A second-generation total synthesis of (+)-phorboxazole A

Article abstract of DOI:10.1021/jo7018152

(Chemical Equation Presented) A highly convergent second-generation synthesis of (+)-phorboxazole A has been achieved. Highlights of the synthetic approach include improved Petasis-Ferrier union/rearrangement conditions on a scale to assemble multigram qu

Full text of DOI:10.1021/jo7018152

A Second-Generation Total Synthesis of (+)-Phorboxazole A
Amos B. Smith, III,* Thomas M. Razler, Jeffrey P. Ciavarri, Tomoyasu Hirose,
Tomoyasu Ishikawa, and Regina M. Meis
Department of Chemistry, The Penn Center for Molecular DiscoVery, and Monell Chemical Senses Center,
UniVersity of PennsylVania, Philadelphia, PennsylVania 19104
smithab@sas.upenn.edu
ReceiVed August 17, 2007
A highly convergent second-generation synthesis of (+)-phorboxazole A has been achieved. Highlights
of the synthetic approach include improved Petasis-Ferrier union/rearrangement conditions on a scale
to assemble multigram quantities of the C(11-15) and C(22-26) cis-tetrahydropyrans inscribed with
the phorboxazole architecture, a convenient method to prepare E- and Z-vinyl bromides from TMS-
protected alkynes utilizing radical isomerization of Z-vinylsilanes, and a convergent late-stage Stille union
to couple a fully elaborated C(1-28) macrocyclic iodide with a C(29-46) oxazole stannane side chain
to establish the complete phorboxazole skeleton. The synthesis, achieved with a longest linear sequence
of 24 steps, proceeded in 4.6% overall yield.
The oceans of the world have provided a rich source of
interesting and important natural products. In addition to their
magnificent architecture, many marine-derived products have
exceedingly potent biological properties. For example, disco-
dermolide,1a the halichondrins,1b and the spongistatins1c display
impressive nanomolar and in some cases subnanomolar cell
growth inhibitory activities when evaluated against a number
of human cancer cell lines. Given their important biological
activities, in conjunction with their often extreme scarcity, total
synthesis comprises the only viable resource to access these
potentially medicinally important compounds.
(+)-phorboxazole A and B [epimeric at C(13)] possess seven
olefins, a 21-membered C(1-26) macrolactone ring, with
four embedded rings, including a C(16-18) oxazole, and one
trans- and two cis-2,6-tetrahydropyran units. In addition, the
C(27-46) side chain display a diverse array of functional motifs,
including two rings, represented by a C(29-31) oxazole and a
C(33-37) tetrahydropyranyl hemiacetal accompanied by four
olefins, terminating in a C(45-46) E-vinyl bromide.
In 1995, Searle and Molinski isolated (+)-phorboxazoles A
and B (1 and 2) from an extract of the sponge Phorbas sp.,
endemic to the Indian Ocean near the western coast of
Australia.2 Through a series of NMR, degradation, and synthetic
correlation experiments, the relative and absolute configura-
tions of both 1 and 2 were determined to possess a novel
macrocyclic architecture.3 In addition to 15 stereogenic centers,
(1) (a) Gunasekera, S. P.; Gunasekera, M.; Longley, R. E.; Schulte, G.
K. J. Org. Chem. 1990, 55, 4912. (b) Pettit, G. R.; Herald, C. L.; Boyd, M.
R.; Leet, J. E.; Dufresne, C.; Doubek, D. L.; Schmidt, J. M.; Cerny, R. L.;
Hooper, J. N. A.; Ru¨tzler, K. C. J. Med. Chem. 1991, 34, 3339. (c) Pettit,
G. R.; Cichacz, Z. A.; Gao, F.; Herald, C. L.; Boyd, M. R.; Schmidt, J. M.;
Hooper, J. N. A. J. Org. Chem. 1993, 58, 1302.
In addition to their impressive architectural features, 1 and 2
have been shown to possess both antifungal and antibiotic
(2) Searle, P. A.; Molinski, T. F. J. Am. Chem. Soc. 1995, 117,
8126.
10.1021/jo7018152 CCC: $40.75 © 2008 American Chemical Society
Published on Web 01/24/2008
1192
J. Org. Chem. 2008, 73, 1192-1200

Second-Generation Total Synthesis of (+)-Phorboxazole A
activity when evaluated against Candida albicans and Saccha-
romyces carlsberensis, respectively. It is their extreme cyto-
toxicity, however, that has attracted the greatest interest from
the scientific community. When evaluated against the National
Cancer Institute’s (NCI) panel of human cancer cell lines, both
phorboxazole A and B displayed extraordinarily potent activity;
a mean GI50 value of 1.58 × 10-9 was observed across the entire
NCI panel. More specifically, when screened against the HCT-
116 (colon tumor) and MCF7 (breast) cancer cell lines, activities
of 4.36 × 10-10 and 5.62 × 10-10 were observed, respectively.
These data place the phorboxazoles in the category of the
halichondrins and spongistatins as some of the most selective
cytotoxic agents known.
Due to the architectural complexity and extraordinary biologi-
cal activity, (+)-phorboxazoles A and B (1 and 2) have attracted
a great deal of attention from the synthetic community. To date,
six total syntheses of (+)-phorboxazole A (1),4 four total
syntheses of (+)-phorboxazole B (2),5 and a number of related
synthetic studies have been reported.6 Determination of the
mechanism of action responsible for the exceedingly potent
cancer cell line growth inhibitory activity of 1 and 2, however,
has only recently been investigated.7
(+)-phorboxazole A.4b,c Improvements would clearly be neces-
sary to facilitate throughput of multigram scale quantities of
the highly complex intermediates. Herein we disclose a full
account of a now highly convergent, second-generation total
synthesis of (+)-phorboxazole A. In the accompanying paper,31
we will provide a full account of the design, synthesis, and
evaluation of a series of side chain and simplified macrocycle
analogues, which provide additional understanding of the
functionality required to retain potent human tumor cell growth
inhibitory activity. Importantly, we have discovered a new,
highly potent subnanomolar analogue, (+)-C(46)-chlorophor-
boxazole A, which has proven to be more readily available
synthetically than the parent (+)-phorboxazole A (1).
To construct meaningful quantities of (+)-phorboxazole A,
we required a highly convergent synthesis wherein fully
elaborated side chain 3 and macrocyclic 4 fragments would be
coupled at a late stage (Scheme 1). To avoid unwanted side
SCHEME 1
Given that only milligram quantities of 1 and 2 were procured
through isolation and total synthesis, we initiated a synthetic
campaign to produce quantities (ca. hundreds of milligrams) of
(+)-phorboxazole A and phorboxazole congeners amenable to
more extensive biological evaluation. In particular, we envi-
sioned a more convergent and efficient synthesis based upon
our successful 2001 first-generation total synthesis of
(3) (a) Searle, P. A.; Molinski, T. F.; Brzezinski, L. J.; Leahy, J. W. J.
Am. Chem. Soc. 1996, 118, 9422. (b) Molinski, T. F. Tetrahedron Lett.
1996, 37, 7879.
(4) (a) Forsyth, C. J.; Ahmed, F.; Cink, R. D.; Lee, C. S. J. Am. Chem.
Soc. 1998, 120, 5597. (b) Smith, A. B., III; Verhoest, P. R.; Minbiole, K.
P.; Schelhaas, M. J. Am. Chem. Soc. 2001, 123, 4834. (c) Smith, A. B., III;
Minbiole, K. P.; Verhoest, P. R.; Schelhaas, M. J. Am. Chem. Soc. 2001,
123, 10942. (d) Gonza´lez, M. A.; Pattenden, G. Angew. Chem., Int. Ed.
