A Pinacol rearrangement/oxidation synthetic route to hydroxyphenstatin

Article abstract of DOI:10.1021/jo000705j

In an attempt to develop biologically active compounds from the inactive trans isomer (3a) of stilbene 1a, after asymmetric dihydroxylation to optically pure (R,R)-diol 8 the unexpected racemic diphenylacetaldehyde (9) was generated via a Pinacol rearrangement. Several derivatives of diphenylacetaldehyde 9 were synthesized (11-15) and reported. Further reaction of aldehyde 9 during desilylation through autoxidative decarbonylation afforded benzophenone 2b, designated hydroxyphenstatin, a potent antitumor and antimitotic agent. Hydroxyphenstatin showed potent inhibition of the tubulin assembly (IC₅₀ 0.82 μM) and exhibited an ED₅₀ of 2.5 μg/mL against the P388 lymphocytic leukemia cell line.

Full text of DOI:10.1021/jo000705j

7438
J . Org. Chem. 2000, 65, 7438-7444
A P in a col Rea r r a n gem en t/Oxid a tion Syn th etic Rou te to
Hyd r oxyp h en sta tin 1
George R. Pettit,* J ohn W. Lippert III, and Delbert L. Herald
Cancer Research Institute and Department of Chemistry, Arizona State University,
Tempe, Arizona 87287-2404
Received May 8, 2000
In an attempt to develop biologically active compounds from the inactive trans isomer (3a ) of stilbene
1a , after asymmetric dihydroxylation to optically pure (R,R)-diol 8 the unexpected racemic
diphenylacetaldehyde (9) was generated via a Pinacol rearrangement. Several derivatives of
diphenylacetaldehyde 9 were synthesized (11-15) and reported. Further reaction of aldehyde 9
during desilylation through autoxidative decarbonylation afforded benzophenone 2b, designated
hydroxyphenstatin, a potent antitumor and antimitotic agent. Hydroxyphenstatin showed potent
inhibition of the tubulin assembly (IC50 0.82 µM) and exhibited an ED50 of 2.5 µg/mL against the
P388 lymphocytic leukemia cell line.
Two of the most promising biologically active lead
selectively within solid tumors.8a The degree of reduction
ranged from 50% with diphenol 1a to 70% with monophe-
nol 1b,8a while the combretastatin A-4 prodrug (1c)8a
induces complete vascular shutdown within cancer mi-
crovascular systems at doses one-tenth of the maximum
tolerated dose.8b Since tumor growth relies on the devel-
opment of new blood vessels, or angiogenesis, cancer-
specific antiangiogenic drugs such as the combretastatin
derivatives discussed above are an attractive way to
approach the cancer problem.8c,d
compounds that we isolated from the African willow tree
Combretum caffrum Kuntze (Combretaceae) are combre-
tastatins A-1 (1a )2a and A-4 (1b),2b reported in 1987 and
1989, respectively. Both stilbenes structurally resemble
the antimitotic and cytotoxic agents colchicine4,6 and
podophyllotoxin.5,6 Monophenol 1b is a potent inhibitor
of microtubule assembly3 (IC50 2-3 µM) and of a broad
range of cancer cell lines,3e e.g., IC50 0.002 µg/mL against
the murine L1210 leukemia.2,3a Diphenol 1a has shown
similar inhibition of microtubule assembly (IC50 2-3 µM),
although it has proved to be less active against the
murine L1210 leukemia cell line (0.2 µg/mL)2a,3a and
other cancer cell lines. However, combretastatin A-1 (1a )
was found to be more potent than combretastatin A-4 (1b)
in its ability to increase intracellular daunorubicin
concentrations in multidrug-resistant cancer cell lines.7
Most importantly, the strong tubulin-binding cis-stil-
benes 1a and 1b elicit irreversible vascular shutdown
* To whom correspondence should be addressed. Phone: (480) 965
3351. Fax: (480) 965 8558.
(1) Antineoplastic Agents. 444. For part 443, see: Pettit, G. R.;
Grealish, M. P.; Herald, D. L.; Boyd, M. R.; Hamel, E.; Pettit, R. K. J .
Med. Chem. 2000, in press.
(2) (a) Pettit, G. R.; Singh, S. B.; Niven, M. L.; Hamel, E.; Schmidt,
J . M. J . Nat. Prod. 1987, 50, 119. (b) Pettit, G. R.; Singh, S. B.; Boyd,
M. R.; Hamel, E.; Pettit, R. K.; Schmidt, J . M.; Hogan F. J . Med. Chem.
1995, 38, 1666. (c) Hamel, E. Med. Res. Rev. 1996, 16, 207.
(3) (a) Lin, C. M.; Singh, S. B.; Ping, S. C.; Dempcy, R. O.; Schmidt,
J . M.; Pettit, G. R.; Hamel, E. Mol. Pharmacol. 1988, 34, 200. (b) Pettit,
G. R.; Singh, S. B.; Schmidt, J . M. J . Nat. Prod. 1988, 51, 517. (c)
Nandy, P.; Samitendu, B.; Gao, H.; Hui, M. B. V.; Lien, E. J . Pharm.
