Design and synthesis of a potent phorboxazole C(11-15) acetal analogue

Article abstract of DOI:10.1021/ol060014v

We disclose here the design, synthesis, and biological evaluation of simplified Z- and E-C(2-3) alkynyl phorboxazole C(11-15) acetals (+)-7Z and (+)-7E, wherein the Z-isomer proved to be a potent nanomolar cytotoxic agent. Reevaluation of (+)-C(45-46) E-c

Full text of DOI:10.1021/ol060014v

ORGANIC
LETTERS
2006
Vol. 8, No. 4
797-799
Design and Synthesis of a Potent
Phorboxazole C(11−15) Acetal Analogue
Amos B. Smith, III,*,† Thomas M. Razler,† Regina M. Meis,† and George R. Pettit‡
Department of Chemistry, Monell Chemical Senses Center, and Laboratory for
Research on the Structure of Matter, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104, and Cancer Research Institute,
Arizona State UniVersity, Tempe, Arizona 85287
smithab@sas.upenn.edu
Received January 3, 2006
ABSTRACT
We disclose here the design, synthesis, and biological evaluation of simplified Z- and E-C(2
and ( )-7E, wherein the Z-isomer proved to be a potent nanomolar cytotoxic agent. Reevaluation of (
A (6) confirms subnanomolar activity across a broad panel of human cancer cell lines.
−
3) alkynyl phorboxazole C(11
−15) acetals (+)-7Z
+
+
)-C(45 46) E-chloroalkenyl phorboxazole
−
(+)-Phorboxazole A and B (1 and 2), two architecturally
complex cytotoxic macrolides, isolated by Searle and
Molinski in 1995, have attracted considerable attention from
the synthetic community.1,2 However, because of their low
natural as well as synthetic availability, biological studies
aimed at defining their mechanism of action and/or cellular
targets remain seriously impaired. In conjunction with our
second generation total synthesis of (+)-phorboxazole A
(1),2f we recently disclosed SAR studies which identified the
C(45-46) alkynyl (3),3 alkenyl (4), alkyl (5), and E-
chloroalkenyl (6) analogues as highly potent congeners, with
activity greater than (+)-phorboxazole A (1) in several
human cancer cell lines (Figure 1).4 Consequently, our goal
became the design and synthesis of congeners also possessing
a simplified, more readily constructed macrocyclic domain,
wherein the C(11-15) tetrahydropyran is replaced with a
conformationally similar acetal, a tactic exploited to great
advantage by the Wender bryostatin program.5 This goal has
now been achieved (vide infra). Moreover, reevaluation of
the C(45-46) E-chloroalkenyl congener (6) reveals subna-
nomolar activity in several human cancer cell lines, rendering
6 to be one of the most potent cytotoxic agents known to
date.6
† University of Pennsylvania.
‡ Arizona State University.
(1) (a) Searle, P. A.; Molinski, T. F. J. Am. Chem. Soc. 1995, 117, 8126.
(b) Searle, P. A.; Molinski, T. F.; Brzezinski, L.; Leahy, J. W. J. Am. Chem.
Soc. 1996, 118, 9422.
(2) (a) Forsyth, C. J.; Ahmed, F.; Cink, R. D.; Lee, C. S. J. Am. Chem.
Soc. 1998, 120, 5597. (b) Evans, D. A.; Fitch, D. M.; Smith, T. E.; Cee, V.
J. J. Am. Chem. Soc. 2000, 122, 10033. (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) Williams, D. R.; Kiryanov, A. A.; Emde, U.; Clark, M. P.; Berliner, M.
A.; Reeves, J. T. Angew Chem., Int. Ed. 2003, 42, 1258. (f) Smith, A. B.,
III; Razler, T. M.; Ciavarri, J. P.; Hirose, T.; Ishikawa, T. Org. Lett. 2005,
7, 4399.
(3) The C(45-46) alkynyl congener (3), first reported by the Forsyth
laboratory, was shown to have potent cytotoxicity. Uckun, F. M.; Forsyth,
C. J. Bioorg. Med. Chem. Lett. 2001, 11, 1181.
(4) Smith, A. B., III; Razler, T. M.; Pettit, G. R.; Chapuis, J.-C. Org.
Lett, 2005, 7, 4403.
(5) Wender, P. A.; DeBrabander, J.; Haran, P. G.; Jimenez, J. M.;
Koehler, M. F. T.; Lippa, B.; Park, C. M.; Shiozaki, M. J. Am. Chem. Soc.
1998, 120, 4534.
10.1021/ol060014v CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/24/2006

