Enantioselective total synthesis of (+)-largazole, a potent inhibitor of histone deacetylase

Article abstract of DOI:10.1021/ol8014623

(Chemical Equation Presented) An enantioselective total synthesis of the cytotoxic natural product (+)-largazole (1) is described. It is a potent histone deacetylase inhibitor. Our synthesis is convergent and involves the assembly of thiazole 3-derived carboxylic acid with amino ester 4 followed by cycloamidation of the corresponding amino acid. The synthesis features an efficient cross-metathesis, an enzymatic kinetic resolution of a β-hydroxy ester, a selective removal of a Boc-protecting group, a HATU/HOAt-promoted cycloamidation reaction, and synthetic manipulations to a sensitive thioester functional group.

Full text of DOI:10.1021/ol8014623

ORGANIC
LETTERS
2008
Vol. 10, No. 17
3907-3909
Enantioselective Total Synthesis of
(+)-Largazole, a Potent Inhibitor of
Histone Deacetylase†
Arun K. Ghosh* and Sarang Kulkarni
Departments of Chemistry and Medicinal Chemistry, Purdue UniVersity,
West Lafayette, Indiana 47907
akghosh@purdue.edu
Received June 27, 2008
ABSTRACT
An enantioselective total synthesis of the cytotoxic natural product (+)-largazole (1) is described. It is a potent histone deacetylase inhibitor.
Our synthesis is convergent and involves the assembly of thiazole 3-derived carboxylic acid with amino ester 4 followed by cycloamidation
of the corresponding amino acid. The synthesis features an efficient cross-metathesis, an enzymatic kinetic resolution of a ꢀ-hydroxy ester,
a selective removal of a Boc-protecting group, a HATU/HOAt-promoted cycloamidation reaction, and synthetic manipulations to a sensitive
thioester functional group.
In January 2008, Luesch and co-workers reported the
isolation of largazole, a novel 16-membered depsipeptide
from Floridian marine cyanobacterium Symploca sp.1 Lar-
gazole’s structure was elucidated by extensive NMR studies
and through chemical degradation. It has shown impressive
growth inhibitory activity of transformed mammary epithelial
cells (MDA-MB-231) in a dose-dependent manner with a
GI50 value of 7.7 nM. In addition, it has shown excellent
selectivity over nontransformed murine mammary epithelial
cells (NMuMG) with a GI50 of 122 nM. More recently,
Luesch, Hong, and co-workers reported the first total
synthesis of largazole. Their synthesis featured a macro-
cyclization at C6 and a late stage addition of the thioester
using cross-metathesis. Subsequently, they determined that
histone deacetylase (HDAC) is the molecular target for
largazole.2 This is very significant as HDAC inhibitors are
emerging as a new and exciting class of antineoplastic agents
for the treatment of solid and nematological malignancies.3
Incidentally, a number of depsipeptides are undergoing
clinical trials for treatment of various cancers.4 Largazole’s
important biological activity, its selectivity for cancer cells,
and its unique structural features led to considerable interest
in its chemistry and biology. To establish structure-activity
relationships and design novel structural variants, we sought
a convergent route to largazole. Herein, we report an
enantioselective synthesis of (+)-largazole.
As shown in Figure 1, our synthetic strategy involves a
late-stage cycloamidation of a sterically less demanding
carboxylic acid and an amine to form the 16-membered ring
from the corresponding amino acid derived from 2. Ester
derivative 2 could be obtained by the formation of an amide
bond between the acid arising from thiazole methyl ester 3
and the 4-derived amine. Our plan is to carry out the
(2) Ying, Y.; Taori, K.; Kim, H.; Hong, J.; Luesch, H. J. Am. Chem.
Soc. 2008, 130, 8455.
† Dedicated to Professor E. J. Corey with profound admiration and
appreciation on the occasion of his 80th birthday.
(1) Taori, K.; Paul, V. J.; Luesch, H. J. Am. Chem. Soc. 2008, 130,
1806.
(3) (a) Vigushin, D. M.; Coombes, R. C. Anti-Cancer Drugs 2002, 13,
1. (b) Marchion, D.; Munster, P. Expert ReV. Anticancer Ther. 2007, 7,
583.
(4) Piekarz, R.; Bastes, S. Curr. Pharm. Des. 2004, 10, 2289.
10.1021/ol8014623 CCC: $40.75
Published on Web 07/29/2008
2008 American Chemical Society