2003, 42, 1255. (e) Pattenden, G.; Gonza´lez, M. A.; Little, P. B.; Millan,
D. S.; Plowright, A. T.; Tornos, J. A.; Ye, T. Org. Biomol. Chem. 2003, 1,
4173. (f) Williams, D. R.; Kiryanov, A. A.; Emde, U.; Clark, M. P.; Berliner,
M. A.; Reeves, J. T. Angew. Chem., Int. Ed. 2003, 42, 1258. (g) Smith, A.
B., III; Razler, T. M.; Ciavarri, J. P.; Hirose, T.; Ishikawa, T. Org. Lett.
2005, 7, 4399. (h) White, J. D.; Kuntiyong, P.; Lee, T. H. Org. Lett. 2006,
8, 6039. (i) White, J. D.; Lee, T. H.; Kunttiyong, P. Org. Lett. 2006, 8,
6043.
(5) (a) Evans, D. A.; Cee, V. J.; Smith, T. E.; Fitch, D. M.; Cho, P. S.
Angew. Chem., Int. Ed. 2000, 39, 2533. (b) Evans, D. A.; Fitch, D. M.
Angew. Chem., Int. Ed. 2000, 39, 2536. (c) Evans, D. A.; Fitch, D. M.;
Smith, T. E.; Cee, V. J. J. Am. Chem. Soc. 2000, 122, 10033. (d) Li, D.-R.;
Zhang, D.-H.; Sun, C.-Y.; Zhang, J.-W.; Yang, L.; Chen, J.; Liu, B.; Su,
C.; Zhou, W.-S.; Lin, G.-Q. Chem. Eur. J. 2006, 12, 1185. (e) Lucas, B.
S.; Gopalsamuthiram, V.; Burke, S. D. Angew. Chem., Int. Ed. 2007, 46,
769.
(6) (a) Rychnovsky, S. D.; Hu, Y.; Ellsworth, B. Tetrahedron Lett.
1998, 39, 7271. (b) Rychnovsky, S. D.; Thomas, C. R. Org. Lett. 2000,
2, 1217. (c) Jasti, R.; Vitale, J.; Rychnovsky, S. D. J. Am. Chem. Soc.
2004, 126, 9904. (d) Wolbers, P.; Hoffmann, H. M. R.; Sasse, F. Synlett
1999, 1808. (e) Wolbers, P.; Misske, A. M.; Hoffmann, H. M. R.
Tetrahedron Lett. 1999, 40, 4527. (f) Wolbers, P.; Hoffmann, H. M. R.
Synthesis 1999, 797. (g) Wolbers, P.; Hoffmann, H. M. R. Tetrahedron
1999, 55, 1905. (h) Misske, A. M.; Hoffmann, H. M. R. Tetrahedron 1999,
55, 4315. (i) Schaus, J. V.; Panek, J. S. Org. Lett. 2000, 2, 469. (j) Huang,
H.; Panek, J. S. Org. Lett. 2001, 3, 1693. (k) Greer, P. B.; Donaldson, W.
A. Tetrahedron Lett. 2000, 41, 3801. (l) Greer, P. B.; Donaldson, W. A.
Tetrahedron 2002, 58, 6009. (m) White, J. D.; Kranemann, C. L.; Kuntiyong,
P. Org. Lett. 2001, 3, 4003. (n) Paterson, I.; Luckhurst, C. A. Tetrahedron
Lett. 2003, 44, 3749. (o) Paterson, I.; Steven, A.; Luckhurst, C. A. Org.
Biomol. Chem. 2004, 2, 3026. (p) Lucas, B. S.; Burke, S. D. Org. Lett.
2003, 5, 3915. (q) Lucas, B. S.; Luther, L. M.; Burke, S. D. Org. Lett.
2004, 6, 2965.
reactions, we envisioned protection of the C(45-46) E-vinyl
bromide as the corresponding E-vinylsilane. After side chain-
macrocycle union, two steps would be required to complete the
synthesis: installation of the C(46) bromide via exposure to an
electrophilic source of bromine and final global deprotection.
Construction of the side chain was projected to arise via a
route similar to our 2001 synthesis. Specifically, Stille union
of vinyl iodide 5 with vinylstannane 6 would furnish a dienyl
lactone intermediate which upon treatment with the Grignard
reagent derived from oxazole 7 would provide the complete
side-chain carbon framework (Scheme 1). However, unlike the
first-generation synthesis, introduction of the C(45-46) E-
(7) Forsyth, C. J.; Ying, L.; Chen, J.; La Clair, J. J. J. Am. Chem. Soc.
2006, 128, 3858.
J. Org. Chem, Vol. 73, No. 4, 2008 1193

Smith et al.
vinylsilane was then projected to arise via a transition-metal-
mediated hydrometalation event involving the free alkyne or
syn-reduction of the TMS-alkyne, followed by isomerization
to the thermodynamically favored E-silane. The synthetic
flexibility of this strategy would thereby permit evaluation of
both Stille and Suzuki coupling tactics with macrocycle 4. That
is, simple palladium-mediated stannylation or boronation of the
bidirectional oxazole triflate developed in our laboratory8 would
permit the opportunity to explore both synthetic tactics.
To construct macrocycle 4, we envisioned utilizing the
Z-selective Still-Gennari modified Horner-Emmons olefination
to fashion the C(2-3) Z-enoate.9 Elaboration of the C(19-20)
E-olefin was then expected to arise via an E-selective Wittig
reaction between aldehyde 8 and the ylide derived from tricycle
9.4b,c As in our first-generation (+)-phorboxazole A total
synthesis, both 2,6-cis-tetrahydropyran moieties (8 and 9) were
ideally suited to derive from a Petasis-Ferrier union/rearrange-
ment tactic.10,11 However, unlike the first-generation campaign,
if tens of grams of valuable intermediates were to be subjected
to the three-step Petasis-Ferrier sequence, improved conditions
would be required to facilitate material advancement.
promoted by Ti(O-i-Pr)4 and (R)-BINOL, followed by treatment
with TFA afforded dihydropyranone (-)-11 in 75% yield with
92% enantiomeric excess (ee). Considerable experimentation
was involved to identify reproducible conditions, including
temperature, solvent, and drying time of the molecular sieves
to define conditions to permit reaction scales at the 50 g level.
Drying the molecular sieves for 2.5 h at 100 °C under vacuum
followed by introduction of Ti(O-i-Pr)4/(R)-BINOL and alde-
hyde 10 and Danishefsky’s diene provided the optimal sequence
of events. Subsequent experimentation revealed that the Jacob-
sen chiral chromium(III) catalyst furnished dihydropyranone
(-)-11 with both improved yield (85%) and selectivity (ca. 95%
ee) on scales as large as 70 g.15 In addition, the Jacobsen
protocol was operationally more straightforward and less sen-
sitive to water; various molecular sieve drying times (ca. 1 to
24 h) delivered both high yields and enantioselectivity.
Condensation of (-)-11 with thio-enol ether 12, promoted
by Sc(OTf)3,16 next furnished the 1,4-addition product, which
17
upon treatment with Cp2TiMe2 followed by 10% Pd/C and
18
HSiEt3 yielded aldehyde (-)-14 in 73% for the three steps
(Scheme 2). Importantly, the Sc(OTf)3-promoted conjugate
addition consistently provided the thio-ester in high yield (ca.
