Total synthesis of phorboxazole A. 2. Assembly of subunits and completion of the synthesis

Article abstract of DOI:10.1021/ol062531j

(Chemical Equation Presented) Subunits of phorboxazole A containing C1-C2, C3-C8, C9-C19, C20-C32, C33-C41, and C42-C46 were connected in a sequence that first linked C32 with C33 and then C41 with C42. A C3-C8 fragment was joined to C9-C19, and the assem

Full text of DOI:10.1021/ol062531j

ORGANIC
LETTERS
2006
Vol. 8, No. 26
6043-6046
Total Synthesis of Phorboxazole A. 2.
Assembly of Subunits and Completion
of the Synthesis
James D. White,* Tae Hee Lee, and Punlop Kuntiyong
Department of Chemistry, Oregon State UniVersity, CorVallis, Oregon 97331
james.white@oregonstate.edu
Received October 13, 2006
ABSTRACT
Subunits of phorboxazole A containing C1
linked C32 with C33 and then C41 with C42. A C3
half of 1. Closure of the macrolide was accomplished by esterification of the C24 alcohol followed by intramolecular Horner
Emmons condensation to set the (E)-C2 C3 alkene.
−
C2, C3
−
C8, C9
−
C19, C20
−
C32, C33
−
C41, and C42
−
C46 were connected in a sequence that first
−
C8 fragment was joined to C9
−C19, and the assembled unit was then joined with the left
−
Wadsworth
−
−
The preceding Letter1 describes the synthesis of four major
subunits of the potent antitumor agent phorboxazole A (1).2
It also reports that attempts to couple two subunits at C32-
C33 through addition of the anion from deprotonation of a
methyl-substituted oxazole representing C20-C32 with a
δ-lactone incorporating C33-C46 encountered difficulties.
Specifically, this reaction resulted in a significant quantity
of a terminal alkyne arising from elimination of HBr from
the bromoalkene. Because we were unsuccessful in attempts
to convert the alkyne byproduct to the desired (E)-bromo-
alkene, we decided to adopt a different strategy for as-
sembling the C20-C46 segment of 1. This Letter reports
an assembly sequence that first combines a C20-C32 unit
2 with C33-C41 (3), adds the remaining segment (C42-
C46) of the phorboxazole side chain, then connects C20 of
2 to a fragment 4 containing C3-C19 before adding the last
two carbons and closing the macrolide (Scheme 1). This
highly convergent strategy takes advantage of certain features
incorporated into other syntheses of phorboxazoles3-8 and
allows us to exploit methodology we developed for the
synthesis of subunits containing tetrahydropyrans A and B.9
To circumvent the problem of HBr elimination encoun-
tered previously in generating the C32-C33 linkage, a
truncated version of the phorboxazole side chain was
(3) Forsyth, C. J.; Ahmed, F.; Cink, R. D.; Lee, C. S. J. Am. Chem. Soc.
1998, 120, 5597.
(4) Smith, A. B., III; Verhoest, P. R.; Minbiole, K. P.; Schelhaas, M. J.
Am. Chem. Soc. 2001, 123, 4834.
(5) (a) Williams, D. R.; Kiryanov, A. A.; Emde, U.; Clark, M. P.;
Berliner, M. A.; Reeves, J. T. Angew. Chem., Int. Ed. 2003, 42, 1258. (b)
Williams, D. R.; Kiryanov, A. A.; Emde, U.; Clark, M. P.; Berliner, M.
A.; Reeves, J. T. Proc. Natl. Acad. Sci. 2004, 101, 12058.
(6) (a) Gonzalez, M. A.; Pattenden, G. Angew. Chem., Int. Ed. 2003,
42, 1255. (b) 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.
(7) Evans, D. A.; Fitch, D. M.; Smith, T. E.; Cee, V. J. J. Am. Chem.
Soc. 2000, 122, 10033.
(8) 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.
(9) White, J. D.; Kranemann, C. L.; Kuntiyong, P. Org. Lett. 2001, 3,
4003.
(1) White, J. D.; Kuntiyoung, P.; Lee, T. H. Org. Lett. 2006, 8, 6039.
(2) (a) Searle, P. A.; Molinski, T. F. J. Am. Chem. Soc. 1995, 117, 8126.
(b) Searle, P. A.; Molinski, T. F.; Brzezinski, L. J.; Leahy, J .W. J. Am.
Chem. Soc. 1996, 118, 9422. (c) Molinski, T. F. Tetrahedron Lett. 1996,
37, 7879. (d) Molinski, T. F.; Brzezinski, L. J.; Leahy, J .W. Tetrahedron:
Asymmetry 2002, 13, 1013.
10.1021/ol062531j CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/22/2006

reduced the ester, cleaved the TIPS ether, blocked the
primary alcohol as its TBS ether, and then masked the C24
hydroxyl group as a p-methoxybenzyl ether (Scheme 3). The
last of these moves was found to be necessary to effect clean
exposure of the C24 hydroxyl group for esterification prior
to the pivotal macrocyclization steps.
Scheme 1. Key Subunits for the C32-C33 and C19-C20
Connections
The anion derived from 7 with lithium diethylamide under
conditions described by Evans7 was reacted with lactone 5
to afford hemiketal 9 as a single epimer in near quantitative
yield (Scheme 4). Treatment of 9 with acidic methanol not
only converted this substance to its methyl ketal but
simultaneously cleaved both TBS ethers, thus allowing
Scheme 4. Union of the C19-C32, C33-C41, and C42-C46
Subunits
employed for coupling with 2. This took the form of lactone
5, obtained in four steps from the previously synthesized
Scheme 2. Synthesis of the C33-C41 Subunit
phenylthio acetal 61 (Scheme 2). The coupling partner 7 for
5 was prepared from tetrahydropyran 8 via a sequence that
Scheme 3. Synthesis of the C19-C32 Subunit
6044
Org. Lett., Vol. 8, No. 26, 2006

