Synthesis and conformational properties of the M(4-6)(5-7) bicyclic tetrapeptide common to the vancomycin antibiotics

Full text of DOI:10.1021/ja964419u

J. Am. Chem. Soc. 1997, 119, 3417-3418
Synthesis and Conformational Properties of the
3417
M(4-6)(5-7) Bicyclic Tetrapeptide Common to the
Vancomycin Antibiotics
David A. Evans,* Christopher J. Dinsmore,
Andrew M. Ratz, Deborah A. Evrard, and James C. Barrow
Department of Chemistry and Chemical Biology
HarVard UniVersity, Cambridge, Massachusetts 02138
ReceiVed December 27, 1996
The vancomycin class of antibiotics, exemplified by vanco-
mycin and orienticin C aglycons (1) and (2), is widely used in
the treatment of infections due to methicillin-resistant Staphy-
lococcus aureus.1 The complexity of these structures, the
interest in their mode of action,2 and the emergence of bacterial
strains resistant to treatment by this family of antibiotics3 warrant
development of reaction methodology and strategies for their
synthesis.4 In pursuit of this objective, we have developed the
relevant asymmetric amino acid syntheses5 and oxidative bond
constructions for the synthesis of the biaryl ether-6 and biaryl-
containing7 vancomycin-related cyclic tripeptide subunits. In
a previous study, we also described the synthesis of the M(2-
4)(4-6)8 bicyclic hexapeptide that was essentially a biaryl bond
construction away from the complete tricyclic heptapeptide.
However, subsequent investigations have revealed the difficul-
ties in integrating these methods into a synthesis of the tricyclic
heptapeptide framework. The purpose of this paper is to
describe the first synthesis and conformational analysis of a fully
functionalized M(4-6)(5-7) bicyclic tetrapeptide (e.g., 3).
With the exception of the chlorination pattern on the
6-position â-hydroxytyrosine constituent, all members of the
vancomycin family share the M(4-6)(5-7) bicyclic tetrapeptide
subunit 3. Accordingly, our efforts have been directed toward
the construction of this moiety followed by attachment of the
N-terminal tripeptide and closure to the tricyclic heptapeptide
related to the vancomycin aglycon. By inspection, there are
two approaches to 3 that differ in the ordering of the macro-
cyclization events. Since prior studies7 demonstrated that the
M(5-7) tripeptide 4 exists as a mixture of biaryl atropisomers
(89:11) as well as (5-6) amide isomers (Vide infra) that
complicate further development,9 attention has been directed
toward a synthesis where macrocycle assemblage follows the
order M(4-6) f M(5-7).
Amino Acid Subunits. The 4-position amino acid was
derived from commercially available D-4-hydroxyphenylglycine,
while the remaining residues were synthesized using chiral imide
enolate methodology previously developed for this purpose.5
A differentially protected o-phenolic substituent on ring 5 was
required to lower the oxidation potential sufficiently to induce
intramolecular oxidative coupling. To suppress the lability of
the C-terminal amino acid constituent, the decision was made
to carry the carboxyl terminus through the synthesis as the
N-methyl amide with the expectation that it might be selectively
cleaved through a site-selective nitrosation/thermolysis10 late in
the synthesis. Preliminary experiments with derivatives of 3
and 4 suggested that this was a viable option.
Synthesis of M(4-6)(5-7) Bicycle. Sequential peptide cou-
pling from the N-methyl amide of the 7-position amino acid
was accomplished using conventional conditions (1-(3-(di-
methylamino)propyl)-3-ethylcarbodiimide hydrochloride (EDCI),
1-hydroxybenzotriazole (HOBt), THF, 0 °C f room temperature
(rt)) to provide tetrapeptide 5 in good overall yield (Scheme
1). Thallium trinitrate-mediated oxidative cyclization6 afforded
the M(4-6) macrocycle 6 in 54-70% yield. The ring-4 phenol
was protected as the acid-stable mesylate to raise the oxidation
potential of ring 4 relative to rings 5 and 7 and to differentially
protect this phenol for the selective functionalization of ring 4
later in the synthesis. Oxidative cyclization of 67a afforded the
highly strained bicyclic tetrapeptide 8 in 72% yield with 97:3
kinetic selectivity for the R (unnatural) atropisomer.11 Inde-
pendent experiments indicated that the expected ring-5 benzyl
ether in 7 is conveniently cleaved under the reaction conditions,
and reaction times were adjusted to effect both steps in one
chemical operation. The ring-5 phenol was then excised by
conversion of 8 to the derived triflate followed by reductive
cleavage12 to give 9 in 86% overall yield. Removal of the
methyl ether protecting groups was followed by atropisomer-
(1) Atkinson, B. A. in Antibiotics in Laboratory Medicine; Lorian, V.,
Ed.; Williams and Wilkins: Baltimore, 1986; pp 995-1162.
