Total synthesis of cryptophycin-24 (arenastatin A) amenable to structural modifications in the C16 side chain

Article abstract of DOI:10.1021/jo000767+

Two efficient protocols for the synthesis of tert-butyl (5S,6R,2E,7E)-5-[(tert-butyldimethylsilyl)-oxy]-6-methyl-8-phenyl-2,7-octadie noate, a major component of the cryptophycins, are reported. The first utilized the Noyori reduction and Frater alkylation of methyl 5-benzyloxy-3-oxopentanoate to set two stereogenic centers, which became the C16 hydroxyl and C1' methyl of the cryptophycins. The second approach started from 3-p-methoxybenzyloxypropanal and a crotyl borane reagent derived from (-)-α-pinene to set both stereocenters in a single step and provided the dephenyl analogue, tert-butyl (5S,6R,2E)-5-[(tert-butyldimethylsilyl)oxy]-6-methyl-2,7-octadienoate, in five steps. This compound was readily converted to the 8-phenyl compound via Heck coupling. The silanyloxy esters were efficiently deprotected and coupled to the C2-C10 amino acid fragment to provide desepoxyarenastatin A and its dephenyl analogue. The terminal olefin of the latter was further elaborated via Heck coupling. Epoxidation provided cryptophycin-24 (arenastatin A).

Full text of DOI:10.1021/jo000767+

7792
J . Org. Chem. 2000, 65, 7792-7799
Tota l Syn th esis of Cr yp top h ycin -24 (Ar en a sta tin A) Am en a ble to
Str u ctu r a l Mod ifica tion s in th e C16 Sid e Ch a in
MariJ ean Eggen, Craig J . Mossman, Suzanne B. Buck, Sajiv K. Nair, Laxminarayan Bhat,
Syed M. Ali, Emily A. Reiff, Thomas C. Boge, and Gunda I. Georg*
Department of Medicinal Chemistry, University of Kansas, Lawrence, Kansas 66045
georg@ukans.edu
Received May 19, 2000 (Revised Manuscript Received September 8, 2000)
Two efficient protocols for the synthesis of tert-butyl (5S,6R,2E,7E)-5-[(tert-butyldimethylsilyl)-
oxy]-6-methyl-8-phenyl-2,7-octadienoate, a major component of the cryptophycins, are reported.
The first utilized the Noyori reduction and Frater alkylation of methyl 5-benzyloxy-3-oxopentanoate
to set two stereogenic centers, which became the C16 hydroxyl and C1′ methyl of the cryptophycins.
The second approach started from 3-p-methoxybenzyloxypropanal and a crotyl borane reagent
derived from (-)-R-pinene to set both stereocenters in a single step and provided the dephenyl
analogue, tert-butyl (5S,6R,2E)-5-[(tert-butyldimethylsilyl)oxy]-6-methyl-2,7-octadienoate, in five
steps. This compound was readily converted to the 8-phenyl compound via Heck coupling. The
silanyloxy esters were efficiently deprotected and coupled to the C2-C10 amino acid fragment to
provide desepoxyarenastatin A and its dephenyl analogue. The terminal olefin of the latter was
further elaborated via Heck coupling. Epoxidation provided cryptophycin-24 (arenastatin A).
In tr od u ction
A novel, cyclic depsipeptide isolated from the blue-
green algae (cyanobacterium) Nostoc sp. ATCC 53789
was reported by Schwartz and co-workers in 1990.1 This
compound demonstrated extremely potent activity against
filamentous fungi of the genus Cryptococcus and thus was
named cryptophycin A (1) (also known as cryptophycin-
1, Figure 1).1 Subsequently, Moore and co-workers iso-
lated the same compound and several structural relatives
from Nostoc sp. GSV 224.2,3 Cryptophycin-1 (1), the most
abundant of the macrolides, was found to have significant
tumor-selective cytotoxicity3-5 and was not an effective
substrate for the P-glycoprotein efflux mechanism in
multiple-drug resistant cells.6 When administered intra-
venously, depsipeptide 1 was also very effective against
mammary, colon, and pancreatic adenocarcinomas in
mouse xenographs.7 Concurrently, Kobayashi and Kita-
gawa isolated a related cytotoxic agent from the Oki-
nawan marine sponge Dysidea arenaria and named it
arenastatin A (2).8-10 It was found that cryptophycin-24
was identical to this compound. Thus far, 25 compounds
of the cryptophycin family have been reported through
isolation.2,7
F igu r e 1. Structures of the cryptophycins.
Cryptophycin-1 (1) blocks the cell cycle at the G2/M
phase apparently through inhibition of tubulin poly-
merization into microtubules.11,12 This compound binds
to a tubulin site distinct from the colchicine site, but one
that may overlap with the vinblastine site.11,13-15 Its
extreme potency (100-1000-fold greater than paclitaxel
and vinblastine) has led to additional studies investigat-
ing other possible modes of action.12,16,17 It was found that
cryptophycin-1 is a highly potent stabilizer of microtubule
dynamics at concentrations (e100 nM) that have no effect
on net microtubule polymerization.16
Semisynthetic analogues of cryptophycin-1 (1) prima-
rily focusing on the reactive epoxide moiety have resulted
in loss of activity except for the halohydrins.18 In par-
ticular, the chlorohydrin derived from cryptophycin-1 (1)
* To whom correspondence should be addressed. Phone: (785) 864-
4498. Fax: (785) 864-5836.
(1) Schwartz, 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.
(2) Subbaraju, G. V.; Golakoti, T.; Patterson, G. M. L.; Moore, R. E.
J . Nat. Prod. 1997, 60, 302.
(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.
(4) Moore, R. E.; Corbett, T. H.; Patterson, G. M. L.; Valeriote, F.
A. Curr. Pharm. Design 1996, 2, 317.
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L.; Panchapor, C.; Pugh, S.; White, K.; Kushner, J .; Rake, J .; Wentland,
M.; Golakoti, T.; Hetzel, C.; Ogino, J .; Patterson, G.; Moore, R. Invest.
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Moore, R. E. Cancer Res. 1994, 54, 3779.
(8) Kobayashi, M.; Kuroso, M.; Ohyabu, N.; Wang, W.; Fujii, S.;
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Iwasaki, S. Chem.-Biol. Interact. 1996, 102, 183.
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R. FEBS Lett. 1995, 377, 59.
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73, 440.
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Cancer Res. 1996, 56, 4398.
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(15) Mooberry, S. L.; Taoka, C. R.; Busquets, L. Cancer Lett. 1996,
107, 53.
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M.; Larsen, L. K.; Patterson, G. M. L.; Moore, R. E.; Mooberry, S. L.;
Corbett, T. H.; Valeriote, F. A. J . Am. Chem. Soc. 1995, 117, 12030.
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10.1021/jo000767+ CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/14/2000

Total Synthesis of Cryptophycin-24
J . Org. Chem., Vol. 65, No. 23, 2000 7793
Sch em e 1
demonstrated higher activity in vivo than the parent
compound. However, comparisons of tubulin assembly
and cell toxicity data indicated that the halohydrins have
reduced or no biological activity and that the observed
activity resulted from the regeneration of the parent
compound 1.19 In pursuit of stable analogues, synthetic
approaches were developed probing the substituents and
stereochemistry of the tyrosine20 and â-amino acid
subunits,21-23 the stereochemical and electronic effects
of the octadienoate ester subunit,24-26 and replacement
of the R-hydroxy acid subunit with an R-amino acid.27
These investigations gave rise to cryptophycin-52 (3), a
synthetic analogue currently in phase II clinical trials.28
This analogue is hydrolytically more stable than 1 due
to the presence of gem-dimethyl substituents on the
â-amino acid moiety and demonstrates similar or im-
proved bioactivity. Cryptophycin-52 (3) has been shown
to stabilize microtubule dynamics28 and be very effective
against numerous human tumor cell lines.29 This com-
pound also accumulated within cells to a concentration
consistent with mitotic arrest without altering microtu-
bule polymer concentration.30 Paclitaxel, which has been
shown to have a second mechanism of action by the
hyperphosphorylation of Bcl2, renders cancer cells sus-
ceptible to apoptosis.31 Cryptophycin-52 (3), likewise, was
recently reported to also have this mechanism of action
and to be the most potent agent known in this respect.32
The outstanding activity, extreme potency, and arrival
of a clinical candidate of the cryptophycins has generated
a large amount of interest in the total synthesis of
arenastatin A (2),33-36 cryptophycin-1 (1),26,37-40 crypto-
phycin-2,41,42 and cryptophycin-52 (3),40,43 as well as the
synthesis of the octadienoate ester fragment.44-48 The
following discussion fully discloses our previously re-
ported fragment syntheses,44,45 as well as the completion
of the synthesis of cryptophycin-24 (arenastatin A, 2).