As shown in Scheme 1, our synthesis starts with the known
azido amide 96 which was treated with Lawesson’s reagent
in THF for 12 h to provide the corresponding thioamide in
67% yield.7 The resulting thioamide was then reacted with
ethyl bromopyruvate in refluxing ethanol for 1 h which
provided thiazole 5 in 82% yield.8 Saponification of ester 5
with 1 M aqueous LiOH gave the acid. The resulting acid
was then coupled with trityl-protected R-methyl cysteine9
under EDC/HOBt conditions in the presence of diisopropyl-
ethylamine to furnish amide 10 in 96% yield over two steps.
The conversion to thiazole-thiazoline fragment 11 was
achieved following a procedure reported by Kelly and co-
workers.10 Accordingly, amide 10 was reacted with 3 equiv
of triphenylphosphine oxide and 1.5 equiv of Tf2O in CH2Cl2
at 0 °C for 10 min to provide ester 11 in 89% yield. The
azide group in 11 was reduced using PPh3 in refluxing
methanol11 to give the amine which was then exposed to
Boc2O to furnish fragment 3 in 95% yield over two steps.
Optically active synthesis of ꢀ-hydroxy ester and its
conversion to ester 15 are shown in Scheme 2. Racemic aldol
Scheme 2. Synthesis of Thio Ester 15
Figure 1. Retrosynthetic analysis of largazole.
remainder of the synthesis with the sensitive thioester
functional group attached. The synthesis of thiazole 3 can
be achieved from a 5-derived thiazole acid and protected
(R)-2-methyl cysteine 6.5 Amino ester 4 could be accessed
by a cross-metathesis reaction between thioester 8 and
optically active allylic alcohol 7 followed by a Yamaguchi
esterification with the appropriately protected L-valine.
Alcohol 7 could be prepared by a lipase-mediated kinetic
resolution of racemic ꢀ-hydroxy ester.
product 12 was prepared by LDA deprotonation of tert-butyl
acetate followed by reaction of the resulting enolate with
acrolein at -78 °C to provide 12 in 81% yield. The racemic
alcohol was then exposed to lipase PS-30 in pentane in the
presence of excess vinyl acetate at 23 °C for 12 h to provide
enantioenriched alcohol 13 and acetate derivative 14 in 45%
and 42% yields, respectively. Selective removal of the acetate
Scheme 1. Synthesis of Segment 3
(5) Pattenden, G.; Thom, S. M.; Jones, M. F. Tetrahedron 1993, 49,
2131.
(6) Dyke, J. M.; Levita, G.; Morris, A.; Ogden, J. S.; Dias, A. A.;
Algarra, M.; Santos, J. P.; Costa, M. L.; Rodrigues, P.; Barros, M. T J.
Phys. Chem. A 2004, 108, 5299.
(7) Ozturk, T.; Ertas, E.; Mert, O. Chem. ReV. 2007, 107, 5210.
(8) Boyce, R. J.; Pattenden, G. Tetrahedron 1995, 51, 7313.
(9) For synthesis of the enantiomer, see: Hara, S.; Makino, K.; Hamada,
Y. Tetrahedron Lett. 2006, 47, 1081.
(10) You, S.-L.; Razavi, H.; Kelly, J. W. Angew. Chem., Int. Ed. 2003,
42, 83.
(11) Pal, B.; Jaisankar, P.; Giri, V. S. Synth. Commun. 2004, 34, 1317.
3908
Org. Lett., Vol. 10, No. 17, 2008