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G. M.; Pettit, G. R.; Von Hoff, D. D. Anti-Cancer Drugs 1993, 4, 19.
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D.; Gelmi, M. L.; Panici, P. B.; Mancuso, S. Anti-Cancer Drug Design
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Stratford, M. R. L.; Hamel, E.; Pettit, R. K.; Chapuis, J .-C.; Oliva, D.
Anti-Cancer Drug Design 1998, 13, 981. (c) Chaplin, D. L.; Pettit, G.
R.; Hill, S. A. Anti-Cancer Drug Design 1999, 19, 189. (d) Chaplin, D.
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10.1021/jo000705j CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/11/2000

Synthetic Route to Hydroxyphenstatin
J . Org. Chem., Vol. 65, No. 22, 2000 7439
An SAR investigation of potential cancer antiangio-
genic agents based on the combretastatin A-series of
structural leads has continued to be of high priority in
our research directed at the discovery of new anticancer
drugs. Our previous SAR analyses of the combretastatins
have indicated that the cis-stilbene geometry is the most
important factor for the inhibition of cancer cell growth.2
With the corresponding (E)-stilbenes, the cancer cell
growth inhibitory and antitubulin activity drops sharply
from that exhibited by the corresponding (Z)-isomers.9,10
Recently, the water-soluble phosphate derivative of com-
bretastatin A-1 (1d ) was synthesized and found to have
improved therapeutic potential, as was earlier shown for
combretastatin A-4 (1b) and its prodrug (1c).11 These
prodrugs are now undergoing preclinical and clinical
development, respectively. As part of the overall SAR
study, benzophenones 2a and 2b, designated phenstatin12a
and hydroxyphenstatin,12b respectively, were synthesized
by our group. These benzophenones were also found to
display strong anticancer and antimitotic activity. The
sp2-hybridized carbonyl group keeps the two aryl rings
in a quasi “cis” orientation, which appears to be necessary
for significant biological activity. These experiments also
showed that the two-carbon ethene unit can be replaced
by a one-carbon ketone, with retention of activity.
Furthermore, the corresponding phosphate prodrugs 2c
and 2d again showed strong anticancer activity.12a,b Other
extensions of these SAR studies led to the synthesis and
biological evaluation of the (S,S) and (R,R)-diols, 4a -
5b.9 Diols 4a -5b, synthesized from the essentially inac-
tive trans isomers 3a and 3b employing the Sharpless
asymmetric dihydroxylation technique,9 showed reduced
cancer cell line inhibitory activity when compared to their
parent cis-stilbenes. Interestingly, (S,S)-derivatives 5a
and 5b exhibited greater overall biological activity (tu-
bulin binding and cancer cell line) when compared to
their (R,R)-enantiomers, thus indicating a favored con-
formation for activity.
for potent anticancer activity, we have synthesized and
evaluated the 1,3-dioxolanes (S,S)-6a (designated diox-
ostatin), its prodrug (S,S)-6b and (R,R)-7.9c The results
again illustrated the importance of the hydroxyl group
in the 2-position of the B ring for tubulin binding9 and
of the cis-diaryl relationship for cancer cell growth
inhibition. Dioxostatin (6a ) was found to be a most potent
inhibitor of microtubule assembly at the colchicine site.9c
During the preparation of other conformationally
restricted combretastatin A-series SAR probes, (R,R)-diol
8 was allowed to react with 2,2-dimethoxypropane in the
presence of boron trifluoride etherate (at room temper-
ature) in an attempt to synthesize the acetonide deriva-
tive. Mass spectral analysis of the major isolated product
indicated the loss of 18 (H2O) from the molecular weight
of (R,R)-diol 8. An aldehyde carbonyl peak at 1726 cm-1
was recorded in the IR spectrum, and the 1H NMR signal
at 9.84 ppm further established the presence of an
aldehyde group. The product was then presumed to arise
from a pinacol rearrangement (8f9) and its structure
was confirmed by an X-ray crystallographic structure
determination.
The X-ray structure determination also helped to
resolve the question as to whether the diphenylac-
etaldhyde product (9) had retained chirality during the
pinacol rearrangement or had been converted to a race-
mic mixture. The former would be the expected outcome
of a concerted type of reaction with inversion of stereo-
chemistry at the carbon atom bearing the hydroxyl
leaving group (i.e., C7 in Figure 1), whereas formation
of a free, stable carbonium ion intermediate at C7 would
result in the loss of chirality at this atom.
Initially, assignment of the correct space group could
not be readily ascertained; the structure solution and
refinement of aldehyde 9 could be accomplished in both
the P1 and P1h space groups and the final R value was
nearly identical in both cases. However, in the case of
(10) (a) Ohsumi, K.; Nakagawa, R.; Yumiko, F.; Hatanaka, T.;
Morinaga, Y.; Nihei, Y.; Ohishi, K.; Suga, Y.; Akiyama, Y.; Tsuji, T. J .