Resu lts a n d Discu ssion
Our interest in structurally flexible fragment syntheses
was primarily for structure-activity relationship (SAR)
studies, biochemical studies,11 and photolabeling experi-
ments with tubulin protein. To achieve this goal, we
targeted cryptophycin-24 (2), which lacks a chiral center
at C6 and the chlorine substituent in the tyrosine moiety.
The activity of 2, though slightly diminished in certain
cancer cell lines in comparison to cryptophycin-1 (1), is
similar with respect to microtubule depolymerization.10,13
The retrosynthetic analysis of cryptophycin-24 (2) is
shown in Scheme 1. The synthesis of 2 can be simplified
by utilizing desepoxyarenastatin 4 as the final precursor
with epoxidation being the last step in the total synthesis.
Further retrosynthetic analysis of desepoxyarenastatin
reveals that 4 can be assembled from ester 5 and amino
acid 6, synthesized from L-leucic acid, â-alanine and D-O-
methyltyrosine.
The structurally more complex fragment, ester 5, was
targeted via two approaches (Scheme 2). The first ap-
proach, which provided flexibility at the C1′ position,
utilized an asymmetric reduction of 8 to set the C16
hydroxyl group and then incorporated the second stereo-
center anti using a Frater alkylation to provide inter-
mediate 7. This alkylation strategy allowed potential
incorporation of alternative substitution at C1′ through
(18) Trimurtulu, G.; Ogino, J .; Heltzel, C. E.; Husebo, T. L.; J ensen,
C. M.; Larsen, L. K.; Patterson, G. M. L.; Moore, R. E.; Mooberry, S.
L.; Corbett, T. H.; Valeriote, F. A. J . Am. Chem. Soc. 1995, 117, 12030.
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H. Bioorg. Med. Chem. Lett. 1998, 8, 1959.
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Med. Chem. Lett. 1999, 9, 69.
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the American Association of Cancer Research, New Orleans, LA;
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Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 9313.
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7794 J . Org. Chem., Vol. 65, No. 23, 2000
Eggen et al.
Sch em e 2
Sch em e 3
and 97% ee (enantiomeric excess was determined through
chiral HPLC: Daicel Chiralcel OD-H). Frater alkyla-
tion53,54 of the dianion of â-hydroxyester 12 with io-
domethane gave anti product 7 in 75% yield and 95%
diastereomeric excess. The minor isomer was easily
removed by flash chromatography. Debenzylation of 7
(95%), followed by silylation of the resulting diol, provided
compound 13 in 98% yield. DIBAL-H reduction of 13 to
the corresponding aldehyde 15 and subsequent Horner-
Emmons reaction (diethyl benzylphosphonate) yielded
styrene 16. It was found, however, that complete reduc-
tion to the primary alcohol 14 with DIBAL-H, followed
by TPAP oxidation55 was experimentally more convenient
and provided a comparable overall yield. Selective cleav-
age of the primary TBS-ether of 16 (HOAc, H2O, THF;
1:1:2)56 was followed by TPAP oxidation to yield aldehyde
18. The aldehyde 18 was converted directly to the methyl
ester 19 in 83% yield or the tert-butyl ester 20 in 89%
yield by Horner-Emmons homologation with the corre-
sponding phosphonate and tetramethylguanidine or DBU
and LiCl,57 respectively. A comparison of the optical
rotation of the methyl ester thus obtained ([R]D ) +66.8),
with the published data for 19 ([R]D ) +68.2)37 verified
the high optical purity and correct absolute stereochem-
istry of our product.
This approach provided the octadienoate ester back-
bone 20 in 10 steps (overall yield of 20%) from readily
available starting materials. The synthesis can be carried
out on a relatively large scale. Although this route
provided an effective method for modification at C1′, we
required a method to more efficiently modify the C3′
aromatic substituent.
A second approach (Scheme 4) to octadienoate ester 5,
amenable to SAR studies at the C3′ phenyl group of the
C16 side chain, was achieved starting from aldehyde 11.58
The key step utilized the crotyl boration of 11 with crotyl
diisopinocampheylborane 10 (prepared from (+)-B-meth-
oxydiisopinocampheylborane) to generate the desired
stereochemistry at the two chiral centers of 21 in 76%
yield (91% ee).59 Silyl protection of the secondary alcohol
with tert-butyldimethylsilyl trifluoromethanesulfonate
the use of electrophiles other than iodomethane. A second
route, in which a dephenyl ester 9 was the targeted
synthon, allowed the probing of the C3′ aromatic region
of the side chain. Ester 9 was derived from a crotyl
boration of aldehyde 11 with borane 10, which incorpo-
rated both backbone stereocenters in a single step. With
9 in hand, we could vary the aryl substituents using a
Heck coupling strategy in the final step of the fragment
synthesis or potentially after the 16-membered crypto-
phycin macrocycle was intact for thorough SAR studies.
Our first approach (Scheme 3) to the octadienate ester
5 targeted flexibility in the C1′ position in the target
compounds. The asymmetric Noyori catalytic hydrogena-
tion49 of â-keto ester 850 with the (R)-Ru(BINAP)Br2
complex51 provided (S)-hydroxy ester 1252 in 97% yield
(53) Frater, G. Helv. Chim. Acta 1979, 62, 2825.
(54) Georg, G. I.; Kant, J .; Gill, H. J . J . Am. Chem. Soc. 1987, 109,
1129.
(55) Ley, S. V.; Norman, J .; Griffith, W. P. Synthesis 1994, 639.
(56) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, 3rd ed.; J ohn Wiley & Sons: New York, 1999; p 779.
(57) Blanchette, M. A.; Choy, W.; Davis, J . T.; Essenfeld, A. P.;
Masamune, S.; Roush, W. R.; Sakai, T. Tetrahedron Lett. 1984, 25,
2183.
(49) Noyori, R. Asymmetric Catalysis in Organic Synthesis; J ohn
Wiley & Sons: New York, 1994; p 56.
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52, 192.
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4801.
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Tetrahedron 1995, 51, 3549.
(58) This aldehyde was conveniently prepared in multigram quanti-
ties from 1,3-propanediol through the following sequence: (a) for
monoprotection see: Urbanek, R. A.; Sabes, S. F.; Forsyth, C. J . J .
Am. Chem. Soc. 1998, 120, 2523. (b) NaOCl (aq), TEMPO, NaHCO3,
KBr, CH2Cl2, 99%.

Total Synthesis of Cryptophycin-24
J . Org. Chem., Vol. 65, No. 23, 2000 7795
Sch em e 4
Sch em e 6
Sch em e 7
Sch em e 5
(TBSOTf) in the presence of 2,6-lutidine proceeded at low
temperature to silyl ether 22 in 92% yield. Short reaction
times were essential for optimal outcome. Rapid depro-
tection of the p-methoxybenzyl ether with DDQ provided
a separable mixture of p-methoxybenzaldehyde and the
desired primary alcohol. However, carrying forward a
mixture of p-methoxybenzaldehyde and the alcohol 23
was experimentally more convenient. This mixture was
subjected to catalytic TPAP oxidation55 conditions in the
presence of NMO producing the easily separable aldehyde
24 after chromatography in 74% yield over two steps. The
Horner-Emmons homologation to form the R,â-unsatur-
ated tert-butyl ester 9 proceeded cleanly using tert-butyl
diethylphosphonoacetate, DBU and LiCl.57 The terminal
olefin of compound 9 is the key moiety necessary for
modification at what becomes the C3′-aromatic position.
Heck coupling60,61 utilizing Pd(OAc)2, PhI and triethy-
lamine provided the aryl synthon 20 in 84% yield (39%
overall in 6 steps from aldehyde 11). Comparison of the
optical rotations of this 20 with the rotation of 20 from
the Noyori reduction/Frater alkylation route confirmed
its optical purity and correct absolute stereochemistry.
Deprotection (Scheme 5) of aryl synthon 20 with 49%
HF in acetonitrile provided hydroxy ester 5 in reasonable
yield. These conditions were also used for the deprotec-
tion of 9, providing hydroxy ester 25. Alternatively, it
was demonstrated that TBAF deprotection of silyl ethers
20 and 9 with a polymer resin workup as described by
Parlow and co-workers62 provided the desired hydroxy
esters 5 and 25 cleanly.
The second key synthon 6 was readily synthesized
starting from N-Boc amino acid 26 (Scheme 6). Activation
of 26 with DCC and N-hydroxysuccinimide (HOSu)
followed by addition of â-alanine provided acid 27. EDCI
activation and subsequent addition of L-leucic acid pro-
vided acid 6 in two steps without the necessity of
extensive protecting group chemistry.
With the key synthons, hydroxy esters 5 and 25 and
acid 6, in hand, we proceeded with macrocycle formation.
Acid 6 was activated (Scheme 7) in a Yamaguchi coupling
reaction63 with 2,4,6-trichlorobenzoyl chloride in the
presence of diisopropylethylamine (DIEA) and catalytic
(59) Brown, H. C.; Bhat, K. S. J . Am. Chem. Soc. 1986, 108, 293.
