On the stereochemistry of palmerolide A

Article abstract of DOI:10.1016/j.tetlet.2007.09.053

Degradative studies of the anticancer macrolide palmerolide A have resulted in re-assignment of the C-7, C-10, and C-11 stereocenters.

Full text of DOI:10.1016/j.tetlet.2007.09.053

Tetrahedron Letters 48 (2007) 8009–8010
On the stereochemistry of palmerolide A
Matthew D. Lebar and Bill J. Baker*
Department of Chemistry, University of South Florida, 4202 E. Folwer Ave CHE205, Tampa, FL 33620, United States
Received 22 April 2007; revised 6 September 2007; accepted 7 September 2007
Available online 12 September 2007
Abstract—Degradative studies of the anticancer macrolide palmerolide A have resulted in re-assignment of the C-7, C-10, and C-11
stereocenters.
Ó 2007 Elsevier Ltd. All rights reserved.
As part of our program to investigate the chemical
diversity of cold-water marine organisms,1 we recently
reported the structure of palmerolide A (1).2 An ena-
mide-bearing macrolide isolated from the Antarctic
tunicate Synoicum adareanum, palmerolide A is a potent
inhibitor (2 nM) of vacuolar-ATPase (V-ATPase). In
the National Cancer Institute’s 60 cell line panel,
palmerolide A displays potent and selective cytotoxicity
toward melanoma and cytostatic activity toward
leukemia, CNS, renal and breast cancer cell lines, as well
as melanoma, with GI50’s <10 nM (the lowest concen-
tration tested). Palmerolide A was active in the NCI hol-
low-fiber assay3 and is currently being evaluated in
xenograft assays.
O
1
O
H
N
7
O
OH
HO
Palmerolide A (1)
O
10
O
NH2
1) O3
2) NaBH4
HO
OH
HO
7
1,2,6-Trihydroxyhexane (2)
The planar structure of palmerolide A was established
on the basis of extensive 2D NMR experimentation
while the stereochemical assignment required a combi-
nation of spectroscopic and derivatization techniques.2
The two secondary alcohols, C-7 and C-10, were amena-
ble to stereochemical analysis by Mosher’s method.4
The remaining stereocenters were determined relative
to C-10 using through-space NMR techniques such as
Figure 1. Reductive ozonolysis of the proposed2 palmerolide
A
structure.
at odds with the value published in the literature
(+3.4),6 both in magnitude as well as sign. We therefore
prepared (R)-1,2,6-hexanetriol, based on a chiral pool
synthesis, for direct comparison.
n
ROESY along with JCH-based analysis.5
In follow up to these spectroscopic techniques, we have
subjected palmerolide A to degradative studies to con-
firm the stereochemical assignments (Fig. 1). Reductive
ozonolysis of palmerolide A produced, among other
fragments, triol 2, which we expected was the R isomer
of 1,2,6-trihydroxyhexane. However, degradation frag-
ment 2 displayed an optical rotation of À9.0, a result
We initially attempted preparation of (R)-1,2,6-trihydr-
oxyhexane (R-2) from the (R)-acetonide of glycerol (3)
(Fig. 2). In the course of this preparation the Wittig
product 4 was reductively ozonized back to 3 to confirm
the stereochemical integrity of the Wittig reaction.
Recovered 3, however, was found to have significantly
epimerized (ee 56% of original).
We subsequently found that a homologue of 3, the ace-
tonide of (R)-1,2,4-trihydroxybutane (R-5) could be sub-
jected to Wittig olefination to R-6 while retaining its
*
Corresponding author. Tel.: +1 813 974 1967; fax: +1 813 974
1733; e-mail: bjbaker@cas.usf.edu
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.09.053