Med. Chem. 1998, 41, 3022. (b) Medarde, M.; Ramos, A.; Caballero,
E.; Pelaez-Lamamie de Clairac, R.; Lopez, J . L.; Gravalos, D. G.;
Feliciano, A. S. Eur. J . Med. Chem. 1998, 33, 71.
(11) Pettit, G. R.; Lippert, J . W., III. Anti-Cancer Drug Design 2000,
in press.
(12) (a) Pettit, G. R.; Toki, B.; Herald, D. L.; Verdier-Pinard, P.;
Boyd, M. R.; Hamel, E.; Pettit, R. K. J . Med. Chem. 1998, 41, 1688.
(b) Pettit, G. R.; Grealish, M. P.; Herald, D. L.; Boyd, M. R.; Hamel,
E.; Pettit, R. K. J . of Med. Chem. 2000, in press.
(13) (a) Swindell, C. S.; Fan, W. M. J . Org. Chem. 1996, 61, 1109.
(b) McKillop, A.; Taylor, R. J . K.; Watson, R. J .; Lewis, N. Synthesis
1994, 31. (c) Hoang, M.; Gadosy, T.; Ghazi, H.; Hou, D. F.; Hopkinson,
A. C.; J ohnston, L. J .; Lee-Ruff, E. J . Org. Chem. 1998, 63, 7168.
To further examine structural requirements involving
the ethylene cis-bridging of the combretastatin A-series
(9) (a) Pettit, G. R.; Toki, B. E.; Herald, D. L.; Boyd, M. R.; Hamel,
E.; Pettit, R. K.; Chapuis, J . C. J . Med. Chem. 1999, 42, 1459. (b) Pettit,
G. R.; Lippert, J . W. III; Herald, D. L.; Pettit, R. K. J . Nat. Prod., in
press. (c) Pettit, G. R.; Lippert, J . W., III; Boyd, M. R.; Hamel, E.; Pettit,
R. K. Anti-Cancer Drug Design, submitted.

7440 J . Org. Chem., Vol. 65, No. 22, 2000
Pettit et al.
Sch em e 1
material, (R,R)-diol 8, was +46° (c 1.24, CHCl3)]9b again
suggesting that a 1:1 enantiomeric ratio (racemic mix-
ture) had been formed in the pinacol reaction.
F igu r e 1. Computer-generated perspective of one enantiomer
of (()-diphenylacetaldehyde derivative (9) (H atoms and
disorder not shown).
Optimization of the pinacol rearrangement13 was ac-
complished through the use of two equivalents of the
Lewis acid in tetrahydrofuran to afford racemic aldehyde
9, in 68% yield. While stereospecific 1,2-rearrangements
for pinacol type reactions using carefully controlled
conditions with a Lewis acid have been reported,14 under
the present reaction conditions (Scheme 1) the 1,2-aryl
shift occurs equally above and below the plane of car-
bocation 10, thus generating racemic aldehyde 9.
the P1 space group, SHELXL17 refinement suggested that
the data was twinned and that the structure should be
treated as a racemic twin, present in the approximate
ratio of 1:1. Since these conditions are the same as those
met by assuming that the crystal is really a racemic
mixture of the same molecule in the P1h space group, the
latter was chosen as being the correct space group
assignment. A final conventional residual index R1 of
0.113 was obtained for the refinement of aldehyde 9 in
P1h space group. The rather poor R value obtained, even
for an ambient temperature (26 °C) data collection, could
be attributed in part to the obvious disorder exhibited
by the OMe groups, as well as the higher than average
residual electron density (+0.73 e/Å3 and -0.75 e/Å3) still
remaining in the final refinement model. The major
proportion of this residual density, however, could be
attributed to the Si atoms themselves (e.g., bond dis-
tances of e1.0 Å to Si atoms), indicating that other,
unaccounted for atoms (such as solvent) were not present.
The unit cell of the X-ray structure was determined to
contain a 1:1 ratio of the two enantiomers of the alde-
hyde, one enantiomer of which is shown in Figure 1. The
presence of a racemic mixture implies that all optical
activity was lost in the pinacol reaction, presumably
owing to the formation of a carbonium ion intermediate
leading to aldehyde 9. Such a conclusion was supported
by the fact that no measurable optical rotation could be
observed for a solution of the pinacol reaction product in
chloroform [the rotation of the optically active starting
Diphenylacetaldehyde 9 was converted to the crystal-
line derivatives shown in Scheme 2. (E)- and (Z)-oximes
11 and 12 (∼3:1) were both obtained owing to the
sterically small oxime hydroxyl group. However, (Z)-
oxime 12 proved to be unstable upon isolation and was
readily converted to the (E)-isomer during crystallization.