(60) Heck, R. F. Palladium Reagents in Organic Syntheses; Aca-
demic: London, 1985.
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Lett. 1998, 8, 2391.

7796 J . Org. Chem., Vol. 65, No. 23, 2000
Eggen et al.
min. The reaction mixture was again cooled to -78 °C, and a
solution of iodomethane (1.73 mL, 27.8 mmol) in HMPA (8.16
mL, 47.0 mmol) was added. The reaction mixture was stirred
at -78 °C for 15 min, warmed slowly to -15 °C over 1.5 h,
and maintained at this temperature overnight. Following
treatment with saturated aqueous NH4Cl and extraction with
Et2O, the organic layer was washed with brine, water and
dried (MgSO4). The crude product was purified by flash column
chromatography (hexane:Et2O 3:1) to give product 7 as an oil
(3.40 g, 77%): 92% diastereomeric excess was determined by
DMAP. Addition of the alcohols 25 or 5 to the mixed
anhydride provided advanced intermediates 28 and 29,
respectively. Deprotection of the tert-butyl ester and the
N-Boc with trifluoroacetic acid (TFA) produced the cy-
clization precursors. Under dilute conditions, HBTU
activation provided the desired macrocycles 30 and 4.
Epoxidation of 4 with dimethyldioxirane64 using reported
conditions36 completed the total synthesis of cryptophy-
cin-24, arenastatin A (2). Through our methodology, we
also synthesized the dephenyl desepoxyarenastatin A (30)
for modification studies on the C16 aromatic side chain.
Heck coupling conditions directly converted compound 30
to 4, albeit in low yield.
In summary, convergent total syntheses of cryptophy-
cin-24 (2) amenable to modifications at the C1′ and C3′
positions of the C16 aromatic side chain were achieved.
Introduction of the aryl moiety at the C3′ position can
be made at a late stage in the synthesis by utilizing
dephenyl desepoxyarenastatin A (30) or dephenyl syn-
thon 9. Application of this methodology toward analogues
for SAR and biochemical studies exploring the crypto-
phycin binding site on tubulin are currently in progress.
1
integration of CHOH in H NMR; [R]20D +14.0° (c 1.49, CHCl3);
1H NMR (500 MHz, CDCl3) δ 7.36-7.26 (m, 5H), 4.52 (s, 2H),
3.94-3.93 (m, 1H), 3.69 (s, 3H), 3.74-3.64 (m, 2H), 3.31-3.30
(d, J ) 4.7 Hz, 1H), 2.58-2.56 (m, 1H), 1.83-1.72 (m, 2H),
1.20 (d, J ) 6 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 175.8,
137.8, 128.3 (2C), 127.53, 127.49 (2C), 73.1, 72.0, 68.2, 51.5,
45.4, 33.6, 13.6; IR (film) 3500,1725 cm-1; HRMS (FAB, NBA)
calcd for C14H21O4 (M + H) 253.1440, found 253.1446.
Meth yl (2S,3S)-3,5-Bis[(ter t-bu tyld im eth ylsilyl)oxy]-2-
m eth ylp en ta n oa te (13). To a solution of 7 (3.70 g, 14.68
mmol) in THF (35 mL) was added 10% Pd/C (0.940 g). The
solution was flushed with argon and hydrogenated (50 psi) for
3 h. After filtration, solvent was removed under reduced
1
pressure to give pure (by H NMR) diol as an oil. The oil was
dissolved in DMF (13 mL) and treated with TBSCl (5.83 g,
38.9 mmol) and imidazole (5.89 g, 77.8 mmol). After 16 h at
room temperature, the reaction was treated with saturated
aqueous NH4Cl (1 mL) and partitioned between Et2O and H2O.
Organic extracts were washed with NaHCO3, H2O, and brine
and dried (Na2SO4). The crude product was purified by flash
Exp er im en ta l Section
Gen er a l Meth od s. 1H and 13C NMR spectra were obtained
in CDCl3 with a 300, 400 or 500 MHz spectrometer. High-
resolution mass spectra were obtained at the University of
Kansas mass spectrometry support laboratory. Optical rota-
tions were recorded on a Perkin-Elmer 241 polarimeter. Chiral
HPLC analysis was performed using a Chiralcel OD-H column.
Column chromatography was carried out on silica gel (230-
400 mesh, Merck). THF and diethyl ether were distilled from
sodium benzophenone ketyl, CH2Cl2 was freshly distilled from
CaH2. DIEA, TEA and 2,6-lutidine were distilled from CaH2
prior to use. All other reagents were commercially available
and used without further purification. All moisture-sensitive
reactions were carried out under inert atmosphere in oven
dried glassware.
column chromatography (hexane/EtOAc 90:10) to give 13 as
1
a clear oil (4.95 g, 93%): [R]20 + 14.11° (c 1.885, CHCl3); H
D
NMR (500 MHz, CDCl3) δ 4.14-4.10 (m, 1H), 3.72-3.61 (m,
2H), 3.64 (s, 3H), 2.70-2.66 (m, 1H), 1.69-1.63 (m, 1H), 1.60-
1.54 (m, 1H), 1.11-1.09 (d, J ) 7 Hz, 3H), 0.87 (s, 9H), 0.86
(s, 9H), 0.05 (s, 3H), 0.035 (s, 3H), 0.025 (s, 3H), 0.022 (s, 3H);
13C NMR (125 MHz, CDCl3) δ 175.0, 70.3, 59.2, 51.4, 45.6, 36.0,
25.9 (3C), 25.7 (3C), 18.2, 18.0, 11.4, -4.6, -4.9, -5.4 (2C); IR
(film) 1725 cm-1; MS (EI) m/z 375(M+ - Me, 1); HRMS (FAB,
TG/G[3:1 thioglycerol/glycerol]) calcd for (M + H) C19H43O4Si2
391.2700, found 391.2680.
(2R,3S)-3,5-Bis[(ter t-bu tyld im eth ylsilyl)oxy]-2-m eth yl-
p en ta n -1-ol (14). DIBALH (13.35 mL, 1.0 M in hexanes) was
slowly added over a period of 15 min to a solution of ester 13
(2.07 g, 5.31 mmol) at -78 °C in THF (50 mL). The reaction
was allowed to warm to -10 °C over 3 h. The reaction was
quenched with MeOH (5 mL) and warmed to room tempera-
ture after gas evolution ceased. The reaction was partitioned
between EtOAc (150 mL) and H2O (50 mL). The aqueous layer
was extracted with EtOAc. Combined organics were dried (Na2-
SO4), concentrated, and subjected to column chromatography
(2:1 hexane:Et2O) to provide the primary alcohol 14 as a
colorless oil (1.74 g, 90%): 1H NMR (400 MHz, CDCl3) δ 3.90-
3.86 (m, 1H), 3.78-3.75 (dd, J ) 4, 11 Hz, 1H), 3.66-3.63 (t,
J ) 6.4 Hz, 2H), 3.53-3.49 (dd, J ) 5.3, 11 Hz, 1H), 1.79-
1.74 (m, 3H), 1.01-1.00 (d, J ) 7 Hz, 3H), 0.88 (s, 9H), 0.87
(s, 9H), 0.075 (s, 3H), 0.070 (s, 3H), 0.03 (s, 6H); 13C NMR (75
MHz, CDCl3) δ 74.1, 65.2, 59.7, 38.6, 37.6, 25.88, 25.86, 18.2,
18.0, 14.3, -4.4, -4.8, -5.3; IR (film) 3440 (br) cm-1; HRMS
(FAB, NBA/LiOAc) calcd for C18H42O3Si2Li (M + Li) 369.2832,
found 369.2842.
(3R,4S,1E)-4,6-Bis[(ter t-bu tyldim eth ylsilyl)oxy]-3-m eth -
yl-6-p h en ylh exen e (16). Alcohol 14 (200 mg, 0.552 mmol)
was dissolved in CH2Cl2 (4 mL) and CH3CN (0.4 mL). TPAP
(9 mg, 5 mol %), NMO (97 mg, 0.83 mmol), and 4 Å molecular
sieves (115 mg) were added, and the mixture was vigorously
stirred for 1 h. The mixture was filtered through Celite. The
filtrate was concentrated and purified by flash column chro-
matography (hexane/EtOAc 95:5) to provide aldehyde 15 (175
mg, 88%). A solution of diethyl benzylphosphonate (219 mg,
0.96 mmol) in THF (4 mL) was cooled to -78 °C. To this was
added n-BuLi (2.5 M in hexanes, 0.37 mL, 0.91 mmol). The
reaction mixture was stirred at - 78 °C for 30 min and treated
with a solution of aldehyde 15 (175 mg, 0.49 mmol) in THF (1
mL). Stirring was continued at -78 °C for another 1 h, and
Meth yl (3R)-5-Ben zyloxy-3-h yd r oxyp en ta n oa te (12).
