A convergent synthesis of (+)-cryptophycin B, a potent antitumor macrolide from Nostoc sp. cyanobacteria.

Article abstract of DOI:10.1021/ol000058i

[structure--see text] An efficient and highly stereoselective synthesis of cryptophycin B (2), a potent cytotoxic agent, is described. The ester-derived titanium-enolate-mediated syn-aldol reaction was employed to generate the stereocenters C(5) and C(6). The route is convergent and provides a convenient access to the synthesis of structural variants of cryptophycin B as well as members of its family.

Full text of DOI:10.1021/ol000058i

ORGANIC
LETTERS
2000
Vol. 2, No. 11
1573-1575
A Convergent Synthesis of
(+)-Cryptophycin B, a Potent Antitumor
Macrolide from Nostoc sp.
Cyanobacteria
Arun K. Ghosh* and Alexander Bischoff
Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street,
Chicago, Illinois 60607
arunghos@uic.edu
Received March 14, 2000
ABSTRACT
An efficient and highly stereoselective synthesis of cryptophycin B (2), a potent cytotoxic agent, is described. The ester-derived titanium-
enolate-mediated syn-aldol reaction was employed to generate the stereocenters C(5) and C(6). The route is convergent and provides a convenient
access to the synthesis of structural variants of cryptophycin B as well as members of its family.
Macrocyclic marine natural products continue to be a rich
source for diverse antitumor agents with significant clinical
potential.1 Of particular interest, the cryptophycins, a group
of depsipeptides isolated recently, have exhibited remarkably
potent antitumor properties. The first cryptophycin (1) was
isolated from terrestrial Nostoc sp. ATCC 53789 by re-
searchers at Merck and was found to be very active against
Cryptococcus, a fungus that often infects persons with
immunodeficiencies.2 In 1994, Moore et al. isolated a host
of cyclic depsipeptides including cryptophycin A (1) and B
(2) from Nostoc. sp. GSV 224 and established their absolute
stereochemistry (Figure 1).3 Cryptophycins A and B exhibited
cytotoxic IC50 values of 5 and 7 pg/mL, respectively, against
KB cells. In addition, the compounds were equally effective
against drug-sensitive and drug-resistant tumor cells.3,4
Arenastatin A (3), another member of the cryptophycin
family, was isolated from the Okinawan marine sponge
Dysidea arenaria.5,6 It has displayed a cytotoxic IC50 value
of 5 pg/mL against KB cells.6
Cryptophycin A and other members of the family have
been shown to promote depletion of microtubules.4 Further-
more, a number of these compounds effectively inhibited in
vitro tubulin polymerization.7 The significant clinical po-
tential of the cryptophycins and their relatively low natural
abundance has attracted immense interest in their synthesis
and structural modification. Several total syntheses and
synthetic approaches to cryptophycins and arenastatin A have
been described in recent years.8,9 As part of our interest in
the structure-function studies of cryptophycins, we sought
a flexible, enantioselective synthesis of cryptophycin B.
Herein we report a convergent and stereocontrolled total
synthesis of cryptophycin B.
As outlined in Figure 1, we planned to assemble crypto-
phycin B in a convergent manner from the protected
(4) Smith, C. D.; Zhang, X.; Mooberry, S. L.; Patterson, G. M. L.; Moore,
R. E. Cancer Res. 1994, 54, 3779.
(5) Kobayashi, M.; Aoki, S.; Ohyabu, N.; Kurosu, M.; Wang, W.;
Kitagawa, I. Tetrahedron Lett. 1994, 35, 7969.
(6) Koiso, Y.; Morita, K.; Kobayashi, M.; Wang, W.; Ohyabu, N.;
Iwasaki, S. Chem.-Biol. Interact. 1996, 102, 183.
(1) Harris, C. R.; Danishefsky, S. J. J. Org. Chem. 1999, 64, 8434.
