Antineoplastic agents. 565. Synthesis of combretastatin D-2 phosphate and dihydro-combretastatin D-2

Article abstract of DOI:10.1021/np800635h

A modified synthetic route to combretastatin D-2 (5) was devised in order to further evaluate its biological activity, for its conversion to phosphate prodrugs (25-28), and as a route to obtaining dihydro-combretastatin D-2 (42). A parallel first total synthesis of dihydro-combretastatin D-2 was completed, proceeding from a saturated 3-phenylpropionic ester intermediate via the Ullmann biaryl ether reaction (39-41). In contrast to the cancer cell growth inhibitory activity exhibited by combretastatin D-2, relatively minor structural modifications (41, 42) caused elimination of those properties.

Full text of DOI:10.1021/np800635h

876
J. Nat. Prod. 2009, 72, 876–883
Antineoplastic Agents. 565. Synthesis of Combretastatin D-2 Phosphate and
Dihydro-combretastatin D-21
George R. Pettit,*,† Peter D. Quistorf,† Jeremy A. Fry,† Delbert L. Herald,† Ernest Hamel,‡ and Jean-Charles Chapuis†
Cancer Research Institute and Department of Chemistry and Biochemistry, Arizona State UniVersity, P.O. Box 871604,
Tempe, Arizona 85287-1604, and DTP, DCTC, NCI, P.O. Box B, Building 469, Room 104, Frederick, Maryland 21702-1201
ReceiVed October 10, 2008
A modified synthetic route to combretastatin D-2 (5) was devised in order to further evaluate its biological activity, for
its conversion to phosphate prodrugs (25-28), and as a route to obtaining dihydro-combretastatin D-2 (42). A parallel
first total synthesis of dihydro-combretastatin D-2 was completed, proceeding from a saturated 3-phenylpropionic ester
intermediate via the Ullmann biaryl ether reaction (39-41). In contrast to the cancer cell growth inhibitory activity
exhibited by combretastatin D-2, relatively minor structural modifications (41, 42) caused elimination of those properties.
The naturally occurring series of cancer cell growth inhibitors
designated combretastatins was isolated from the South African
bush willow tree Combretum caffrum.2 We reported the first of
the series, designated combretastatin (1), in 1982.3,4 Bibenzyl 1
inhibited growth of the murine P388 lymphocytic leukemia cell
line (ED50 1.1 × 10-2 µg/mL), inhibited tubulin polymerization
(IC50 5-7 µM), and competitively inhibited the binding of
colchicine (2) to tubulin.3,4 Subsequently, we found the C. caffrum
tree to contain a series of stilbenes designated the combretastatin
A series,5-7 and two of these, combretastatins A-1 (3a)5 and A-4
(3d),7 exhibited substantial promise on the basis of antineoplastic
activity and inhibition of tubulin polymerization. Owing to the
remarkable cancer vascular targeting activity of combretastatin A-4
(3d, P388 ED50 3.4 × 10-3 µg/mL),7 the phosphate prodrug
(CA4P)2,8a of cis-stilbene 3d has been undergoing extended clinical
trials8b in the United States and Europe. Those advances toward
improving human cancer treatments have recently been augmented
by initiation of the phase 1 human cancer clinical trial of
combretastatin A-1 (3a, aka OXI4503) phosphate prodrug (CA1P).2,8c
Two macrocyclic lactones, combretastatin D-1 (4)9 and com-
bretastatin D-2 (5),10 were also isolated from C. caffrum. Com-
bretastatin D-1 (4, P388 ED50 3.3 µg/mL)9 possesses an epoxide
ring, while the D-2 (5) contains the corresponding cis-olefin and
exhibits a P388 ED50 of 5.2 µg/mL.10 Since the initial cancer cell
growth inhibitory activities of the D-series did not seem impressive
when compared to the A-series, development of useful syntheses
to provide enough for further biological evaluations was placed at
a lower priority than that of the A-series. We reassessed the priority
in 1999 when it was reported11 that the D-series exhibits a different
mode of action from the A-series during cell division. The A-series
combretastatins bind to the colchicine binding site on tubulin and
prevent microtubule assembly (destabilize), which ultimately
prevents cell division. Combretastatins D-1 (CD1) and D-2 (CD2)
were shown to allow assembly of the microtubules (stabilize) but
not allow the microtubules to disassemble later in cell division.11
That led us to reinvestigate the potential of combretastatin D-2 (5)
by first completing a useful synthesis, followed by conversion to a
phosphate prodrug to increase aqueous solubility/transport and
reduction to the dihydro derivative for SAR purposes, and to
examine the tubulin stabilization effects. At this point in 1999, six
syntheses of lactone 5 had already been reported,12a-h and it was
clear from these studies, and others, that cyclization of biaryl ethers
to form relatively small macrocyclic lactones is a challenge and
usually results in low yields.
Results and Discussion
As was previously demonstrated,12 combretastatin D-2 (5) was,
surprisingly, a synthetic challenge. One of the initial goals in
developing a new synthesis of CD2 was to reduce the number of
synthetic steps while optimizing the yield of the ring closure
reaction, a macrocyclization using the Ullmann ether synthesis
(Figure 1). A benzyl protecting group for the phenol was utilized
in hopes of improving the overall yield from that achieved with
the methyl protection used in previous syntheses, when regeneration
of the phenol group of CD2 proved problematic.12a
Commercially available 3,4-dihydroxybenzaldehyde (6, Scheme
1) was selectively protected at the 4-position to afford in good yield
aldehyde 7,13 which was then subjected to a Wittig elongation to
afford R,ꢀ-unsaturated ester 9.12g,h,14 Hydrogenation of the olefin
* Corresponding author. Tel: 480-965-3351. Fax: 480-965-8558. E-mail:
bpettit@asu.edu.
† Cancer Research Institute, Arizona State University.
‡ National Cancer Institute, Frederick, MD.
10.1021/np800635h CCC: $40.75  2009 American Chemical Society and American Society of Pharmacognosy
Published on Web 03/25/2009

Synthesis of Combretastatin D-2 Phosphate
Journal of Natural Products, 2009, Vol. 72, No. 5 877
Scheme 2. Synthesis of Allylic Alcohol 19
Figure 1. Retrosynthesis of combretastatin D-2.
Scheme 1. Synthesis of 3-Phenylpropionic Acid Derivatives
synthetic strategy was implemented owing to the moderate yields
and the difficulty in separation of the cis-olefin from the trans-
isomer and large quantities of benzil. Employment of the
Still-Gennari modification of the Wadsworth-Horner-Emmons
reaction18 led to synthesis of the cis-R,ꢀ-unsaturated ester 18 in
high yield and high stereoselectivity from 4-bromobenzaldehyde
(15) (Scheme 2). The Horner-Emmons reaction is the traditional
method for the synthesis of unsaturated esters, but it tends to form
the more stable trans-olefin. The Still-Gennari modification
employs electrophilic bistrifluoroethylphosphonoesters and strongly
dissociated base systems such as KHMDS and 18-crown-6 to
convert aldehydes to cis-unsaturated esters in high stereoselectiv-
ity.18 DIBAL reduction12a,h of the cis-R,ꢀ-unsaturated ester 18
provided propenol derivative 19 in high yield.
Upon construction of the two subunits of combretastatin D-2,
formation of the ester and biaryl ether was next undertaken. The
route of choice was to first use the Mitsunobu-type reaction for
ester formation, followed by the biaryl ether synthesis employing
Ullmann conditions. Carboxylic acid 13 or 14 and alcohol 19 were
coupled using Mitsunobu conditions19a-e to afford the new esters
20 and 21 (Scheme 3). Subsequent intramolecular biaryl ether
formation to give 22 and 23, respectively, was accomplished in
low yields (0-25%) utilizing conditions (varying equivalents of
CuBr-dimethyl sulfide complex and K2CO3 or methylcopper in
pyridine) outlined by Boger.12a The methyl protecting group was
removed from 23 using aluminum bromide and ethanethiol to afford
CD2 (5) in 19% yield.12d Removal of the benzyl group from 22
using hydrogenation was not practical in the presence of the olefin,
so instead neat trifluoroacetic acid was utilized.20 These conditions
removed the benzyl group but also opened the lactone at the ester
group, preventing good yields of CD2 (5) from being realized. The
structure of combretastatin D-2 was further confirmed by X-ray
crystal structure determination (Figure 2).
Once the synthesis of CD2 was achieved, a series of phosphate
salts was synthesized to investigate the effects of different cations
on both the anticancer activity of the prodrug (Table 1) and its
solubility characteristics (Table 2). Synthesis of the sodium
phosphate (25) and other prodrugs (26-28) resulted from first
treating phenol 5 with dibenzyl phosphite to yield the corresponding
bis(benzyl)phosphate. The benzyl protecting groups were cleaved
with bromotrimethylsilane, and subsequent treatment of the phos-
phoric acid with the appropriate cation precursor (NaOCH3, KOH,
LiOH, and morpholine, respectively) yielded prodrugs 25-28. As
recorded in Table 2, the prodrug salts had substantially increased
water solubility, which is very important for transport to metastatic
cancer,2,8a compared to the very sparingly soluble CD2. However,
presumably owing to lack of the phosphatases needed to regenerate
the drug in the isolated cancer cells, the prodrug salts did not
with 5% palladium on carbon in benzene provided saturated ester
11.12h The choice of solvent was vital to selectivity of the reduction.
