Synthesis of Cryptophycin 52 Using the Sharpless Asymmetric Dihydroxylation: Diol to Epoxide Transformation Optimized for a Base-Sensitive Substrate

Article abstract of DOI:10.1021/jo9919862

A synthesis of cryptophycin 52 (2) is reported using a Sharpless asymmetric dihydroxylation (AD) strategy to install the epoxide moiety. The high stereoselectivity of the AD reaction that allows for an efficient means of preparing the epoxide is in contrast to the standard direct epoxidation of cryptophycin substrates, which proceeds with poor diastereoselectivity. Methodology for conversion of the diol AD product to the requisite epoxide is disclosed. The transformation has been optimized to proceed in high yield in the presence of base sensitive functionality.

Full text of DOI:10.1021/jo9919862

J . Org. Chem. 2000, 65, 3143-3147
3143
Syn th esis of Cr yp top h ycin 52 Usin g th e Sh a r p less Asym m etr ic
Dih yd r oxyla tion : Diol to Ep oxid e Tr a n sfor m a tion Op tim ized for a
Ba se-Sen sitive Su bstr a te
J ian Liang,† Eric D. Moher,*,‡ Richard E. Moore,*,† and David W. Hoard‡
Department of Chemistry, 2545 The Mall, University of Hawaii, Honolulu, Hawaii 96822, and
Chemical Process Research and Development, Lilly Research Laboratories, Eli Lilly and Company,
Indianapolis, Indiana 46285
Received December 28, 1999
A synthesis of cryptophycin 52 (2) is reported using a Sharpless asymmetric dihydroxylation (AD)
strategy to install the epoxide moiety. The high stereoselectivity of the AD reaction that allows for
an efficient means of preparing the epoxide is in contrast to the standard direct epoxidation of
cryptophycin substrates, which proceeds with poor diastereoselectivity. Methodology for conversion
of the diol AD product to the requisite epoxide is disclosed. The transformation has been optimized
to proceed in high yield in the presence of base sensitive functionality.
The cryptophycins, exemplified by cryptophycin 1 (1),
are cytotoxic macrocyclic depsipeptides isolated from
blue-green algae (Nostoc sp. strains ATCC 5371891 and
GSV 2242). Cryptophycin 1 (1) has been shown by Moore
and co-workers to be a potent tumor-selective cytotoxin
in vivo.2,3 Noteworthy is the broad spectrum of antitumor
activity exhibited by 1 across a variety of tumors im-
planted in mice. Particularly compelling is the observa-
tion that 1 significantly reduced the mean tumor burden
from a Taxol-resistant mammary adenocarcinoma tumor
implanted in mice.2 The remarkable antiproliferative
effects of 1 are believed to be derived from reversible high
affinity binding to microtubules, making it one of the
most potent suppressors of microtubule dynamics known.4
antitumor activity, is in clinical development for the
treatment of solid tumors.
Additionally, several research groups have adopted
programs aimed at addressing the synthetic challenges
associated with this structurally intriguing class of
molecules. The first reports on the total synthesis of
cryptophycins appeared in 1994 from the research groups
of Kitigawa,7 Moore, and Tius.8 These were soon followed
by numerous approaches comprised of both formal and
total syntheses.9 A common strategical thread running
through the majority of syntheses of epoxide-containing
cryptophycins (e.g., 1) has been the introduction of the
epoxide pharmacophore in a single late-stage operation
(vide infra). The epoxidation proceeds with a diastereo-
selectivity of 2-3:1 necessitating a tedious chromato-
graphic separation of the desired (major) â-isomer.
Typical isolated yields are reported in the 50-55% range,
(5) (a) de Muys, J .-M.; Rej, R.; Nguyen, R. R.; Go, B.; Fortin, S.;
Lavalee, J .-F. Bioorg. Med. Chem. Lett. 1996, 6, 1111. (b) Norman, B.
H.; Hemscheidt, T.; Schultz, R. M.; Andis, S. L.; J . Org. Chem. 1998,
63, 5288. (c) Georg, G. I.; Ali, S. M.; Valentino, J . S.; Waugh, W. N.;
Himes, R. H.; Bioorg. Med. Chem. Lett. 1998, 8, 1959. (d) Varie, D. L.;
Shih, C.; Hay, D. A.; Andis, S. L.; Corbett, T. H.; Gossett, L. S.; J anisse,
S. K.; Martinelli, M. J .; Moher, E. D.; Schultz, R. M.; Toth, J . E. Bioorg.
Med. Chem. Lett. 1999, 9, 369. (e) Shih, C.; Gossett, L. S.; Gruber, J .
M.; Grossman, C. S.; Andis, S. L.; Schultz, R. M.; Worzalla, J . F.;
Corbett, T. H.; Metz, J . T. Bioorg. Med. Chem. Lett. 1999, 9, 69. (f)
Patel, V. F.; Andis, S. L.; Kennedy, J . H.; Ray, J . E.; Schultz, R. M.; J .
Med. Chem. 1999, 42, 2588.