Under a N2 atmosphere, a mixture of degassed methyl 5-ben-
zyloxy-3-oxopentanoate (0.5 g, 2.12 mmol) and methanol (10
mL) was placed in a Parr hydrogenation bottle. To this was
added Ru(BINAP)Br2 (prepared from bis-(2-methylallyl)cy-
cloocta-1,5-diene ruthenium (II) (4 mg, 0.013 mmol) and (S)-
BINAP (8 mg, 0.013 mmol). Hydrogenation was carried out
at 50 psi and 50 °C for 5 h. The catalyst was precipitated with
Et2O (100 mL) and filtered through a plug of Celite. The
filtrate was concentrated, and the residue was purified by flash
column chromatography (hexane/ether 65:35) to give pure
alcohol 12 as an oil (0.49 g, 97%): 97% enantiomeric excess
was determined by chiral HPLC (Daicel Chiralcel OD-H, 254
nm, 25:1 hexane/2-propanol, 254 nm, 1 mL/min, retention time
12 ) 17.2 min, ent-12 ) 21.6 min); [R]20 +11.8° (c 1.07,
D
CHCl3); 1H NMR (300 MHz, CDCl3) δ 7.32 (m, 5H), 4.52 (s,
2H), 4.27-4.23 (m, 1H), 3.70 (s, 3H), 3.74-3.62 (m, 2H), 3.41-
3.40 (d, J ) 5.6 Hz, 1H), 2.51-2.50 (d, J ) 10.5 Hz, 2H), 1.86-
1.76 (m, 2H); 13C NMR (75 MHz, CDCl3) δ 172.8, 138.0, 128.5,
128.4, 127.73, 127.68, 73.3, 68.0, 67.0, 51.7, 41.4, 36.0; IR (film)
3500 (br), 1725 cm-1; HRMS (FAB, NBA) calcd for C13H19O4
(M + H) 239.1283, found 239.1307.
Meth yl (2S,3S)-5-Ben zyloxy-3-h yd r oxy-2-m eth ylp en -
ta n oa te (7). To a solution of diisopropylamine (6.8 mL, 49
mmol) in THF (75 mL) at -78 °C was added n-BuLi (2.5 M in
hexanes, 17.4 mL, 43.5 mmol). After the mixture was stirred
at -78 °C for 20 min, 12 (4.40 g, 18.5 mmol) was added. The
reaction mixture was stirred at -78 °C for 20 min and slowly
warmed to -20 °C and maintained at this temperature for 90
(63) Inanaga, J .; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguch, M.
Bull. Chem. Soc. J pn. 1979, 52, 1989.
(64) Adam, W.; Bialas, J .; Hadjiarapoglou, L. Chem. Ber. 1991, 124,
2377.

Total Synthesis of Cryptophycin-24
J . Org. Chem., Vol. 65, No. 23, 2000 7797
then the reaction was gradually warmed to room temperature
over 6 h. The reaction was quenched with NH4Cl solution and
extracted with Et2O. The combined organic layers were washed
with water and brine and dried (MgSO4). Column chromatog-
raphy (hexane/EtOAc 99:1) gave styrene 16 as a colorless oil
(155 mg, 74%): [R]20D +23° (c 0.77, CHCl3); 1H NMR (300 MHz,
CDCl3) δ 7.4 (m, 5H), 6.41-6.36 (d, J ) 16 Hz, 1H), 6.23-
6.15 (dd, J ) 16, 7.7 Hz, 1H), 3.84 (m, 1H), 3.73-3.65 (m, 2H),
2.49 (m, 1H), 1.70-1.63 (m, 2H), 1.12 (d, J ) 6.9 Hz, 3H), 0.93
(s, 9H), 0.90 (s, 9H), 0.091 (s, 6H), 0.056 (s, 6H); 13C NMR (75
MHz, CDCl3) δ 137.8, 132.8, 129.8, 128.4, 126.9, 126.0, 72.7,
60.1, 42.7, 36.7, 25.9 (6C), 18.3, 18.1, 15.5, -4.5 (2C), -5.3 (2C);
IR (film) 3020 cm-1; MS (EI) m/z 303 (M+ - OTBS, 12); HRMS
(3S,4R)-1-(4-Met h oxyb en zyloxy)-4-m et h yl-5-h exen -3-
ol (21). To a stirred mixture of potassium tert-butoxide (7.75
g, 69.06 mmol), THF (72 mL), and trans-2-butene (25.7 mL,
89.8 mmol) was added n-BuLi (1.6 M in hexanes, 43.2 mL,
69.1 mmol) at -78 °C. After the addition was complete, the
mixture was stirred at -45 °C for 10 min, producing a bright
canary yellow/orange solution. This solution was recooled to
-78 °C, and to it was added dropwise a solution of (+)-B-
methoxydiisopinocampheylborane (26.23 g, 82.87 mmol) in
Et2O (89 mL). After the reaction mixture was stirred at -78
°C for 30 min, BF3‚Et2O (11.74 mL, 92.54 mmol) was added
dropwise followed immediately by aldehyde 1158 (16.11 g, 82.87
mmol). The reaction was stirred for an additional 3 h at -78
°C. The mixture was then concentrated, redissolved in dry
Et2O (150 mL), and cooled to 0 °C, and ethanolamine (4.2 mL)
was added with vigorous stirring. The reaction was warmed
to room temperature and stirred for 72 h. The mixture was
then filtered and the filtrate concentrated. Column chroma-
tography of the filtrate (95:5 to 80:20 hexane/EtOAc) provided
the pure alcohol 21 (13.20 g, 76%): 91% enantiomeric excess
was determined by chiral HPLC comparing alcohol 21 with
alcohol ent-21 derived from reaction with the (-)-borane (Daicel
Chiralcel OD-H, 254 nm, 99:1 hexanes/2-propanol, 0.5 mL/min,
retention time: ent-21 ) 32.2 min, 21 ) 34.6 min); [R]20D +2.7°
(FAB, NBA) calcd for
377.2303.
C
25H46O2Si2 (M+) 377.2332, found
(3S,4R,5E)-3-[(ter t-Bu tyld im eth ylsilyl)oxy]-4-m eth yl-6-
p h en yl-5-h exen -1-ol (17). A solution of 16 (0.123 g, mmol)
in AcOH and aqueous THF (AcOH/H2O/THF 1:1:2, 6 mL) was
stirred at room temperature. After 72 h, the reaction mixture
was neutralized with saturated NaHCO3 solution to pH ) 7
and extracted with EtOAc. The combined organic layers were
washed with water and dried (MgSO4). Flash column chro-
matography (hexane/EtOAc 90:10) gave pure product as color-
less oil (0.071 g, 82%): [R]20D +28.3° (c 0.675, CHCl3); 1H NMR
(300 MHz, CDCl3) δ 7.3 (m, 5H), 6.42-6.37 (d, J ) 16 Hz, 1H),
6.18-6.10 (dd, J ) 16, 7.8), 3.91-3.87 (m, 1H), 3.77-3.73 (m,
2H), 2.62-2.51 (m, 1H), 2.05 (br s, 1H), 1.76-1.72 (m, 2H),
1.12-1.10 (d, J ) 6.8 Hz, 3H), 0.91 (s, 9H), 0.12 (s, 3H), 0.10
(s, 3H); 13C NMR (75 MHz, CDCl3) δ 137.5, 132.5, 130.0, 128.5,
128.4, 127.0, 126.0, 74.6, 60.1, 42.7, 35.0, 25.9, 18.0, 14.8, -4.3,
-4.6; IR (film) 3350 (br), 3020 cm-1; HRMS (FAB, NBA/LiOAc)
calcd for (M + Li) C19H32O2SiLi 327.2332, found 327.2323.
(3S,4R,5E)-3-[(ter t-Bu tyld im eth ylsilyl)oxy]-4-m eth yl-6-
p h en yl-5-h exen a l (18). To alcohol 17 (65 mg, 0.2 mmol) in
CH2Cl2 (2 mL) and CH3CN (0.2 mL) were added TPAP (3.5
mg, 5 mol %), NMO (35 mg, 0.3 mmol), and 4 Å molecular
sieves (50 mg). The mixture was stirred at room temperature
for 1 h. The reaction mixture was filtered through Celite and
subjected directly to flash column chromatography (hexane:
Et2O 95:5 to 90:10) which gave aldehyde 18 as an oil (51 mg,
78%). Aldehyde 18 was used as obtained in the next reaction.