A one-flask Wittig salt formation/olefination,2b involving
exposure of (-)-13 to tri-n-butylphosphine, followed by
introduction of aldehyde (+)-14 and DBU generated the
C(19-20) E-olefin (+)-15 in 91% yield with excellent
configurational control (ca. 19:1, E/Z).
Completion of macrocycles 9 entailed removal of the tert-
butyldiphenylsilyl (BPS) group (TBAF), Dess-Martin oxi-
dation, and removal of the 3,4-dimethoxybenzyl (DMB)
group (DDQ) to furnish alcohol (+)-16 in 75% yield (three
steps; Scheme 2). Union with phosphonate acid 17 promoted
Scheme 2
Figure 1. Phorboxazole analogues.
The Z- and E-C(2-3)-C(11-15) acetal alkynyl analogues
(7Z and 7E) were envisioned to arise via Stille union of the
advanced alkynyl side chain stannane 8 and the correspond-
ing Z- and E-macrocycles 9Z and 9E, similar to the strategy
employed in our second generation total synthesis of (+)-
phorboxazole A.2f
We began with construction of the vinyl iodides 9Z and
9E exploiting the TMSOTf-promoted condensation of the
silylated diol derived from (-)-10 with oxazole aldehyde
11 (Scheme 1) to furnish (-)-12 as a separable diastereo-
by EDCI‚MeI/HOBT, followed by an intramolecular Stille
modified Horner-Emmons olefination, led to (+)-9 in 88%
yield as a mixture (2.1:1, Z/E), which proved readily
separable via column chromatography (only the Z-isomer is
shown).8
Recognizing that acidic hydrolysis of a C(33) mixed
methyl acetal, as employed in our recently reported phor-
boxazole analogue synthesis, would prove problematic given
the C(11-15) acetal functionality,4 we opted to protect this
position as the triethylsilyl (TES) ether.2b Global deprotection
employing a fluoride source would then permit the
C(11-15) acetal to remain intact.
Scheme 1
Construction of the requisite stannyl side chain began with
addition of the Grignard reagent derived from oxazole 19 to
dienyl-lactone (-)-18 to furnish the corresponding hemi-
acetal (Scheme 3). Protection as the corresponding TES ether
(buffered TESOTf) proceeded in modest yield. Final elabora-
tion of the stannyl side chain (-)-8 entailed a palladium-
catalyzed exchange of the oxazole enol-triflate for trimeth-
ylstannane; the overall yield for the three steps was 30%.
Pleasingly, Stille union of both the Z- and E-acetals (9Z
and 9E) with side chain (-)-8 furnished the corresponding
Z- and E-macrocycles in good yield (Scheme 3). Global
deprotection employing 4 equiv of tetrabutylammonium
(6) Previous biological screening of (+)-6 on two occasions revealed
subnanomolar activity across the panel of human cancer cell lines; however,
one assay resulted in only low nanomolar activity. This discrepancy is now
known to be due to sample degradation.
(7) Noyori, R.; Murata, S.; Suzuki, M. Tetrahedron 1981, 37, 3899.
(8) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405.
meric mixture at C(15) (ca. 7:1).7 Removal of the p-
methoxybenzyl (PMB) group with DDQ, followed by
mesylation, afforded tricycle (-)-13 in 82% yield (two steps).
798
Org. Lett., Vol. 8, No. 4, 2006

the cancer cell panel. Importantly, the simplified, Z-acetal
analogue (+)-7Z displayed significant activity across the six
cell lines with potent activities of 11.8 and 18.3 nM,
respectively, against the KM20L2 and MCF-3 cell lines. As
expected from our earlier results, the isomeric E-acetal
analogue (+)-7E was significantly less active. The enhanced
activity of the Z-acetal congener (+)-7Z clearly reinforces
both the importance of the Z-geometry imparted by the
macrocyclic enoate and the ability of the acetal to mimic
the conformation of the (+)-phorboxazole A C(11-15)
tetrahydropyran. Equally pleasing, the C(45-46) chloro-
alkenyl congener (+)-6 displayed subnanomolar activity
when screened against the six cancer cell lines. These
biological data place (+)-6 within the potency range of the
spongistatins,9 bryostatins,10 and halichondrins11 as one of
the most cytotoxic agents known to date.
Scheme 3
In summary, we have achieved the design, synthesis, and
biological evaluation of the simplified Z- and E-phorboxazole
acetals (+)-7Z and (+)-7E, possessing the terminal C(45-
46) side chain alkyne. The Z-isomer proved to be a potent
cytotoxic agent, due presumably to the ability of the C(11-
15) acetal to mimic the conformation imparted by the
corresponding tetrahydropyran functionality in (+)-phor-
boxazole A (1). Importantly, (+)-7Z represents the first
macrocycle-modified phorboxazole analogue to retain potent
tumor cell growth inhibitory activity. Reevaluation of (+)-
6, the C(45-46) E-chloroalkenyl congener of phorboxazole
A, revealed subnanomolar activity across a broad panel of
human tumor cell lines. Studies to identify additional
simplified side chain/macrocycle constructs, which possess
significant cytotoxic activity, as well as a campaign to secure
large quantities of the chloro congener (+)-6 by total
synthesis in anticipation of further biological studies continue
in our laboratory.
fluoride (TBAF) removed the protecting groups to furnish
alkynyl phorboxazole analogues (+)-7Z and (+)-7E in 54%
and 49% yields, respectively (Scheme 3, only the Z-isomer
is shown).
Biological evaluation of (+)-7Z and (+)-7E, along with
the previously reported C(45-46) alkyl phorboxazole A (-)-5
as a control and the E-chloroalkenyl congener (+)-6, against
a panel of six human tumor cancer cell lines, [BXP-3
(pancreatic), MCF-3 (breast), F-268 (CNS), NCI-H460
(nonsmall lung), KM20L2 (colon), and DU-145 (prostate)]
revealed impressive levels of cytotoxicity (Table 1). The
Table 1. Human Cancer Cell Line Screening
Acknowledgment. Financial support was provided by the
National Institutes of Health through Grant CA-19033.
Supporting Information Available: Experimental data
and experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL060014V
(9) 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.
(10) Pettit, G. R. J. Nat. Prod. 1996, 59, 812.
(11) 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.
control C(45-46) alkyl analogue (-)-5, as previosuly
reported, displayed an average GI50 value of 5.3 nM across
Org. Lett., Vol. 8, No. 4, 2006
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