Significant cleavage of the benzyl group resulted when ethanol was
the solvent choice, but yields of ester 11 improved by switching to
benzene. Magnesium in methanol12c,d is specifically used for the
reduction of R,ꢀ-unsaturated esters in the presence of other reducible
functionalities,15,16 but no reaction occurred on several attempts
with this method. Hydrolysis of ester 11 using NaOH in THF-water
afforded carboxylic acid 13 in nearly quantitative yield. The Wittig
elongation, olefin reduction, and ester hydrolysis were also con-
ducted on 3-hydroxy-4-methoxybenzaldehyde (8) to afford car-
boxylic acid 14 in high overall yield.
The initial synthesis of the allylic alcohol intermediate 19
proceeded through a photochemical isomerization of the trans-R,ꢀ-
unsaturated ester 16 to generate the necessary cis-olefin moiety
(Scheme 2). Commercially available 4-bromobenzaldehyde (15) was
subjected to Wittig elongation using the stabilized ylide (carbe-
thoxymethylene)triphenylphosphorane to yield the trans-R,ꢀ-
unsaturated ester 16,12h which was then photochemically isomerized
(254 nm, 450 W mercury lamp) to the cis-olefin 17 using benzil in
benzene.17 The cis-olefin was typically generated in 45-65% yields
and allowed for the recovery of the trans-isomer. A change in

878 Journal of Natural Products, 2009, Vol. 72, No. 5
Pettit et al.
Scheme 3. Synthesis of Combretastatin D-2 (5) and Prodrug
Salts
Table 1. Results of Human Cancer Cell Line (GI50, µg/mL) and
Murine P388 Lymphocytic Leukemia Cell Line Inhibitory
(ED50, µg/mL) Evaluations
structure no.
cell line
21
23
5
25
26
27
28
P388a
pancreas
breast
brain
lung
colon
>100
8.1
7.3
0.83
2.2
6.6
9.4
5.6
16.8
4.8
6.6
4.7
6.0
23.9
22.1
30.5
36.3
13.3
25.5
18.2
23.9
40.1
34.5
45.2
27.8
40.6
14.4
27.1
35.9
26.1
37.9
14.4
27.5
4.3
86.2
>10
>10
>10
>10
>10
>10
16.4
16.4
18.5
16.5
17.1
8.9
>10
2.7
prostate
a Compounds 20, 22, 32, and 40-42 led to ED50 > 100 and 33 gave
ED50 27.0 µg/mL.
Table 2. Solubility Comparison of Combretastatin D-2 and
Prodrugs in Water (mg/mL) at 25 °C
structure no.
mg/mL
5
0.5
>70
>50
20
25
26
27
28
5
Scheme 4. Attempted Synthesis of Nitro-combretastatin D-2
Benzyl and Methyl Ethers
enhance inhibition of the cancer cell line growth compared to
combretastatin D-2 (Table 1). Cancer tissue in vivo has greatly
increased concentrations of the necessary phosphatases for cleavage
of the prodrug ester bond.
The low yields of the intramolecular Ullmann ether synthesis
during cyclization and the inability to cyclize using Mitsunobu
conditions prompted a change in conditions for the synthesis of
CD2 (5). A review of the literature led to a significant volume of
work devoted to biaryl ether synthesis using 4-fluoro-3-nitrophenyl
derivatives.21a-k To explore this approach, commercially available
4-fluoro-3-nitrobenzaldehyde (29) was subjected to the Still-Gennari
modification of the Wadsworth-Horner-Emmons reaction18 to
afford the cis-R,ꢀ-unsaturated ester 30 in moderate yield (Scheme
4). DIBAL reduction of the ester provided allylic alcohol 31, which
was then subjected to Mitsunobu conditions in the presence of
carboxylic acids 13 and 14 to afford esters 32 and 33, respectively.
However, attempts to cyclize 32 and 33 utilizing different reaction
conditions failed (K2CO3, 18-crown-6, DMF; Cs2CO3, DMF; CsF,
DMF, Scheme 4). Although use of the nitroaryl moiety initially
Figure 2. X-ray crystal structure of combretastatin D-2 (5) with
50% thermal ellipsoids.

Synthesis of Combretastatin D-2 Phosphate
Journal of Natural Products, 2009, Vol. 72, No. 5 879
Scheme 6. Synthesis of Dihydro-combretastatin D-2 (42)
Scheme 5. Synthetic Routes to 3-Phenylpropyl Alcohol 38
Evaluation of combretastatin D-2 (5), dihydro-combretastatin D-2
(42), and their synthetic intermediates against the following
microorganisms did not reveal any significant antibacterial or
antifungal activity: Candida albicans (ATCC 90028), Cryptococcus
neoformans (ATCC 90112), Micrococcus luteus (Presque Isle 456),
Staphylococcus aureus (ATCC 29213), Streptococcus pneumoniae
(ATCC 6303), Escherichia coli (ATCC 25922), Stenotrophomonas
maltophilia (ATCC 13637), Enterobacter cloacae (ATCC 13047),
Enterococcus faecalis (ATCC 29212), and Neisseria gonorrhoeae
(ATCC 49226). In addition, both combretastatins D-1 (4) and D-2
(5) were found to be inactive (IC50 > 40 µM) as inhibitors of tubulin
assembly.5
looked promising for effecting ring closure and for obtaining higher
yields of CD2 (5) than the procedure outlined by Boger,12a the
cyclization step proved difficult owing to the presence of the cis-
double bond.
The difficulty of obtaining acceptable yields in the macrocy-
clization step leading to combretastatin D-2 led to examination of
the necessity of the cis-olefin group to maintain cancer cell growth
inhibition. It was expected that the ring closure would proceed in
much higher yields in the absence of the cis-olefin, as seen in
previous syntheses of combretastatin D-2,12d,f,h and that cyclization
could proceed with either an Ullmann ether synthesis or a
Mitsunobu ester formation. First, the trans-R,ꢀ-unsaturated ester
16 was hydrogenated using 5% palladium on carbon to yield ester
36 (Scheme 5). Hydrogenation of the olefin gave low yields owing
to partial cleavage of the bromide group, as ascertained by
spectroscopic analyses (mass spectrometry and IR). An alternative
pathway via reduction of the ester to allylic alcohol 37 was next
utilized. Hydrogenation of olefin 37 to produce saturated alcohol
38 also resulted in low yields, again owing to the removal of the
bromide group. However, a one-step reaction was discovered that
converted R,ꢀ-unsaturated ester 16 directly to alcohol 38 in good
yield, using sodium borohydride and polyethylene glycol (PEG
400).22 Next, propanol 38 and carboxylic acids 13 and 14 were
subjected to Mitsunobu esterification to afford esters 39 and 40,
respectively. Intramolecular biaryl ether synthesis from 39 using
Ullmann conditions afforded dihydro-combretastatin D-2 benzyl
ether (41, 11% yield, Scheme 6), although ester 40 failed to cyclize
under Ullmann conditions. Unfortunately, the yield of the macro-
lactonization applied to ester 39 was unexpectedly low, comparable
to yields obtained in the presence of the double bond. Subsequent
cleavage (hydrogenation) of the benzyl protecting group from 41
afforded dihydro-combretastatin D-2 (42) in good yield.
Experimental Section
General Experimental Procedures. DCM refers to dichlo-
romethane, DMF to dimethylformamide, THF to tetrahydrofuran,
DIBAL to diisobutylaluminum hydride, DEAD to diethyl azodicar-
boxylate, and KHMDS to potassium hexamethyldisilazane; CC refers
to silica gel column chromatography, and room temperature (rt) refers
to 22-25 °C. All solvents were anhydrous. Reactions that required
dry conditions were carried out under argon in flame-dried glassware
equipped with a magnetic stir bar and a rubber septum. Distilled water
was used in all aqueous solutions. Reactions were monitored by thin-
layer chromatography using Analtech silica gel GHLF uniplates
visualized with short-wavelength UV (254 nm) and stained with
phosphomolybdic acid solution. Organic extracts were dried with
anhydrous magnesium sulfate. Solvents for chromatography were
redistilled prior to use, and column chromatography was achieved with
either gravity (70-230 mesh ASTM) or flash (230-400 mesh ASTM)
silica from EM Science. Melting points are uncorrected and were
1
measured with a digital Electrothermal 9100 apparatus. The H and
13C NMR spectra were referenced to TMS or the deuterated solvent
(CDCl3). APT, HMQC, and HMBC techniques were utilized to assist
in peak assignments.
3-Hydroxy-4-benzyloxybenzaldehyde (7). To a stirred solution of
3,4-dihydroxybenzaldehyde (6) (2.32 g, 16.8 mmol) in anhydrous DMF
(84 mL) was added NaHCO3 (2.13 g, 25.4 mmol), benzyl chloride (3.8
mL, 33 mmol), and NaI (0.80 g, 5.3 mmol). The mixture was stirred
at 40 °C for 24 h. After cooling to rt, 1 N HCl (100 mL) was added
and the solution extracted with EtOAc (4 × 75 mL). The combined
organic extracts were washed with brine (2 × 75 mL), and the solvent
was removed in vacuo to yield a brown oil. Separation by gravity CC
(elution with 9:1-4:1 hexane-acetone) provided aldehyde 7 as a
colorless solid, which crystallized from EtOH as colorless needles (2.46
g, 64%): mp 120.9-122.9 °C (lit.13 mp 120-121 °C); spectroscopic
data identical to literature data.13
Although the absence of the olefin did not lead to an improved
yield of the biaryl ether synthesis, dihydro-CD2 (42) was important
for structure-activity relationship studies. Dihydro-CD2 (42) proved
to be inactive (>100 µg/mL, P388 cell line, Table 1), indicating
that the olefin was necessary for cancer cell growth inhibition by
combretastatin D-2 (2). The same lack of significant P388 cancer
cell line activity held for all the intermediates from the different
synthetic pathways utilized in the synthesis of combretastatin D-2.