Meth yl (5S,6R,2E,7E)-5-[(ter t-Bu tyld im eth ylsilyl)oxy]-
1
(c 1.5, CHCl3); H NMR (400 MHz, CDCl3) δ 7.28-7.26 (d, J
) 8.7 Hz, 2H), 6.91-6.88 (d, J ) 8.7 Hz, 2H), 5.87-5.78 (m,
1H), 5.11 (br s, 1H), 5.08-5.06 (br d, J ) 5 Hz, 2H), 4.47 (s,
2H), 3.82 (s, 3H), 3.74-3.61 (m, 3H), 2.82-2.81 (d, J ) 3 Hz,
1H), 2.28-2.22 (m, 1H), 1.76-1.69 (m, 2H), 1.07-1.05 (d, J )
6.9 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 159.2, 140.5, 130.1,
129.3 (2C), 115.4, 113.8 (2C), 74.3, 73.0, 68.9, 55.3, 44.0, 33.5,
15.8; IR (film) 3420, 3040 cm-1; HRMS (FAB, NBA) calcd for
C
15H22O3 (M+) 250.1569, found 250.1577.
(3S,4R)-3-[(ter t-Bu tyld im eth ylsilyl)oxy]-1-(4-m eth oxy-
ben zyloxy)-4-m eth yl-5-h exen e (22). Alcohol 21 (0.150 g,
0.599 mmol) was dissolved in CH2Cl2 (6 mL) and cooled to -78
°C. 2,6-Lutidine (0.14 mL, 1.20 mmol) was added, followed by
TBSOTf (0.165 mL, 0.719 mmol) dropwise via syringe. After
10 min, the reaction was quenched with saturated aqueous
NaHCO3. The aqueous layer was extracted with CH2Cl2. The
combined organics were dried (MgSO4), filtered, and concen-
trated. Flash column chromatography (90:10 hexane/EtOAc)
provided a colorless oil (0.201 g, 92%): 1H NMR (400 MHz,
CDCl3) δ 7.25-7.23 (d, J ) 8.6 Hz, 2H), 6.88-6.86 (d, J ) 8.6
Hz, 2H), 5.80-5.71 (ddd, J ) 7, 11, 16 Hz, 1H), 5.00 (br s,
1H), 4.97-4.95 (br d, J ) 7 Hz, 1H), 4.44-4.41 (d, J ) 11 Hz,
1H), 4.39-4.36 (d, J ) 11 Hz, 1H), 3.80 (s, 3H), 3.77-3.73 (m,
1H), 3.48-3.45 (dt, J ) 2, 7 Hz, 2H), 2.29-2.26 (m, 1H), 1.70-
1.63 (m, 2H), 1.00-0.98 (d, J ) 6.9 Hz, 3H), 0.88 (s, 9H), 0.04
(s, 3H), 0.03 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 159.1, 140.7,
130.7, 129.2 (2C), 114.5, 113.7 (2 C), 72.5, 67.1, 55.3, 43.4, 33.2,
25.9, 18.1, 14.5, -4.4, -4.5; IR (film) 3080 cm-1; HRMS (FAB,
NBA) calcd for C21H36O3Si (M+) 364.2434, found 364.2442.
(3S)-3-[(ter t-Bu tyldim eth ylsilyl)oxy]-4-m eth yl-5-h exen -
1-ol (23). The p-methoxybenzyl ether 22 (1.600 g, 4.388 mmol)
was dissolved in CH2Cl2 (20.7 mL). Water (1.3 mL) and solid
DDQ (0.996 g, 4.388 mmol) were added. After 1 h, CH2Cl2 (200
mL) and saturated aqueous NaHCO3 (40 mL) were added. The
organic layer was further washed with NaHCO3 and brine
until the organics were pale yellow in color. The organics were
dried (MgSO4), filtered and concentrated. Flash column chro-
matography (80:20 hexane/EtOAc) provided a mixture of
desired primary alcohol 23 and p-methoxybenzaldehyde which
was used without further purification in the next reaction: 1H
NMR (400 MHz, CDCl3) δ 5.78-5.70 (ddd, J ) 7, 10.6, 17 Hz,
1H), 5.04-5.02 (br d, J ) 7 Hz, 1H), 4.99 (br s, 1H), 3.83-
3.77 (m, 1H), 3.73-3.69 (m, 2H), 2.39-2.35 (m, 1H), 1.68-
1.63 (m, 2H), 1.00-0.99, (d, J ) 6.9 Hz, 3H), 0.89 (s, 9H), 0.08
(s, 3H), 0.07 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 132.0, 114.3,
74.3, 60.5, 43.2, 34.6, 25.8, 18.0, 13.9, -4.4, -4.6.
6-m eth yl-8-p h en yl-2,7-octa d ien oa te (19). Procedure as pre-
1
viously reported:37 [R]20 +67° (c 0.63, CHCl3); H NMR (500
D
MHz, CDCl3) δ 7.2-7.4 (m, 5H), 6.96 (ddd, J ) 15.6, 7.8, 7.5
Hz, 1H), 6.37 (d, J ) 16 Hz, 1H), 6.16 (dd, J ) 15.9, 8.1 Hz,
1H), 5.84 (d, J ) 15.7 Hz, 1H), 3.75 (m, 1H), 3.72 (s, 3H), 2.44
(m, 1H), 2.36 (m, 2H), 1.10 (d, J ) 6.9 Hz, 3H), 0.91 (s, 9H),
0.06 (s, 3H), 0.05 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 166.8,
146.4, 137.6, 131.9, 130.4, 128.5, 127.0, 126.0, 122.9, 75.0, 51.4,
42.8, 37.6, 25.8, 18.1, 16.2, - 4.4, - 4.5; IR (film) 2950, 1720,
1650 cm-1; MS (EI) m/z 374 (M+, 1); HRMS (FAB, NBA) calcd
for C22H35O3Si (M + H) 375.2355, found 375.2367.
ter t-Bu tyl (5S,6R,2E,7E)-5-[(ter t-Bu tyld im eth ylsilyl)-
oxy]-6-m eth yl-8-p h en yl-2,7-octa d ien oa te (20). A solution
of tert-butyldiethylphosphonoacetate (0.864 mL, 3.68 mmol),
DBU (0.330 mL, 2.21 mmol), and LiCl (0.109 g, 2.58 mmol)
was stirred vigorously at room temperature for 30 min in CH3-
CN (35 mL) and then added dropwise to a solution of aldehyde
18 (0.586 g, 1.84 mmol) in CH3CN (4.4 mL). After 75 min, the
reaction mixture was washed with saturated aqueous NH4Cl
solution, H2O, and brine. The organic phase was dried (Na2-
SO4), filtered, and concentrated. Flash column chromatography
(hexane/EtOA, 95:5) provided tert-butyl ester 20 as an oil
(0.582 g, 76%): [R]20D +69° (c 0.73, CHCl3); 1H NMR (300 MHz,
CDCl3) δ 7.37-7.17 (m, 6H), 6.86-6.76 (dt, J ) 8, 16 Hz, 1H),
6.38-6.33 (d, J ) 16 Hz, 1H), 6.18-6.10 (dd, J ) 8, 16 Hz,
1H), 5.74-5.69 (d, J ) 16 Hz, 1H), 3.74-3.69 (app q, J ) 6
Hz, 1H), 2.47-2.40 (m, 1H), 2.32-2.27 (m, 2H), 1.45 (s, 9H),
1.09-1.06 (d, J ) 7 Hz, 3H), 0.88 (s, 9H), 0.033 (s, 6H); 13C
NMR (100 MHz, CDCl3) δ 165.8, 144.8, 137.6, 132.0, 130.4,
128.5 (2 C), 127.0, 126.0 (2 C), 125.1, 80.0, 75.1, 42.8, 37.3,
28.1 (3 C), 18.1, 16.1, -4.4, -4.5; IR (film) 2910, 1695, 1645
cm-1; MS (CI) m/z 417 (M + 1); HRMS (FAB, NBA) calcd for
(3S)-3-[(ter t-Bu t yld im et h ylsilyl)oxy]-4-m et h yl-5-h ex-
en a l (24). To alcohol 23 was added CH2Cl2 (15 mL) and freshly
activated crushed 4 Å molecular sieves (0.380 g), NMO (0.617
g, 5.27 mmol) and TPAP (0.077 g, 0.219 mmol). After the
reaction was complete by TLC (15-30 min), the mixture was
C
25H41O3Si (M + H) 417.2825, found 417.2848.

7798 J . Org. Chem., Vol. 65, No. 23, 2000
Eggen et al.
concentrated and directly purified by flash chromatography
(95:5 hexane/EtOAc) to provide aldehyde 24 as an oil (0.784
g, 74% over two steps): 1H NMR (300 MHz, CDCl3) δ 9.78-
9.72 (t, J ) 2 Hz, 1H), 5.79-5.70 (ddd, J ) 7, 10.5, 17 Hz,
1H), 5.06-5.04 (m, 1H), 5.05-4.99 (m, 1H), 4.20-4.17 (m, 1H),
2.50-2.43 (app dt, J ) 2, 6 Hz, 2H), 2.39-2.33 (m, 1H), 1.03-
1.01 (d, J ) 6.9 Hz, 3H), 0.87 (s, 9H), 0.08 (s, 3H), 0.04 (s,
extracted with CH2Cl2. Organics were dried (MgSO4), filtered,
and concentrated. Flash column chromatography (90:10 to 85:
15 hexane/EtOAc) provided the desired alcohol as a colorless
oil (45 mg, 68%).