95%) as a single diastereomer on scales up to 55 g. Here, solvent
water content proved inconsequential as both CH2Cl2 with and
without drying over molecular sieves consistently provided both
excellent yields and diastereoselectivity. In order to orchestrate
the C(11) hydroxyl stereogenicity responsible for directing the
Petasis-Ferrier rearrangement, a Sn(OTf)2-promoted Nagao
acetate aldol reaction was enlisted to provide the corresponding
â-hydroxy thioimide in 91% yield with good diastereoselectivity
(ca. 10:1 dr).19 In our hands, the reaction was critically
dependent on the purity of Sn(OTf)2. Commercial material from
several sources proved highly variable, affording a range of
yields and diastereoselectivities. With no clear trend identifiable,
we opted to prepare Sn(OTf)2 from SnCl2 and TfOH. Immediate
use of the snow-white Sn(OTf)2 proved optimal, as prolonged
storage afforded both depressed yields and selectivity. Following
diastereomer separation, treatment with LiOOH furnished â-hy-
droxy carboxylic acid (-)-15 in near quantitative yield.
Results and Discussion
We began the second-generation synthesis with construction
of the C(3)-C(19) segment tricycle 9 (Scheme 1). Utilizing
the Keck hetero-Diels-Alder conditions,12 treatment of known
aldehyde 1013 with the Danishefsky diene (Scheme 2),14
SCHEME 2
With (-)-15 in hand, we were poised to explore the large-
scale, three-step Petasis-Ferrier union/rearrangement proto-
col to construct the requisite C(11-15) cis-tetrahydropyran
(9). Following the lead from our 2001 synthesis, treatment of
(-)-15 with HMDS20 followed by known oxazole aldehyde
164b,f and TMSOTf furnished dioxanone (-)-17 in 90-95%
yield with good diastereomeric control (ca. 10:1, Scheme 3).
Importantly, this reaction was found to be reliable on scales as
large as 20 g. Residual HMDS was, however, found to have a
detremental effect on the reaction. To avoid quenching the
catalytic TfOH arising via hydrolysis of TMSOTf during the
reaction, complete removal of HMDS under vacuum at 60 °C
was paramount for high conversion.
Treatment of dioxanone (-)-17 with the Petasis-Tebbe
reagent (Cp2TiMe2) in THF at reflux for 48 h afforded the
desired enol-acetal, albeit in a disappointing yield of 58%
(Scheme 3). To facilitate material throughput, we undertook an
optimization study. We reasoned that the extended reaction times
allowed the excess Cp2TiMe2 to react with enol-acetal (-)-18
(8) Smith, A. B., III; Minbiole, K. P.; Freeze, B. S. Synlett, 2001, 1543.
(9) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405.
(10) (a) Smith, A. B., III; Verhoest, P. R.; Minbiole, K. P.; Lim, J. J.
Org. Lett. 1999, 1, 909. (b) Smith, A. B., III; Minbiole, K. P.; Verhoest, P.
R.; Beauchamp, T. J. Org. Lett. 1999, 1, 913.
(11) Smith, A. B., III; Fox, R. J.; Razler, T. M. Acc. Chem. Res., in
press.
(12) Keck, G. E.; Li, X.-Y.; Krishnamurthy, D. J. Org. Chem. 1995, 60,
5998.
(13) Boeckman, R. K., Jr.; Charette, A. B.; Asberom, T.; Johnston, B.
H. J. Am. Chem. Soc. 1987, 109, 7553.
(15) (a) Dossetter, A. G.; Jamison, T. F.; Jacobsen, E. N. Angew. Chem.,
Int. Ed. 1999, 38, 2398. (b) Joly, G. D.; Jacobsen, E. N. Org. Lett. 2002,
4, 1795.
(16) Yamashita, Y.; Saito, S.; Ishitani, H.; Kobayashi, S. J. Am. Chem.
Soc. 2003, 125, 3793.
(14) Danishefsky, S. Acc. Chem. Res. 1981, 14, 400.
1194 J. Org. Chem., Vol. 73, No. 4, 2008

Second-Generation Total Synthesis of (+)-Phorboxazole A
SCHEME 3
yield of the tetrahydropyranone product without the C(20)
p-methoxybenzyl (PMB) protecting group. After considerable
experimentation, we found that addition of cesium carbonate
(Cs2CO3) completely suppressed PMB loss during the 2 h
reaction time. While the exact nature of this beneficial outcome
is still under investigation, a scan of the literature points to the
formation of an aluminum-carbonate complex,22 which slowed
both the rate of the Petasis-Ferrier rearrangement as well as
the Lewis acid mediated PMB removal. Pleasingly, cis-
tetrahydropyranone (-)-19 was obtained in 66% yield for the
two steps on scales approaching 10 g. Three steps were then
utilized to complete the synthesis of 9. Reduction of the C(15)
ketone to the axial alcohol and protection as the TBS-ether
followed by removal of the PMB-group with DDQ furnished
(-)-9 in 92% for the three steps.
Construction of the C(22-26) tetrahydropyranone in our first-
generation synthesis also relied upon an efficient Petasis-Ferrier
rearrangement in this case of a mixed ethylidene enol-acetal
(1:1, E:Z) to construct simultaneously the tetrahydropyranone
and install the C(25) methyl.4b,c Construction of the ethylidene
enol-acetal, however, required a somewhat cumbersome four-
step sequence from the corresponding dioxanone involving a
Julia protocol.23 At the outset of our second-generation synthesis,
it was not clear if this reaction sequence would be well suited
for scales as large as 10-15 g. To facilitate material throughput,
a more direct approach was desired wherein installation of the
C(25) methyl after the Petasis-Ferrier rearrangement would
avoid the first-generation ethylidene enol-acetal construction.
In addition, we envisioned direct installation of the vinyl iodide
during dioxanone formation to avoid the multistep carbometa-
lation sequence utilized in the 2001 campaign.
in a [2 + 2] fashion, leading to decomposition. In addition, a
more concentrated reaction solution would provide an added
benefit toward increasing the reaction rate. Indeed, surveying
0.6, 0.7, 0.8, and 1.0 M solutions of the Petasis-Tebbe reagent
revealed that the 0.7 M solution of Cp2TiMe2 in THF instead
of the prescribed 0.5 M improved the yield to 65% in 24 h.
Both the 0.8 and 1.0 M conditions provided faster reaction rates;
however, reagent stability upon storage was found to be
problematic after 48 h freezer storage at 0 °C.
Ethyl pivalate has been utilized as a Cp2TiMe2 scavenger in
olefination reactions.21 In the presence of ethyl pivalate,
Cp2TiMe2 reacts first with the desired carbonyl in preference
to the hindered ethyl pivalate. However, once the olefinated
product is generated, ethyl pivalate becomes a more attractive
target for Cp2TiMe2 and thereby prevents secondary product
formation. When applied to the phorboxazole system, ethyl
pivalate in conjunction with the higher concentration of the
Petasis-Tebbe reagent21 provided the greatest improvement in
yield (ca. 80%, Scheme 3).
With this scenario in mind, direct condensation of vinyl iodide
aldehyde 21 with â-hydroxy carboxylic acid (+)-20 afforded
dioxanone (+)-22 (Scheme 4). We first explored direct instal-
lation of the C(25) methyl after the Petasis-Ferrier rearrange-
SCHEME 4
Purification of the unstable enol-acetal also proved to be
scale dependent. On a scale less than 1 g, silica gel purification
of (-)-18 was straightforward; however, on larger scales (ca.
> 15 g), we began to observe decomposition of (-)-18 to the
corresponding â-hydroxy ketone. Presumably, hydrolysis of the
enol-acetal was occurring from the extended exposure to acidic
silica gel due to the larger scales. To circumvent enol-acetal
decomposition, the titanium byproducts were quickly removed
by filtration through a short plug of silica gel, followed by
immediate execution of the Petasis-Ferrier rearrangement.