However, very important SAR results were provided by these cancer
cell line evaluations and suggested that even some minor changes
in the CD1 and CD2 natural product’s structure can cause loss of
activity.
Ethyl 3-(3′-Hydroxy-4′-benzyloxyphenyl)-2E-propenoate (9). Al-
dehyde 7 (2.29 g, 10.0 mmol) was treated with (carbethoxymethy-
lene)triphenylphosphorane (4.51 g, 12.9 mmol) as described in the
literature,12h with toluene (80 mL) replacing benzene, to afford R,ꢀ-

880 Journal of Natural Products, 2009, Vol. 72, No. 5
Pettit et al.
unsaturated ester 9 as a clear oil that solidified upon further drying
under high vacuum. Recrystallization from EtOH provided a colorless,
fluffy solid (2.95 g, 99%): mp 83.0-84.7 °C (lit.12h mp 80-85 °C);
spectroscopic data identical to literature data.12h
3′, 5′, 6′); 13C NMR (100 MHz, CDCl3) δ 14.1 (OCH2CH3), 60.3
(OCH2CH3), 120.4 (2), 123.1 (4′), 131.0 (2′, 6′), 131.2 (3′, 5′), 133.5
(1′), 141.7 (3), 165.7 (1); HRMS (FAB+) m/z 257.0008, 255.0019 [M
+ H]+ (calcd for C11H12O2Br, 257.0000, 255.0021); anal. C 52.16%,
H 4.36%, calcd for C11H11O2Br, C 51.79%, H 4.35%.
Methyl 3-(4′-Bromophenyl)-2Z-propenoate (18). Treatment of 15
(2.31 g, 12.5 mmol) with bis(2,2,2-trifluoroethyl)(methoxycarbon-
ylmethyl)phosphonate (3.2 mL, 15 mmol), 18-crown-6 ether (13.63 g,
51.6 mmol), and KHMDS (0.5 M solution in toluene, 31 mL, 16 mmol)
as described in the literature23 yielded ester 18 as a colorless solid (2.84
g, 94%): mp 43.3-44.8 °C (lit.23 mp 40-42 °C).
Ethyl 3-(3′-Hydroxy-4′-methoxyphenyl)-2E-propenoate (10). A
solution of 3-hydroxy-4-methoxybenzaldehyde (8, 8.01 g, 52.6 mmol)
and (carbethoxymethylene)triphenylphosphorane (24.33 g, 69.84 mmol)
in toluene (200 mL) at rt was stirred for 8 h. The solution was
concentrated in vacuo to afford a yellow oil, which was subjected to
gravity CC (4:1 hexane-acetone) to yield 10 as a clear oil that solidified
upon further drying under high vacuum. Recrystallization from
EtOAc-hexane provided colorless needles (11.67 g, 99%): mp
57.4-58.7 °C; 1H NMR (CDCl3, 300 MHz) δ 1.33 (3H, t, J ) 7.2 Hz,
OCH2CH3), 3.91 (3H, s, OCH3), 4.25 (2H, q, J ) 7.2 Hz, OCH2CH3),
5.75 (1H, s, OH), 6.28 (1H, d, J ) 15.9 Hz, 2), 6.83 (1H, d, J ) 8.1
Hz, 5′), 7.02 (1H, dd, J ) 8.1, 2.1 Hz, 6′), 7.13 (1H, d, J ) 2.1 Hz,
2′), 7.59 (1H, d, J ) 15.9 Hz, 3); HRMS (EI+) m/z, [M]+ 222.0890
(calcd for C12H14O4, 222.0892).
Ethyl 3-(3′-Hydroxy-4′-benzyloxyphenyl)propanoate (11). Ester
11 was prepared12h from olefin 9 (40.6 g, 136 mmol) as a colorless,
crystalline solid (29.5 g, 72%): mp 60.4- 62.0 °C (lit.12h oil).
Ethyl 3-(3′-Hydroxy-4′-methoxyphenyl)propanoate (12). Pal-
ladium on carbon (5%, 704 mg) was added to a stirred solution of
olefin 10 (9.48 g, 42.7 mmol) in CH3OH (75 mL) at rt. Hydrogen gas
was bubbled through the suspension until TLC analysis showed
complete consumption of starting material. The catalyst was removed
by filtration of the solution through a plug of Celite, and the solution
was concentrated to a colorless solid that crystallized from EtOAc-
hexane as colorless cubic crystals (9.45 g, 99%): mp 71.3-72.8 °C;
1H NMR (CDCl3, 300 MHz) δ 1.24 (3H, t, J ) 7.2 Hz, OCH2CH3),
2.57 (2H, t, J ) 8.1 Hz, 2), 2.86 (2H, t, J ) 8.1 Hz, 3), 3.86 (3H, s,
OCH3), 4.12 (2H, q, J ) 7.2 Hz, OCH2CH3), 5.60 (1H, s, OH), 6.67
(1H, dd, J ) 8.1, 2.1 Hz, 6′), 6.76 (1H, d, J ) 8.1 Hz, 5′), 6.77 (1H,
d, J ) 1.5 Hz, 2′); HRMS (FAB+) m/z 225.1124 [M + H]+ (calcd for
C12H17O4, 225.1127).
3-(3′-Hydroxy-4′-benzyloxyphenyl)propionic Acid (13). To a
solution of ester 11 (29.38 g, 97.8 mmol) in THF-H2O (1:1, 150 mL)
was added NaOH (7.95 g, 199 mmol), and the resulting yellow solution
was stirred at rt for 5 h. The reaction mixture was acidified to pH 1
with 1.0 N HCl and extracted with DCM (3 × 200 mL). The combined
organic extracts were washed with brine (100 mL), dried, filtered, and
concentrated in vacuo to afford a tan powder. Crystallization from EtOH
provided colorless needles (26.2 g, 98%): mp 128.8-130.5 °C; 1H NMR
(CDCl3, 300 MHz) δ 2.64 (2H, t, J ) 7.8 Hz, 2), 2.87 (2H, t, J ) 7.8
Hz, 3), 5.08 (2H, s, ArCH2), 6.67 (1H, dd, J ) 8.1, 2.1 Hz, 6′), 6.81
(1H, d, J ) 2.1 Hz, 2′), 6.84 (1H, d, J ) 8.1 Hz, 5′), 7.40 (5H, bs,
2′′- 6′′); HRMS (APCI+) m/z 273.1112 [M + H]+ (calcd for C16H17O4,
273.1127).
3-(4′-Bromophenyl)-2Z-propenol (19). To a cooled (-78 °C)
solution of 18 (7.46 g, 30.9 mmol) in DCM (56 mL) under argon was
added DIBAL (1 M solution in DCM, 80 mL, 80 mmol). After stirring
for 2 h the reaction was terminated with H2O (80 mL) and acidified to
pH 1 with H2SO4 (18 M). The solution was extracted with DCM (5 ×
75 mL), and the combined organic extracts were concentrated in vacuo
to afford a solid. Gravity CC (9:1 hexane-acetone) provided 19 as a
1
colorless solid (6.18 g, 94%): mp 69.3-70.6 °C; H NMR (CDCl3,
300 MHz) δ 1.60 (1H, s, OH), 4.39 (2H, dd, J ) 6.6, 1.5 Hz, 1), 5.90
(1H, dt, J ) 12.0, 6.6 Hz, 2), 6.49 (1H, d, J ) 12.0 Hz, 3), 7.08 (2H,
d, J ) 8.4 Hz, 2′, 6′), 7.46 (2H, d, J ) 8.1 Hz, 3′, 5′); 13C NMR (CDCl3,
100 MHz) δ 59.5 (1), 121.2 (4′), 129.9 (2), 130.3 (2′, 6′), 131.3 (3′,
5′), 131.7 (3), 135.2 (1′); HRMS (EI+) m/z 213.9813, 211.9832 [M]+
(calcd for C9H9OBr, 213.9816, 211.9837); anal. C 50.57%, H 4.55%,
calcd for C9H9OBr, C 50.73%, H, 4.26%.
3′-(4′′′-Bromophenyl)-2′Z-propenyl 3-(3′′-Hydroxy-4′′-benzylox-
yphenyl)propanoate (20). A solution of allylic alcohol 19 (6.12 g,
28.7 mmol), carboxylic acid 13 (7.86 g, 28.9 mmol), and Ph3P (8.39
g, 32.0 mmol) in THF (43 mL) at rt was treated dropwise with DEAD
(5.0 mL, 32 mmol). The orange solution was stirred rapidly for 66 h
and then concentrated to an orange tar. Gravity CC (9:1 hexane-acetone)
provided ester 20 as a colorless solid that crystallized from EtOH (9.75
g, 73%): mp 80.1-81.7 °C; 1H NMR (CDCl3, 500 MHz) δ 2.61 (2H,
t, J ) 8.5 Hz, 2), 2.86 (2H, t, J ) 8.0 Hz, 3), 4.78 (2H, dd, J ) 6.5,
2.0 Hz, 1′), 5.06 (2H, s, ArCH2), 5.64 (1H, s, OH), 5.80 (1H, dt, J )
12.0, 6.5 Hz, 2′), 6.56 (1H, d, J ) 12.0 Hz, 3′), 6.65 (1H, dd, J ) 8.5,
2.0 Hz, 6′′), 6.79 (1H, d, J ) 2.0 Hz, 2′′), 6.82 (1H, d, J ) 8.5 Hz,
5′′), 7.07 (2H, d, J ) 8.5 Hz, 2′′′, 6′′′), 7.34-7.39 (5H, m, 2′′′′-6′′′′),
7.46 (2H, d, J ) 8.5 Hz, 3′′′, 5′′′); 13C NMR (CDCl3, 125 MHz) δ
30.3 (3), 35.9 (2), 61.1 (1′), 71.2 (ArCH2), 112.2 (5′′), 114.7 (2′′), 119.6
(6′′), 121.6 (4′′′), 126.6 (2′), 127.8 (2′′′′, 6′′′′), 128.3 (4′′′′), 128.7 (3′′′′,
5′′′′), 130.3 (2′′′, 6′′′), 131.5 (3′′′, 5′′′), 131.8 (3′), 134.1 (1′′), 134.9
(1′′′), 136.4 (1′′′′), 144.3 (4′′), 145.8 (3′′), 172.6 (1); HRMS (APCI+)
m/z 469.0817, 467.0810 [M + H]+ (calcd for C25H24O4Br, 469.0838,
467.0858); anal. C 64.09%, H 4.93%, calcd for C25H23O4Br, C 64.25%,
H 4.96%.