was carried out by exposure of 14 to potassium carbonate
in methanol at -30 °C to afford optically active ꢀ-hydroxy
ester 7 in high enantiomeric purity (93% ee). The enantio-
meric excess was determined by formation of the corre-
sponding Mosher ester of alcohol 15 followed by analysis
of 19F NMR.12 The kinetic resolution of ꢀ-hydroxy ester has
provided a convenient access to optically active esters.13 For
preparation of alcohol 15 we planned a cross-metathesis of
alcohol 7 and thioester 8. The requisite thioester was prepared
by reaction of 3-butenethiol14 and octanoyl chloride in the
presence of DMAP. A cross-metathesis reaction of alcohol
7 and thioester 8 in the presence of 3 mol % of Grubbs’
second-generation catalyst afforded E-olefin 15 exclusively
in 67% yield.15
Reaction of the resulting anhydride with alcohol 15 and
DMAP furnished thioester 4 in 91% yield. Selective depro-
tection of the Boc group in the presence of a tert-butyl ester
was carried out by exposure of 4 to 30% trifluoroacetic acid
in CH2Cl2 at 0 °C for 20 min to provide amine 17. For
assembly of the largazole subunits, saponification of methyl
ester 3 was carried out with 1 M aqueous LiOH to give acid
18. Coupling of acid 18 with amine 17 was accomplished
by using HATU and HOAt in the presence of diisopropyl-
ethylamine to furnish the requisite protected amino ester 2
in 66% yield. Formation of the 16-membered cycloamide
was carried out in a two-step sequence involving (1) exposure
of 2 to trifluoroacetic acid at 23 °C for 3 h to remove both
the Boc and the tert-butyl groups and (2) treatment of the
resulting amino acid with 2 equiv of HATU and 2 equiv of
HOAt in the presence of diisopropylethylamine under dilute
conditions to provide synthetic (+)-largazole (1) in 40%
isolated yield (two steps). The spectral data (1H and 13C
The final assembly of the largazole fragment is shown in
Scheme 3. N-Boc-valine 16 was subjected to esterification
with alcohol 15 using Yamaguchi’s protocol.16 Accordingly,
reaction of 16 with 2,4,6-trichlorobenzoyl chloride in the
presence of diisopropylethylamine gave the anhydride.
NMR) of synthetic (+)-largazole (1, [R]23 +24, c 0.13,
D
MeOH (lit.1 [R]20 +22, c 0.1, MeOH)) is identical with
D
that reported for the natural (+)-largazole.1
Scheme 3. Synthesis of Largazole
In summary, we have accomplished an enantioselective
synthesis of (+)-largazole (1). The synthesis will provide a
convenient access to a variety of largazole derivatives.
Structural modifications are currently in progress.17
Acknowledgment. Partial financial support by the Na-
tional Institute of Health is gratefully acknowledged. We
thank Mr. David D. Anderson of Purdue University for his
help with the HPLC analysis.
Supporting Information Available: Experimental pro-
1
cedures and H and 13C NMR spectra for compounds 1-5,
7-11, and 15. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL8014623
(12) Dale, J. A.; Dull, D. L.; Mosher, H. S. J. Org. Chem. 1969, 34,
2543.
(13) (a) Vrielynck, S.; Vandewalle, M.; Garc´ıa, A. M.; Mascaren˜as, J. L.;
Mourin˜o, A. Tetrahedron Lett. 1995, 36, 9023. (b) Pollini, G. P.; Risi, C. D.;
Lumento, F.; Marchetti, P.; Zanirato, V. Synlett 2005, 164.
(14) Minozzi, M.; Nanni, D.; Walton, J. C. Org. Lett. 2003, 5, 901.
(15) (a) Scholl, M.; Ding, S.; Lee, C.; Grubbs, R. H. Org. Lett. 1999, 1,
953. (b) Chatterjee, A. K.; Choi, T.; Sanders, D. P.; Grubbs, R. H. J. Am.
Chem. Soc. 2003, 125, 11360.
(16) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989.
(17) During the review of this manuscript, two other syntheses have
appeared in the literature; see: (a) Nasveschuk, C. G.; Ungermannova, D.;
Liu, X.; Phillips, A. J. Org. Lett. 2008, 10, 3595–3598. (b) Seiser, T.;
Kamena, F.; Cramer, N. Angew. Chem. DOI:, 10.1002/anie.200802043.
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