(2) Groves, P.; Searle, M. S.; Waltho, J. P.; Williams, D. H. J. Am. Chem.
Soc. 1995, 117, 7958-7964 and references cited therein.
(3) Walsh, C. T.; Fisher, S. L.; Park, I.-S.; Prahalad, M.; Wu, Z. Chem.
Biol. 1996, 3, 21-28 and references cited therein.
(4) For reviews of synthetic approaches, see: (a) Evans, D. A.; DeVries,
K. D. In Glycopeptide Antibiotics; Nagarajan, R., Ed.; Marcel Dekker,
Inc.: New York; 1994; pp. 63-103. (b) Rama Rao, A. V.; Gurjar, M. K.;
Reddy, K. L.; Rao, A. S. Chem. ReV. 1995, 95, 2135-2168.
(5) (a) Evans, D. A.; Britton, T. C.; Ellman, J. A.; Dorow, R. L. J. Am.
Chem. Soc. 1990, 112, 4011-4030. (b) Evans, D. A.; Evrard, D. A.;
Rychnovsky, S. D.; Fru¨h, T.; Whittingham, W. G.; DeVries, K. M.
Tetrahedron Lett. 1992, 33, 1189-1192. (c) Evans, D. A.; Weber, A. E. J.
Am. Chem. Soc. 1987, 109, 7151-7157.
(6) Evans, D. A.; Ellman, J. A.; DeVries, K. M. J. Am. Chem. Soc. 1989,
111, 8912-8914.
(7) (a) Evans, D. A.; Dinsmore, C. J.; Evrard, D. A.; DeVries, K. M. J.
Am. Chem. Soc. 1993, 115, 6426-6427. (b) Evans, D. A.; Dinsmore, C. J.
Tetrahedron Lett. 1993, 34, 6029-6032.
(8) The seven amino acid residues are numbered consecutively, starting
from the amino terminus. The M(X-Y) nomenclature refers to the macrocycle
containing an oxidative crosslink between aryl groups of residues X and Y.
Bicyclic moieties will be identified as M(X-Y)(Y-Z).
(10) (a) White, E. H. J. Am. Chem. Soc. 1955, 77, 6008-6010, 6011-
6014, 6014-6021. (b) Garcia, J.; Gonzalez, J.; Segura, R.; Vilarrasa, J.
Tetrahedron 1984, 40, 3121-3127.
(9) None of the unnatural biaryl atropisomer has been detected in any
of the vancomycin glycopeptides, and there is only one example, the
antibiotic UK69542, that exhibits 5,6-amide cis/trans-isomerization, but only
in DMSO. Skelton, N. J.; Williams, D. H.; Rance, M. J.; Ruddock, J. C. J.
Am. Chem. Soc. 1991, 113, 3757-3765.
(11) The selectivity for the R (unnatural) atropisomer has previously been
demonstrated to be the result of an A(1,3) interaction between the ring-5
o-oxygen substituent and the proximal stereogenic center (ref 7b).
(12) Cacci, S.; Ciattini, P. G.; Morera, E.; Ortar, G. Tetrahedron Lett.
1986, 27, 5541-5544.