Thiosemicarbazone 13, 2,4-dinitrophenylhydrazone 14,
and semicarbazone 15 were all isolated as their (E)-
isomers in good yield. Attempts at cleavage of the silyl
protecting group from derivatives 11-15 afforded mix-
tures15 and were not further pursued. However, upon
desilylation of diphenylacetaldehyde 9 using tetrabuty-
lammonium fluoride (TBAF) another unexpected reaction
occurred. Benzophenone 2b was consistently obtained in
yields greater than 45%. Loss of the aldehyde group was
established by NMR analyses. Mass spectral analysis
indicated that a CH2 unit had been lost in addition to
the silyl groups, and a conjugated carbonyl absorption
at 1631 cm-1 was seen in the IR spectrum. In addition,
the physical properties exhibited by this new product led
us to believe that it was identical to that of a compound
previously prepared in our laboratories,12b designated
hydroxyphenstatin. We anticipated that confirmation of
its identity could most readily be accomplished by the
simple comparison of cell parameters of the new product
with those of the previously prepared, authentic sample
of hydroxyphenstatin (for which a complete X-ray crystal
structure determination had already been performed).
Since both samples had been crystallized from the same
solvent (CH3OH), it was assumed they would adopt the
same crystalline habit and have identical cell parameters.
Surprisingly, this was not the case. Instead, the newly
prepared hydroxyphenstatin 2b crystallized in space
group Pbca, assuming nearly twice the cell volume as
(14) (a) Suzuki, K. J . Synth. Org. Chem. J pn 1988, 365, 365. (b)
Suzuki, K.; Katayama, E.; Tsuchihashi, G. Tetrahedron Lett. 1983, 24,
4997; 1984, 25, 1817. (c) Tsuchihaski, G.; Tomooka, K.; Suzuki, K.
Tetrahedron Lett. 1984, 25, 4253. (d) Suzuki, K.; Tomooka, K.;
Shimczaki, M.; Tsuchihashi, G. Tetrahedron Lett. 1985, 26, 4781.
(15) (a) Huffman, J . W.; Elliot, R. P. Chem. Ind. 1963, 650. (b)
Bornstein, J .; J oseph, M. A.; Shields, J . E. J . Org. Chem. 1965, 20,
801. (c) Walling, C. Free Radicals in Solution; J ohn Wiley and Sons:
New York, 1957; pp 398-466.
(16) (a) Haines, A. H. In Methods for Oxidation of Organic Com-
pounds; Academic Press: New York, 1988; Vol. 2, pp 305-323, 438-
447. (b) Sinhababu, A. K.; Kawase, M.; Borchardt, R. T. Synthesis 1988,
710. (c) Nelson, T. D.; Crouch, D. Synthesis 1996, 1031.
(17) Olah, G. A.; Kuhn, S. J . J . Am. Chem. Soc. 1960, 82, 2381.

Synthetic Route to Hydroxyphenstatin
J . Org. Chem., Vol. 65, No. 22, 2000 7441
Sch em e 2a
that noted for the previously prepared, authentic sample
of hydroxyphenstatin, which had crystallized in space
group Pc from the same solvent. Since such a phenom-
enon is an extremely unusual event, complete data
collection and structure solution was required on the new
sample of hydroxyphenstatin, to positively confirm its
identity. An X-ray structural model of the results is
shown in Figure 2a, along with a comparative view of
the previously obtained hydroxyphenstatin X-ray struc-
tural results in Figure 2b. Comparison of parts a and b
of Figure 2 shows the slight conformational differences
assumed by the p-O-methyl substituent in the tri-
methoxybenzene rings of the two structures, thus ex-
plaining the difference in crystalline habit adopted by
each of these two different crystallization specimens of
hydroxyphenstatin 2b.
Authentication of the new compound as hydroxyphen-
statin confirmed that a base-catalyzed autoxidative de-
carbonylation (Scheme 3) had occurred. This is analogous
to the reaction of diphenylacetaldehyde to yield ben-
zophenone, as reported in the literature.16 Initiation of
the chain reaction leading to ketone 2b would appear to
begin with formation of free radical 16. The conjugation
to the two aryl rings and carbonyl group would allow
favorable delocalization and hence stabilization of the
radical. Propagation of radical 16 through reaction with
molecular oxygen would then afford radical 17. Although
this reaction was performed under argon and anhydrous
conditions, one can assume the tetrahydrofuran solvent
was the source of oxygen owing to the solubility of oxygen
in tetrahydrofuran and peroxide formation. Radical
propagation would continue when radical 17 abstracts a
hydrogen from another molecule of diphenylacetaldehyde
9, to yield radical 16 and hydroperoxide 18. Finally,
termination of radical propagation would occur when
hydroperoxide 18 reacts with a fluoride ion (from 2.2
equivalents of TBAF) at the carbonyl carbon on aldehyde
18, thereby eliminating formyl fluoride.17
In summary, trans-stilbene 3a was converted (overall
33% yield) into the strong cancer cell growth inhibitory
and antimitotic agent hydroxyphenstatin, 2b, via a
pinacol rearrangement and base-catalyzed autooxidative
decarbonylation sequence.