TBAF P r oced u r e. Ester 9 (25 mg, 0.073 mmol) was
dissolved in THF (650 µL). TBAF solution (1 M in THF, 81
µL, 0.081 mmol) was added dropwise. After 2 h, Amberlyst-
15 (220 mg), the calcium salt of Amberlyst-15 (220 mg),62 and
1 mL of THF were added and the mixture stirred for an
additional 2.5 h. The resin was removed by filtration and
thoroughly rinsed with CH2Cl2. The filtrate was concentrated
and subjected to a short plug of silica (80:20 hexane/EtOAc)
to provide an oil (11 mg, 69%): 1H NMR (400 MHz, CDCl3) δ
6.93-6.85, 5.85-5.80, 5.78-5.69, 5.15-5.14 (m, 1H), 5.13-
5.10 (m, 1H), m (1), dddd (1.5, J ) 4, 6.9, 14 Hz, 1H), m (2H),
1.67-1.66 (d, J ) 4 Hz, 1H), 1.47 (s, 9H), 1.05-1.04 (d, J )
6.9 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 166.5, 144.0, 139.6,
125.5, 116.9, 80.2, 73.4, 43.8, 36.9, 28.1 (3 C), 16.2; IR (film)
3460, 3100, 1655 cm-1; MS (CI) m/z 244 (M + NH3, 15), 227
(M + H, 8), 188 (100); HRMS (FAB, NBA) calcd for C13H23O3
(M + H) 227.1647, found 227.1669.
3H); IR (film) 3090, 1715 cm-1
.
ter t-Bu tyl (5S,6R,2E)-5-[(ter t-Bu tyld im eth ylsilyl)oxy]-
6-m eth yl-2,7-octa d ien oa te (9). tert-Butyldiethylphospho-
noacetate (1.16 mL, 4.95 mmol), DBU (0.44 mL, 2.97 mmol),
and LiCl (0.147 g, 3.47 mmol) were stirred vigorously at room
temperature for 30 min in CH3CN (10.8 mL). The solution was
added dropwise via syringe to a solution of aldehyde 24 (0.600
g, 2.48 mmol) in CH3CN (5.7 mL). After 2 h, saturated aqueous
NH4Cl solution was added. The organics were further washed
with brine. The aqueous layers were cross-extracted with
EtOAc. The combined organics were dried (MgSO4), filtered
and concentrated. Flash chromatography (95:5 hexane/EtOAc)
provided product as a colorless oil (0.750 g, 89%): [R]20D +0.7°
1
(c 2.0, CHCl3); H NMR (400 MHz, CDCl3) δ 6.82-6.75 (dt, J
) 8, 16 Hz, 1H), 5.80-5.71 (buried, 1H), 5.74-5.70 (br d, J )
16 Hz, 1H), 5.03 (br s, 1H), 5.01-4.99 (br d, J ) 9 Hz, 1H),
3.67-3.63 (m, 1H), 2.29-2.23 (m, 3H), 1.46 (s, 9H), 1.00-0.99
(d, J ) 6.8 Hz, 3H), 0.88 (s, 9H), 0.033 (s, 3H), 0.027 (s, 3H);
13C NMR (100 MHz, CDCl3) δ 165.7, 145.1, 140.1, 125.0, 115.1,
80.0, 74.9, 43.3, 36.8, 28.1 (3 C), 25.8 (3 C), 18.1, 15.3, -4.4,
-4.6; IR (film) 3060, 1695 cm-1; HRMS (FAB, NBA) calcd for
3-[(2R)-(ter t-Bu toxyca r bon yla m in o)-3-(4-m eth oxyp h e-
n yl)p r op a n oyla m in o]p r op a n oic Acid (27). DCC (4.37 g,
21.2 mmol) in DME (25 mL) was added dropwise to a solution
of Boc-D-Tyr(Me)OH 26 (5.00 g, 16.9 mmol) and N-hydrox-
ysuccinimide (2.92 g, 25.4 mmol) in DME (75 mL) at 0 °C. The
reaction mixture was stirred at 0 °C for 30 min and at room
temperature for 24 h. A solution of 3-aminopropionic acid (2.10
g, 20.3 mmol) and TEA (4.72 mL, 34.0 mmol) in water (50 mL)
was slowly added in 10 portions. After 2 h, the reaction
mixture was filtered and the residue washed with water. The
combined filtrates were concentrated, and a K2CO3 (aq, 5%)
solution was added to the residue until pH 9. The aqueous
layer was washed with CH2Cl2, acidified to pH 2 with 10%
aqueous HCl, and extracted with CH2Cl2. This organic phase
was then washed with brine, dried (Na2SO4), and concentrated
to an oily residue. The residue was triturated with pentane
and the resulting precipitate was filtered. The product was
recrystallized (ether/pentane) to give pure acid 27 as a
C
19H37O3Si (M + H) 341.2512, found 341.2514.
ter t-Bu tyl (5S,6R,2E,7E)-5-[(ter t-Bu tyld im eth ylsilyl)-
oxy]-6-m eth yl-8-p h en yl-2,7-octa d ien oa te (20).36 Olefin 9
(75 mg, 0.22 mmol) was weighed into a dry seal tube and
flushed with argon. CH3CN (2 mL) was added, followed by
iodobenzene (49 mg, 0.24 mmol), Pd(OAc)2 (2.5 mg, 0.011
mmol), and TEA (0.31 mL, 2.2 mmol). The tube was sealed
and heated at 80-85 °C for 24 h. The mixture was concen-
trated and subjected directly to column chromatography (90:
10 hexane/EtOAc) to provide the product 20 as an oil (77 mg,
83%): [R]20D +64° (c ) 0.73, CHCl3); [R]20D +69° (c 0.73, CHCl3)
via the Noyori reduction/Frater alkylation route; spectral data
consistent with other route.
crystalline solid (5.50 g, 88%): mp 103-105 °C; [R]20 -7.9°
D
(c 1.5, CHCl3); 1H NMR (400 MHz MHz, CDCl3) δ 7.06 (d, J )
8.6 Hz, 2H), 6.98 (br s, 1H), 6.83 (d, J ) 8.6 Hz, 2H), 5.48 (br
d, J ) 8.5 Hz, 1H), 4.45 (br d, J ) 7.6 Hz, 1H), 3.73 (s, 3H),
3.47 (br s, 1H), 3.45 (br s, 1H), 2.89 (d, J ) 6.7 Hz, 2H), 2.44
(m, 2H), 1.36 (s, 9H); 13C NMR (100 MHz, CDCl3) δ 175.3,
171.7, 158.5, 155.8, 130.2(2C), 128.4, 113.9(2C), 80.5, 55.6,
55.1, 38.2, 34.4, 33.5, 28.2(3C); IR (KBr) 3336, 1710, 1684, 1649
cm-1; HRMS (FAB, TG/G) calcd for C18H27N2O6 (M + H)
367.1881, found 367.1875.
ter t-Bu tyl (5S,6R,2E,7E)-5-Hyd r oxy-6-m eth yl-8-p h en yl-
2,7-octa d ien oa te (5).36 HF P r oced u r e. HF (48% w/w, aq,
13 µL) was added to ester 20 (0.077 g, 0.19 mmol) dissolved
in CH3CN (1.9 mL) at 0 °C. After 1.5 h, CH2Cl2 (10 mL) and
saturated aqueous NaHCO3 solution were added until the pH
was neutral. The aqueous layer was extracted with CH2Cl2.
Combined organics were dried (MgSO4), filtered, and concen-
trated. Flash chromatography (90:10 to 85:15 hexane/EtOAc)
provided a pale yellow oil (30 mg, 54%).
(2S)-2-[3-(2R)-2-ter t-Bu toxyca r bon yla m in o-3-(4-m eth -
oxyp h en yl)p r op a n oyla m in o]p r op a n oyloxy]-4-m et h yl-
p en ta n oic Acid (6).36 To a stirred solution of acid 27 (5.50 g,
15 mmol) and N-hydroxysuccinimide (3.40 g, 30 mmol) in DME
(100 mL) and CH2Cl2 (100 mL) was added 1-(3-dimethylami-
nopropyl)-3-ethylcarbodiimide hydrochloride (4.80 g, 25 mmol)
at room temperature. After 24 h, the reaction mixture was
diluted with EtOAc (200 mL) and washed with water, 1% HCl,
5% aqueous NaHCO3 solution, H2O, and brine. The organic
phase was then dried (Na2SO4) and concentrated to give the
activated ester. The intermediate was dissolved in CH3CN (100
mL), and DMAP (9.16 g, 75 mmol) and L-leucic acid (4.95 g,
37.5 mmol) were added at room temperature. The reaction
mixture was stirred for 18 h and then acidified with HCl (0.1
N) at 0 °C until pH 2. The solution was extracted with EtOAc,
and the combined extracts were washed with H2O and brine.