Scheme 5. Union of C3-C19 with C20-C46 and Completion of the Synthesis
selective oxidation of the allylic alcohol to an aldehyde.
major fragment of 1, the primary silyl ether was cleaved and
the resulting alcohol was oxidized to aldehyde 13.
Reprotection of the remaining primary alcohol then gave 10.
Julia-Kocienski condensation10 of aldehyde 10 with known
sufone 115 afforded a quantitative yield of 12 representing
a fully functionalized and protected version of the C19-
C46 sector of 1. In preparation for uniting 12 with a second
Synthesis of 4 for its linkage to 13 began with reduction
of previously prepared ester 141 to aldehyde 15 (Scheme 5).
The latter was reacted with allylsilane 16 in the presence of
stannic chloride11 to yield a 1:1 mixture of stereoisomeric
alcohols 17 and 18 that were easily separated by chroma-
tography. The (9S) configuration of 17 was proven through
(10) Blakemore, P. R.; Cole, W. J.; Kocienski, P. J.; Morley, A. Synlett
1998, 26.
Org. Lett., Vol. 8, No. 26, 2006
6045

NMR analysis (1H and 19F) of its Mosher ester,12 and 18
was converted to 17 by oxidation to a ketone followed by
reduction with L-Selectride.
aldehyde which was lactonized via an intramolecular Hor-
ner-Wadsworth-Emmons reaction under conditions de-
scribed by Williams.5 The resulting (E/Z) mixture of alkenes
22 (1:4, respectively) was treated with TBAF to remove the
remaining pair of TBDPS ethers and then with aqueous
hydrochloric acid to hydrolyze the methyl acetal. After
chromatographic purification to remove the minor (E) isomer,
synthetic phorboxazole A (1) was obtained which was
Wittig-Schlosser coupling13 of the ylide prepared from
the tri-n-butylphosphonium salt of 17 with aldehyde 13
produced trans C19-C20 alkene 19 accompanied by only a
trace of the cis alkene. Alcohol 19 was converted to its
mesylate. The TES ether at C5 was cleaved selectively, and
the resulting hydroxy mesylate was exposed to triethylamine.
This sequence closed tetrahydropyran C to afford 20. The
p-methoxybenzyl ether of 20 was cleaved under carefully
controlled conditions with DDQ, and the C24 alcohol was
esterified with dimethoxyphosphonylacetic acid to give 21.
At this point, closure of the macrolactone of 1 required
formation of a C3 aldehyde which mandated selective
cleavage of the primary TBDPS ether in the presence of two
secondary TBDPS ethers. The reagent TAS-F was partly
successful in this case,14 giving a mixture of the primary
alcohol and two diols, but although it was possible to
selectively oxidize the primary alcohol with TEMPO and
sodium hypochlorite, we sought an improved method for
advancing 21 to a primary alcohol. This was accomplished
with an excess of ammonium fluoride in warm methanol;
oxidation of the resulting primary alcohol then gave an
1
identical, by comparison of its H NMR spectrum, with
natural material isolated by Molinski.2a Our synthetic phor-
boxazole A was further identified by comparison of its 13C
NMR spectrum with that published by Williams.5
In summary, a highly convergent synthesis of phorbox-
azole A was completed in which the longest linear sequence
is 37 steps and the overall yield is 0.4%. Two of the three
tetrahydropyrans within the macrolide framework were
formed using intramolecular alkoxycarbonylation in the
presence of a palladium(II) species, exemplifying useful
methodology for the stereoselective construction of this
ubiquitous heterocycle.
Acknowledgment. Financial support from the National
Institute of General Medical Science through grants GM-
50574 and GM-58889 is gratefully acknowledged.
(11) Dias, L. C.; dos Santos, D. R.; Steil, L. J. Tetrahedron Lett. 2003,
44, 6861.
(12) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem.
Soc. 1991, 113, 4092.
(13) Schlosser, M.; Schaub, B. J. Am. Chem. Soc. 1982, 104, 5821.
(14) White, J. D.; Blakemore, P. R.; Green, N. J.; Hauser, E. B.;
Holoboski, M. A.; Keown, L. E.; Nylund Kolz, C. S.; Phillips, B. W. J.
Org. Chem. 2002, 67, 7750.
Supporting Information Available: Detailed experi-
mental procedures and characterization data for new com-
pounds; 1H and 13C NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL062531J
6046
Org. Lett., Vol. 8, No. 26, 2006