Exp er im en ta l Section
Gen er a l Exp er im en ta l P r oced u r es. Methanol (CH3OH),
ethyl acetate (EtOAc), hexane, and dichloromethane (DCM)
were distilled prior to use. TBAF refers to tetrabutylammo-
nium fluoride. Thiosemicarbazide and sodium chloride were
purchased from the Fisher-Acros Chemical Co., sodium hy-
droxide was purchased from VWR Scientific, hydroxylamine
hydrochloride was purchased from the Eastman Chemical Co.,
2,4-dinitrophenylhydrazine, semicarbazide hydrochloride, and
hydrochloric acid were purchased from the J . T. Baker Chemi-
cal Co., anhydrous sodium acetate was purchased from the
Mallinckrodt Chemical Co., and all other reagents were
purchased from the Sigma-Aldrich Chemical Co. Solvent
extracts of aqueous solutions were dried over anhydrous
sodium sulfate. Column chromatography was performed using
either flash silica (230-400 mesh ASTM) or gravity silica (70-
230 mesh ASTM) from E. Merck. Analtech silica gel GHLF
plates were used for TLC. All compounds were visible with
fluorescent short-wave light (254 nm).
Melting points are uncorrected. Elemental analyses were
determined by Galbraith Laboratories, Inc., Knoxville, TN.
(()-2-(2′,3′-Di[(ter t-bu tyldim eth ylsilyl)oxy]-4′-m eth oxy-
p h en yl)-2-(3,4,5-tr im eth oxyp h en yl)a ceta ld eh yd e (9). In i-
tia l Syn th esis. BF3‚OEt2 (0.10 mL; 0.81 mmol; 4 equiv) was
added dropwise to a stirred solution of (R,R)-diol 8 (0.11 g;
0.18 mmol) and 2,2-DMP (0.19 mL; 1.5 mmol; 8 equiv) in
acetone (2 mL) under argon at room temperature. After 1 h,
saturated NaHCO3 (aq) was added to the mixture before
extraction with DCM (4 × 5 mL) and drying of the combined

7442 J . Org. Chem., Vol. 65, No. 22, 2000
Pettit et al.
reflection data set using the XPREP18 program indicated the
space group was either P1 or P1h. Subsequent structure
refinements allowed the final correct assignment of the space
group as P1h. Crystal data: C30H48O7Si2, a ) 7.787(2) Å, b )
11.379(2) Å, c ) 19.791(4) Å, R ) 94.43(3)°, â ) 100.55(3)°, γ
) 98.95(3)°, V ) 1693.0(6) Å3, λ (Cu KR) ) 1.541 78 Å, Fc )
1.132 g cm-3 for Z ) 2 and FW ) 576.86, F(000) ) 624. After
Lorentz and polarization corrections, merging of equivalent
reflections and rejection of systematic absences, 5594 unique
reflections (R(int) ) 0.0417) remained, of which 4292 were
considered observed (Io > 2σ(Io)) and were used in the
subsequent structure solution and refinement. Linear and
anisotropic decay corrections were applied to the intensity data
as well as an empirical absorption correction (based on a series
of psi-scans).19 Structure determination was accomplished with
SHELXS.18 All non-hydrogen atoms for 9 were located using
the default settings of that program. The remaining hydrogen
atom coordinates were calculated at optimum positions using
the SHELXTL18 software suite . The latter atoms were
assigned thermal parameters equal to either 1.2 or 1.5
(depending upon chemical type) of the Uiso value of the atom
to which they were attached and then both coordinates and
thermal values were forced to ride that atom during final cycles
of refinement. All non-hydrogen atoms were refined anisotro-
pically in a full-matrix least-squares refinement process with
SHELXL.18 Disorder was noted for several of the methoxyl
groups, with splitting occurring for the O3, O4, C26, and C27
atoms. During final anisotropic refinement, these atoms were
treated as if they were each residing over two, separate, half-
occupied sites. The final standard residual R1 value for the
model shown in Figure 1 was 0.1126 (for observed data) and
0.1291 (for all data). The corresponding Sheldrick R values
were wR2 of 0.2973 and 0.3182, respectively. The difference
Fourier map exhibited some residual electron density; the
largest difference peak and hole being +0.727 and -0.752 e/Å3
, respectively. However, nearly all of this residual density sites
were proximate (∼1.0 Å) to, and attributed to, the Si atoms.
The unit cell was found to contain two molecules of the parent
molecule, related as enantiomeric mirror images. As a conse-
quence, the diphenylacetaldehyde derivative 9 existed as a
racemic mixture.
F igu r e 2. (a) X-ray crystal structure conformation of hydroxy-
phenstatin 2b, showing contents in an asymmetric unit of the
cell, for space group Pbca. (b) X-ray crystal structure confor-
mation of hydroxyphenstatin 2b, showing one of the two
independent molecules present in an asymmetric unit of the
cell, for space group Pc.
organics. Evaporation of the solvent in vacuo afforded a light
brown oil, which was subject to flash column chromatography
(15:1 hexanes-EtOAc) to generate a clear oil that crystallized
from CH3OH as a colorless solid (0.040 g; 39%): mp 103-104
°C; Rf 0.15 (10:1, hexanes-EtOAc); EIMS m/z 576 (M+, 20%);
IR (film) νmax 1726 cm-1 [HC(O)R]; 1H NMR (300 MHz, CDCl3)
δ 0.11 (6H, s), 0.16 (6H, s), 0.96 (9H, s), 1.03 (9H, s), 3.77
(6H, s), 3.78 (3H, s), 3.84 (3H, s), 5.24 (1H, d, J ) 1.8 Hz),
6.34 (2H, s), 6.56 (1H, d, J ) 8.1 Hz), 6.64 (1H, d, J ) 8.7 Hz),
9.84 (1H, d, J ) 1.5 Hz). Anal. Calcd for C30H48O7Si2: C, 62.46;
H, 8.39. Found: C, 62.14; H, 8.61. Op t im ized Syn t h esis.