(2) (a) Schwarz, R. E.; Hirsch, C. F.; Sesin, D. F.; Flor, J. E.; Chartrain,
M.; Fromtling, R. E.; Harris, G. H.; Salvatore, M. J.; Liesch, J. M.; Yudin,
K. J. Ind. Microbiol. 1990, 5, 113. (b) Schwarz, R. E.; Hirsch, C. F.; Sesin,
D. F. U.S. Patent 4,946,835, Aug 7, 1990.
(3) Trimurtulu, G.; Ohtani, I.; Patterson, G. M. L.; Moore, R. E.; Corbett,
T. H.; Valeriote, F. A.; Demchik, L. J. Am. Chem. Soc. 1994, 116, 4729.
(7) Kerksiek, M. R.; Mejillano, R. E.; Schwartz, R. E.; Georg, G. I.;
Himes, R. H. FEBS Lett. 1995, 377, 59.
10.1021/ol000058i CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/11/2000

Scheme 1a
Figure 1.
a (a) TiCl4, iPr2NEt, 0 to 23 °C, 1 h then BnO(CH2)2CHO, -78
°C, 20 min (98%); (b) LAH, THF, 0 °C, 1 h (92%); (c) PhLi, THF,
-78 °C, 30 min, TsCl, -20 °C, 30 min, then LAH, 0 °C, 20 min
(96%); (d) TIPSOTf, 2,6-lutidine (99%); (e) K2CO3, BBr3, CH2Cl2,
0 °C, (83%); (f) PCC, MS 4 Å, 23 °C, 10 min (98%); (g) NaH,
triethylphosphonoacetate, THF, 0 °C, 30 min (92%); (h) aq LiOH,
EtOH (1:1), 23 °C, 2 h (94%); (i) 5, DCC, DMAP, 23 °C, 12 h
(79%); (j) TBAF, THF, 23 °C (99%).
octadienoic acid 4, D-tyrosine ester 5, and protected â-amino
acid derivative 6. The fragments were planned to be
connected by Yamaguchi esterfication and cycloamidation
reactions. Introduction of the sensitive epoxide functionality
was planned at the final stage of the synthesis. The key
elements of our synthesis involved an ester-derived titanium-
enolate-mediated syn-aldol reaction to set the stereocenters
at C(5) and C(6) of fragment 410a,b Thus, acylation of trans-
4-phenyl-3-butenoic acid and (+)-(1R,2S)-1-(N-tosylamino)-
2-indanol with DCC and DMAP afforded the ester 7 in 98%
yield.10c As shown in Scheme 1, exposure of 7 to TiCl4 and
N,N-diisopropylethylamine in CH2Cl2 at 23 °C generated the
corresponding titanium enolate. Subsequent reaction of the
enolate with (benzyloxy)propionaldehyde at -78 °C fur-
nished the aldol adduct 8 as a single diastereomer in 98%
yield after silica gel chromatography. Reduction of 8 by
lithium aluminum hydride in THF at 0 °C for 1 h afforded
the diol in 92% yield. The primary hydroxyl group of the
resulting diol was selectively converted to the corresponding
methyl group in an one-pot, two-step sequence. Thus,
treatment of the diol with phenyllithium and p-toluenesulfo-
nyl chloride at -78 °C to -20 °C followed by reduction of
the resulting tosylate with lithium aluminum hydride pro-
vided the alcohol 9 in 96% yield.
Protection of the secondary alcohol using triisopropylsilyl
triflate and 2,6-lutidine in CH2Cl2 gave 10 in 99% yield.11
Selective removal of the benzyl protecting group was
effectively achieved by treatment of 10 with boron tribromide
in CH2Cl2 in the presence of potassium carbonate to provide
the corresponding alcohol in 83% yield.12 Initial attempts
with other conditions such as TMSCl/nBu4N+I-, FeCl3, or
Na/NH3 were unsuccessful.13 Oxidation of the resulting
alcohol with PCC in CH2Cl2 followed by Horner-Emmons
olefination of the aldehyde with sodium hydride and triethyl
phosphonoacetate provided the R,â-unsaturated ester 11 in
92% yield. Ester hydrolysis with aqueous lithium hydroxide
in a mixture (1:1) of EtOH and H2O gave the octadienoic
acid fragment 4 in 94% yield. Coupling of this acid with
(8) (a) Norman, B. H.; Hemscheidt, T.; Schultz, R. M.; Andis, S. L. J.