3′-(4′′′-Bromophenyl)-2′Z-propenyl 3-(3′′-Hydroxy-4′′-methox-
yphenyl)propanoate (21). DEAD (0.90 mL, 5.2 mmol) was added
dropwise to a stirring solution of acid 14 (1.03 g, 5.25 mmol), alcohol
19 (1.00 g, 4.69 mmol), and triphenylphosphine (1.39 g, 5.30 mmol)
in THF (10 mL) in a flask covered in aluminum foil. After stirring for
25 h, the reaction was terminated by the addition of brine (10 mL) and
extracted with EtOAc (3 × 15 mL), and the combined organic extract
was concentrated in vacuo. Purification of the residue by gravity CC
(9:1 hexane-EtOAc) provided ester 21 as an off-white solid (1.43 g,
3-(3′-Hydroxy-4′-methoxyphenyl)propionic Acid (14). To a stirred
solution of ester 12 (63.2 g, 282 mmol) in THF-H2O (1:1, 300 mL)
was added NaOH (22.76 g, 569.0 mmol), and the resulting red-brown
solution was heated to reflux for 1.5 h. The dark brown solution was
then cooled to rt while stirring continued, and the THF was removed
in vacuo. The resulting aqueous solution was acidified and extracted
(see 13), and the extract was washed and dried to afford an off-white
solid. Recrystallization from EtOH provided colorless needles (43.9 g,
1
79%): mp 149.9-151.8 °C; H NMR (CDCl3, 300 MHz) δ 2.64 (2H,
1
78%): mp 61.5-61.9 °C; H NMR (CDCl3, 300 MHz) δ 2.61 (2H, t,
t, J ) 8.1 Hz, 2), 2.87 (2H, t, J ) 8.1 Hz, 3), 3.87 (3H, s, -OCH3),
6.68 (1H, dd, J ) 8.1, 2.1 Hz, 6′), 6.77 (1H, d, J ) 8.1 Hz, 5′), 6.78
(1H, d, J ) 1.5 Hz, 2′); HRMS (FAB+) m/z 197.0876 [M + H]+ (calcd
for C10H13O4, 197.0814).
J ) 8.1 Hz), 2.86 (2H, t, J ) 8.1 Hz), 3.83 (3H, s), 4.78 (2H, dd, J )
1.5 Hz), 5.64 (1H, s), 5.81 (1H, dt, J ) 12.0 Hz), 6.56 (1H, d, J )
11.4 Hz), 6.66 (1H, dd, J ) 1.8 Hz), 6.73 (1H, s), 6.77 (1H, t, J ) 2.1
Hz), 7.06 (2H, d, J ) 8.4 Hz), 7.45 (2H, d, J ) 8.7 Hz); 13C NMR
(CDCl3, 75 MHz) δ 29.8, 35.5, 55.5, 60.7, 110.2, 114.0, 119.1, 121.1,
126.1, 129.8, 131.0, 131.4, 133.2, 134.4, 144.6, 145.1, 172.2; HRMS
(APCI+) m/z 391.0356 [M + H]+ (calcd for C19H20O4Br, 391.0545);
anal. C 58.07%, H 5.10%, calcd for C19H19O4Br, C 58.33%, H 4.89%.
Combretastatin D-2 Benzyl Ether (22). Phenol 20 (2.39 g, 5.11
mmol), CuBr-S(CH3)2 (3.85 g, 18.7 mmol), and K2CO3 (5.71 g, 41.3
mmol) were dissolved in pyridine (1 L). The resulting orange solution
was heated to reflux under argon for 24 h. The solvent was removed
in vacuo to afford a black tar, which was taken up in EtOAc (200 mL)
and washed with 1.0 N HCl (3 × 100 mL) to remove copper salts and
excess K2CO3. The combined organic extracts were concentrated in
vacuo to a brown oil, which was separated by gravity CC (9:1
hexane-acetone) to afford ether 22 as a yellow oil that solidified on
Ethyl 3-(4′-Bromophenyl)-2E-propenoate (16). Ester 16 (7.63 g,
100%, trace of Z-isomer) was prepared from 4-bromobenzaldehyde (15,
5.55 g, 30.0 mmol) and (carbethoxymethylene)triphenylphosphorane
(15.68 g, 45.0 mmol) as described in the literature,12h toluene (100
mL) replacing benzene as solvent.
Ethyl 3-(4′-Bromophenyl)-2Z-propenoate (17). A solution of 16
(9.99 g, 39.2 mmol) and benzil (41.26 g, 196.3 mmol) in benzene
(2 L) was stirred under argon at rt for 25.5 h. The resulting yellow
solution was irradiated with a UV lamp (254 nm) for 4 h. The orange
solution was concentrated in vacuo to afford a yellow solid, which
was purified by flash CC (19:1 hexane-EtOAc) to yield the Z-isomer
as a clear oil (5.92 g, 59%): 1H NMR (CDCl3, 300 MHz) δ 1.26 (3H,
t, J ) 7.2 Hz, OCH2CH3), 4.18 (2H, q, J ) 7.2 Hz, OCH2CH3), 5.97
(1H, d, J ) 12.6 Hz, 2), 6.86 (1H, d, J ) 12.6 Hz, 3), 7.47 (4H, s, 2′,

Synthesis of Combretastatin D-2 Phosphate
Journal of Natural Products, 2009, Vol. 72, No. 5 881
standing and crystallized from acetone-hexane as fine colorless needles
(0.19 g, 10%): mp 107.2-108.8 °C (lit.12h oil); 1H NMR (CDCl3, 500
MHz) δ 2.29 (2H, t, J ) 5.5 Hz, 2), 2.87 (2H, t, J ) 5.5 Hz, 3), 4.66
(2H, d, J ) 6.5 Hz, 1′), 5.13 (1H, d, J ) 2.5 Hz, 2′′), 5.24 (2H, s,
ArCH2), 6.05 (1H, dt, J ) 11.0, 6.5 Hz, 2′), 6.61 (1H, dd, J ) 8.5, 2.0
Hz, 6′′), 6.84 (1H, d, J ) 8.5 Hz, 5′′), 7.10 (1H, d, 3′), 7.10 (2H, d, J
) 8.5 Hz, 3′′′, 5′′′), 7.32 (2H, d, J ) 7.0 Hz, 2′′′, 6′′′), 7.32 (1H, d, J
) 8.0 Hz, 4′′′′), 7.39 (2H, t, J ) 7.5 Hz, 3′′′′, 5′′′′), 7.51 (2H, d, J )
7.5 Hz, 2′′′′, 6′′′′); 13C NMR (CDCl3, 125 MHz) δ 26.7 (3), 31.1 (2),
59.1 (1′), 71.8 (ArCH2), 113.6 (2′′), 115.4 (5′′), 121.1 (6′′), 124.1 (3′′′,
5′′′), 125.4 (2′), 127.4 (2′′′′, 6′′′′), 127.9 (4′′′′), 128.6 (3′′′′, 5′′′′), 128.9
(2′′′, 6′′′), 133.1 (1′′), 135.0 (1′′′), 137.3 (1′′′′), 137.8 (3′), 145.1 (4′′),
152.2 (3′′), 156.1 (4′′′), 173.2 (1).
Combretastatin D-2 Methyl Ether (23). Compound 23 was
synthesized as described in the literature.12a Treatment of ester 21 (4.08
g, 10.4 mmol) with CuCH3, prepared from reaction of CuI-(SBu2)2
(12.57 g, 26.02 mmol) and CH3Li (1.6 M, 16.0 mL, 25.6 mmol),
afforded ether 23 as a pale yellow solid (0.80 g, 25%): mp 132.4-132.8
°C (lit.12a mp 130-132 °C); 13C NMR (CDCl3, 75 MHz) δ 26.2, 30.7,
55.7, 58.5, 111.7, 112.7, 120.7, 123.5, 124.9, 128.4, 131.9, 134.5, 137.3,
145.6, 150.9, 155.4, 172.7.