BF3‚OEt2 (0.82 mL; 6.7 mmol; 2 equiv) was added dropwise
to a stirred solution of (R,R)-diol 8 and (1S,2S)-1-[2′,3′-
di[(tert-butyldimethylsilyl)oxy]-4′methoxyphenyl]-2-(3,4,5-tri-
methoxyphenyl)ethane-1,2-diol (2.1 g; 3.3 mmol) in anhydrous
THF (20 mL) under argon at room temperature. After 1 h,
saturated NaHCO3 (aq) was added to the mixture before
extraction with EtOAc (4 × 15 mL) and drying of the combined
organics. Evaporation of the solvent in vacuo afforded a light
brown oil, which was subjected to flash column chromatogra-
phy (15:1 hexanes-EtOAc) to generate a clear oil that crystal-
lized from CH3OH as a colorless solid (1.3 g; 68%) that was
spectroscopically identical to product afforded from the above
procedure.
2′,3′-Dih yd r oxy-3,4,4′,5-t et r a m et h oxyb en zop h en on e
(Hyd r oxyp h en sta tin ) (2b). TBAF (0.88 mL; 0.88 mmol; 2.2
equiv; 1 M in THF solution) was added to a stirred solution of
diphenylacetaldehyde 9 (0.23 g; 0.40 mmol) in anhydrous THF
(5 mL) under argon. After 20 min, ice-cold 2 N HCl was added,
and the product was extracted with EtOAc (4 × 25 mL). The
combined extract was washed with saturated NaCl (aq) and
dried. Evaporation of the solvents in vacuo resulted in a light
brown oil, which was subjected to column chromatography (50:
50:1 hexane/EtOAc/AcOH) to afford a yellow oil that was
recrystallized from CH3OH as a yellow solid (65 mg; 49%): mp
178-179 °C; Rf 0.69 (hexanes-EtOAc-AcOH, 50:50:1); EIMS
m/z 334 (M+, 40%); IR (film) νmax 1631 cm-1 [ArC(O)Ar]; 1H
NMR (300 MHz, CDCl3) δ 3.90 (6H, s), 3.94 (3H, s), 3.99 (3H,
s), 5.56 (1H, s), 6.51 (1H, d, J ) 9.0 Hz), 6.92 (2H, s), 7.26
(1H, d, J ) 9.3 Hz), 12.22 (1H, s); 13C NMR (75 MHz, CDCl3)
δ 199.59, 152.92, 152.09, 150.93, 141.33, 133.66, 133.12,
125.59, 114.02, 106.79, 102.55, 60.98, 56.32, 56.24. Anal. Calcd
for C17H18O7: C, 61.08; H, 5.43. Found: C, 61.00; H, 5.54.
X-r ay Cr ystal Str u ctu r e Deter m in ation . Hydr oxyph en -
sta tin , 2b. Colorless crystals of this compound were obtained
from methanol solution. A specimen, with approximate dimen-
sions of 0.54 × 0.44 × 0.18 mm, was mounted on the tip of a
glass fiber with Super Glue. Data collection was performed at
X-r a y Cr ysta l Str u ctu r e Deter m in a tion . Dip h en yla c-
eta ld eh yd e Silyl d er iva tive 9. Very large, colorless crystals
of this compound were obtained from hexane solution. A
crystalline specimen, with approximate dimensions of 0.42 ×
0.40 × 0.10 mm, was obtained by cleavage from a larger crystal
and mounted on the tip of a glass fiber with Super Glue. Data
collection was performed at 26 ( 1° for a triclinic system, with
all reflections corresponding to slightly more than a complete
hemisphere (2θ e 130°) being measured using an ω/2θ scan
technique. Subsequent statistical analysis of the complete
(18) SHELXTL, Version 5.101, 1998, an integrated software system
for the determination of crystal structures from diffraction data, Bruker
Analytical X-ray Systems, Inc., Madison, WI 53719. This package
includes, among others: XPREP, an automatic space group determi-
nation program; XS, the Bruker SHELXS module for the solution of
X-ray crystal structures from diffraction data; XL, the Bruker SHELXL
module for structure refinement; XP, the Bruker interactive graphics
display module for crystal structures.
(19) North, A. C.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351-359.