Turning next to the Petasis-Ferrier rearrangement, the con-
ditions utilized in the first-generation synthesis of (+)-phor-
boxazole A afforded only a modest 37% yield of the desired
C(11-15) tetrahydropyranone (-)-19, accompanied by a 20%
(17) Petasis, N. A.; Bzowej, E. I. J. Am. Chem. Soc. 1990, 112,
6392.
(18) Fukayama, T.; Tokuyama, H. Aldrichim. Acta 2004, 37, 87.
(19) Nagao, Y.; Yamada, S.; Kumagai, T.; Ochiai, M.; Fujita, E. J. Chem.
Soc., Chem. Commun. 1985, 1418.
(20) Noyori, R.; Murata, S.; Suzuki, M. Tetrahedron 1981, 37, 3899.
(21) Huffman, M. PCT. Int. Appl. WO 2000024723 A1, 2000.
(22) Evans, W. J.; Giarikos, D. G.; Ziller, J. W. Organometallics 2001,
20, 5751.
(23) Smith, A. B., III; Doughty, V. A.; Lin, Q.; Zhuang, L.; McBriar,
M. D.; Boldi, A. M.; Moser, W. H.; Murase, N.; Nakayama, Sobukawa, K.
M. Angew. Chem., Int. Ed. 2001, 40, 191.
J. Org. Chem, Vol. 73, No. 4, 2008 1195

Smith et al.
ment. Treatment of dioxanone (+)-22 with Cp2TiMe2 furnished
the corresponding enol-acetal in 79% yield after purification
by deactivated silica gel.24 Pleasingly, exposure of the enol-
acetal to Me2AlCl without a carbonate additive provided
tetrahydropyranone (+)-23 in nearly quantitative yield. Both
the BPS-protecting group and the vinyl iodide were unaffected
by the unbuffered Me2AlCl conditions. Analysis of the proposed
C(25) methylation process revealed that upon kinetic enolization
of (+)-23, the C(23) methyl should block the bottom face of
the molecule, thereby precluding formation of the undesired
C(25) axial methyl (Scheme 4). In the event, enolization with
LHMDS and HMPA as an additive, preventing higher order
lithium aggregates, followed by addition of CH3I pleasingly
afforded the desired C(25) equatorial methyl tetrahydropyranone
(+)-24 as a single diastereomer.25 A four-step protocol was
implemented to complete the synthesis of aldehyde (+)-8.4g
We next turned attention to union of (+)-8 with (-)-9 and
ultimately to completion of macrocycle 4 (Scheme 5). On a
yield, and selectivity. Conversion of alcohol (-)-9 to mesylate
(-)-27 proceeded in excellent yield (ca. 99%, Scheme 6). In a
SCHEME 6
SCHEME 5
one-flask operation, mesylate (-)-27 was then treated with tri-
n-butylphosphine (PBu3) in DMF to furnish the corresponding
ylide without formation of the undesired methyl oxazole C
(monitored by TLC). Without isolation, introduction of aldehyde
(+)-8, followed by DBU led to (+)-26 in excellent yield (96%)
with greater than 20:1 E:Z selectivity. Importantly, this one-
flask protocol reproducibly provided (+)-26 in ca. 96% yield
on scales as large as 2 g.
Closure of the macrocyle was accomplished using a five
step procedure that relied upon a final Still-Gennari modi-
fied Horner-Emmons olefination to fashion the requisite
Z-C(2-3) enoate in excellent yield, albeit modest selectivity
(ca. 96% yield, 2.5:1 Z:E, Scheme 6).4g Pleasingly, construction
of macrocycle (+)-4 was achieved with a longest linear sequence
of 20 steps and an overall yield of 20%.
With an efficient synthesis of macrocycle (+)-4 in hand, we
next turned attention to elaboration of the C(29-46) side chain
3. Beginning with lactone (-)-28 (Scheme 7), an intermediate
small scale, conversion of alcohol (-)-9 to the corresponding
chloride, followed by ylide formation and Wittig olefination
with aldehyde (+)-8, furnished the E-C(19-20) olefin (+)-26
in 92% yield, with >20:1 E:Z selectivity. However, conducting
the reaction on a gram scale afforded a disappointing 67% yield
of (+)-26, accompanied by 30% yield of a byproduct identi-
fied as methyl oxazole C. A similar reductive dehalogena-
tion was observed by Evans et al. in their total synthesis of
(+)-phorboxazole B,5a-c presumably involving nucleophilic
attack of PBu3 at chloride.
SCHEME 7
To circumvent this problem, we adopted the Evans protocol
to construct the E-C(19-20) olefin, for ease of operation, high
(24) Deactivated silica gel (prepared as a 10% water/silica gel (v/v) slurry)
was utilized to mitigate the acidity of silica gel, preventing hydrolysis of
the enol-acetal.
(25) Comparison of the 1HNMR spectrum of (+)-24 with that of the
first generation synthesis confirmed the stereochemical outcome of the
methylation.
1196 J. Org. Chem., Vol. 73, No. 4, 2008

Second-Generation Total Synthesis of (+)-Phorboxazole A
SCHEME 9
from our first-generation synthesis, we envisioned installation
of an E-vinylsilane as a surrogate for the phorboxazole vinyl
bromide. After union with macrocycle (+)-4, bromo-desilylation
was expected to afford protected phorboxazole A. To this end,
semi-reduction of (-)-28 under Lindlar conditions, afforded
Z-silane (-)-29 in 87% yield with 15:1 selectivity (Z:E, Scheme
7). Initial attempts to isomerize (-)-29 to the desired E-silane
under acidic conditions met with little success. Careful examina-
tion of the literature revealed that Utimoto had reported the
isomerization of both Z-vinyl trimethylgermanium and the
corresponding silane substrates to the corresponding E-isomers
employing radical conditions initiated by triethylborane (Et3B)
and HSnBu3.26 Exposure of (-)-29 to catalytic Et3B and HSnBu3
for 24 h furnished the desired E-silane (-)-30 in 89% yield
as an inseparable mixture (ca. 5:1 E:Z) of isomers. Extending
the reaction time to 72 h led to (-)-30 in 83% yield with
excellent E-selectivity (>20:1 E:Z). The critical bromo-desily-
lation event was then examined as a model for the late stage
bromide installation on the phorboxazole system. Both the Z-
and E-silanes, (-)-29 and (-)-30, were independently exposed
to NBS/CH3CN to afford the corresponding vinyl bromides
[(-)-31 and (-)-32, respectively].
