheme 7, entry 2). Our initial
structural assignment for this product (epi-26) was confirmed
by carrying it through the three-step sequence of saponification,
removal of the N-terminal carbamate (→27), and macro-
cyclization using the mixed phosphonic anhydride method20
employed by Wenger12c (Scheme 8). The compound so
Scheme 8. Synthesis of Cyclosporin H
2381
dx.doi.org/10.1021/ja2103372 | J. Am. Chem.Soc. 2012, 134, 2378−2384

Journal of the American Chemical Society
Article
resoundingly, yielding only a trace of product. This trend is
certainly not the case for typical type II reactions. Indeed, in
other cases (vide supra), the use of tert-butyl isonitrile afforded
the highest yields. Though the reasons for this striking
difference in reactivity are still not fully explained, we offer
this observation for its potential usefulness to future
practitioners of this chemistry: sterically demanding type II
couplings can sometimes be enabled by a less hindered
isonitrile.
Scheme 9. Synthesis of Cyclosporin A by Type IIb
Macrocyclization
Furthermore, there does appear to be something unique
about 30 as a macrocyclization substrate. In our previous
report,7l we surmised that the substrate preorganization
proposed by Wenger12c facilitated the mild conversion of the
seco-acid to cyclosporin A using our chemistry. To test whether
perturbing the stereochemistry of the substrate would have an
effect on our isonitrile-mediated macrocyclization, we subjected
27the Val(11) epimer of 30to the same type IIa
conditions. We were able to isolate a product in 41% yield
with a mass corresponding to the expected product, cyclosporin
1
H (28), but to our surprise, the H NMR spectrum did not
While the type IIb cyclization did indeed proceed at ambient
temperature, the overall yield from 26 was lower than what was
previously obtained7l using the type IIa cyclization, due to the
manipulations that were required to install the C-terminal
thioacid. Thus, it was synthetically more expedient to complete
the synthesis according to the previously determined endgame
(Scheme 10). We were pleased to obtain an improved yield
match that of the natural sample.22 Since we are very confident
in our structural assignment of epi-26 (the precursor to 27, vide
supra), we can only surmise that another epimerization event
occurred, this time at the C-terminal L-Ala(7) residue, prior to
cyclization. While further studies would be required to
rigorously assign the structure as [D-Ala(7),D-MeVal(11)]-
cyclosporin A, it is interesting to speculate on the implications
of the experiment for the mechanism of 30 → 1 if indeed the
C-terminus is prone to epimerization. The fact that we cleanly
obtain cyclosporine and not epi-cyclosporine in the case of 30
may simply reflect a kinetic advantage induced by substrate
preorganization that enables macrolactamization to proceed at a
faster rate than C-terminal epimerization. By contrast, 27,
which may lack this predisposition for cyclization, undergoes
prior epimerization at Ala(7) via its epimerization-prone
FCMA. Supporting this proposition is our observation that,
when the less epimerization-prone phosphonic anhydride
method is used to effect the cyclization of 27, cyclosporin H
(28) is indeed obtained (vide supra, Scheme 8). This
explanation is also consistent with our qualitative observation
that the type IIa macrolactamization of 27 requires more time
to proceed to complete conversion than 30.
Scheme 10. Synthesis of Cyclosporin A by Type IIa
Macrocyclization
There is a second possibility that may be relevant, especially
in light of our experience with the coupling of 3 and 4. The rate
of C-terminal epimerization under type II conditions may in
fact be much faster than the rate of macrolactamization for both
30 and 27. In the case of 30, however, substrate
preorganization may enable L-Ala(7)-30 to cyclize faster than
D-Ala(7)-30. Such a dynamic kinetic resolution would imply
that the “faithful” conversion of 30 to cyclosporin A (1) may
not reflect any stereochemical vigilance on the part of the
putative FCMA or HOBt intermediates. It may instead reflect a
proclivity of the substrate to cyclize onto the L- rather than D-
Ala(7)-activated ester, as a consequence of preorganization.
over the final three steps (67% versus 54%)7l by incorporating a
simple ether rinse to partially purify intermediate 30 prior to
the final macrocyclization.22 In the event, the type IIa
macrolactamization proceeded without the formation of
detectable byproducts, allowing the desired compound to be
isolated in high purity by simple flash chromatography. The
characterization data for our synthetic cyclosporine were fully
consistent with an authentic sample of natural cyclosporine, as
well as with published values.7l,11a,12c
Two additional points regarding the remarkable trans-
formation of 30 to the final target 1 are worthy of note here.
First, the reaction is very sensitive to the steric properties of the
mediating isonitrile. While cyclohexyl isonitrile proved exceed-
ingly capable in the cyclization, tert-butyl isonitrile failed
CONCLUSION
■
In summary, the utility of isonitriles in forming various amide
and peptidic bonds has been well demonstrated in this second
generation total synthesis of cyclosporin A. Construction of
each of the secondary and tertiary amide linkages was
accomplished with good efficiency, although the 4 + 7 coupling
of 3 and 4 under these conditions led to inversion at position
11 to the D-amino acid. Most impressively, the final
2382
dx.doi.org/10.1021/ja2103372 | J. Am. Chem.Soc. 2012, 134, 2378−2384

Journal of the American Chemical Society
Article
2008, 130, 13222−13224. (d) Li, X.; Yuan, Y.; Kan, C.; Danishefsky, S.
