Kedarcidin chromophore: Synthesis of its proposed structure and evidence for a stereochemical revision

Article abstract of DOI:10.1021/ja071205b

A convergent, enantioselective synthesis of the proposed structure of kedarcidin chromophore (1) is described. The route is 24 steps in the longest linear sequence (beginning with the commercial reagent 2,3-O-isopropylidene-d-erythronolactone) with an ave

Full text of DOI:10.1021/ja071205b

Published on Web 04/07/2007
Kedarcidin Chromophore: Synthesis of Its Proposed Structure and Evidence
for a Stereochemical Revision
Feng Ren, Philip C. Hogan, Alan J. Anderson, and Andrew G. Myers*
Department of Chemistry and Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138
Received February 23, 2007; E-mail: myers@chemistry.harvard.edu
The structure of the chromophore component of the cytotoxic
protein-chromophore complex kedarcidin was determined by
researchers at BMS in 1992 on the basis of an extensive
spectroscopic analysis of the isolated chromophore and its degrada-
tion products, an undertaking that was greatly complicated by the
reactivity of the chromophore and its low natural abundance.1 The
original structural proposal was modified (as 1, Scheme 1 and
Figure 1) by Hirama and co-workers in 1997, thereby transforming
the originally proposed R-azatyrosyl residue of the ansa-bridge to
the corresponding â-amino acid derivative and reversing the
handedness of the molecule.2 Here, we describe an unambiguous,
enantioselective synthesis of structure 1. Our spectroscopic data
show that the structure of kedarcidin chromophore must be further
revised, we suggest by epimerization of the mycarose-bearing
carbon, C10 (structure 2, Figure 1, see below).
We synthesized structure 1 using a highly convergent route
(Scheme 1). The sequence was initiated with the azatyrosyl
dibromide 3 as starting material (>95% ee), prepared by extension
of our published route to the corresponding tert-butyldimethylsilyl
(TBS)-alkynyl derivative3 using a different, auxiliary-based route
to the â-amino acid component4 (see Supporting Information; use
of the triethylsilyl (TES)-alkynyl group within 3 is a refinement
that allows for a simultaneous deprotection reaction later, 8f9,
vide infra). The methyl ester group of 3 was transformed into the
corresponding 9-fluorenylmethyl (Fm) ester5 by saponification with
lithium hydroperoxide and esterification of the resulting carboxylic
acid with 9-fluorenemethanol using 2-methyl-6-nitrobenzoic an-
hydride as an activating agent (93%, two steps).6 Cleavage of the
N-tert-butoxycarbonyl (BOC) protective group with trimethylsilyl
triflate-2,6-lutidine7 and, in the same flask, addition of saturated
aqueous sodium bicarbonate solution and a solution of the pivaloyl-
protected, hydroxybenzotriazole-activated 2-naphthoic acid deriva-
tive 48 in dichloromethane afforded the coupled product 5 in 97%
yield (22-g scale). Selection of the pivaloyl group to protect the
naphthol group in the latter step allowed for a mild, fluoride-based
deprotection reaction in the final step of the sequence (vide infra).
Sonogashira coupling of the dibromoolefin 5 with the R-kedarosyl-
ated dialkyne component 69 formed the cis-bromoolefin 7 in 50-
60% yield.10 Although the stereoisomeric trans-coupling product
was never observed, the (chromatographically separable) bis-
coupling product was detected as a side product, one whose
formation was never completely suppressed. Selective cleavage of
the Fm ester in the presence of the pivaloate ester within 7 (Et3N,
THF) cleanly afforded the corresponding carboxylic acid, which
was cyclized using the Shiina macrolactonization protocol11 to
provide the macrolactone 8 (atropisomeric mixture; see Scheme 1
for details concerning the conformations of this and subsequent
macrocyclic intermediates). The macrolactonization reaction could
be performed on the gram-scale without diminishing its yield (66%).
Both TES-alkynyl protective groups of 8 were selectively cleaved
in the presence of the two silyl ethers by the Hirama procedure
(AgNO3, H2O, THF),12 providing the substrate for intramolecular
oxidative acetylenic coupling (9, 73% yield). At this point, a second
macrocyclization reaction was performed employing modified
Eglinton conditions (Cu(OAc)2, CuI, THF, pyridine),3,13 producing
the macrobicyclic intermediate 10. The latter product was found
to be extremely unstable and, for this reason, was directly subjected
to transannular cyclization (10f11).14 Thus, sequential treatment
of a solution of the macrobicyclic vinyl bromide 10 in THF/toluene
(1:1, prestirred at 23 °C with 4 Å MS for 15 min prior to cooling
to -78 °C) with LHMDS (1.0 M in THF, 6.0 equiv) then, after 2