Final elaboration of side chain 3 entailed treatment of oxazole
78 with isopropylmagnesium chloride to induce a facile mag-
nesium-halogen exchange (Scheme 8). Exposure of the derived
SCHEME 8
provided a 76% yield of the coupled product (+)-34 on a 5 mg
scale. Importantly, isolation entailed direct addition of the
reaction mixture to a silica gel column. The additive Ph2PO2NBu4
has been shown by Liebeskind to sequester the transmetalation
byproduct XSnR3 and prevent entry into the Stille catalytic
cycle.27 In our case, Ph2PO2NBu4 afforded a dramatic increase
in yield, indicating the involvement of ISnMe3 in the catalytic
cycle.28,29 To complete the second-generation total synthesis of
(+)-phorboxazole A, all that remained was bromodesilylation
at C(46), followed by global deprotection. However, upon
treatment of (+)-34 with NBS/MeCN, conditions that proved
successful in the side-chain model study, the expected protected
(+)-phorboxazole A was not obtained. Instead, bromination
occurred at the C(27-28) trisubstituted olefin as defined by 1H
NMR. Unable to circumvent the reactivity of the C(27-28)-
trisubstituted olefin, we turned to our first-generation route to
complete phorboxazole A (1), as by this time we recognized
C(46)-chlorophorboxazole A to be the more important target
for large-scale synthesis; see the accompanying paper.31
Stille union of macrocycle vinyl iodide (+)-4 with the
C(45-46) TMS-alkyne side chain (-)-36 delivered (+)-37
comprising the full phorboxazole carbon backbone (Scheme 10);
the yield was 68%. Treatment with NBS and AgNO3 then
provided the corresponding bromoalkyne in excellent yield
(95%), which upon exposure to PdCl2(PPh3)2 and HSnBu3
delivered an inseparable mixture of external and internal vinyl
stannanes (ca. 6:1).30
Grignard reagent to dienyl lactone (-)-30 led to the corre-
sponding hemiacetal. Immediate treatment with p-TsOH and
MeOH provided the expected mixed methyl hemiacetal (-)-33
in 73% yield for the two steps. Completion of the side chain
then called upon a palladium-catalyzed C(29) triflate-trimeth-
ylstannane exchange to furnish (-)-3 in 64% yield. The longest
linear sequence to (-)-3 entailed 17 steps and proceeded in 6.5%
overall yield.
Having achieved the syntheses of both the macrocycle and
side-chain fragments, we turned to their union, employing a
palladium-mediated Stille tactic (Scheme 9).4g Exploring a
variety of ligands, bases, solvents, and temperature ranges, we
found that a catalyst system comprising 1.5 mol % of
Pd2(dba)3‚CHCl3 with AsPh3 in combination with diisopropy-
lamine, Ph2PO2NBu4, and DMF at room temperature for 15 h
(27) Alfred, G. D.; Liebeskind, L. S. J. Am. Chem. Soc. 1996, 118, 2748.
(28) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113, 9585.
(29) (a) Casado, A. L.; Espinett, P. J. Am. Chem. Soc. 1998, 120, 8978.
(b) Amatore, C.; Bahsoun, A. A.; Jutand, A.; Meyer, G.; Nededi Ntepe,
A.; Ricard, L. J. Am. Chem. Soc. 2003, 125, 4212.
(30) Zhang, H. X.; Guibe´, F.; Balavoine, G. J. Org. Chem. 1990, 55,
1857.
(26) Taniguchi, M.; Nozaki, K.; Miura, K.; Oshima, K.; Utimoto, K.
Bull. Chem. Soc. Jpn. 1992, 65, 349.
(31) Smith, A. B., III; Razler, T. M.; Meis, R. M.; Pettit, G. R. J. Org.
Chem. 2008, 74, 1201.
J. Org. Chem, Vol. 73, No. 4, 2008 1197

Smith et al.
SCHEME 10
Experimental Section
In the sections to follow, key transformations are described; for
full details of this synthetic venture, see the Supporting Information.
Preparation of Cyclic Enone (-)-11 (Keck Procedure).
Trifluoroacetic acid (TFA, 0.029 mL, 0.384 mmol) was added
to a solution of R-(+)-BINOL (5.50 g, 19.2 mmol), distilled
Ti(O-i-Pr)4 (2.83 mL, 9.60 mmol), and 4 Å molecular sieves (36.9
g) in Et2O (320 mL) at rt under an atmosphere of argon.32 After
the reaction mixture was heated for 1 h at 35 °C, the slurry was
cooled to rt, where aldehyde 10 (30.0 g, 96.0 mmol) in Et2O (107
mL) was added via cannula as a steady stream. The reaction mixture
was then cooled to -78 °C, and Danishefsky’s diene (28.0 mL,
144 mmol) was added via cannula. After being stirred for 10 min,
the reaction mixture was warmed to rt and stirred for 24 h. A
solution of saturated NaHCO3 (20 mL) was added, and the reaction
mixture was stirred for 1 h. At this point, the reaction mixture was
filtered through a pad of Celite and the filtrate was washed with
saturated NaHCO3 (200 mL, 1×) and brine (200 mL, 1×), dried
over Na2SO4, filtered, and concentrated under reduced pressure.
The crude product was dissolved in CH2Cl2 (600 mL) and cooled
to 0 °C. TFA (40 mL) was added via syringe and the reaction
mixture stirred for 1 h, quenched with saturated NaHCO3 (800 mL),
and extracted with CH2Cl2 (1 L, 3×). The combined organic extracts
were dried over Na2SO4, filtered, and concentrated. Removal of
BINOL was accomplished via silica gel chromatography (90/10/1,
CHCl3/Et2O/Et3N). Further purification of the resultant oil via silica
gel chromatography (20% EtOAc/hexanes) afforded enone (-)-11
(27.4 g, 75% yield, 92% ee) as a clear oil. Enantiomeric excess
was determined by chiral HPLC: Chiralcel OD, 98:2 hexanes/
iPrOH, 0.75 mL/min, 50 µL injection loop); [R]20 -35.8 (c 1.0,
D
CHCl3); IR (CHCl3) 2980 (s), 1670 (s), 1595 (w), 1280 (s), 1100
(s) cm-1; HNMR (500 MHz, CDCl3) δ 7.64 (m, 4H), 7.30 (m,
1
Without purification, treatment with NBS/MeCN furnished
the desired brominated product, which again proved inseparable
via silica gel chromatography. Global deprotection employing
6% HCl/THF at room temperature, conditions developed in our
first-generation synthesis, led to the fully deprotected product
as a 6:1 external/internal mixture of vinyl bromides. Separation
by reversed-phase chromatography delivered (+)-phorboxazole
A (1) in a combined yield of 35% for the three steps.4g
6H), 7.26 (d, J ) 6.0 Hz, 1H), 5.37 (dd, J ) 6.0, 1.0 Hz, 1H), 4.66
(m, 1H), 3.85 (ddd, J ) 13.0, 10.5, 8.2 Hz, 1H), 3.78 (ddd, J )
10.7, 5.5, 5.3 Hz, 1H), 2.46 (m, 2H), 2.00 (m, 1H), 1.88 (m, 1H),
1.02 (s, 9H); 13CNMR (125 MHz, CDCl3) δ 192.4, 163.0, 135.5,
133.5, 129.8, 127.7, 107.1, 59.2, 41.9, 37.2, 29.8, 19.1; high-
resolution mass spectrum (ES, NH3) m/z 381.1888 [(M + H)+; calcd
for C23H29O3Si: 381.1885].
Preparation of Cyclic Enone (-)-11 (Jacobsen Procedure).
A solution of aldehyde 10 (70.5 g, 226.0 mmol) in acetone (100
mL) was added via cannula to a slurry of 4 Å molecular sieves (45
g) and Jacobsen’s Cr(III) catalyst (1.53 g, 3.38 mmol) in acetone
(350 mL). The reaction mixture was stirred at rt for 3 h, at which
point Danishefsky’s diene (65.8 mL, 338.0 mmol) was added and
the reaction mixture was stirred for 15 h. The reaction mixture was
diluted with THF (400 mL) and cooled to 0 °C, and TFA (23.5
mL) was added over 1.5 h via syringe. After the addition was
complete, the reaction mixture was stirred for an additional 30 min,
warmed to rt, and diluted with 2/1 hexanes/ether (1 L). The reaction
mixture was then washed with water (1 L, 2×), followed by
saturated aqueous NaHCO3 (1 L, 1×) and saturated aqueous NaCl
(1 L, 1×). The organic extracts were dried over MgSO4, filtered,
and concentrated under reduced pressure. Purification via silica gel
chromatography (10% EtOAc/hexanes) afforded cyclic enone
(-)-11 (73.1 g, 85% yield, 95% ee) as a clear oil. Enantiomeric
excess was determined by chiral HPLC: Chiralcel OD, 98:2
hexanes/iPrOH, 0.75 mL/min, 50 µL injection loop).