S0002-7863(96)04419-8 CCC: $14.00 © 1997 American Chemical Society

3418 J. Am. Chem. Soc., Vol. 119, No. 14, 1997
Communications to the Editor
Scheme 1a
a
Reagents and conditions: (a) Tl(NO3)3‚3H2O, pyridine, THF/MeOH, 0 °C, then CrCl2, THF/MeOH, 0 °C; (b) MeSO2Cl, i-Pr2NEt, THF, 0 °C;
(c) TFA, DMS, CH2Cl2, then TFAA, 2,6-lutidine, CH2Cl2, 0 °C; (d) VOF3, BF3‚OEt2, AgBF4, TFA, 0 °C, then Zn; (e) PhNTf2, K2CO3, DMF,
0 °C; (f) Pd(dppf)Cl2‚CH2Cl2, Et3N, HCO2H, DMF, 75 °C; (g) AlBr3, NaI, ClCH2CH2Cl; (h) MeOH, 55 °C, 96 h; (i) BnBr, Cs2CO3, Bu4NI, DMF,
0 °C; (j) NaBH4, EtOH, 0 °C; (k) MeMgCl, THF, 0 °C; (l) Boc2O, NaHCO3, dioxane/H2O, rt; (m) NIS, DMF, rt.
ization (MeOH, 55 °C, 55 h) which afforded 11 as a single
1
isomer as judged by H NMR analysis.
Bicyclic tetrapeptide 11, as the desired S (natural) atropiso-
mer, was further modified in anticipation of eventual fragment
coupling and final M(2-4) macrocyclization.13 The ring-4
halogen, lost during removal of the ring-5 triflate, was selec-
tively reinstalled, and a set of protecting groups suitable for
the final stages of the synthesis was incorporated. Thus,
reprotection of the three phenols as their derived benzyl ethers,
achieved without detectable epimerization, afforded bicyclic
intermediate 12. Subsequent reductive removal of the trifluo-
roacetamide with NaBH4 provided the intermediate amine in
78% yield. Finally, removal of the mesylate and reprotection
of the amine as its tert-butyl carbamate now allowed for
selective iodination of ring 4 with N-iodosuccinimide in DMF
to afford 13 in 57% overall yield from 12. This represents the
first synthesis of an appropriately functionalized M(4-6)(5-7)
vancomycin bicycle which could, in principle, serve as an
intermediate in the synthesis of any member of this family of
natural products.
Figure 1. Comparison of conformational equilibria and barriers for
biaryl atropisomerization in MeOH.
appears to be derived largely from the entropy of activation for
the process which requires a more complex set of bond
reorganizations.14
The integration of this M(4-6)(5-7) bicyclic tetrapeptide into
the orienticin C aglycon is presented in the following paper.13
Conformational Properties. On the basis of the method of
synthesis, the preceding route provides access to both biaryl
atropisomers 10 and 11 (Scheme 1). In a series of closely
related biaryl-containing cyclic peptides, we have determined
the conformational properties and activation parameters for the
unnatural f natural, R f S, atropisomerization (Figure 1).
Comparison of the kinetic and equilibrium data on the unnatural
atropisomers of peptides 147 and 15 reveals the profound
influence of the M(4-6) macrocyclic ring on the M(5-7)
macrocycle conformation. The data indicate that the presence
of the M(4-6) macrocycle reinforces both the stability of the S
biaryl atropisomer (>98:2) and the bias for the cis-configuration
of the (5-6) amide bond that is found in the vancomycin
structure. The differences in the rates of atropisomerization are
significant. The longer half-life for the isomerization of 15
Acknowledgment. Support has been provided by the NIH, NSF,
and Merck. The NIH BRS Shared Instrumentation Grant Program has
provided support for NMR facilities.
Supporting Information Available: Characterization data for all
compounds, NOE data for 9, 12, and 15, and kinetic data for the
isomerization to 15 (7 pages). See any current masthead page for
ordering and Internet access instructions.
JA964419U
(14) (a) Brooks, J. W.; Harris, M. M.; Howlett, K. E. J. Chem. Soc.
1957, 1934-1935. (b) Hall, D. M.; Harris, M. M. J. Chem. Soc. 1960,
490-494. (c) Cagle, F. W.; Eyring, H. J. Am. Chem. Soc. 1951, 73, 5628-
5630.
(13) Evans, D. A.; Barrow, J. C.; Watson, P. S.; Ratz, A. M.; Dinsmore,
C. J.; Evrard, D. A.; DeVries, K. M.; Ellman, J. A.; Rychnovsky, S. D.;
Lacour, J. J. Am. Chem. Soc. 1997, 119, 3419-3420.