Combretastatin D-2 (5). To a cooled (-15 °C) solution of AlBr3
(1.0 M in CH2Br2, 19.5 mL, 19.5 mmol) and ethanethiol (5.75 mL,
77.6 mmol) was added a cooled (-15 °C for 30 min) solution of lactone
23 (1.20 g, 3.87 mmol) in DCM (120 mL), and the mixture was stirred
for 40 min at -13 °C. The reaction was terminated by the addition of
H2O (100 mL), and the mixture was acidified with 1.0 N HCl and
extracted with DCM (3 × 50 mL). The organic layer was washed with
brine (150 mL) and concentrated in vacuo. Separation by gravity CC
(4:1 hexane-EtOAc) followed by crystallization from acetone-
hexane afforded lactone 5 as a colorless crystalline solid (0.22 g, 19%):
mp 160.4-160.7 °C (lit.10 mp 148-151 °C, lit.12a mp 152-154.5 °C,
lit.12d mp 154.5-155 °C); 1H NMR (CDCl3, 500 MHz) δ 2.28 (2H, t,
J ) 5.5 Hz), 2.86 (2H, t, J ) 5.5 Hz), 4.63 (2H, d, J ) 6.5 Hz), 5.06
(1H, d, J ) 1.5 Hz), 5.48 (1H, s), 6.03-6.08 (1H, m), 6.61-6.63 (1H,
m), 6.84 (1H, d, J ) 7.5 Hz), 7.07-7.11 (3H, m), 7.32 (2H, d, J ) 8.0
Hz); 13C NMR (CDCl3, 125 MHz) δ 26.8, 31.3, 60.0, 112.5, 115.3,
121.8, 123.9, 125.6, 129.0, 131.9, 135.4, 137.7, 142.4, 149.5, 155.5,
173.3; HRMS (APCI+) m/z 297.1107 [M + H]+ (calcd for C18H17O4,
297.1127); anal. C 72.82%, H 5.58%, calcd for C18H16O4, C 72.96%,
H 5.44%.
Combretastatin D-2 (5) X-ray Crystal Structure Determination.
A small, block-shaped crystal (∼0.10 × 0.11 × 0.19 mm), grown from
an acetone-hexane solution, was mounted on the tip of a glass fiber.
Cell parameter measurements and data collection were performed at
123 ( 1 K with a Bruker SMART 6000 diffractometer system using
Cu KR radiation. A sphere of reciprocal space was covered using the
Multirun technique.24 Thus, six sets of frames of data were collected
with 0.396° steps in ω and a last set of frames with 0.396° steps in ꢁ
so that 98.2% coverage of all unique reflections to a resolution of 0.84
Å was accomplished.
and -0.174 e/ Å3, respectively. Final bond distances and angles were
all within expected and acceptable limits. The Flack absolute structure
parameter for the model shown in Figure 2 is 0.0018(9).28
8-Bis(benzyl)phosphorylcombretastatin D-2 (24). A solution of
combretastatin D-2 (5, 0.10 g, 0.34 mmol) in CH3CN (5 mL) was cooled
to -10 °C, and CCl4 (0.38 mL, 3.9 mmol) was added dropwise with
stirring. After 10 min, diisopropylethylamine (0.12 mL, 0.69 mmol)
and dimethylaminopyridine (6.0 mg, 0.049 mmol) were added, and the
solution was stirred for another 5 min. Dibenzyl phosphite (0.15 mL,
0.67 mmol) was then added dropwise, and the reaction mixture was
stirred for 2 h. The phosphorylation was terminated by the addition of
KH2PO4 (0.5 M, 2 mL), and the mixture was stirred for an additional
30 min. DCM (5 mL) was added and the phases were separated. The
aqueous layer was extracted with DCM (3 × 5 mL), and the extract
was concentrated in vacuo. The residue was separated by gravity CC
(7:3 hexane-EtOAc) to yield phosphate 24 as a colorless oil (0.16 g,
1
83%): H NMR (CDCl3, 300 MHz) δ 2.28 (2H, t, J ) 5.4 Hz), 2.89
(2H, t, J ) 5.1 Hz,), 4.63 (2H, d, J ) 6.6 Hz), 5.15 (1H, d, J ) 1.2
Hz), 5.23 (4H, d, J ) 8.4 Hz), 5.99-6.08 (1H, m), 6.64 (1H, dd, J )
1.5, 8.1 Hz), 6.96 (2H, d, J ) 9.0 Hz), 7.07 (1H, d, J ) 10.8 Hz), 7.12
(1H, dd, J ) 1.2, 8.1 Hz), 7.26 (2H, dd, J ) 8.1 Hz), 7.31-7.36 (10H,
m); 13C NMR (CDCl3, 125 MHz) δ 26.5, 30.5, 58.5, 69.5, 69.5, 113.8,
121.0, 123.3, 125.0, 127.5, 128.0, 128.1, 128.5, 134.7, 135.2, 135.3,
136.2, 137.2, 152.3, 155.2, 172.4; HRMS (APCI+) m/z 557.1712 [M
+ H]+ (calcd for C32H30O7P, 557.1729); anal. C 69.02%, H 5.61%,
calcd for C32H29O7P, C 69.06%, H 5.25%.
Sodium Combretastatin D-2 8-O-Phosphate (25). A solution of
24 (0.040 g, 0.07 mmol) and bromotrimethylsilane (0.020 mL, 0.15
mmol) in DCM (1 mL) was stirred at rt for 40 min. The reaction was
terminated by the addition of CH3OH (1 mL), and the solvent was
removed in vacuo to yield an oil, which was then dissolved in CH3OH
(1 mL). After the addition of CH3ONa (7.6 mg, 0.14 mmol), the solution
was stirred for 30 min. Removal of solvent and trituration of the residue
with hexane (3 × 1 mL) afforded sodium salt 25 as a colorless solid
1
(28 mg, 93%): mp 145.3-146.0 °C; H NMR (CD3OD, 300 MHz) δ
2.15-2.18 (2H, m), 2.72 (2H, d, J ) 4.8 Hz), 4.52 (2H, d, J ) 6.0
Hz), 5.03-5.08 (1H, m), 5.92-5.99 (1H, m), 6.51 (1H, d, J ) 8.1
Hz), 6.98 (2H, d, J ) 7.8 Hz), 7.22 (3H, d, J ) 8.4 Hz).
Potassium Combretastatin D-2 8-O-Phosphate (26). Treatment of
24 (0.040 g, 0.07 mmol) with bromotrimethylsilane (0.020 mL, 0.15
mmol) in DCM (1 mL) was carried out as above (see 25) to yield an
oil, which was dissolved in CH3OH (1 mL). Addition of KOH (9.0
mg, 0.16 mmol), followed by stirring for 30 min and concentration to
a residue that was triturated with hexane (3 × 1 mL), afforded potassium
salt 26 as a colorless solid (31 mg, 95%): mp (dec) 160 °C.
Lithium Combretastatin D-2 8-O-Phosphate (27). Reaction of
bromotrimethylsilane (0.020 mL, 0.15 mmol) and 24 (0.040 g, 0.07
mmol) in DCM (1 mL) was conducted as summarized above (see 25).
The resultant oil was dissolved in CH3OH (1 mL), and LiOH (6.8 mg,
0.16 mmol) was added. The solution was stirred for 30 min and then
concentrated to a residue that was triturated with hexane (3 × 1 mL)
to afford lithium salt 27 as a colorless solid (27 mg, 96%): mp (dec)
230 °C.
Crystal Data: C18H16O4, fw ) 296.31, monoclinic, P21, a ) 8.3872(1)
Å, b ) 10.9512(1) Å, c ) 8.9106(1) Å, ꢀ ) 117.2690(10)°, V )
727.481(14) Å3, Z ) 2, Fc ) 1.353 mg/m3, µ(Cu KR) ) 0.782 mm-1
λ ) 1.54178.
,
Morpholine Combretastatin D-2 8-O-Phosphate (28). Reaction
of bromotrimethylsilane (0.020 mL, 0.15 mmol) and 24 (0.040 g, 0.07
mmol) in DCM (1 mL) was carried out as described above (see 25),
and the product was dissolved in CH3OH (1 mL). Morpholine (0.016
mL, 0.18 mmol) was added, and removal of solvent after 30 min
provided an oil that was triturated with acetone-Et2O to yield
morpholine salt 28 as a tan solid (9 mg, 24%): mp (dec) 195 °C; HRMS
(APCI+) m/z 547.3941 [M + H]+ (calcd for C26H31N2O9P, 547.1845).
Methyl 3-(3′-Nitro-4′-fluorophenyl)-2Z-propenoate (30). A solu-
tion of 18-crown-6 (38.62 g, 146.1 mmol) and bis(2,2,2-trifluoroethyl)
(methoxycarbonylmethyl)phosphonate (9.2 mL, 44 mmol) in THF (340
mL) at -78 °C was treated dropwise with KHMDS (0.5 M solution in
toluene, 85 mL, 43 mmol), followed by 3-nitro-4-fluorobenzaldehyde
(29, 6.01 g, 35.5 mmol). After stirring for 4 h the dark purple solution
was treated with saturated NH4Cl (30 mL), diluted with H2O (100 mL),
and extracted with EtOAc (5 × 100 mL). The combined organic extracts
were concentrated in vacuo to an orange viscous oil. Separation by
gravity CC (9:1 hexane-EtOAc) provided olefin 30 as an off-white
A total of 5690 reflections was collected, of which 2309 reflections
were independent (R(int) ) 0.0521). Subsequent statistical analysis of
the data set with the XPREP25 program indicated the space group was
P21. Final cell constants were determined from the set of the 2309
observed (>2σ(I)) reflections that were used in structure solution and
refinement. An absorption correction was applied to the data with
SADBS.26 Structure determination and refinement was readily ac-
complished with the direct-methods program SHELXTL.27 All non-
hydrogen atom coordinates were located in a routine run using default
values for that program. The remaining H atom coordinates were
calculated at optimum positions. All non-hydrogen atoms were refined
anisotropically in a full-matrix least-squares refinement procedure. The
H atoms were included, their Uiso thermal parameters fixed at either
1.2 or 1.5 (depending on atom type) of the value of the Uiso of the
atom to which they were attached and forced to ride that atom. The
final standard residual R1 value for 5 was 0.0338 for observed data
and 0.0351 for all data. The goodness-of-fit on F2 was 1.003. The
corresponding Sheldrick R values were wR2 ) 0.0864 and 0.0870,
respectively. A final difference Fourier map showed minimal residual
electron density, the largest difference peak and hole being +0.164
1
solid (6.00 g, 75%): mp 67.5-68.9 °C; H NMR (CDCl3, 300 MHz)
δ 3.74 (3H, s, OCH3), 6.10 (1H, d, J ) 12.6 Hz, 2), 6.91 (1H, d, J )
12.6 Hz, 3), 7.27 (1H, dd, J ) 10.5, 8.7 Hz, 5′), 7.90 (1H, qd, J ) 8.1,
3.9, 2.1 Hz, 6′), 8.36 (1H, dd, J ) 7.2, 2.1 Hz, 2′); 13C NMR (CDCl3,

882 Journal of Natural Products, 2009, Vol. 72, No. 5
Pettit et al.