In summary, an effective second-generation total synthesis
of (+)-phorboxazole A (1) has been achieved, with an improved
longest linear sequence of 24 steps and an overall yield of 4.6%,
compared to our first-generation route (27 steps and 3.1%).
Importantly, improved Petasis-Ferrier union/rearrangement
conditions were developed, permitting access to multigram
quantities of the C(11-15) tetrahydropyranone (-)-19. In
addition, a convenient method to access both E- and Z-vinyl
bromides from TMS-protected alkynes was developed employ-
ing a highly stereoselective radical vinylsilane isomerization.
Robust Stille coupling conditions were also identified which
enabled the efficient union of the advanced fragments, vinyl
iodide macrocycle (+)-4, and oxazole stannane sidechains (-)-3
and (-)-36.
Importantly, our second-generation synthesis of (+)-phor-
boxazole A provided the resources to initiate an analogue
program. In the following publication,31 we will detail the
design, total synthesis, and biological evaluation of a series of
C(45-46), C(11-15), and C(2-3) phorboxazole congeners.
Through these studies, the more readily available C(46)-
chlorophorboxazole A was identified as an analogue possessing
picomolar tumor cell growth inhibitory avtivity. The C(46)
chloro congener thus became our target for preparative-scale
elaboration.
Preparation of trans-Tetrahydropyranone Thio-ester (-)-SI1.
To a solution of enone (-)-11 (50.4 g, 132.4 mmol) and scandium
triflate [Sc(OTf)3] (0.65 g, 1.32 mmol, added as a solid) in
dichloromethane (662 mL) under argon was added dropwise the
TMS-enol ether derived from ethyl thioacetate (35.0 g, 198.6 mmol)
at -78 °C over 2 h. Once the addition was complete, the reaction
(32) The 4 Å molecular sieves were purchased as an activated powder
(< 50 µm). Activation at 100 °C for 2.5 h under atmospheric pressure proved
optimal.
1198 J. Org. Chem., Vol. 73, No. 4, 2008

Second-Generation Total Synthesis of (+)-Phorboxazole A
mixture was stirred for an additional 15 min at -78 °C. When the
starting enone was consumed (as monitored by TLC), a solution
of methanol/pyridine (1/1, 750 mL) was added dropwise over 3 h.
Once the addition was complete, the -78 °C bath was removed
and the reaction was warmed to rt. After 1 h and 30 min at rt, the
layers of the biphasic mixture were separated and the aqueous layer
was extracted with dichloromethane (300 mL, 3×). The combined
organic extracts were dried over Na2SO4, filtered, and concentrated
under reduced pressure. Purification via silica gel chromatography
(10% EtOAc/hexanes) afforded trans-tetrahydropyranone thio-ester
47.2, 46.3, 39.6, 39.5, 39.4, 36.4, 26.9, 19.2; high-resolution mass
spectrum (ES, NH3) m/z 732.3359 [(M + Na)+, calcd for
C42H51O7NSiNa 732.3333].
Preparation of Methylated Tetrahydropyranone (+)-24.
Under an argon atmosphere in a flame-dried 250 mL round-bottom
flask at rt, lithium hexamethyldisilazane (LiHMDS) (1.19 M/THF)
(9.92 mL, 11.8 mmol) was diluted with THF (9.92 mL). and the
solution was cooled to -78 °C with stirring. After 20 min, a solution
of tetrahydropyranone (+)-23 (6.03 g, 10.7 mmol) in THF (53.5
mL) was added dropwise via cannula over 45 min to the diluted
LHMDS solution. After 30 min at -78 °C, the solution was warmed
to -20 °C and stirred for 1 h. A cooled solution (-20 °C) of methyl
iodide (2.00 mL, 32.2 mmol) and HMPA (5.60 mL, 32.2 mmol) in
THF (21.5 mL) was then added dropwise via cannula over 1 h,
and the reaction mixture was stirred at this temperature. After 2 h,
the reaction was quenched at -20 °C with saturated aqueous NH4Cl
(130 mL), warmed to rt, stirred for 30 min, and then extracted with
Et2O (150 mL, 3×). The combined organic extracts were washed
with saturated aqueous NH4Cl (200 mL) followed by saturated
aqueous NaCl (200 mL), dried over Na2SO4, filtered, and concen-
trated under reduced pressure. Purification via silica gel chroma-
tography (2.5% EtOAc/hexanes) furnished methylated tetrahydro-
pyranone (+)-24 (4.20 g, 68%; 91% based on recovered starting
material), as a colorless oil and starting material (+)-23 (1.53 g,
25%). (+)-24: [R20]D +12.6 (c 1.1, CHCl3); IR (CHCl3) 2930 (m),
2857 (m), 1715 (s), 1457 (w), 1428 (m), 1282 (w), 1197 (w), 1112
(-)-SI1 (60.9 g, 95%) as a light yellow oil: [R]20 -9.1 (c 2.3,
D
CHCl3); IR (CHCl3) 2931 (m), 2857 (m), 1719 (s), 1685 (s), 1472
(w), 1428 (m), 1112 (s), 998 (w), 823 (w), 739 (m), 614 (m) cm-1
;
1HNMR (500 MHz, CDCl3) δ 7.65 (m, 4H), 7.38 (m, 6H), 4.50
(m, 1H), 4.41 (ddd, J ) 8.6, 5.7, 5.5 Hz, 1H), 3.79 (ddd, J ) 10.5,
7.8, 5.4 Hz, 1H), 3.71 (m, 1H), 2.84 (m, 3H), 2.65 (dd, J ) 14.9,
5.8 Hz, 1H), 2.58 (ddd, J ) 14.5, 5.3, 1.1 Hz, 1H), 2.53 (ddd, J )
14.5, 4.7, 1.3 Hz, 1H), 2.33 (ddd, J ) 14.7, 7.6, 1.1 Hz, 1H), 2.30
(ddd, J ) 14.5, 6.2, 1.3 Hz, 1H), 1.89 (m, 1H), 1.68 (m, 1H), 1.20
(app t, J ) 7.4 Hz, 3H), 1.05 (s, 9H); 13CNMR (125 MHz, CDCl3)
δ 206.2, 195.7, 135.5, 133.7, 133.6, 129.6, 129.5, 127.7, 70.0, 68.7,
59.9, 48.7, 46.5, 46.2, 36.7, 26.9, 23.5, 19.2, 14.5; high-resolution
mass spectrum (ES+) m/z 507.2024 [(M + Na)+; calcd for
C27H36O4SSiNa 507.2001].
Preparation of Tetrahydropyranone (-)-19. Under an argon
atmosphere, Cp2TiMe2 (0.7 M/THF, 206.5 mL, 144.6 mmol) was
added to a solution of dioxanone (-)-17 (17.2 g, 24.1 mmol) and
ethyl pivalate (1.94 mL, 12.0 mmol) in THF (240 mL). The orange
solution was heated to 65 °C with stirring in the absence of light
and monitored by TLC. After 15 h, an additional charge of
Cp2TiMe2 (0.7 M/THF, 137.7 mL, 96.4 mmol) was added along
with ethyl pivalate (1.94 mL, 12.0 mmol), and the dark orange
solution was allowed to stir at 65 °C. After an additional 10 h of
stirring, Cp2TiMe2 (0.7 M/THF, 68.9 mL, 48.1 mmol) was added
in addition to ethyl pivalate (1.94 mL, 12.0 mmol). After 15 h of
additional stirring at 65 °C, the reaction was deemed complete by
TLC. The dark orange reaction mixture was cooled to rt and diluted
with hexanes (0.5 L) in order to precipitate Cp2TiO. The orange
reaction slurry was filtered through a pad of silica gel and washed
with 40% EtOAc/hexanes (2.5 L). The resultant filtrate was
concentrated under reduced pressure to afford enol-acetal (-)-18
(14.5 g) as a light orange, clear oil that was used as is in the
subsequent Petasis-Ferrier rearrangement.