(6) (a) Moore, R. E.; Tius, M. A.; Barrow, R. A.; Liang, J .; Corbett,
T. H.; Valeriote, F. A.; Hemscheidt, T. K. PCT Int. Appl. 1996, WO
9640184 A1 19961219. (b) Wagner, M. M.; Paul, D. C.; Shih, C.; J ordan,
M. A.; Wilson, L.; Williams, D. C. Cancer Chemother. Pharmacol. 1999,
43, 115 and references therein.
As a result of these findings, interest in the crypto-
phycins has blossomed in recent years with numerous
reports highlighting the biological evaluation of new
synthetic analogues.5 From our efforts, in collaboration
with Wayne State University, aimed at discovering and
developing cryptophycin analogues possessing refined
biological properties, has emerged cryptophycin 52 (2).6
Cryptophycin 52, designed to enhance hydrolytic stabil-
ity3 relative to 1 while maintaining a favorable profile of
(7) Kobayashi, M.; Kurosu, M.; Wang, W.; Kitigawa, I. Chem.
Pharm. Bull. (J apan) 1994, 42, 2394.
(8) Barrow, R. A.; Hemscheidt, T.; Liang, J .; Paik, S.; Moore, R. E.;
Tius, M. A.J . Am. Chem. Soc. 1995, 117, 2479.
† University of Hawaii.
(9) (a) Lian, J .; 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) White,
J . D.; Hong, J .; Robarge, L. A.; J . Org. Chem. 1999, 64, 6206. (c)
Furuyama, M.; Shimizu, I.; Tetrahedron: Asymmetry, 1998, 9, 1351.
(d) Eggen, M.; Georg, G. I. Bioorg. Med. Chem. Lett. 1998, 8, 3177. (e)
Varie, D. L.; Brennan, J .; Briggs, B.; Cronin, J . S.; Hay, D. A.; Rieck,
J . A.; Zmijewski, M. J . Tetrahedron Lett. 1998, 39, 8405. (f) Dhokte,
U. P.; Khau, V. V.; Hutchison, D. R.; Martinelli, M. J . Tetrahedron
Lett. 1998, 39, 8771. (g) Gardinier, K. M.; Leahy, J . W. J . Org. Chem.
1997, 62, 7098. (h) Ali, S. M.; Georg, G. I. Tetrahedron Lett. 1997, 38,
1703. (i) Salomonczyk, G. M.; Han, K.; Guo, Z-w.; Sih, C. J . J . Org.
Chem. 1996, 61, 6893. (j) Rej, R. R.; Nguyen, D.; Go, B.; Fortin, S.;
Lavallee, J .-F. J . Org. Chem. 1996, 61, 6289.
‡
Eli Lilly and Company.
(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) 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.
(3) Golakoti, T.; 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.
(4) Panda, D.; Himes, R. H.; Moore, R. E.; Wilson, L.; J ordan, M. A.
Biochemistry, 1997, 36, 12948.
10.1021/jo9919862 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/12/2000

3144 J . Org. Chem., Vol. 65, No. 10, 2000
Liang et al.
Sch em e 1
Sch em e 2
Sch em e 3
Ta ble 1. Stoich iom etr y Stu d y for th e AD of 3
making this transformation the Achilles heal of the
synthesis in terms of efficiency and throughput. One
notable exception is Leahy’s elegant synthesis of 1
featuring an auxiliary based diastereoselective aldol
reaction leading to a syn-1,2-â-diol as an epoxide
precursor.9g Elaboration of the diol to epoxide 1 proceeded
by way of a novel variation on the Sharpless ortho ester
method.10
As part of our initial efforts to generate supplies of 2
and synthetic intermediates to fuel clinical development
and SAR, respectively, we adopted the Moore-Tius
synthesis of 2,6a,8 making necessary adjustments for
multikilo operations. The final synthetic sequence11
(Scheme 1) culminates in an epoxidation employing
m-CPBA to produce a 2:1 mixture of epoxides 2 and 5
from which 2 is isolated in 51% yield by reversed-phase
HPLC. Clearly we required a more efficient means for
generating the epoxide since the transformation, due to
its poor diastereoselectivity, was a bottleneck that ham-
pered our ability to generate sufficient quantities of 2.
Sharpless has shown that syn-vicinal diols serve as
epoxide precursors capable of being masked and carried
through several synthetic operations prior to revealing
the potentially sensitive oxirane.10,12 With the advent of
the Sharpless asymmetric dihydroxylation (AD) protocol,
a wide variety of olefins can be enantio- and diastereo-
selectively transformed into syn-1,2-diols and thus to
epoxides in good yield over a three- to four-step se-
quence.13 In analogy to the Leahy synthesis,9g we envi-
sioned the conversion of diol 6 to epoxide 2 (Scheme 2);
however, we were intrigued by the potential for a
stereoselective dihydroxylation of styrene 4, an overall
strategy in keeping with the familiar protocols of the
Moore-Tius route yet obviating the inefficient epoxida-
tion reaction.
K2OsO2(OH)4 ligand
K2CO3
(equiv)
time
7
entry
(mol %)
(mol %)
(h) (% yield)a
1
2
3
4
5
6
7
8
2
2
1
1
2
1
2
2
4
2
2
1
2
2
2
2
3
3
3
3
2
3.5
4
5.5
27
20.5
9
55
55
53
30
61
57
48
60
2.3
4
3.5
3 (+3 NaHCO3) 20
a
Isolated yield after flash chromatography.