75 MHz) δ 51.2 (OCH3), 117.3 and 117.6 (5′), 121.5 (2), 127.0 (2′),
131.2 (1′ or 3′), 136.3 and 136.5 (6′), 139.4 (3), 153.1 (3′ or 1′), 156.6
(4′), 165.2 (1); HRMS (APCI+) m/z 226.0715 [M + H]+ (calcd for
C10H9FNO4, 226.0516); anal. C 53.40%, H 3.53%, N 6.18%, calcd for
C10H8FNO4, C 53.34%, H 3.58%, N 6.22%.
(4′), 129.7 (2′, 6′), 130.9 (3′, 5′), 140.2 (1′); HRMS (EI+) m/z 215.9967,
213.9991 [M]+ (calcd for C9H11OBr, 215.9973, 213.9993).
3′-(4′′′-Bromophenyl)propanyl 3-(3′′-Hydroxy-4′′-benzyloxyphe-
nyl)propanoate (39). A solution of propanol 38 (3.45 g, 16.0 mmol),
carboxylic acid 13 (4.40 g, 16.2 mmol), and Ph3P (4.70 g, 17.9 mmol)
in THF (25 mL) at rt was treated dropwise with DEAD (2.9 mL, 18
mmol), and the mixture was stirred rapidly for 63 h. The solution was
concentrated to a viscous yellow oil, and fractionation by gravity CC
(17:3 hexane-acetone) provided ester 39 as a clear oil (4.04 g, 54%):
1H NMR (CDCl3, 500 MHz) δ 1.88 (2H, tt, J ) 8.0, 6.5 Hz, 2′), 2.57
(2H, t, J ) 8.0 Hz, 3′), 2.59 (2H, t, J ) 7.5 Hz, 2), 2.85 (2H, t, J )
7.5 Hz, 3), 4.06 (2H, t, J ) 6.5 Hz, 1′), 5.04 (2H, s, ArCH2), 5.63 (1H,
s, OH), 6.65 (1H, dd, J ) 8.5, 2.0 Hz, 6′′), 6.80 (1H, d, J ) 2.0 Hz,
2′′), 6.82 (1H, d, J ) 8.5 Hz, 5′′), 7.01 (2H, d, J ) 8.0 Hz, 2′′′, 6′′′),
7.38 (5H, s, 2′′′′, 6′′′′), 7.38 (2H, d, J ) 8.0 Hz, 3′′′, 5′′′); 13C NMR
(CDCl3, 100 MHz) δ 30.0 (2′), 30.4 (3), 31.5 (3′), 35.9 (2), 63.5 (1′),
71.2 (ArCH2), 112.2 (5′′), 114.7 (2′′), 119.6 (6′′), 119.7 (4′′′), 127.8
(2′′′′, 6′′′′), 128.3 (4′′′′), 128.7 (3′′′′, 5′′′′), 130.1 (2′′′, 6′′′), 131.4 (3′′′,
5′′′), 134.1 (1′′), 136.4 (1′′′′), 140.1 (1′′′), 144.3 (4′′), 145.8 (3′′), 172.9
(1); HRMS (APCI+) m/z 471.0997, 469.1055, [M + H]+ (calcd for
C25H26O4Br, 471.0994, 469.1014); anal. C 63.84%, H 5.46%, calcd
for C25H25O4Br, C 63.97%, H 5.37%.
3-(3′-Nitro-4′-fluorophenyl)-2Z-propenol (31). To a cooled (-78
°C) solution of Z-R,ꢀ-unsaturated ester 30 (5.97 g, 26.5 mmol) in DCM
(47 mL) under argon was added DIBAL (1 M solution in DCM, 68
mL, 68 mmol). After the mixture was stirred for 4 h, the reaction was
terminated with cold H2O (60 mL), and the mixture was acidified to
pH 1 with H2SO4 (18 M) before extraction with DCM (5 × 80 mL).
The extract was concentrated in vacuo to an orange oil, and fractionation
by gravity CC (4:1 hexane-EtOAc) provided propenol 31 as a tan oil
1
(4.05 g, 77%): H NMR (CDCl3, 300 MHz) δ 1.88 (1H, s, OH), 4.39
(2H, dd, J ) 6.6, 1.8 Hz, 1), 6.03 (1H, dt, J ) 11.4, 6.6 Hz, 2), 6.53
(1H, d, J ) 11.4 Hz, 3), 7.26 (1H, dd, J ) 10.2, 9.0 Hz, 5′), 7.49 (1H,
qd, J ) 8.4, 3.9, 2.1 Hz, 6′), 7.90 (1H, dd, J ) 7.2, 2.1 Hz, 2′).
3′-(3′′′-Nitro-4′′′-fluorophenyl)-2′Z-propenyl 3-(3′′-Hydroxy-4′′-
benzyloxyphenyl)propanoate (32). A solution of allylic alcohol 31
(3.98 g, 20.2 mmol), carboxylic acid 13 (5.50 g, 20.2 mmol), and Ph3P
(5.86 g, 22.3 mmol) in THF (41 mL) at rt was treated dropwise with
DEAD (3.6 mL, 23 mmol). After rapid stirring for 76.5 h, the solution
was concentrated in vacuo to an orange solid. Separation using gravity
CC (4:1 hexane-EtOAc) yielded ester 32 as a yellow crystalline solid,
which recrystallized from EtOH (5.16 g, 57%): mp 81.6-83.0 °C; 1H
NMR (CDCl3, 500 MHz) δ 2.63 (2H, t, J ) 8.0 Hz, 2), 2.87 (2H, t, J
) 8.0 Hz, 3), 4.75 (2H, dd, J ) 7.5, 1.5 Hz, 1′), 5.07 (2H, s, ArCH2),
5.61 (1H, s, OH), 5.94 (1H, dt, J ) 12.0, 7.0 Hz, 2′), 6.61 (1H, d, J )
12.0 Hz, 3′), 6.65 (1H, dd, J ) 8.5, 2.0 Hz, 6′′), 6.79 (1H, d, J ) 2.5
Hz, 2′′), 6.83 (1H, d, J ) 8.5 Hz, 5′′), 7.27 (1H, dd, J ) 11.0, 9.0 Hz,
5′′′), 7.34-7.40 (5H, m, 2′′′′-6′′′′), 7.47 (1H, qd, J ) 8.5, 4.0, 2.5 Hz,
6′′′), 7.92 (1H, dd, J ) 7.0, 2.5 Hz, 2′′′); 13C NMR (CDCl3, 125 MHz)
δ 30.3 (3), 35.8 (2), 60.5 (1′), 71.2 (ArCH2), 112.2 (5′′), 114.7 (2′′),
118.4 and 118.6 (5′′′), 119.7 (6′′), 126.00 and 126.03 (2′′′), 127.8 (2′′′′,
6′′′′), 128.4 (4′′′′), 128.66 (2′), 128.70 (3′′′′, 5′′′′), 129.9 (3′), 132.98
and 133.01 (1′′′ or 3′′′), 134.0 (1′′), 135.35 and 135.42 (6′′′), 136.4
(1′′′′), 144.3 (4′′), 145.8 (3′′), 153.5 (3′′′ or 1′′′), 155.7 (4′′′), 172.5
(1); HRMS (EI+) m/z 433.1358 [M - H2O]+ (calcd for C25H20FNO5,
433.1325); anal. C 66.49%, H 5.11%, N 3.02%, calcd for C25H22FNO6,
C 66.51%, H 4.91%, N 3.10%.