Org. Chem. 1998, 63, 5288. (b) Gardinier, K. M.; Leahy, J. W. J. Org.
Chem. 1997, 62, 7098. (c) Salamonczyk, G. M.; Han, K.; Guo, Z.-W.; Sih,
C. J. J. Org. Chem. 1996, 61, 6893. (d) Rej, R.; Nguyen, D.; Go, B.; Fortin,
S.; Lavalle´e, J.-F. J. Org. Chem. 1996, 61, 6289. (e) Barrow, R. A.;
Hemscheidt, T.; Liang, J.; Paik, S.; Moore, R. E.; Tius, M. A. J. Am. Chem.
Soc. 1995, 117, 2479. (f) Dhokte, U. P.; Khau, V. V.; Hutchison, D. R.;
Martinelli, M. J. Tetrahedron Lett. 1998, 39, 8771. (g) White, J. D.; Hong,
J.; Robarge, L. A. J. Org. Chem. 1999, 64, 6206. (h) Kobayashi, M.; Wang,
W.; Ohyabu, N.; Kurosu, M.; Kitagawa, I. Chem. Pharm. Bull. 1995, 43,
1598. (i) Kobayashi, M.; Wang, W.; Ohyabu, N.; Kurosu, M.; Kitagawa, I.
Chem. Pharm. Bull. 1994, 42, 2394.
(9) (a) Liang, L.; Hoard, D. W.; Khau, V. V.; Martinelli, M. J.; Moher,
E. D.; Moore, R. E.; Tius, M. A. J. Org. Chem. 1999, 64, 1459. (b)
Furuyama, M.; Shimizu, I. Tetrahedron: Asymmetry 1998, 9, 1351. (c)
Ali, S. M.; Georg, G. I. Tetrahedron Lett. 1997, 38, 1703.
(10) (a) Ghosh, A. K.; Fidanze, S.; Senanayake, C. H. Synthesis 1998,
937. (b) Ghosh, A. K.; Fidanze, S.; Onishi, M.; Hussain, K. A. Tetrahedron
Lett. 1997, 38, 7171. (c) Ghosh, A. K.; Onishi, M. J. Am. Chem. Soc. 1996,
118, 2527.
(11) Ru¨cker C. Chem. ReV. 1995, 95, 1009 and references cited therein.
(12) Treatment of 10 with boron tribromide in CH2Cl2 afforded the
corresponding alcohol in only 32% yield. The major side product was the
desilylation product.
(13) (a) Vedejs, E.; Buchanan, R. A.; Watanabe, Y. J. Am. Chem. Soc.
1989, 111, 8430. (b) Oikawa, Y.; Tanaka, T.; Yonemitsu, O. Tetrahedron
Lett. 1986, 27, 3647. (c) Park, M. H.; Takeda, R.; Nakanishi, K. Tetrahedron
Lett. 1987, 28, 3823.
1574
Org. Lett., Vol. 2, No. 11, 2000

O-methyl-D-tyrosine tert-butyl ester14 in the presence of DCC
and DMAP in CH2Cl2 resulted in the amide 12 in 79%
yield.15 Exposure of 12 with nBu4N+F- in the presence of a
trace of water in THF furnished the alcohol 13 in near
quantitative yield.16
protecting group to furnish the requisite amino acid precursor
for the macrocyclization. Thus, subjection of the resulting
amino acid to Yamaguchi conditions with 2,4,6-trichloroben-
zoyl chloride, N,N-diisopropylethylamine, and DMAP at 23
°C for 12 h furnished the cycloamide 18 ([R]23 +36.2, c
D
The synthesis of â-amino acid derivative 6 was carried
out in two steps as shown in Scheme 2. Esterification of
0.72, MeOH; lit.3 [R]D +36.7, c 1.93, MeOH) in 74% yield
(from 17). Epoxidation of 18 with dimethyldioxirane in CH2-
Cl2 at -30 °C to 23 °C for 12 h provided a 3:1 mixture of
epoxides in 87% yield. The major epoxide was separated
by reverse-phase HPLC on a C-18 column20 (eluent, 3:1
MeOH/H2O) to provide the synthetic cryptophycin B (2,
Scheme 2a
[R]23 +20.6, c 0.24, MeOH; lit.3 [R]D +20.4, c 0.54,
D
1
MeOH). Spectral data (500 MHz H NMR, IR, and 13C
NMR) for the synthetic cryptophycin B are in full agreement
with that reported by Moore et al. for the natural crypto-
phycin B.3
In summary, a stereocontrolled synthesis of cryptophycin
B has been accomplished in 14 steps from 7 in 22% overall
yield. The present synthetic route is easily amenable to the
synthesis of other members of the cryptophycin family.