Initial attempts to stereoselectively dihydroxylate 4
using OsO4, catalytic14 or stoichiometric, gave approxi-
mately equal mixtures of four diastereomers resulting
from indiscriminate stereofacial and regiochemical attack
of the reagent on the two olefins. Ruthenium-catalyzed
cis-dihydroxylation according to the method of Shing15
led to rapid degradation of 4 with production of alde-
hydic components as evidenced by 1H NMR. Equally
discouraging was the AD using the dihydroquinidine
based phthalazine ligand (DHQD)2PHAL, which proved
completely ineffective due to the insolubility of 4 in the
standard AD solvent system (t-BuOH/H2O).
With the selective dihydroxylation of 4 appearing
unfeasible, we moved to a modified strategy employing
a more tractable substrate. To our satisfaction, the AD
of styrene 3 (Scheme 3) employing (DHQD)2PHAL in the
presence of catalytic K2OsO2(OH)4 proceeded in moderate
yield to produce diols 7 and 8 with high diastereoselec-
tivity (29:1) favoring â-diol 7. Table 1 summarizes the
results of a study aimed at optimizing the yield of 7 while
minimizing the amount of ligand and osmium used. The
reactions were conducted in t-BuOH/H2O (1:1, 0.1 M in
olefin) at room temperature with added MeSO2NH2 (1
equiv) as an accelerant, K3Fe(CN)6 (3 equiv) as stoichio-
metric oxidant, and K2CO3 as base to facilitate osmate
(10) Kolb, H. C.; Sharpless, K. B. Tetrahedron 1992, 48, 10515.
(11) Fray, A. H.; Tetrahedron: Asymmetry, 1998, 9, 2777.
(12) For a discussion of various methods, see ref 13, pp 2518-2524
and references therein.
(14) Van Rheenen, V.; Kelly, R. C.; Cha, D. Y.; Tetrahedron Lett.
1976, 1973.
(13) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem.
Rev. 1994, 94, 2483.
(15) Shing, T. K. M.; Tam, E. K. W.; Tai, V. W.-F.; Chung, I. H. F.;
J iang, Q. Chem. Eur. J . 1996, 2, 50.

Synthesis of Cryptophycin 52
J . Org. Chem., Vol. 65, No. 10, 2000 3145
Sch em e 4
ester hydrolysis.13 From the results in Table 1 two
general features of the reaction become apparent: (1)
styrene 3 reacts sluggishly with low osmium and ligand
loading (entry 1-3 vs 4), in contrast to many olefins that
require as little as 0.2 mol % Os and 1 mol % ligand for
high conversion; (2) the yield is compromised by the base-
sensitive nature of the substrate (entry 5 vs 7). Decreas-
ing both osmium and ligand loading results in a sluggish
reaction giving rise to inferior yields since prolonged
exposure of the product and starting material to the basic
reaction conditions is detrimental (entry 4). These two
trends, which work in concert, dictate loading of the
expensive osmium and cinchona alkaloid-based ligand to
a lower limit of 2-4 mol %. Unfortunately, buffering the
reaction (entry 8), employing a slow addition of base (not
shown), or adding excess MeSO2NH2 (not shown) did not
lead to significantly increased yields.
The carbonate approach was abandoned for the alter-
native three-step procedure involving the ionization of
an ortho ester and opening of the resulting 1,3-dioxolan-
2-ylium ion intermediate (e.g., 12) by halide ion to provide
a vicinal acetoxy halide species.12 Subsequent saponifica-
tion with spontaneous cyclization would lead to the
desired epoxide. Toward this end, orthoacetate 10 was
treated with TMSCl giving rise (via 12) to acetoxy
chloride 14 in nearly quantitative yield as a single isomer
(Scheme 4). In keeping with Leahy’s observations,9g
exposure of 14 to a variety of basic hydrolysis conditions
(K2CO3-methanol, LiOH, n-Bu4NOH) resulted in decom-
position of the material owing to indiscriminate ester
cleavage. Best results were obtained using in situ gener-
ated lithium hydroperoxide19 giving 2 directly in 50%
isolated yield. The base sensitive nature of the crypto-
phycins necessitated an alternative yet concise and
economical method for the diol to epoxide conversion.
Critical to enabling a successful ortho ester strategy
was to employ an ester more labile than acetate or any
of the other ester linkages comprising the cryptophycin
framework. It was believed that a formate ester would
meet this requirement and allow efficient access to
epoxide 2. Treatment of orthoformate 11 with TMSCl
failed to produce any of the desired formyloxy chloride
15, but instead only hydroxy formates 18 and 19 were
formed presumably by kinetic trapping of the putative
intermediate 1,3-dioxolan-2-ylium ion 13 at the 2-position
by chloride ion with eventual hydrolysis by adventitious
water.
With seco-diol 7 in hand, we were ready to carry out
the macrolactamization. This was conducted uneventfully
according to a modification of the Fray protocol11 for
cyclization of seco-styrene 3. Namely, 7 was treated with
TFA (15 equiv) to effect tert-butyl carbamate cleavage
followed by workup with K2CO3, which afforded the
intermediate amine as an amorphous solid. Dissolution
of the amine in acetonitrile-toluene and treatment with
2-hydroxypyridine (2HP, 2 equiv) afforded an 82% yield
of macrocycle 6 after flash chromatography. It is note-
worthy that exposure of unprotected 7 to TFA does not
result in pinacol rearrangement or epimerization due to
SN1 reactions at the benzylic hydroxyl-bearing methine.