J. J. Am. Chem. Soc. 2008, 130, 13225−13227. (e) Li, X.; Danishefsky,
S. J. Nat. Protoc. 2008, 3, 1666−1670. (f) Zhu, J.; Wu, X.; Danishefsky,
S. J. Tetrahedron Lett. 2009, 50, 577−579. (g) Wu, X.; Li, X.;
Danishefsky, S. J. Tetrahedron Lett. 2009, 50, 1523−1525. (h) Yuan,
Y.; Zhu, J.; Li, X.; Wu, X.; Danishefsky, S. J. Tetrahedron Lett. 2009, 50,
2329−2333. (i) Wu, X.; Yuan, Y.; Li, X.; Danishefsky, S. J. Tetrahedron
Lett. 2009, 50, 4666−4669. (j) Stockdill, J. L.; Wu, X.; Danishefsky, S.
J. Tetrahedron Lett. 2009, 50, 5152−5155. (k) Rao, Y.; Li, X.;
Danishefsky, S. J. J. Am. Chem. Soc. 2009, 131, 12924−12926. (l) Wu,
X.; Stockdill, J. L.; Wang, P.; Danishefsky, S. J. J. Am. Chem. Soc. 2010,
132, 4098−4100. (m) Wu, X.; Park, P. K.; Danishefsky, S. J. J. Am.
Chem. Soc. 2011, 133, 7700−7703.
macrocyclization was achieved with high efficiency using
isonitrile chemistry. Another important finding concerned the
final macrolactamization step. In the precyclosporine series,
which manifests a high level of preorganization, cyclization
occurs smoothly to provide the spectroscopically well-ordered
1. In the case of the D-MeVal(11) system 27 derived from epi-
26, cyclization does not seem to benefit as much from
preorganization. Depending on the conditions, this ring closure
can provide either cyclosporin H (i.e., [D-MeVal(11)]-cyclo-
sporin A) or, under epimerization-prone FCMA conditions, a
new cyclosporin, tentatively assigned as the [D-Ala(7),D-
MeVal(11)] compound. Neither cyclosporin H nor the [D-
Ala(7),D-MeVal(11)] cyclosporin exhibit well-ordered struc-
tures at room temperature by NMR criteria. It is interesting to
note that the relative configurations at the α-stereocenters
needed for rapid macrolactam formation resulting in a well-
ordered molecule may correlate well with the bioactivity of the
resultant product.
(8) We use this term as a mnemonic device to convey the idea that
none of the elements of the isonitrile appear in the product, in
distinction to the situation in type I. Of course, the isonitrile becomes
the corresponding formamide, which can, in principle, be recycled to
regenerate the isonitrile.
(9) Dreyfuss, M.; Haerri, E.; Hofmann, H.; Kobel, H.; Pache, W.;
Tscherter, H. Eur. J. Appl. Microbiol. 1976, 3, 125−133.
(10) (a) Stahelin, H. F. Experientia 1996, 52, 5−13. (b) Heusler, K.;
Pletscher, A. Swiss Med. Wkly. 2001, 131, 299−302. (c) Borel, J. F.
Wien. Klin. Wochenschr. 2002, 114, 433−437.
ASSOCIATED CONTENT
* Supporting Information
■
S
General experimental procedures, including spectroscopic and
analytical data for new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
(11) (a) Ruegger, A.; Kuhn, M.; Lichti, H.; Loosli, H.; Huguenin, R.;
̈
Quiquerez, C.; von Wartburg, A. Helv. Chim. Acta 1976, 59, 1075−
1092. (b) Petcher, T. J.; Weber, H.; Ruegger, A. Helv. Chim. Acta
̈
1976, 59, 1480−1488.
(12) (a) Wenger, R. M. Helv. Chim. Acta 1983, 66, 2308−2321.
(b) Wenger, R. M. Helv. Chim. Acta 1983, 66, 2672−2702. (c) Wenger,
R. M. Helv. Chim. Acta 1984, 67, 502−525. (d) For syntheses of
related cyclosporine structures, see: Rich, D. H.; Sun, C.-Q.;
Guillaume, D.; Dunlap, B.; Evans, D. A.; Weber, A. E. J. Med. Chem.
1989, 32, 1982−1987. (e) Tantry, S. J.; Venkataramanarao, R.;
Chennakrishnareddy, G.; Sureshbabu, V. V. J. Org. Chem. 2007, 72,
9360−9363. (f) Xu, J. C.; Li, P. J. Org. Chem. 2000, 65, 2951−2958.
(13) For a historical retrospective on the clinical impact of
cyclosporine, see: Kahan, B. D. Transplant. Proc. 2009, 41, 1423−1437.