Further studies of cryptophycins are ongoing in our labora-
tory.
Acknowledgment. Financial support for this work was
provided by the National Institutes of Health (GM 55600).
We thank Mr. Alexander Ho for preliminary experimental
assistance.
Supporting Information Available: 1H NMR and 13C
NMR spectra for compounds 2 and 4-18. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL000058I
(14) O-Methyl-D-tyrosine tert-butyl ester was prepared in a three-step
sequence involving (1) treatment of commercially available N-Cbz-D-
tyrosine with Me2SO4 and NaOH in aqueous EtOH at reflux; (2) reaction
of the resulting O-methyltyrosine with 2,4,6-trichlorobenzoyl chloride, iPr2-
NEt, DMAP, and tBuOH; and (3) removal of Cbz group by hydrogenation
over 10% Pd-C in MeOH (55% overall).
(15) Hassner A.; Alexanian V. Tetrahedron Lett. 1978, 46, 4475.
(16) Reaction rate was accelerated in the presence of a trace amount of
water.
a (a) DCC, benzyl (S)-(-)-2-hydroxyisocaproate, DMAP, CH2Cl2,
23 °C, 12 h (91%); (b) H2, 10% Pd-C, EtOAc, 5 h (95%); (c)
2,4,6-(Cl3)PhCOCl, iPr2NEt, DMAP, alcohol 13, 23 °C; (d)
CF3CO2H, 23 °C, 2 h; (e) dimethyldioxirane, CH2Cl2, -30 to 23
°C, 12 h (87%, 3:1 mixture).
(17) Amino acid 14 was prepared by tosylation of commercially available
methyl (R)-(-)-3-hydroxy-2-methylpropionate with TsCl and NEt3 in CH2-
Cl2 for 1 h. Displacement of tosylate with NaN3 in DMSO at 23°C for 6 h.
Hydrogenation of the resulting azide over 10% Pd/C in the presence of
Boc2O in EtOAc followed by saponification with aqueous LiOH at 23°C
for 30 min (90% overall).
(18) Benzyl (S)-(-)-2-hydroxyisocaproate was prepared from com-
mercially available (S)-(-)-2-hydroxyisocaproate by treatment with CsCO3
in MeOH/H2O (6:1) for 30 min followed by reaction with BnBr in DMF at
0 °C (91%).
(19) Inanaga J.; Hirata K.; Saeki H.; Katsuki T.; Yamaguchi M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989.
(20) The major epoxide was cleanly separated by an isocratic reverse
phase HPLC (see ref 8g for similar separation) on a YMC-Pack ODS-AQ
5S 120 Å column (4.6 × 250 mm) with flow rate 1 mL/min and UV
detection at 254 nm. Retention times for 2: 31.58 min; and diastereomer:
37.20 min.
optically active N-Boc-3-amino-2-methylpropionic acid 1417
and benzyl (S)-(-)-2-hydroxyisocaproate18 with dicyclo-
hexylcarbodiimide and DMAP in CH2Cl2 at 23 °C for 12 h
afforded the diester 15 in 91% yield. Catalytic hydrogenation
of diester 15 over 10% Pd/C in ethyl acetate under a
hydrogen-filled balloon for 5 h furnished the acid 6 in 95%
yield. The acid 6 was subjected to esterification by reaction
with 2,4,6-trichlorobenzoyl chloride, N,N-diisopropylethyl-
amine, and alcohol 13 in the presence of DMAP under
Yamaguchi conditions to provide diester 17 in 84% yield.19
Exposure of 17 to trifluoroacetic acid at 23 °C for 2 h resulted
in the removal of both the tert-butyl ester and the Boc-
Org. Lett., Vol. 2, No. 11, 2000
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