Having access to macrocyclic diol 6, the stage was set
for the critical diol to epoxide conversion and elaboration
to 2. Initial studies focused on carbonate 9 drawing from
literature precedent provided by Sharpless,16 who showed
that heating the carbonate derived from stilbene diol in
the presence of LiCl smoothly provided stilbene oxide in
84% yield. In the event, 9 and LiCl (1-2 equiv) were
heated in DMF for 9 h to afford a 25% yield of epoxide 2
and 25% recovered 9 with the balance comprised of un-
known decomposition products. Clearly, at elevated tem-
peratures the molecule suffers decomposition at a rate
competitive with chloride ion displacement of CO2. Other
attempts were made using LiI17 (DMF and pyridine as
solvents), TMSI,18 and TMSCl resulting primarily in
recovered carbonate.
To our great satisfaction, reaction of 11 with TMSI
produced formyloxy iodide 16 in 93% yield over the two
steps (Scheme 4). Formate 16 could subsequently be
saponified and cyclized producing epoxide 2 with high
efficiency. The conversion is best accomplished with
KHCO3 or K2CO3 in methanol-tetrahydrofuran (93% and
98% yield, respectively) while lithium hydroperoxide gave
(16) Chang, H.-T.; Sharpless, K. B. J . Org. Chem. 1996, 61, 6456.
(17) Fisher, J . W.; Trinkle, K. L. Tetrahedron Lett. 1994, 35, 2505
and references therein.
(18) Fisher, J . W.; Dunigan, J . M.; Hatfield, L. D.; Hoying, R. C.;
Ray, J . E.; Thomas, K. L. Tetrahedron Lett. 1993, 34, 4755.
(19) Evans, D. A.; Britton, T. C.; Ellman, J . A. Tetrahedron Lett.
1987, 28, 6141.

3146 J . Org. Chem., Vol. 65, No. 10, 2000
Liang et al.
1.03-0.99 (m, 6H), 0.97 (d, 3H, J ) 6.5 Hz); 125 MHz 13C NMR
(CDCl3) δ 177.42, 172.28, 170.37, 165.63, 156.72, 154.52,
140.58, 139.64, 131.59, 129.29, 129.13, 128.85, 127.40, 125.49,
122.68, 112.54, 94.75, 79.59, 76.38, 75.64, 75.03, 74.88, 71.72,
56.50, 53.64, 49.03, 44.39, 39.95, 38.09, 36.96, 33.94, 28.80,
25.40, 23.57, 23.20, 22.80, 21.88, 9.94; FTIR (KBr) 3390 (s),
2974 (s), 1725 (s), 1678 (m), 1641 (m), 1504 (s), 1259 (s), 1151
(s) cm-1. Anal. Calcd for C43H58Cl4N2O12: C, 55.13; H, 6.24;
N, 2.99. Found: C, 55.38; H, 6.39; N, 2.99.
an inferior yield of 67%. The expense of TMSI led us to
look for an alternative reagent for the rearrangement of
11. We were pleased to find that acetyl bromide is
sufficiently reactive to provide formyloxy bromide 17 in
85% yield for the two-step sequence. Conversion of 17 to
2 follows in a similar high-yielding manner to that
described for 16.
Similar diol to vic-formyloxy halide transformations
have appeared in the literature; however, they have
rarely been applied in organic synthesis.20 For example,
Hartman20c and Baganz20d have reported on the prepara-
tion of formyloxy chlorides by heating orthoformates
derived from simple 1,2-diols in the presence of acetyl
chloride. Additionally, limited examples exist where the
treatment of orthoformates with PCl5 affords fomyloxy
chlorides in good yield.20a,b Thus, borrowing from the work
of Nicolaou,20b 11 was treated with PCl5 giving rise to
formyloxy chloride 15, however, in a disappointingly low
yield (18%).
In conclusion, a synthesis of cryptophycin 52 (2) has
been demonstrated on the basis of the Sharpless asym-
metric dihydroxylation (AD), which provides an efficient
method of installing the cryptophycin epoxide moiety.
The route circumvents the commonly employed direct last
step epoxidation which exhibits poor diastereoselectivity
and necessitates a preparative HPLC isolation. Experi-
ments are currently underway exploring the scope and
generality of the methodology disclosed herein for the
mild diol to epoxide transformation.
Ma cr ocyclic Diol 6. To a solution of 7 (1.32 g, 1.41 mmol)
in 2.8 mL of CH2Cl2 at 0 °C was added TFA (1.62 mL, 21.1
mmol). After being stirred for 1.5 h, the reaction mixture was
poured into a solution of K2CO3 (4.8 g, 34.7 mmol) in 8.4 mL
of water. The layers were separated, and the aqueous layer
was washed with ethyl acetate. The combined organics were
washed with H2O, dried (Na2SO4), filtered, and concentrated
in vacuo to a faint brown oil. The crude amine was dissolved
in 22.5 mL of acetonitrile, diluted with 22.5 mL of toluene,
and treated with 2-hydroxypyridine (268 mg, 2.82 mmol). The
mixture was heated to 40 °C and stirred for 21 h, after which
time it was diluted with 15 mL of ethyl acetate, washed with
saturated aqueous NaHCO3, washed with brine, dried (Na2SO4),
filtered, and concentrated in vacuo to a faint brown oil.