3′-(4′′′-Bromophenyl)propanyl 3-(3′′-Hydroxy-4′′-methoxyphe-
nyl)propanoate (40). A solution of propanol 38 (2.09 g, 9.72 mmol),
carboxylic acid 14 (1.93 g, 9.84 mmol), and Ph3P (2.86 g, 10.9 mmol)
in THF (20 mL) at rt was treated dropwise with DEAD (1.7 mL, 11
mmol), and the mixture was stirred rapidly for 64 h. The solution was
concentrated, and separation by gravity CC (17:3 hexane-acetone)
provided ester 40 as a yellow oil that crystallized upon standing and
recrystallized from EtOAc-hexane as colorless crystals (2.15 g, 56%):
1
mp 66.8-68.7 °C; H NMR (CDCl3, 400 MHz) δ 1.89 (2H, tt, J )
8.4, 6.8 Hz, 2′), 2.57 (2H, t, J ) 6.8 Hz, 3′), 2.59 (2H, t, J ) 7.6 Hz,
2), 2.85 (2H, t, J ) 7.6 Hz, 3), 3.83 (3H, s, OCH3), 4.07 (2H, t, J )
6.8 Hz, 1′), 5.60 (1H, s, OH), 6.67 (1H, dd, J ) 8.0, 2.0 Hz, 6′′), 6.75
(1H, d, J ) 8.0 Hz, 5′′), 6.78 (1H, d, J ) 2.0 Hz, 2′′), 7.01 (2H, d, J
) 8.4 Hz, 2′′′, 6′′′), 7.38 (2H, d, J ) 8.4 Hz, 3′′′, 5′′′); 13C NMR (CDCl3,
100 MHz) δ 30.0 (2′), 30.3 (3), 31.5 (3′), 35.9 (2), 55.9 (OCH3), 63.5
(1′), 110.6 (5′′), 114.5 (2′′), 119.6 (6′′), 119.7 (4′′′), 130.1 (2′′′, 6′′′),
131.4 (3′′′, 5′′′), 133.7 (1′′), 140.1 (1′′′), 145.0 (4′′), 145.5 (3′′), 172.9
(1); HRMS (APCI+) m/z 395.0657, 393.0751 [M + H]+ (calcd for
C19H22O4Br, 395.0681, 393.0701); anal. C 57.92%, H 5.52%, calcd
for C19H21O4Br, C 58.03%, H 5.38%.
3′-(3′′′-Nitro-4′′′-fluorophenyl)-2′Z-propenyl 3-(3′′-Hydroxy-4′′-
methoxyphenyl)propanoate (33). To a solution of 31 (5.78 g, 29.3
mmol), 14 (5.75 g, 29.3 mmol), and Ph3P (8.52 g, 32.5 mmol) in THF
(60 mL at rt) was added DEAD (5.2 mL, 33 mmol, dropwise), and the
mixture was stirred rapidly for 71 h. Removal of solvent afforded a
viscous orange oil, which was separated by gravity CC (3:1
hexane-EtOAc) to provide ester 33 as a yellow oil (7.10 g, 65%): 1H
NMR (CDCl3, 500 MHz) δ 2.63 (2H, t, J ) 7.0 Hz, 2), 2.87 (2H, t, J
) 7.5 Hz, 3), 3.85 (3H, s, OCH3), 4.75 (2H, dd, J ) 7.0, 1.5 Hz, 1′),
5.56 (1H, s, OH), 5.94 (1H, dt, J ) 12.0, 7.0 Hz, 2′), 6.61 (1H, d, J )
12.0 Hz, 3′), 6.67 (1H, dd, J ) 8.0, 2.5 Hz, 6′′), 6.76 (1H, d, J ) 2.5
Hz, 2′′), 6.76 (1H, d, J ) 8.0 Hz, 5′′), 7.27 (1H, dd, J ) 10.5, 8.5 Hz,
5′′′), 7.47 (1H, qd, J ) 8.5, 4.0, 2.5 Hz, 6′′′), 7.93 (1H, dd, J ) 7.0,
2.5 Hz, 2′′′); 13C NMR (CDCl3, 125 MHz) δ 30.3 (3), 35.9 (2), 56.0
(OCH3), 60.4 (1′), 110.7 (5′′), 114.5 (2′′), 118.4 and 118.6 (5′′′), 119.6
(6′′), 126.00 and 126.03 (2′′′), 128.7 (2′), 129.9 (3′), 132.98 and 133.02
(1′′′ or 3′′′), 133.5 (1′′), 135.35 and 135.42 (6′′′), 145.1 (4′′), 145.5
(3′′), 153.5 (3′′′ or 1′′′), 155.7 (4′′′), 172.6 (1); HRMS (EI+) m/z
447.1506 [M - H + Si(CH3)3]+ (calcd. for C22H26FNO6Si, 447.1513);
anal. C 60.67%, H 4.96%, N 3.83%, calcd for C19H18FNO6, C 60.80%,
H 4.83%, N 3.73%.
3-(4′-Bromophenyl)propanol (38). To a stirred solution of ester
16 (3.88 g, 15.2 mmol) in PEG 400 (90 mL) was added slowly NaBH4
(1.72 g, 45.5 mmol). The reaction mixture was heated to 80 °C (oil
bath temperature) and stirred for 16.5 h, following an initial evolution
of H2 gas. The reaction was terminated with 1 N HCl (90 mL), followed
by extraction with EtOAc (3 × 100 mL). The extract was washed with
H2O (100 mL) and concentrated in vacuo to an oil. Separation by gravity
CC (4:1 hexane-acetone) yielded alcohol 38 as a clear oil (2.61 g,
80%): 1H NMR (CDCl3, 300 MHz) δ 1.85 (2H, tt, J ) 7.8, 6.6 Hz, 2),
2.65 (2H, t, J ) 7.8 Hz, 3), 2.84 (1H, bs, OH), 3.65 (2H, t, J ) 6.6
Hz, 1), 7.06 (2H, d, J ) 8.1 Hz, 2′, 6′), 7.39 (2H, d, J ) 8.4 Hz, 3′,
5′); 13C NMR (CDCl3, 75 MHz) δ 30.9 (3), 33.4 (2), 61.4 (1), 119.1
Dihydro-combretastatin D-2 Benzyl Ether (41). Phenol 39 (3.83
g, 8.16 mmol), K2CO3 (9.19 g, 66.5 mmol), and CuBr-S(CH3)2 (5.23
g, 25.4 mmol) were dissolved in pyridine (1.1 L), and the solution was
heated to reflux for 24 h. Removal of solvent afforded a viscous black
oil, which was dissolved in EtOAc (150 mL). The solution was washed
with 1 N HCl (3 × 150 mL) and concentrated to a brown oil, which
was separated using gravity CC (9:1 hexane-acetone) to yield biaryl
ether 41 as a colorless solid. Crystallization from acetone-hexane
provided fine colorless needles (0.36 g, 11%): mp 136.5-137.6 °C;
1H NMR (CDCl3, 500 MHz) δ 2.09 (2H, tt, J ) 6.5, 2.5 Hz, 2′), 2.25
(2H, t, J ) 5.5 Hz, 2), 2.80 (2H, t, J ) 6.5 Hz, 3′), 2.83 (2H, t, J )
5.0 Hz, 3), 4.07 (2H, t, J ) 4.5 Hz, 1′), 5.23 (2H, s, ArCH2), 5.36 (1H,
d, J ) 1.5 Hz, 2′′), 6.57 (1H, dd, J ) 8.5, 2.0 Hz, 6′′), 6.81 (1H, d, J
) 8.0 Hz, 5′′), 7.03 (2H, d, J ) 8.5 Hz, 3′′′, 5′′′), 7.29 (2H, d, J ) 8.5
Hz, 2′′′, 6′′′), 7.31 (1H, d, J ) 7.5 Hz, 4′′′′), 7.38 (2H, t, J ) 7.5 Hz,
3′′′′, 5′′′′), 7.50 (2H, d, J ) 7.5 Hz, 2′′′′, 6′′′′); 13C NMR (CDCl3, 125
MHz) δ 26.9 (3), 28.6 (2′), 32.6 (2), 34.0 (3′), 63.9 (1′), 71.8 (ArCH2),
113.7 (2′′), 115.2 (5′′), 120.8 (6′′), 123.7 (3′′′, 5′′′), 127.4 (2′′′′, 6′′′′),
127.8 (4′′′′), 128.5 (3′′′′, 5′′′′), 131.0 (2′′′, 6′′′), 133.9 (1′′), 137.3 (1′′′,
1′′′′), 145.2 (4′′), 152.1 (3′′), 154.7 (4′′′), 173.8 (1); HRMS (APCI+)
m/z 389.1779 [M + H]+ (calcd for C25H25O4, 389.1753); anal. C
76.80%, H 6.27%, calcd for C25H24O4, C 77.30%, H 6.23%.
Dihydro-combretastatin D-2 (42). Palladium on carbon (5%, 41
mg) was added to a stirred solution of benzyl ether 41 (125 mg, 0.322
mmol) in EtOH-EtOAc (1:1, 10 mL). Hydrogen gas was bubbled
through until reaction was complete (by TLC). The catalyst was
removed, and concentration of the solution provided a colorless
crystalline solid that recrystallized from acetone-hexane as colorless
1
needles (79 mg, 82%): mp 175.4-177.7 °C; H NMR (CDCl3, 400
MHz) δ 2.07-2.12 (2H, m, 2′), 2.25 (2H, t, J ) 5.6 Hz, 2), 2.81 (2H,
t, J ) 6.8 Hz, 3′), 2.83 (2H, t, J ) 5.4 Hz, 3), 4.06 (2H, t, J ) 4.4 Hz,
1′), 5.30 (1H, d, J ) 2.0 Hz, 2′′), 5.52 (1H, s, OH), 6.60 (1H, dd, J )

Synthesis of Combretastatin D-2 Phosphate
Journal of Natural Products, 2009, Vol. 72, No. 5 883
33–43. (i) Cousin, D.; Mann, J.; Nieuwenhuyzen, M.; van den Berg,
H. Org. Biomol. Chem. 2006, 4, 54–62.
(13) Banwell, M. G.; Chand, S.; Savage, G. P. Tetrahedron: Asymmetry
2005, 16, 1645–1654.
(14) Murphy, P. J.; Lee, S. E. J. Chem. Soc., Perkin Trans. 1 1999, 3049–
3066.