Synthetic Route to Hydroxyphenstatin
J . Org. Chem., Vol. 65, No. 22, 2000 7443
Sch em e 3
23 ( 1° for an orthorhombic system, with all reflections
corresponding to slightly more than a octant of data (2θ e
130°) being measured using an ω/2θ scan technique. Subse-
quent statistical analysis of the complete reflection data set
using the XPREP18 program indicated the space group was
Pbca. Crystal data: C17H18O7, a ) 10.807(2) Å, b ) 14.630(3)
Å, c ) 19.876(4) Å, V ) 3142.5(11) Å3, λ (Cu KR) ) 1.541 78
Å, Fc ) 1.413 g cm-3 for Z ) 8 and FW ) 334.31, F(000) )
1408. After Lorentz and polarization corrections, merging of
equivalent reflections and rejection of systematic absences,
2650 unique reflections (R(int) ) 0.0339) remained, of which
2381 were considered observed (Io > 2σ(Io)) and were used in
the subsequent structure solution and refinement. Linear and
anisotropic decay corrections were applied to the intensity data
as well as an empirical absorption correction (based on a series
of psi-scans).19 Structure determination was accomplished with
SHELXS.18 All non-hydrogen atoms for 2b were located using
the default settings of that program. The remaining hydrogen
atom coordinates were calculated at optimum positions using
the SHELXTL18 software suite. The latter atoms were assigned
thermal parameters equal to either 1.2 or 1.5 (depending upon
chemical type) of the Uiso value of the atom to which they were
attached and then both coordinates and thermal values were
forced to ride that atom during final cycles of refinement. All
non-hydrogen atoms were refined anisotropically in a full-
matrix least-squares refinement process with SHELXL.18 The
final standard residual R1 value for the model shown in Figure
2a was 0.0738 (for observed data) and 0.0783 (for all data).
The corresponding Sheldrick R values were wR2 of 0.1836 and
0.1891, respectively. The largest difference peak and hole in
the final difference Fourier map were +0.551 and -0.377 e/Å3
, respectively. All bond distances and angles were within
generally accepted limits.
m/z 591 (M+, 10); IR (film) νmax 3450 cm-1 (OH); 1H NMR (300
MHz, CDCl3) δ 0.05 (3H, s), 0.13 (6H, s), 0.17 (3H, s), 0.95
(9H, s), 1.01 (9H, s), 3.74 (6H, s), 3.77 (3H, s), 3.82 (3H, s),
5.23 (1H, d, J ) 6.6 Hz), 6.32 (2H, s), 6.53 (1H, d, J ) 8.7 Hz),
6.69 (1H, d, J ) 8.1 Hz), 7.66 (1H, s), 7.74 (1H, d, J ) 6.6 Hz).
Anal. Calcd for C30H49O7Si2N: C, 60.88; H, 8.35; N, 2.37.
Found: C, 61.16; H, 8.44; N, 2.36. Compound 12 [(Z)-oxime,
87 mg; 17%): mp 162-163 °C; Rf 0.54 (7:3, hexanes-EtOAc);
EIMS m/z 591 (M+, 10); IR (film) νmax 3439 cm-1 (OH); 1H NMR
(300 MHz, CDCl3) δ 0.06 (3H, s), 0.09 (6H, s), 0.15 (3H, s),
0.20 (3H, s), 0.96 (9H, s), 1.02 (9H, s), 3.74 (6H, s), 3.76 (3H,
s), 3.80 (3H, s), 5.85 (1H, d, J ) 6.6 Hz), 6.36 (2H, s), 6.53
(1H, d, J ) 8.7 Hz), 6.71 (1H, d, J ) 8.1 Hz), 7.13 (1H, d, J )
7.2 Hz), 8.04 (1H, brs). Anal. Calcd for C30H49O7Si2N.1/2H2O:
C, 59.97; H, 8.39; N, 2.33. Found: C, 60.09; H, 8.30; N,
2.40.
Th iosem ica r ba zon e 13. Aldehyde 9 (0.10 g; 0.17 mmol)