Chromatography (30 g of flash SiO2) eluting with ethyl acetate
provided 793 mg (82%) of 6 as a white solid: Rf 0.36 (ethyl
acetate); 500 MHz 1H NMR (CDCl3) δ 7.51-7.31 (m, 5H), 7.22
(d, 1H, J ) 2.0 Hz), 7.19-7.15 (m, 1H), 7.07 (dd, 1H, J ) 8.4,
2.0 Hz), 6.86 (d, 1H, J ) 8.4 Hz), 6.74 (ddd, 1H, J ) 15, 11,
3.7 Hz), 5.72, (dd, 1H, J ) 15, 1.2 Hz), 5.58 (d, 1H, J ) 7.9
Hz), 5.12-5.08 (m, 1H), 6.18 (dd, 1H, J ) 9.9, 3.7 Hz), 4.77-
4.72 (m, 1H), 4.60 (d, 1H, J ) 8.4 Hz), 3.90 (s, 3H), 3.80 (d,
1H, J ) 8.4 Hz), 3.38 (dd, 1H, J ) 13.5, 8.1 Hz), 3.20 (dd, 1H,
J ) 13.5, 3.9 Hz), 3.13 (dd, 1H, J ) 14.5, 5.2 Hz), 3.05 (dd,
1H, J ) 14.5, 7.6 Hz), 2.83 (br s, 1H), 2.78 (br s, 1H), 2.52-
2.47 (m, 1H), 2.28-2.20 (m, 1H), 1.86-1.80 (m, 1H), 1.72-
1.65 (m, 1H), 1.54-1.44 (m, 2H), 1.25 (s, 3H), 1.19 (s, 3H), 1.03
(d, 3H, J ) 7.0 Hz), 0.97 (d, 3H, 6.7 Hz), 0.91 (d, 3H, J ) 6.5
Exp er im en ta l Section
Gen er a l P r oced u r es. 1H and 13C NMR spectra were
recorded on a 500 MHz spectrometer. Melting points are
uncorrected. Reactions were monitored by TLC (p-anisalde-
hyde stain) and HPLC equipped with a 4.6 mm × 250 mm
Zorbax SB-C18 column (1 mL/min, acetonitrile-water both
with 0.5% TFA) with detection at 220 nm. Elemental analyses
and infrared (FTIR) spectra were performed at the Structural
and Organic Chemistry Research Laboratory, Eli Lilly and
Company, Indianapolis, IN.
1
Hz); 500 MHz H NMR (CD3OD) δ 7.40-7.35 (m, 5H), 7.34-
7.30 (m, 1H), 7.27 (d, 1H, J ) 2.1 Hz), 7.16 (dd, 1H, J ) 8.4,
2.1 Hz), 6.98 (d, 1H, J ) 8.4 Hz), 6.69 (ddd, 1H, J ) 15, 13,
3.8 Hz), 5.85 (dd, 1H, J ) 15, 1.6 Hz), 5.10 (br t, 1H, J ) 9.7
Hz), 4.93 (dd, 1H, J ) 10, 3.5 Hz), 4.55 (d, 1H, J ) 8.3 Hz),
4.51 (dd, 1H, J ) 11, 3.7 Hz), 3.85 (s, 3H), 3.73 (dd, 1H, 8.3,
1.6 Hz), 3.46 (d, 1H, 14 Hz), 3.18 (dd, 1H, J ) 14, 3.7 Hz),
3.10 (d, 1H, J ) 14 Hz), 2.73 (dd, 1H, J ) 14, 11 Hz), 2.66-
2.59 (m, 1H), 2.18-2.09 (m, 1H), 1.85-1.78 (m, 1H), 1.75-
1.66 (m, 1H), 1.60-1.52 (m, 1H), 1.49-1.41 (m, 1H), 1.22 (s,
3H), 1.19 (s, 3H), 1.02-0.97 (m, 6H), 0.95 (d, 3H, J ) 6.6 Hz);
125 MHz 13C NMR (CD3OD) δ 177.83, 172.70, 170.78, 167.23,
154.37, 143.23, 142.32, 131.19, 130.47, 128.52, 128.25, 128.00,
127.13, 124.04, 122.28, 112.53, 76.20, 76.12, 74.88, 71.47,
56.44, 55.61, 46.41, 43.09, 39.92, 38.77, 36.52, 35.44, 25.08,
22.41, 22.39, 22.29, 20.98, 8.67; FTIR (KBr) 3419 (m), 3280
(m), 1752 (s), 1721 (s), 1663 (s), 1503 (s), 1257 (s), 1196 (s),
1151 (s) cm-1. An analytical sample was prepared by crystal-
seco-Diol 7. To a mixture of seco-styrene 3 (304 mg, 0.34
mmol), K2OsO2(OH)4 (2.5 mg, 0.0067 mmol), (DHQD)2PHAL
(5.2 mg, 0.0067 mmol), K3Fe(CN)6 (333 mg, 1.01 mmol), K2CO3
(93 mg, 0.67 mmol), and MeSO2NH2 (32 mg, 0.34 mmol) was
added 0.615 mL of t-BuOH and 0.615 mL of H2O. The
heterogeneous mixture was allowed to stir rapidly at room
temperature for 20.5 h. The reaction was treated with 304 mg
of Na2SO3. After being stirred for 25 min, the mixture was
diluted with 3 mL of H2O and washed with ethyl acetate. The
combined organic extracts were dried (Na2SO4), filtered, and
concentrated in vacuo to an off-white oil. Chromatography (34
g of flash SiO2) eluting with ethyl acetate-hexanes (1:1 then
2:1) provided 193 mg (61%) of 7 as a white amorphous solid:
Rf 0.54 (2:1/ethyl acetate/hexanes); [R]25D -52° (c 1.14, CHCl3);
lization from ethyl acetate: mp 144-145 °C; [R]25 -34° (c
D
1.00, MeOH). Anal. Calcd for C36H47ClN2O9: C, 62.92; H, 6.89;
N, 4.08. Found: C, 62.66; H, 6.73; N, 4.00.