(15) Youn, I. K.; Yon, G. H.; Pak, C. S. Tetrahedron Lett. 1986, 27, 2409–
2410.
8.4, 2.0 Hz, 6′′), 6.82 (1H, d, J ) 8.4 Hz, 5′′), 7.01 (2H, d, J ) 8.4 Hz,
3′′′, 5′′′), 7.30 (2H, d, J ) 8.4 Hz, 2′′′, 6′′′); 13C NMR (CDCl3, 100
MHz) δ 27.0 (3), 28.7 (2′), 32.8 (2), 34.0 (3′), 63.9 (1′), 112.6 (2′′),
115.0 (5′′), 121.5 (6′′), 123.5 (3′′′, 5′′′), 131.1 (2′′′, 6′′′), 132.7 (1′′),
137.9 (1′′′), 142.5 (4′′), 149.1 (3′′), 154.2 (4′′′), 173.9 (1); HRMS
(APCI+) m/z 299.1299 [M + H]+ (calcd for C18H19O4, 299.1283); anal.
C 72.50%, H 6.17%, calcd for C18H18O4, C 72.47%, H 6.08%.
(16) Hudlicky, T.; Sinai-Zingde, G.; Natchus, M. G. Tetrahedron Lett. 1987,
28, 5287–5290.
Acknowledgment. We are pleased to acknowledge the financial
support provided by Grant RO1 CA 90441-01-05 with the Division of
Cancer Treatment and Diagnosis, NCI, DHHS; the Arizona Disease
Control Research Commission; the Fannie E. Rippel Foundation; the
Robert B. Dalton Endowment Fund; and Gary L. and Diane R. Tooker,
Dr. John C. Budzinski, and Sally Schloegel. For other helpful assistance
we thank Drs. J. C. Knight, V. J. R. V. Mukku, R. K. Pettit, and M. S.
Hoard, as well as M. Dodson, F. Craciunescu, and C. A. Weber.
(17) Pettit, G. R.; Anderson, C. R.; Herald, D. L.; Jung, M. K.; Lee, D. J.;
Hamel, E.; Pettit, R. K. J. Med. Chem. 2003, 46, 525–531.
(18) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405–4408.
(19) (a) Mitsunobu, O. Synthesis 1981, 1–28. (b) Hughes, D. L.; Reamer,
R. A.; Bergan, J. J.; Grabowski, E. J. J. J. Am. Chem. Soc. 1988, 110,
6487–6491. (c) Ahn, C.; Correia, R.; DeShong, P. J. Org. Chem. 2002,
67, 1751–1753. (d) Ahn, C. J.; DeShong, P. J. Org. Chem. 2002, 67,
1754–1759. (e) McNulty, J.; Capretta, A.; Laritchev, V.; Dyck, J.;
Robertson, A. J. J. Org. Chem. 2003, 68, 1597–1600.
(20) Bhatt, M. V.; Kulkarni, S. U. Synthesis 1983, 249–282.
(21) (a) Bois-Choussy, M.; Beugelmans, R.; Bouillon, J. P.; Zhu, J.
Tetrahedron Lett. 1995, 36, 4781–4784. (b) Bois-Choussy, M.;
Neuville, L.; Beugelmans, R.; Zhu, J. J. Org. Chem. 1996, 61, 9309–
9322. (c) Rama Rao, A. V.; Gurjar, M. K.; Lakshmipathi, P.; Reddy,
M. M.; Nagarajan, M.; Pal, S.; Sarma, B. V. N. B. S.; Tripathy, N. K.
Tetrahedron Lett. 1997, 38, 7433–7436. (d) Evans, D. A.; Wood,
M. R.; Trotter, B. W.; Richardson, T. I.; Barrow, J. C.; Katz, J. L.
Angew. Chem., Int. Ed. Engl. 1998, 37, 2700–2704. (e) Boger, D. L.;
Castle, S. L.; Miyazaki, S.; Wu, J. H.; Beresis, R. T.; Loiseleur, O. J.
Org. Chem. 1999, 64, 70–80. (f) Neuville, L.; Bois-Choussy, M.; Zhu,
J. Tetrahedron Lett. 2000, 41, 1747–1751. (g) Beugelmans, R.;
Neuville, L.; Bois-Choussy, M.; Zhu, J. Tetrahedron Lett. 1995, 36,
8787–8790. (h) Rama Rao, A. V.; Reddy, K. L.; Rao, A. S.; Vittal,
T.; Reddy, M. M.; Pathi, P. L. Tetrahedron Lett. 1996, 37, 3023–
3026. (i) Bois-Choussy, M.; Vergne, C.; Neuville, L.; Beugelmans,
R.; Zhu, J. Tetrahedron Lett. 1997, 38, 5795–5798. (j) Evans, D. A.;
Watson, P. S. Tetrahedron Lett. 1996, 37, 3251–3254. (k) Crowley,
B. M.; Mori, Y.; McComas, C. C.; Tang, D.; Boger, D. L. J. Am.
Chem. Soc. 2004, 126, 4310–4317.
(22) Santaniello, E.; Ferraboschi, P.; Sozanni, P. J. Org. Chem. 1981, 46,
4584–4585.
(23) Jacobsen, E. N.; Deng, L.; Furukawa, Y.; Martinez, L. E. Tetrahedron
1994, 50, 4323–4334.
(24) SMART for Windows NT v5.605; Bruker AXS Inc.: Madison, WI.
(25) XPREP, The automatic space group determination program in
SHELXTL (see ref 27).
(26) Blessing, R. H. Acta Crystallogr. 1995, A51, 33–38.
(27) SHELXTL-Version 5.1; Bruker AXS Inc.: Madison, WI, 1997. An
integrated suite of programs for the determination of crystal structures
from diffraction data. This package includes, among others, XPREP (an
automatic space group determination program), SHELXS (a structure
solution program via Patterson or direct methods), and SHELXL (structure
refinement software).
References and Notes
(1) For the preceding contribution (AA 564) refer to: Monk, K. A.; Siles,
R.; Hadimani, M. B.; Mugabe, B. E.; Ackley, J. F.; Studerus, S. W.;
Edvardsen, K.; Trawick, M. L.; Garner, C. M.; Rhodes, M. R.; Pettit,
G. R.; Pinney, K. G. Bioorg. Med. Chem. 2006, 14, 3231–3244.
(2) Pinney, K. G.; Jelinek, C.; Edvardsen, K.; Chaplin, D. J.; Pettit, G. R.
In Anticancer Agents from Natural Products; Cragg, G. M.; Kingston,
D. G. I.; Newman, D. J., Eds.; CRC Press: Boca Raton, FL, 2005;
Chapter 3, pp 23-46.
(3) Pettit, G. R.; Cragg, G. M.; Herald, D. L.; Schmidt, J. M.; Lohavani-
jaya, P. Can. J. Chem. 1982, 60, 1374–1376.
(4) Pettit, G. R.; Singh, S. B.; Cragg, G. M. J. Org. Chem. 1985, 50,
3404–3406.
(5) Pettit, G. R.; Singh, S. B.; Niven, M. L.; Hamel, E.; Schmidt, J. M. J.
Nat. Prod. 1987, 50, 119–131.
(6) Pettit, G. R.; Singh, S. B. Can. J. Chem. 1987, 65, 2390–2396.
(7) Pettit, G. R.; Singh, S. B.; Hamel, E.; Lin, C. M.; Alberts, D. S.; Garcia-
Kendall, D. Experientia 1989, 45, 209–211.
(8) (a) Kanthou, C.; Greco, O.; Stratford, A.; Cook, I.; Knight, R.;
Benzakour, O.; Tozer, G. Am. J. Pathol. 2004, 165, 1401–1411. (b)
Cooney, M. M.; Ortiz, J.; Bukowski, R. M.; Remick, S. C. Curr. Oncol.
Rep. 2005, 7, 90–95. (c) Sheng, Y.; Hua, J.; Pinney, K. G.; Garner,
C. M.; Kane, R. R.; Prezioso, J. A.; Chaplin, D. J.; Edvardsen, K. Int.
J. Cancer 2004, 111, 604–610.
(9) Pettit, G. R.; Singh, S. B.; Niven, M. L. J. Am. Chem. Soc. 1988,
110, 8539–8540.
(10) Singh, S. B.; Pettit, G. R. J. Org. Chem. 1990, 55, 2797–2800.
(11) Couladouros, E. A.; Li, T.; Moutsos, V. I.; Pitsinos, E. N.; Soufli,
I. C. Bioorg. Med. Chem. Lett. 1999, 9, 2927–2928.
(12) (a) Boger, D. L.; Sakya, S. M.; Yohannes, D. J. Org. Chem. 1991,
56, 4204–4207. (b) Deshpande, V. H.; Gokhale, N. J. Tetrahedron
Lett. 1992, 33, 4213–4216. (c) Gangakhedkar, K. K. Synth. Commun.
1996, 26, 1887–1896. (d) Rychnovsky, S. D.; Hwang, K. J. Org. Chem.
1994, 59, 5414–5418. (e) Rychnovsky, S. D.; Hwang, K. Tetrahedron
Lett. 1994, 35, 8927–8930. (f) Couladouros, E. A.; Soufli, I. C.
Tetrahedron Lett. 1994, 35, 4409–4412. (g) Couladouros, E. A.; Soufli,
I. C. Tetrahedron Lett. 1995, 36, 9369–9372. (h) Couladouros, E. A.;
Soufli, I. C.; Moutsos, V. I.; Chadha, R. K. Chem.-Eur. J. 1998, 4,
(28) CCDC 714059 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
NP800635H