(14) (a) Handschumacher, R.; Harding, M.; Rice, J.; Drugge, R.;
Speicher, D. Science 1984, 226, 544−547. (b) Liu, J.; Farmer, J. D. Jr.;
Lane, W. S.; Friedman, J.; Weissman, I.; Schreiber, S. L. Cell 1991, 66,
807−815.
AUTHOR INFORMATION
Corresponding Author
s-danishefsky@ski.mskcc.org
■
ACKNOWLEDGMENTS
■
The authors are grateful for support from the NIH (CA28824
to S.J.D., 1F32CA142002-01A1 and K99GM097095-01 to
J.L.S.) and the American Cancer Society (PF-11-014-01-CDD
to P.K.P.), spectroscopic assistance from Dr. George Sukenick,
Hui Fang, and Sylvi Rusli of SKI’s NMR core facility, and Laura
Wilson and Rebecca Wilson for assistance with the preparation
of the manuscript. Dr. Ping Wang is thanked for the gift of
compound 15. We also thank Professor Gregory Verdine of
Harvard University for first calling our attention to the
cyclosporin problem in the context of isonitriles.
(15) For a review of the calcium-calcineurin-NFAT pathway, see:
Crabtree, G. R.; Olson, E. N. Cell 2002, 109, S67−S79.
(16) Evans, D. A.; Weber, A. E. J. Am. Chem. Soc. 1986, 108, 6757−
6761.
(17) Crich, D.; Sana, K.; Guo, S. Org. Lett. 2007, 9, 4423−4426.
(18) Direct conversion of 3a to 3b results in epimerization of the
substrate, presumably at Val(11).
(19) Confirmation of 19 as the major epimer formed in the coupling
of 18 and 17 was accomplished by independent synthesis of 19 via N-
terminal extension of 17 using O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-
tetramethyluronium hexafluorophosphate (HATU) for the amide
bond formations (in modest yields). See the Supporting Information
for details.
REFERENCES
■
(1) (a) Nilsson, B. L.; Soellner, M. B.; Raines, R. T. Annu. Rev.
Biophys. Biomol. Struct. 2005, 34, 91−118. (b) Kimmerlin, T.; Seebach,
D. J. Pept. Res. 2005, 65, 229−260. (c) Dawson, P. In Chemical Biology:
From Small Molecules to Systems Biology and Drug Design; Schreiber, S.
L., Kapoor, T. M., Wess, G., Eds.; Wiley-VCH Verlag GmbH:
Weinheim, Germany, 2008; Vol. 2, pp 567−592.
(2) (a) Montalbetti, C. A. G. N.; Falque, V. Tetrahedron 2005, 61,
10827−10852. (b) El-Faham, A.; Albericio, F. Chem. Rev. 2011, 111,
6557−6602. (c) Pattabiraman, V. R.; Bode, J. W. Nature 2011, 480,
471−479.
(3) See: Albericio, F.; Bofill, J. M.; El-Faham, A.; Kates, S. A. J. Org.
Chem. 1998, 63, 9678−9683 and references therein.
(4) Dawson, P. E.; Muir, T. W.; Clark-Lewis, I.; Kent, S. B. Science
1994, 266, 776−779.
(5) Humphrey, J. M.; Chamberlin, A. R. Chem. Rev. 1997, 97, 2243−
2266.
(20) Wissmann, H.; Kleiner, H. Angew. Chem., Int. Ed. 1980, 19,
133−134.
(21) Traber, R.; Loosli, H.; Hofmann, H.; Kuhn, M.; Wartburg, A. V.
Helv. Chim. Acta 1982, 65, 1655−1677.
(22) See the Supporting Information for details.
(23) McDermott, J. R.; Benoiton, N. L. Can. J. Chem. 1973, 51,
2562−2570.
(24) This hypothesis makes no assumptions regarding the
mechanism of epimerization. The equilibration between 3a and epi-
3a could occur, in principle, by enolization of the activated carboxylate
species or the oxazolonium ion. However, if oxazolonium
intermediates are involved, they cannot be the predominant acylating
species in both types of couplings (isonitrile and PyBOP); otherwise,
both would give the same product. To allow for the observed
(6) Chatterjee, J.; Gilon, C.; Hoffman, A.; Kessler, H. Acc. Chem. Res.
2008, 41, 1331−1342.
(7) (a) Li, X.; Danishefsky, S. J. J. Am. Chem. Soc. 2008, 130, 5446−
5448. (b) Jones, G. O.; Li, X.; Hayden, A. E.; Houk, K. N.;
Danishefsky, S. J. Org. Lett. 2008, 10, 4093−4096. (c) Li, X.; Yuan, Y.;
Berkowitz, W. F.; Todaro, L. J.; Danishefsky, S. J. J. Am. Chem. Soc.
2383
dx.doi.org/10.1021/ja2103372 | J. Am. Chem.Soc. 2012, 134, 2378−2384

Journal of the American Chemical Society
Article
selectivity differences, there must be a more reactive acyl donor in
solution in at least one of the two cases.
2384
dx.doi.org/10.1021/ja2103372 | J. Am. Chem.Soc. 2012, 134, 2378−2384