Acetoxy Ch lor id e 14. To a slurry of 6 (33 mg, 0.048 mmol)
and pyridinium p-toluenesulfonate (1.2 mg, 0.0048 mmol) in
0.096 mL of CH2Cl2 at room temperature was added trimethyl
orthoacetate (0.061 mL, 0.48 mmol). After being stirred for 1
h, the reaction was applied directly to a chromatography
column (13 g of flash SiO2) eluting with ethyl acetate-hexanes
(5:1) to provide 34 mg (94%) of 10 as a white amorphous solid.
To a solution of 10 (33 mg, 0.044 mmol) in 0.222 mL of CH2Cl2
at room temperature was added TMSCl (0.0068 mL, 0.053
mmol). After being stirred for 2.5 h, the reaction was concen-
trated in vacuo to provide 32 mg (97%) of 14 as a white
amorphous solid: Rf 0.43 (5:1/ethyl acetate/hexanes); 500 MHz
1H NMR (CDCl3) δ 7.38-7.31 (m, 5H), 7.24 (d, 1H, J ) 2.1
Hz), 7.22-7.18 (m, 1H), 7.10 (dd, 1H, J ) 8.5, 2.1 Hz), 6.88 (d,
1H, J ) 8.5 Hz), 6.75 (ddd, 1H, J ) 15, 13, 4.6 Hz), 5.78 (dd,
1H, J ) 15, 1.0 Hz), 5.55 (d, 1H, J ) 7.9 Hz), 5.46 (dd, 1H, J
) 9.8, 1.2 Hz), 4.95 (dd, 1H, J ) 11, 2.9 Hz), 4.89 (ddd, 1H, J
1
500 MHz H NMR (CDCl3) δ 7.41-7.31 (m, 5H), 7.18 (s, 1H),
7.07 (d, 1H, J ) 8.2 Hz), 6.85 (d, 1H, J ) 8.2 Hz), 6.60-6.63
(m, 1H), 6.53 (d, 1H, J ) 7.8 Hz), 5.74 (d, 1H, J ) 16 Hz), 5.28
(t, 1H, J ) 6.2 Hz), 5.07-4.99 (m, 2H), 4.94 (dd, 1H, J ) 9.9,
3.2 Hz), 4.80 and 4.72 (AB quartet, 2H, J ) 12 Hz), 4.63 (d,
1H, J ) 8.1 Hz), 3.88 (s, 3H), 3.65-3.60 (m, 1H), 3.30 (d, 2H,
J ) 6.5 Hz), 3.21 (dd, 1H, J ) 14, 6.8 Hz), 3.07 (dd, 1H, J )
14, 5.8 Hz), 3.00 (d, 1H, J ) 3.5 Hz), 2.96 (s, 1H), 2.54-2.60
(m, 1H), 2.41-2.34 (m, 1H), 1.90-1.77 (m, 2H), 1.65-1.58 (m,
1H), 1.57-1.41 (m, 1H), 1.46 (s, 9H), 1.23 (s, 3H), 1.18 (s, 3H),
(20) (a) Konopelski, J . P.; Boehler, M. A.; Tarasow, T. M. J . Org.
Chem. 1989, 54, 4966. (b) Nicolaou, K. C.; Papahatjis, D. P.; Claremon,
D. A.; Magolda, R. L.; Dolle, R. E. J . Org. Chem. 1985, 50, 1440. (c)
Hartman, W.; Heine, H.-G.; Wendisch, D. Tetrahedron Lett. 1977, 2263.
(d) Baganz, H.; Domaschke, L. Chem. Ber. 1958, 653.