The mixture was dried (Na2SO4) and concentrated. The residue
was purified by column chromatography (CH2Cl2/EtOAc 3:1)
TBAF P r oced u r e. The ester 20 (25 mg, 0.060 mmol) was
dissolved in THF (530 µL). TBAF solution (1 M in THF, 66
µL, 0.066 mmol) was added dropwise. After 2 h, Amberlyst-
15 (180 mg), the calcium salt of Amberlyst-15 (180 mg),62 and
1 mL of THF were added and the mixture stirred for an
additional 2.5 h. The resin was removed by filtration and
rinsed with CH2Cl2. The filtrate was concentrated and sub-
jected to a short plug of silica (80:20 hexane/EtOAc), providing
a pale yellow oil (14 mg, 78%): [R]20D +66° (c 0.80, CHCl3); 1H
NMR (400 MHz, CDCl3) δ 7.37-7.36 (d, J ) 7 Hz, 2H), 7.32-
7.28 (app t, J ) 7 Hz, 2H), 7.25-7.20 (t, J ) 7 Hz, 1H), 6.95-
6.88 (dt, J ) 7.5, 15.6 Hz, 1H), 6.48-6.44 (d, J ) 16 Hz, 1H),
6.17-6.11 (dd, J ) 8.6, 16 Hz, 1H), 5.86-5.82 (br d, J ) 15.6
Hz, 1H), 3.67-3.62 (m, 1H), 2.48-2.38 (m, 2H), 2.36-2.28 (m,
1H), 1.86 (br s, 1H), 1.47 (s, 9H), 1.15 (s, 3H); 13C NMR (100
MHz, CDCl3) δ 165.7, 144.0, 137.0, 131.8, 130.9, 128.5 (2C),
127.4, 126.1 (2C), 125.4, 80.2, 73.8, 43.2, 37.1, 28.1 (3C), 16.8;
IR (film) 3420, 1690, 1635 cm-1; MS (CI) m/z 320 (M + NH3,
9), 303 (M + H, 3), 264 (100); HRMS (FAB, NBA) calcd for
to give acid 6 (5.90 g, 82%) as a foam: [R]20 -21° (c 0.77,
D
CHCl3); 1H NMR (400 MHz, CDCl3) δ 7.05 (d, J ) 8.5 Hz, 2H),
6.85 (br s, 1H), 6.77 (br d, J ) 7.6 Hz, 2H), 5.36 (m, 1H), 5.12
(m, 1H), 4.28 (m, 1H), 3.74 (s, 3H), 3.45 (br s, 2H), 2.95 (br d,
J ) 6.2 Hz, 2H), 2.52 (m, 2H), 1.75 (m, 3H), 1.36 (s, 9H), 0.94
(d, J ) 6.2 Hz, 3H), 0.91 (d, J ) 6.2 Hz, 3H); 13C NMR (100
MHz, CDCl3) δ 173.2, 171.7, 158.5, 155.8, 130.3, 128.4, 113.9,
80.5, 71.2, 55.2, 39.4, 37.9, 33.9, 28.2, 24.6, 23.0, 21.5, 14.1;
C
19H27O3 (M + H) 303.1960, found 303.1980.
ter t-Bu tyl (5S,6R,2E)-5-Hyd r oxy-6-m eth yl-2,7-octa d i-
en oa te (25). HF P r oced u r e. Ester 9 (100 mg, 0.294 mmol)
was dissolved in CH3CN (0.294 mL) and cooled to 0 °C. HF
(48% w/w, aq, 24 µL) was added. After 25 min, saturated
aqueous NaHCO3 solution was added. The aqueous layer was

Total Synthesis of Cryptophycin-24
J . Org. Chem., Vol. 65, No. 23, 2000 7799
MS (FAB, TG/G) m/z 481.3 (M + H); HRMS (FAB, TG/G) calcd
for C24H39N2O8 (M + H) 481.2550, found 481.2573.
m, 1H), 6.41-6.37 (d, J ) 15.9 Hz, 1H), 6.02-5.95 (dd, J )
8.6, 15.9 Hz, 1H), 5.83-5.80 (d, J ) 15.6 Hz, 1H), 5.45-5.43
(d, J ) 8 Hz, 1H), 5.03-5.00 (m, 1H), 4.96-4.93 (dd, J ) 4, 10
Hz, 1H), 4.36-4.31 (m, 1H), 3.75 (s, 3H), 3.54-3.52 (m, 1H),
3.48-3.41 (m, 1H), 3.15-3.10 (dd, J ) 5.5, 14 Hz, 1H), 2.93-
2.88 (m, 1H), 2.60-2.40 (m, 5H), 1.71-1.57 (m, 3H), 1.47 (s,
9H), 1.34 (s, 9H), 1.33 (buried, 1H), 1.10-1.08 (d, J ) 6.9 Hz,
3H), 0.83-0.81 (d, J ) 6.5 Hz, 3H), 0.78-0.76 (d, J ) 6.5 Hz,
3H); other data as previously reported.36
ter t-Bu tyl (5S,6R,2E)-5-[(2S)-2-[3-[(2R)-2-ter t-Bu toxy-
ca r b on yla m in o-3-(4-m et h oxyp h en yl)p r op a n oyla m in o]-
p r op a n oyloxy]-4-m eth ylp en ta n oyloxy]-6-m eth ylocta -2,7-
d ien oa te (28). N-Boc amino acid 6 (314 mg, 0.65 mmol) was
dissolved in THF (12.5 mL). DIEA (142 µL, 0.82 mmol), 2,4,6-
trichlorobenzoyl chloride (97 µL, 0.62 mmol), and DMAP (5
mg) were added. After 30 min, the alcohol 25 (74 mg, 0.33
mmol) in THF (300 µL) was added dropwise via syringe. After
45 min, saturated aqueous NaHCO3 solution was added. The
aqueous layer was then extracted with CH2Cl2. The combined
organics were dried (MgSO4), filtered, and concentrated. Flash
chromatography (80:20 to 70:30 hexanes/EtOAc) provided the
Desep oxya r en a sta tin A (4) fr om 29.36 Ester 29 (50 mg,
0.065 mmol) was dissolved in CH2Cl2 (6 mL), and TFA (200
µL) was added. After 1 h, toluene (2 mL) was added, and the
reaction mixture was concentrated in vacuo. The resulting acid
was dissolved in CH3CN (6 mL), and DIEA (33 µL, 0.19 mmol)
and HBTU (28 mg, 0.075 mmol) were added. The reaction was
stirred at room temperature for 45 min. Saturated aqueous
NaHCO3 solution was added, and the aqueous phase was
extracted with CH2Cl2. The combined organics were dried
(MgSO4), filtered, and concentrated. Column chromatography
(7:1 to 4:1 CH2Cl2/acetone) provided 4 as an oil (24 mg, 65%):
[R]20D +27° (c 0.80, CHCl3); 1H NMR (400 MHz, CDCl3) δ 7.33-
7.15 (m, 5H), 7.11-7.09 (d, J ) 8.6 Hz, 2H), 7.05-7.02 (t, J )
5.5 Hz, 1H), 6.81-6.78 (d, J ) 8.6 Hz, 2H), 6.73-6.66 (ddd, J
) 4.7, 10.5, 15 Hz, 1H), 6.41-6.37 (d, J ) 15.8 Hz, 1H), 6.03-
5.96 (dd, J ) 8.8, 15.8 Hz, 1H), 5.77-5.75 (d, J ) 7.9 Hz, 1H),
5.75-5.71 (d, J ) 15 Hz, 1H), 5.06-5.01 (ddd, J ) 2, 6.6, 11
Hz, 1H), 4.91-4.88 (dd, J ) 3.6, 10 Hz, 1H), 4.73-4.67 (m,
1H), 3.76 (s, 3H), 3.54-3.48 (m, 1H), 3.46-3.39 (m, 1H), 3.15-
3.11 (dd, J ) 6, 14.4, 1H), 3.04-2.98 (dd, J ) 7.5, 14.4 Hz,
1H), 2.57-2.52 (m, 3H), 2.39-2.30 (m, 1H), 1.78-1.57 (m, 3H),
1.35-1.27 (m, 1H), 1.13-1.11 (d, J ) 6.8 Hz, 3H), 0.73-0.72
(d, J ) 6.4 Hz, 3H), 0.70-0.69 (d, J ) 6.4 Hz, 3H); 13C NMR
(100 MHz, CDCl3) δ 172.8, 170.9, 170.8, 165.6, 158.5, 141.7,
136.7, 131.8, 130.2 (2C), 128.6 (2C), 128.5, 127.5, 126.1 (2C),
125.0, 114.1 (2C), 76.6, 71.5, 55.2, 54.3, 42.2, 39.7, 36.4, 35.2,
34.2, 32.4, 24.3, 22.6, 21.2, 17.2; IR (film) 3365, 3260, 2940,
1725, 1710, 1665 cm-1; MS (FAB, TG/G) m/z 591.3 (M + H);
HRMS (FAB, TG/G) calcd for C34H43N2O4 (M + H) 591.3070,
found 591.3069.