min, with a solution of t-BuLi in pentane (1.7 M, 8.0 equiv) and
immediately thereafter (<5 s) with a quenching solution of acetic
acid in THF (1:3, 50 equiv), all at -78 °C, afforded the tricyclic
kedarcidin core structure 11 in 50% yield.3 The TBS ether group
of 11 was selectively removed in the presence of the diethyl-iso-
propylsilyl ether of the kedarose sugar using o-nitrophenol-buffered
tetra-N-butylammonium fluoride (TBAF);3a this simultaneously
cleaved the pivaloate ester, which was re-introduced (75%, two
steps 11f12). Vanadium-directed epoxidation15 of the resulting
allylic alcohol 12 was successful only with the hindered oxygen
atom source 1,1-diphenylethyl hydroperoxide,16 and afforded the
epoxy alcohol 13 (28% yield) as well as recovered starting material
(12, 30%). The position- and stereoselectivity of the epoxidation
reaction as well as the conformation of the ansa-bridge were
unambiguously established by several reinforcing NOE measure-
ments within 13, as well as by chemical shift and coupling constant
based arguments (see Supporting Information). An R-glycosidic
linkage was readily formed between 13 and the thioglycoside donor
14 (a mixture of anomers) using AgPF6 (2.5 equiv) as activator in
the presence of 2,6-di-tert-butyl-4-methylpyridine (DTBMP, 3.5
equiv),17 methodology previously developed by the Hirama group
in their studies directed toward a synthesis of 1.18
The R-mycarosylated product 15 was formed in 59% yield;
approximately 15% of the starting material (13) was also recovered.
Completion of the synthesis of 1 was achieved by dehydration of
15 using the Martin sulfurane in benzene (65% yield), followed
by global deprotection of the dehydration product 16 (TBAF,
o-nitrophenol, CH3CN;3a then Et3N‚3HF, 50% yield). Synthetic 1
provided spectroscopic data in complete accord with the proposed
structure, as did its cycloaromatization product (formed in the
presence of 1,4-cyclohexadiene at 23 °C, 12 h, ∼50% yield), but
our 1H NMR measurements differed substantially from those
reported for the natural product.1 After rigorous reevaluation of
our synthetic process, in which we reconfirmed all stereochemistry
and each reaction outcome, as reported herein,19 we were led to
carefully reconsider the original spectroscopic data for kedarcidin
chromophore, leading us to identify inconsistencies between these
data and both the original and the revised (1) structural assign-
ments.1,2 In particular, the observation of a nonzero coupling
between protons bound to C10 and C11 (3.1 Hz in DMSO-d6), the
lack of an NOE between the pyridyl C4′ proton and H10, and the
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J. AM. CHEM. SOC. 2007, 129, 5381-5383
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C O M M U N I C A T I O N S
Scheme 1 a
a Conditions: (a) 3, LiOH, H2O2, THF, H2O, 23 °C; (b) 9-fluorenemethanol (FmOH), 2-methyl-6-nitrobenzoic anhydride (MNBA), DMAP, CH2Cl2,
23 °C, 93% (two steps); (c) TMSOTf, 2,6-lutidine, CH2Cl2, 23 °C; then NaHCO3, 4, 97%; (d) 6, Pd(PPh3)4, CuI, Et3N, Et2O, 23 °C, 61%; (e) Et3N, THF;
MNBA, DMAP, benzene, 23 °C, 66%; (f) AgNO3, THF, H2O, 23 °C; 2,6-lutidine, 73%; (g) Cu(OAc)2, CuI, pyridine, THF, 60 °C, 59%; (h) LHMDS, THF,
toluene, -78 °C; t-BuLi; HOAc, 52%; (i) TBAF, o-nitrophenol, THF, 23 °C; (j) PivCl, Et3N, CH2Cl2, 23 °C, 76%; (k) CH3C(Ph)2OOH, VO(acac)2, benzene,
23 °C, 28%; (l) AgPF6, DTBMP, 14, CH2Cl2, 0 °C; pyridine, 59%; (m) Martin sulfurane, benzene, 23 °C, 65%; (n) TBAF, o-nitrophenol, CH3CN, 23 °C;
then Et3N‚3HF, 50%.
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C O M M U N I C A T I O N S
kedarcidin chromophore is a single atropisomer, whereas the trans-
oriented molecule we synthesized (1) is a mixture of atropisomers,
in two different NMR solvents (see Scheme 1). Presumably,
kedarcidin chromophore (2, as proposed herein) cannot exist in the
atropisomeric form that positions the chlorine atom toward the C10-
mycarosyl substituent.
Acknowledgment. Generous financial support from the National
Institutes of Health is gratefully acknowledged. Eli Lilly and Co.
provided critical support in the form of a graduate fellowship to
F.R. We are especially grateful to Dr. John Leet and his colleagues
at BMS, who were most helpful and accommodating in providing
assistance to us throughout our investigations.
Supporting Information Available: Detailed experimental pro-
cedures and tabulated spectroscopic data (1H and 13C NMR, FT-IR,
and MS) for all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Figure 1. Previous (1, ref 2) and current (2) structural revisions of
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borohydride-induced cycloaromatization of kedarcidin chromo-
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