Synthesis of Cryptophycin 52
J . Org. Chem., Vol. 65, No. 10, 2000 3147
) 9.9, 9.9, 1.7 Hz), 4.81 (d, 1H, J ) 9.8 Hz), 4.79-4.74 (m,
1H), 3.91 (s, 3H), 3.39 (dd, 1H, J ) 13, 8.1 Hz), 3.22 (dd, 1H,
J ) 13, 4.1 Hz), 3.16 (dd, 1H, J ) 14, 5.1 Hz), 3.07 (dd, 1H, J
) 14, 7.6 Hz), 2.65-2.55 (m, 2H), 2.47-2.39 (m, 1H), 1.95 (ddd,
1H, J ) 14, 13, 4.6 Hz), 1.86-1.77 (m, 1H), 1.73-1.66 (m, 1H),
1.68 (s, 3H), 1.27 (s, 3H), 1.19 (s, 3H), 1.09 (d, 3H, J ) 7.1
Hz), 1.03 (d, 3H, J ) 6.7 Hz), 0.97 (d, 3H, J ) 6.6 Hz).
F or m yloxy Iod id e 16. To a slurry of 6 (144 mg, 0.210
mmol) and pyridinium p-toluenesulfonate (2.6 mg, 0.010 mmol)
in 0.698 mL of CH2Cl2 at room temperature was added
trimethyl orthoformate (0.229 mL, 2.10 mmol). After being
stirred for 2.5 h, the reaction was treated with 1 mL of
saturated aqueous NaHCO3 and washed with ethyl acetate.
The combined organics were dried (Na2SO4), filtered, and
concentrated in vacuo to provide 153 mg of 11 as a white
amorphous solid that was used directly in the next experiment.
To a solution of crude 11 from above in 1.4 mL of CH2Cl2 at 0
°C was added TMSI (0.045 mL, 0.315 mmol). After being
stirred for 45 min, the reaction was quenched with 1 mL of
saturated aqueous NaHCO3 followed by 0.5 mL of 10% aqueous
Na2S2O3 and washed with ethyl acetate. The combined organ-
ics were dried (Na2SO4), filtered, and concentrated in vacuo
to a faint yellow amorphous solid. Chromatography (15 g of
flash SiO2) eluting with ethyl acetate:hexanes (3:1) gave 161
mg (93%) of 16 as a faint yellow foam: Rf 0.42 (3:1/ethyl
acetate/hexanes); 500 MHz 1H NMR (CDCl3) δ 7.61 (s, 1H),
7.41-7.18 (m, 7H), 7.10 (dd, 1H, J ) 8.4 Hz, 2.0 Hz), 6.87 (d,
1H, J ) 8.4 Hz), 6.76 (ddd, 1H, J ) 15, 11, 4.6 Hz), 5.88 (d,
1H, J ) 10 Hz), 5.81 (d, 1H, J ) 15 Hz), 5.66 (d, 1H, J ) 7.9
Hz), 5.07 (d, 1H, J ) 10 Hz), 4.99 (dd, 1H, J ) 11, 3.0 Hz),
4.93-4.87 (m, 1H), 4.79-4.72 (m, 1H), 3.90 (s, 3H), 3.39 (dd,
1H, J ) 13, 8.0 Hz), 3.23 (dd, 1H, J ) 13, 3.9 Hz), 3.16 (dd,
1H, J ) 14, 5.1 Hz), 3.07 (dd, 1H, J ) 14, 7.8 Hz), 2.90-2.83
(m, 1H), 2.63-2.58 (m, 1H), 2.51-2.43 (m, 1H), 2.02-1.96 (m,
1H), 1.86-1.53 (m, 2H), 1.28 (s, 3H), 1.20 (s, 3H), 1.05 (d, 6H,
J ) 6.7 Hz), 0.98 (d, 3H, J ) 6.4 Hz); 125 MHz 13C NMR
(CD3OD) δ 178.22, 170.80, 170.48, 165.51, 159.12, 154.46,
142.08, 139.86, 131.33, 130.22, 129.04, 128.98, 128.81, 128.68,
125.18, 122.93, 112.77, 75.49, 73.88, 71.63, 56.57, 54.88, 47.00,
43.27, 40.22, 40.06, 37.12, 35.73, 31.29, 25.35, 23.59, 23.32,
23.19, 21.92, 10.52; FTIR (KBr) 3421 (m), 3275 (m), 1758 (s),
1727 (s), 1674 (s), 1538 (s), 1504 (s), 1151 (s) cm-1. An
analytical sample was prepared by crystallization from ethyl
acetate/heptane: mp 144-146 °C; [R]25D +84° (c 1.09, CHCl3).
Anal. Calcd for C37H46ClIN2O9: C, 53.86; H, 5.62; N, 3.39.
Found: C, 53.76; H, 5.56; N, 3.39.
and pH 7 buffer (1 mL). The layers were separated, and the
aqueous layer was washed with ethyl acetate. The combined
organics were dried (Na2SO4), filtered, and concentrated in
vacuo to a yellow foam. Chromatography (13 g of flash SiO2)
eluting with ethyl acetate/heptane (3:1) afforded 58 mg (98%)
of 26a as a white solid.
F or m yloxy Br om id e 17. To a slurry of 6 (144 mg, 0.210
mmol) and pyridinium p-toluenesulfonate (2.6 mg, 0.010 mmol)
in 0.698 mL of CH2Cl2 at room temperature was added
trimethyl orthoformate (0.229 mL, 2.10 mmol). After being
stirred for 2.5 h, the reaction was treated with 1 mL of
saturated aqueous NaHCO3 and washed with ethyl acetate.