desired ester 28 as a thick oil (173 mg, 77%): [R]20 -24.6° (c
D
1.00, CHCl3); 1H NMR (400 MHz, CDCl3) δ 7.12-7.10 (d, J )
8.5 Hz, 2H), 6.80-6.78 (d, J ) 8.5 Hz, 2H), 6.73 (buried m,
1H), 5.81-5.77 (br d, J ) 15.6 Hz, 1H), 5.69-5.60 (ddd, J )
8, 10.8, 16.6 Hz, 1H), 5.44-5.42 (br d, J ) 8 Hz, 1H), 5.07 (s,
1H), 5.05-5.03 (d, J ) 7 Hz, 1H), 4.98-4.94 (dd, J ) 4, 10
Hz, 1H), 4.96-4.92 (buried, 1H), 4.38-4.32 (m, 1H), 3.74 (s,
3H), 3.61-3.55 (m, 1H), 3.48-3.43 (m, 1H), 3.14-3.09 (dd, J
) 5.5, 14 Hz, 1H), 2.93-2.88 (m, 1H), 2.51-2.38 (m, 5H), 1.78-
1.68 (m, 3H), 1.59-1.52 (m, 1H), 1.46 (s, 9H), 1.34 (s, 9H),
1.00-0.98 (d, J ) 7 Hz, 3H), 0.92-0.91 (d, J ) 6.5 Hz, 3H),
0.90-0.88 (d, J ) 6.5 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ
171.3, 170.6, 165.7, 158.3, 155.4, 141.8, 138.3, 130.3 (2C),
129.1, 126.0, 116.6, 113.8, 80.4, 79.5, 76.2, 71.2, 55.9, 55.1, 41.2,
39.6, 37.7, 35.1, 34.6, 34.3, 28.2 (3C), 28.1 (3C), 24.6, 23.0, 21.4,
16.4; MS (FAB, TG/G) m/z 689.4 (M+H); HRMS (FAB, TG/G)
calcd for C37H57N2O10 (M + H) 689.4013, found 689.4030.
Dep h en yld esep oxya r en a sta tin A (30). The starting ester
28 (170 mg, 1.48 mmol) was dissolved in CH2Cl2 (5 mL), and
TFA (300 µL) was added. After 1 h, toluene (5 mL) was added
and the reaction mixture was concentrated in vacuo. The
mixture was then redissolved in CH3CN (23 mL), and DIEA
(123 µL, 0.706 mmol) and HBTU (107 mg, 0.282 mmol) were
added. After the mixture was stirred for 1 h, saturated aqueous
NaHCO3 and CH2Cl2 were added. The aqueous layer was
extracted with CH2Cl2. The combined organics were dried
(MgSO4), filtered,and concentrated. Flash chromatography (7:1
Desep oxya r en a sta tin A (4) fr om 30. Olefin 30 (17 mg,
0.033 mmol) was dissolved in CH3CN (0.33 mL) in a dry sealed
tube. The solution was flushed with argon, and then iodoben-
zene (4 mL, 0.036 mmol), Pd(OAc)2 (1.1 mg, 0.005 mmol), and
TEA (46 mL, 0.330 mmol) were added. The tube was sealed
and placed in a 80-85 °C oil bath with vigorous stirring
overnight. After 20 h, the solution was filtered and purified
by silica gel chromatography to obtain product 4 as a solid (3
mg, 31% based on recovered starting material) and unreacted
starting material (6 mg).
to 4:1 CH2Cl2/acetone) provided the desired macrocycle 30 as
1
a colorless oil (88 mg, 73%): [R]20 +24° (c 0.80, CHCl3); H
D
NMR (400 MHz, CDCl3) δ 7.11-7.09 (d, J ) 8.6 Hz, 2H), 7.05-
7.02 (t, J ) 5.7 Hz, 1H), 6.81-6.79 (d, J ) 8.6 Hz, 2H), 6.71-
6.63 (ddd, J ) 5, 10.3, 15 Hz, 1H), 5.84-5.82 (d, J ) 8 Hz,
1H), 5.75-5.71 (d, J ) 14 Hz, 1H), 5.69-5.65 (m, 1H), 5.08
(br s, 1H), 5.05-5.04 (d, J ) 6 Hz, 1H), 5.02-4.97 (ddd, J ) 2,
5, 11 Hz, 1H), 4.94-4.91 (dd, J ) 4, 9.5 Hz, 1H), 4.72-4.66
(m, 1H), 3.76 (s, 3H), 3.54-3.47 (m, 1H), 3.47-3.40 (m, 1H),
3.16-3.11 (dd, J ) 6, 14.4 Hz, 1H), 3.03-2.97 (dd, J ) 7.6,
14.4 Hz, 1H), 2.44-2.28 (m, 2H), 1.76-1.66 (m, 1H), 1.48-
1.40 (m, 1H), 1.04-1.02 (d, J ) 7 Hz, 3H), 0.92-0.90 (d, J )
6.4 Hz, 3H), 0.88-0.87 (d, J ) 6.4 Hz, 3H); 13C NMR (100 MHz,
CDCl3) δ 172.8, 170.82, 170.76, 165.7, 158.5, 141.8, 138.6, 130.2
(2C), 128.5, 124.9, 116.5, 114.1 (2C), 76.8, 71.4, 55.2, 54.3, 42.4,
39.8, 36.1, 35.2, 34.2, 32.5, 24.5, 22.9, 21.5, 16.6; IR (film) 3420,
3300, 2990, 1740, 1675, 1525 cm-1; MS (FAB+, TG/G) 515.3
(M + H); HRMS (FAB, TG/G) calcd for C28H39O7N2 (M + H)
515.2757, found 515.2775.
Ar en a sta tin A (2). Olefin 4 (5.0 mg, 8.5 µmol) was reacted
as previously reported36 with dimethyldioxirane64 to obtain a
2:1 mixture (de was determined by HPLC Phenomenex Hypersil
5 µm, C18, 150 × 3.2 mm, 254 nm, 3:2 CH3CN/H2O, 0.5 mL/
min, retention time: â) 7.2 min, R ) 7.9 min) of epoxide
diastereomers (3.9 mg, 76%): [R]20 +37° (c 0.10, CHCl3).7,36
D
Ack n ow led gm en t. We thank the National Insti-
tutes of Health for financial support (RO1 CA 70369).
The Department of the Army is acknowledged by M.E.
for a postdoctoral fellowship (DAMD 17-97-1-7051) and
by S.B.B. for a predoctoral fellowship (DAMD17-98-1-
8200). C.J .M., S.K.N., S.M.A., and T.C.B. thank the
Kansas Health Foundation for postdoctoral fellowships.
T.C.B. thanks the Scientific Education Partnership of
Hoechst Marion Roussel Inc. for a postdoctoral fellow-
ship. S.B.B. and E.A.R. also thank the NIH for a
predoctoral training grant (NIH-GM-07775).
ter t-Bu tyl (5S,6R,2E,7E)-5-[(2S)-2-[3-[(2R)-2-ter t-Bu tox-
yca r bon yla m in o-3-(4-m eth oxyp h en yl)p r op a n oyla m in o]-
pr opan oyloxy]-4-m eth ylpen tan oyloxy]-6-m eth yl-8-ph en yl-
2E,7E-octa d ien oa te (29).36 The N-Boc amino acid 6 (95 mg,
0. 198 mmol) was dissolved in THF (3.8 mL), and then DIEA
(43 µL, 0.248 mmol), 2,4,6-trichlorobenzoyl chloride (34 µL,
0.218 mmol), and DMAP were added. After 1.5 h, the alcohol
5 (30 mg, 0. 099 mmol) dissolved in THF (200 µL) was added
dropwise. After 1 h, saturated aqueous NaHCO3 solution was
added. The mixture was extracted with CH2Cl2. The combined
organics were dried (MgSO4), filtered, and concentrated.
Column chromatography (80:20 to 70:30 hexanes/EtOAc)
provided the desired ester 29 as a colorless oil (54 mg, 71%):
1H NMR (400 MHz, CDCl3) δ 7.32-7.18 (m, 5H), 7.13-7.10
(d, J ) 8.5 Hz, 2H), 6.80-6.78 (d, J ) 8.5 Hz, 2H), 6.76 (buried
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
compounds 2, 4-7, 9, 13, 14, 16, 18-25, and 27-30. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O000767+