was added to a solution of thiosemicarbazide (16 mg; 0.18
mmol; 1 equiv) in H2O (2 mL) and CH3OH (3 mL). The mixture
was heated for 15 min and stirred for an additional 6 h before
the reaction mixture was concentrated in vacuo and the
product recrystallized from H2O-CH3OH to afford a colorless
solid (91 mg; 81%): mp 160-161 °C; Rf 0.33 (7:3, hexanes-
EtOAc); EIMS m/z 649 (M+, 10); IR (film) νmax 1452 cm-1 [Cd
S]; 1H NMR (300 MHz, CDCl3) δ 0.09 (3H, s), 0.10 (3H, s),
0.16 (3H, s), 0.16 (3H, s), 0.95 (9H, s), 1.02 (9H, s), 3.76 (6H,
s), 3.77 (3H, s), 3.83 (3H, s), 5.26 (1H, d, J ) 6.3 Hz), 6.14
(1H, brs), 6.29 (2H, s), 6.51 (1H, d, J ) 8.7 Hz), 6.57 (1H, d, J
) 9.0 Hz), 6.91 (1H, brs), 7.41 (1H, d, J ) 6.0 Hz), 8.91 (1H,
brs). Anal. Calcd. for C31H51O6Si2N3S: C, 57.28; H, 7.91; N,
6.46. Found: C, 57.55; H, 8.02; N, 6.40.
2,4-Din itr op h en ylh yd r a zon e 14. Aldehyde 9 (99 mg; 0.17
mmol) was added to a solution of 2,4-dinitrophenylhydrazine
(35 mg; 0.17 mmol; 1 equiv) in CH3OH (5 mL) and concen-
trated HCl (0.50 mL), that had been previously heated and
filtered. The mixture was stirred for 6 h before the reaction
mixture was concentrated in vacuo and the product recrystal-
lized from EtOH to afford a yellow crystalline solid (69 mg;
54%): mp 190-191 °C; Rf 0.71 (7:3, hexanes-EtOAc); EIMS
m/z 757 (M+, 5); IR (film) νmax 1504 cm-1 (NO2); 1H NMR (300
MHz, CDCl3) δ 0.09 (3H, s), 0.14 (3H, s), 0.21 (6H, s), 0.96
(9H, s), 1.08 (9H, s), 3.78 (6H, s), 3.80 (3H, s), 3.85 (3H, s),
5.24 (1H, d, J ) 6.5 Hz), 6.38 (2H, s), 6.58 (1H, d, J ) 9.0 Hz),
Oxim es 11 a n d 12. Aldehyde 9 (0.50 g; 0.87 mmol) was
added to a mixture of hydroxylamine hydrochloride (1.0 g; 42
mmol; 48 equiv) and sodium acetate (2.0 g, 25 mmol; 28 equiv)
in H2O (10 mL) and CH3OH (20 mL). The mixture was heated
for 30 min and stirred for an additional 6 h before being
absorbed onto silica gel and subjected to flash column chro-
matography (10:1 hexanes-EtOAc) to afford two products as
clear oils that each crystallized from H2O-CH3OH as a
colorless crystalline solid. Compound 11 [(E)-oxime, 0.31 g;
60%]: mp 171-172 °C; Rf 0.77 (7:3, hexanes-EtOAc); EIMS

7444 J . Org. Chem., Vol. 65, No. 22, 2000
Pettit et al.
6.71 (1H, d, J ) 8.5 Hz), 7.80 (1H, d, J ) 6.5 Hz), 7.84 (1H, d,
J ) 9.0 Hz), 8.29 (1H, d, J ) 9.5 Hz), 9.12 (1H, s), 11.13 (1H,
s). Anal. Calcd for C36H52O10Si2N4: C, 57.12; H, 6.92; N, 7.40.
Found: C, 56.85; H, 6.95; N, 7.25.
Ack n ow led gm en t. We are pleased to thank for
support of this research Outstanding Investigator Grant
CA 44344-07-11 with the Division of Cancer Treatment
and Diagnosis, NCI, DHHS; the Arizona Disease Con-
trol Research Commission; Gary L. and Diane R.
Tooker; Captain George Brereton; the Caitlin Robb
Foundation; Dr. J ohn C. Budzinski; Robert and Sharon
Witzer; and the Robert B. Dalton Endowment. For other
assistance, we thank Drs. Cherry L. Herald, Fiona
Hogan, J ames McNulty, and Michael D. Williams, as
well as Laura Crews and Lee Williams.
Sem ica r ba zon e 15. Aldehyde 9 (0.50 g; 0.88 mmol) was
added to a mixture of semicarbazide hydrochloride (1.1 g; 9.7
mmol; 11 equiv) and sodium acetate (1.5 g, 18 mmol; 21 equiv)
in H2O (10 mL) and CH3OH (10 mL). The mixture was heated
for 15 min and stirred for an additional 30 min before the
reaction mixture was absorbed onto silica gel and subjected
to flash column chromatography (1:1 EtOAc-hexane) to afford
a clear oil that crystallized from H2O-CH3OH as a colorless
crystalline solid (442 mg; 79%): mp 145-146 °C; Rf 0.09 (1:1,
hexanes-EtOAc); EIMS m/z 633 (M+, 10); IR (film) νmax
1691 cm-1 [C(O)NH2]; 1H NMR (300 MHz, CD3OD) δ 0.11 (3H,
s), 0.18 (3H, s), 0.21 (3H, s), 0.25 (3H, s), 1.00 (9H, s), 1.09
(9H, s), 3.76 (6H, s), 3.77 (3H, s), 3.81 (3H, s), 5.27 (1H, d, J )
6.6 Hz), 6.42 (2H, s), 6.70 (1H, d, J ) 8.7 Hz), 6.77 (1H, d, J
) 8.7 Hz), 7.61 (1H, d, J ) 7.2 Hz). Anal. Calcd for C31H51O7-
Si2N3: C, 58.73; H, 8.10; N, 6.62. Found: C, 58.64; H, 8.27; N,
6.66.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, bond lengths and angles, atomic coordinates,
and anisotropic thermal parameters are available for struc-
tures 9 and 2b. This material is available free of charge via
the Internet at http://pubs.acs.org.
J O000705J