The combined organics were dried (Na2SO4), filtered, and
concentrated in vacuo to provide 166 mg of 11 as a white
amorphous solid that was used directly in the next experiment.
To a solution of crude 11 from above in 1.4 mL of CH2Cl2 at
room temperature was added acetyl bromide (0.031 mL, 0.420
mmol). After being stirred for 140 min, the reaction was
quenched with saturated NaHCO3 (1 mL) and washed with
ethyl acetate. The combined organics were dried (Na2SO4),
filtered, and concentrated in vacuo to an off-white foam.
Chromatography (15 g of flash SiO2) eluting with ethyl acetate/
hexanes (3:1) gave 139 mg (85%) of 17 as a white solid: Rf
1
0.42 (3:1/ethyl acetate/hexanes); 500 MHz H NMR (CDCl3) δ
7.62 (s, 1H), 7.39-7.28 (m, 5H), 7.24 (d, 1H, J ) 1.9 Hz), 7.21-
7.18 (m, 1H), 7.10 (dd, 1H, J ) 8.4, 1.9 Hz), 6.88 (d, 1H, J )
8.4 Hz), 6.76 (ddd, 1H, J ) 15, 11, 4.7 Hz), 5.79 (d, 1H, J ) 15
Hz), 5.78 (d, 1H, J ) 10 Hz), 5.53 (d, 1H, J ) 7.9 Hz), 4.97
(dd, 1H, J ) 11, 3.0 Hz), 4.93-4.87 (m, 1H), 4.90 (d, 1H, J )
10 Hz), 4.79-4.72 (m, 1H), 3.91 (s, 3H), 3.38 (dd, 1H, J ) 13,
7.9 Hz), 3.24 (dd, 1H, J ) 13, 3.9 Hz), 3.17 (dd, 1H, J ) 14,
5.1 Hz), 3.08 (dd, 1H, J ) 14, 7.6 Hz), 2.81-2.76 (m, 1H), 2.63-
2.58 (m, 1H), 2.51-2.42 (m, 1H), 2.01-1.93 (m, 1H), 1.86-
1.68 (m, 2H), 1.27 (s, 3H), 1.20 (s, 3H), 1.10 (d, 3H, J ) 7.0
Hz), 1.04 (d, 3H, J ) 6.7 Hz), 0.99 (d, 3H, J ) 6.5 Hz); 125
MHz 13C NMR (CDCl3) δ 178.18, 170.81, 170.50, 165.52,
158.99, 154.46, 142.07, 137.87, 131.33, 130.24, 129.48, 128.97,
128.88, 128.67, 125.19, 122.92, 112.77, 75.37, 73.31, 71.60,
56.56, 54.90, 52.27, 47.00, 43.26, 40.03, 39.13, 37.16, 35.73,
25.32, 23.57, 23.32, 23.28, 21.88, 10.56; FTIR (KBr) 3421 (m),
3265 (m), 1758 (s), 1728 (s), 1674 (s), 1504 (s), 1150 (s) cm-1
.
An analytical sample was prepared by crystallization from
ethyl acetate/heptane: mp 153-155 °C; [R]25 +73° (c 1.05,
D
CHCl3). Anal. Calcd for C37H46ClBrN2O9: C, 56.82; H, 5.91;
N, 3.65. Found: C, 57.11; H, 5.96; N, 3.60.
Cr yp top h ycin 52 (2) fr om 17 w ith P ota ssiu m Bica r -
bon a te. The procedure was identical to the analogous method
for the convesion of 16 to 2. Here, 47 mg (0.060 mmol) of 17
afforded 37 mg (93%) of 26a as a white solid.
Cr yp top h ycin 52 (2) fr om 17 w ith P ota ssiu m Ca r bon -
a te. The procedure was identical to the analogous method for
the convesion of 16 to 2. Here, 56 mg (0.072 mmol) of 17
afforded 45 mg (94%) of 26a as a white solid.
Cr yp top h ycin 52 (2) fr om 16 w ith K2CO3. To a solution
of 16 (55 mg, 0.067 mmol) in 0.138 mL of THF and 0.412 mL
of MeOH was added KHCO3 (33 mg, 0.333 mmol). The
heterogeneous mixture was allowed to stir at 43 °C for 14.5 h.
The reaction was diluted with H2O (0.5 mL) and washed with
ethyl acetate. The combined organics were dried (Na2SO4),
filtered, and concentrated in vacuo to a yellow foam. Chroma-
tography (12 g of flash SiO2) eluting with ethyl acetate:heptane
(3:1) afforded 42 mg (93%) of 26a as a white solid.
Cr yp top h ycin 52 (2) fr om 16 w ith P ota ssiu m Ca r bon -
a te. To a solution of 16 (73 mg, 0.088 mmol) in 0.295 mL of
THF and 0.295 mL of MeOH was added K2CO3 (18 mg, 0.133
mmol). The heterogeneous mixture was allowed to stir at 0
°C for 6 h. The reaction was diluted with ethyl acetate (1 mL)
Ack n ow led gm en t. We thank Mr. David Robbins for
LC-MS analysis. Additionally, we are gratefully in-
debted to Professor William Roush for helpful discussions.
J O9919862