1
(s), 822 (w), 738 (m), 701 (s) cm-1; HNMR (500 MHz, CDCl3)
δ 7.64 (m, 4H), 7.40 (m, 6H), 6.25 (d, J ) 0.6 Hz, 1H), 3.93 (ddd,
J ) 8.8, 4.0, 2.4 Hz, 1H), 3.77 (ddd, J ) 10.0, 10.0, 4.7 Hz, 1H),
3.70 (ddd, J ) 10.0, 4.9, 4.9 Hz, 1H), 3.66 (d, J ) 10.5 Hz, 1H),
2.65 (dddd, J ) 14.4, 10.5, 6.6, 3.9 Hz, 1H), 2.43 (dddd, J ) 11.8,
7.1, 4.7, 2.4 Hz, 1H), 1.90 (d, J ) 0.5 Hz, 3H), 1.85 (m, 1H), 1.64
(m, 1H), 1.15 (d, J ) 7.1 Hz, 3H), 1.04 (s, 9H), 0.86 (d, J ) 6.6
Hz, 3H); 13CNMR (125 MHz, CDCl3) δ 211.8, 145.5, 135.5, 133.7,
133.6, 129.6, 129.4, 127.7, 88.0, 81.7, 75.7, 60.0, 49.3, 43.2, 34.6,
26.8, 19.2, 18.8, 11.2, 9.3; high-resolution mass spectrum (ES+)
m/z 599.1467 [(M + Na)+, calcd for C28H37O3SiINa 599.1454].
Preparation of Vinyl Iodide (+)-26. To a solution of mesylate
(-)-27 (2.02 g, 2.57 mmol) in anhydrous DMF (549 mL) at rt under
an argon atmosphere was added dropwise tri-n-butylphosphine (2.57
mL, 10.3 mmol). After 36 h of stirring at rt, aldehyde (+)-8 (1.25
g, 2.57 mmol) in anhydrous DMF (295 mL) was added dropwise
via cannula over 10 min followed immediately by dropwise addition
of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (0.77 mL, 5.14 mmol)
via syringe over 15 min. After being stirred at rt for 3 h, the reaction
mixture was diluted with diethyl ether (400 mL) and poured into
water (400 mL). The layers were separated, and the aqueous layer
was extracted with diethyl ether (150 mL, 5×). The combined
organic extracts were washed with saturated aqueous NaCl (350
mL), dried over MgSO4, filtered, and concentrated under reduced
pressure. Purification via silica gel chromatography (20% EtOAc/
hexanes) afforded vinyl iodide (+)-26 (2.86 g, 96%, 20:1, E:Z) as
Me2AlCl (1 M/hexanes, 26.5 mL, 26.5 mmol) was added
dropwise over 30 min to a stirring slurry of Cs2CO3 (10.6 g, 32.6
mmol) in CH2Cl2 (500 mL) at rt under an argon atmosphere. After
stirring for 1 h, a solution of enol-acetal (-)-18 (14.5 g, 20.4
mmol) in CH2Cl2 (317 mL) was added to the Me2AlCl/Cs2CO3
solution at rt via cannula as a steady stream over 10 min. After 2
h, the reaction was quenched by dropwise addition of a saturated
aqueous solution of NaHCO3 (263 mL), and the bisphasic mixture
was stirred vigorously at rt. After 15 min, the layers were separated,
and the aqueous layer was extracted with CH2Cl2 (150 mL, 2×).
The combined organic extracts were washed with a saturated
solution of NaCl (1×, 150 mL), dried over MgSO4, filtered, and
concentrated under reduced pressure to afford a clear, colorless oil.
Purification via silica gel chromatography (40% EtOAc/hexanes)
afforded tetrahydropyranone (-)-19 (11.2 g, 66% yield over two
a white foam: [R]20 +22.7 (c 1.0, CHCl3); IR (neat) 2967 (b),
D
2930 (s), 1487 (m), 1258 (s), 1110 (s), 776 (s) cm-1; 1HNMR (500
MHz, CDCl3) δ 7.64 (m, 4H), 7.35 (m, 6H), 6.89 (d, J ) 1.9 Hz,
1H), 6.82 (dd, J ) 8.2, 1.9 Hz, 1H), 6.58 (m, 1H), 6.32 (d, J )
16.0 Hz, 1H), 6.23 (s, 1H), 4.82 (dd, J ) 11.2, 1.9 Hz, 1H), 4.70
(s, 2H), 4.56 (d, J ) 11.2 Hz, 1H), 4.26 (d, J ) 11.2 Hz, 1H), 4.24
(m, 1H), 3.95 (m, 3H), 3.87 (s, 3H), 3.65 (m, 2H), 3.50 (d, J )
10.4 Hz, 1H), 3.45 (ddd, J ) 14.1, 7.1, 1.8 Hz, 1H), 3.12 (dd, J )
10.4, 4.5 Hz, 1H), 2.52 (m, 1H), 2.34 (m, 3H), 2.08 (m, 1H), 2.00
(dd, J ) 13.0, 7.0 Hz, 1H), 1.93 (dd, J ) 13.0, 7.0 Hz, 1H), 1.86
(m, 2H), 1.82 (s, 3H), 1.80 (m, 2H), 1.75 (m, 4H), 1.61 (m, 3H),
1.48 (m, 3H), 1.06 (s, 9H), 0.97 (d, J ) 6.7 Hz, 3H), 0.91 (s, 9H),
0.76 (d, J ) 6.3 Hz, 3H), 0.06 (s, 6H); 13CNMR (125 MHz, CDCl3)
δ 160.7, 149.0, 148.6, 146.3, 143.3, 142.3, 135.6, 135.5, 135.1,
134.2, 133.9, 133.8, 130.9, 129.5, 127.6, 120.2, 118.8, 111.1, 110.1,
87.5, 82.8, 80.9, 77.4, 77.2, 69.9, 69.0, 68.8, 67.4, 64.7, 60.6,
55.9, 55.7, 39.6, 39.3, 39.2, 38.9, 38.2, 36.6, 36.2, 33.7, 33.5, 33.3,
steps) as a clear oil: [R]20 -14.7 (c 0.20, CHCl3); IR (CHCl3)
D
2940 (s), 1720 (s), 1105 (s) cm-1; HNMR (500 MHz, CDCl3) δ
1
7.65 (m, 4H), 7.49 (s, 1H), 7.38 (m, 6H), 7.25 (m, 2H), 6.87 (dd,
J ) 6.6, 2.1 Hz, 2H), 4.72 (s, 2H), 4.52 (s, 2H), 4.51 (s, 2H), 4.50
(m, 1H), 4.00 (m, 1H), 3.92 (m, 1H), 3.82 (m, 1H), 3.79 (s, 3H),
3.74 (m, 1H), 3.67 (m, 1H), 2.69 (dd, J ) 14.4, 11.8 Hz, 1H), 2.58
(dt, J ) 14.3, 2.6 Hz, 1H), 2.49 (ddd, J ) 14.5, 2.3, 2.1 Hz, 1H),
2.33 (m, 3H), 2.16 (m, 1H), 2.02 (m, 2H), 1.78 (m, 1H), 1.65 (m,
2H), 1.04 (s, 9H); 13CNMR (125 MHz, CDCl3) δ 205.6, 161.3,
159.5, 141.7, 140.4, 135.9, 135.5, 133.9, 129.7, 129.6, 129.2, 127.7,
113.9, 110.5, 74.3, 72.3, 72.8, 71.7, 69.0, 68.2, 63.5, 60.5, 55.3,
J. Org. Chem, Vol. 73, No. 4, 2008 1199

Smith et al.
30.3, 26.8, 25.8, 19.1, 18.0, 13.5, 5.7, -4.8, -4.9; high-resolution
mass spectrum (ES+) m/z 1182.4735 [(M + Na)+, calcd for
C61H86INO9Si2Na 1182.4886].
Hz, 3H), 0.24 (s, 9H), 0.03 (s, 3H), 0.01 (s, 3H); 13CNMR (125
MHz, C6D6) δ 165.8, 161.7, 160.0, 145.5, 143.8, 143.0, 139.2,
138.2, 137.3, 136.8, 135.0, 134.4, 133.8, 133.5, 121.4, 120.1, 119.5,
110.5, 104.6, 100.8, 90.0, 86.8, 81.0, 80.2, 78.7, 74.7, 74.1, 73.7,
72.7, 69.9, 69.3, 67.7, 65.9, 56.7, 55.6, 48.3, 42.4, 40.5, 40.4, 40.0,
37.9, 36.4, 36.3, 34.9, 33.1, 32.4, 31.2, 30.5, 27.9, 26.3, 18.7, 18.6,
14.6, 14.1, 13.7, 13.2, 6.6, 1.7, 0.6, -4.5, -4.4; high-resolution
mass spectrum (ES+) m/z 1321.7641 [(M + Na)+, calcd for
C72H114N2O13Si3Na 1321.7626].
Preparation of Protected C(45-46)-TMS-alkyne-phorbox-
azole (+)-37. Vinyl iodide macrocycle (+)-4 (13.8 mg, 0.017
mmol) and C(45-46)-TMS-alkyne side chain (-)-36 (21.0 mg,
0.026 mmol) were combined in a flame-dried round-bottom flask
(5 mL), azeotroped from benzene (2 mL, 3×), and dried under
vacuum for 2 h. To the flask under an argon atmosphere were
added tris(dibenzylideneacetone)dipalladium-chloroform adduct
[Pd2(dba)3‚CHCl3] (3.6 mg, 0.003 mmol), triphenylarsine (AsPh3)
(6.4 mg, 0.021 mmol), and Ph2PO2NBu4. (12 mg, 0.026 mmol)
followed by introduction of DMF (0.17 mL, sparged with argon,
30 min) and diisopropylethylamine (DIPEA) (0.003 mL, 0.017
mmol). After being stirred for 16 h at rt, the light brown reaction
mixture was introduced directly onto a silica gel column (20%
EtOAc/hexanes f 30% EtOAc/hexanes) to afford protected
C(45-46)-TMS-alkynyl-phorboxazole (+)-37 (14.9 mg, 68%) as
a light yellow oil: [R]20D +1.3 (c 0.2, CHCl3); IR (neat) 2925 (b),
1718 (s), 1456 (b), 1250 (s), 1187 (s), 1091 (s), 1053 (b), 840 (s)
Preparation of Phorboxazole A (+)-1. Protected phorboxazole
A, under an argon atmosphere, was dissolved in freshly distilled
THF (1.7 mL) followed by the dropwise addition of 6% HCl (0.67
mL) at rt. After 96 h, the reaction mixture was cooled to 0 °C and
poured into saturated aqueous sodium bicarbonate (5 mL) and
extracted with dichloromethane (5 mL, 3×) followed by ethyl
acetate (5 mL, 3×). The combined organic extracts were dried over
MgSO4, filtered, and concentrated under reduced pressure. Purifica-
tion via silica gel chromatography (100% EtOAc f 10% methanol/
EtOAc) afforded phorboxazole A (+)-1 (1.1 mg, 6:1 C(46)/C(45)
vinyl bromide). Further purification via reverse phase HPLC
(Zorbax C18 column, acetonitrile/H2O (55/45) eluent) afforded
phorboxazole A (+)-1 (0.90 mg, 35% over three steps after HPLC
purification) which matched the 1HNMR, [R]20D, and high-resolution
1
cm-1; HNMR (500 MHz, C6D6) δ 6.96 (s, 1H), 6.90 (m, 1H),
6.37 (s, 1H), 6.32 (d, J ) 15.7 Hz, 1H), 6.20 (d, J ) 15.9 Hz, 1H),
5.79 (dd, J ) 11.2, 2.3 Hz, 1H), 5.69 (dd, J ) 15.7, 7.3 Hz, 1H),
5.57 (d, J ) 8.9 Hz, 1H), 5.47 (ddd, J ) 10.7, 10.5, 2.9 Hz, 1H),
5.18 (s, 1H), 4.96 (dd, J ) 11.3, 2.0 Hz, 1H), 4.77 (m, 2H), 4.62
(dd, J ) 11.2, 4.4 Hz, 1H), 4.39 (m, 1H), 4.25 (app t, J ) 10.4 Hz,
1H), 4.08 (m, 1H), 4.03 (d, J ) 2.6 Hz, 1H), 3.95 (m, 1H), 3.76
(m, 1H), 3.69 (app t, J ) 6.9 Hz, 1H), 3.68 (m, 1H), 3.52 (dd, J )
9.4, 4.6 Hz, 1H), 3.45 (d, J ) 10.1 Hz, 1H), 3.41 (s, 3H), 3.34 (d,
J ) 14.7 Hz, 1H), 3.28 (app t, J ) 4.8 Hz, 1H), 3.23 (app t, J )
5.0 Hz, 1H), 3.09 (s, 3H), 3.08 (s, 3H), 3.04 (d, J ) 11.4 Hz, 1H),
2.95 (d, J ) 14.8 Hz, 1H), 2.66 (app t, J ) 6.3 Hz, 1H), 2.60 (m,
1H), 2.58 (dd, J ) 16.7, 5.7 Hz, 1H), 2.44 (dd, J ) 16.7, 6.8 Hz,
1H), 2.42 (m, 2H), 2.37 (dd, J ) 12.8, 5.5 Hz, 1H), 2.17 (dd, J )
10.0, 7.7 Hz, 1H), 2.10 (m, 1H), 2.09 (d, J ) 0.6 Hz, 3H), 2.02 (d,
J ) 11.8 Hz, 1H), 1.98 (d, J ) 13.0 Hz, 1H), 1.77 (d, J ) 0.8 Hz,
3H), 1.65 (m, 3H), 1.49 (d, J ) 13.2 Hz, 1H), 1.35 (m, 3H), 1.17
(m, 22H), 1.05 (d, J ) 6.9 Hz, 3H), 0.94 (s, 9H), 0.77 (d, J ) 6.5
mass spectrum of natural (+)-phorboxazole A: [R]20 +43.4 (c
D
0.04, CH2Cl2); high-resolution mass spectrum (ES+) m/z 1045.4029
[(M + Na)+, calcd for C53H71N2O13BrNa 1045.4037].
Acknowledgment. Financial support was provided by the
National Institutes of Health (National Cancer Institute) through
Grant No. CA-19033. We also thank Drs. G. Furst and R. Kohli
for assistance in obtaining NMR spectra and high-resolution
mass spectra, respectively.
Supporting Information Available: Experimental procedures
1
and analytical data for all new compounds. Copies of H and 13C
NMR spectra for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO7018152
1200 J. Org. Chem., Vol. 73, No. 4, 2008