8010
M. D. Lebar, B. J. Baker / Tetrahedron Letters 48 (2007) 8009–8010
O
stereocenter as bearing the S configuration, rather than
the originally published 7R.
OH
O
a, b
c
This stereochemical revision took us back to the data
from our Mosher’s analysis to establish whether our
procedures were in error or the method made the wrong
prediction. Our notebooks indicated the correct conver-
sion of the acid chloride stereochemistry to the corre-
sponding ester stereochemistry ((R)-acid chloride
results in the (S)-ester) was made, ruling out a not-
uncommon mistake.7 We therefore repeated the esterifi-
cation of palmerolide A with (R)-methoxytrifluoro-
methylphenylacetoyl chloride, which we found to bear
1H NMR shifts that matched the data originally
assigned to the (R)-MTPA ester instead of the (S)-
MTPA ester, suggesting they had been transposed.8
O
O
O
O
4
3
Figure 2. Reagents and conditions: (a) (COCl)2, DMSO, TEA, DCM,
À60 °C–rt, 63%; (b) Ph3P(CH2)2CH(OCH2)2Br, BuLi, THF, À78 °C,
60–80%; (c) O3, À78 °C, then NaBH4, 44% yield, 56% ee.
O
OH
OEt
a, b
c
O
O
O
O
d
Palmerolide A’s C-10 configurational assignment was
made from the same MTPA products, necessitating that
we re-evaluate the C-7/C-10 MTPA diester. We found
that C-10 had been subject to the same sample or data
transposition. Because the C-11 configuration is based
R
-6
R
-5
2
3
on JCH and JCH conformational analysis of the
C-10/C-11 spin system, both C-10 and C-11 must be
revised to the S configuration. Thus, palmerolide stereo-
centers at positions 7, 10, and 11 must be revised from
R to S.
OH
OH
e
HO
OH
O
O
Synthetic 1,2,6-Trihydroxy-
R
-7
hexane ( -8)
R
Acknowledgement
Figure 3. Reagents and conditions: (a) Dess–Martin periodinane,
pyridine, DCM, rt, 2 h, 74–82%; (b) Ph3PCH2CO2EtBr, Et3N, rt, 15–
17 h, 45–52%; (c) O3, À78 °C, then NaBH4 in MeOH, 55–60%; (d)
LiAlH4, THF, rt, 2 h, 37–45%; (e) 50% AcOH, rt, 30 min, 89–93%.
This research was supported by a grant from the US
National Science Foundation (OPP-0442857).
optical purity, based on a similar reductive ozonolysis
back to R-5 (Fig. 3). Degradation of product 2 could
then be obtained in two additional steps from 6. 1,4-
reduction of the conjugated ester with LAH produced
the terminal alcohol 7, which was hydrolyzed to triol
R-8 under mild acid conditions. Spectral and chromato-
graphic data (1H NMR, 13C NMR, GC/MS, and
ESIMS) of (R)-1,2,6-trihydroxyhexane matched that of
commercially available ( )-1,2,6-trihydroxyhexane.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.09.053.
References and notes
1. Lebar, M. D.; Heimbegner, J. L.; Baker, B. J. Nat. Prod.
Rep. 2007, 24, 774–797.
With (R)-1,2,6-hexatriene in hand, we compared the
optical rotation of synthetic product (R-8, +11.1) to
degradation product (2, À9.0). We were satisfied to find
the magnitude of the optical rotation from the synthetic
product more closely matched the degradation product,
but the sign of the rotation was opposite that of the
degradation product.
2. Diyabalanage, T.; Amsler, C. D.; McClintock, J. B.; Baker,
B. J. J. Am. Chem. Soc. 2006, 128, 5630–5631.
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7. Erickson, K. L.; Beutler, J. A.; Cardellina, J. H., II; Boyd,
M. R. J. Org. Chem. 2001, 66, 1532.
Final verification of the C-7 stereochemistry was there-
fore achieved by preparation of (S)-1,2,6-trihydroxyhex-
ane (S-8), starting from the acetonide of (S)-1,2,4-
trihydroxybutane (S-5). The optical rotation of the
S-triol (S-8, À11.6) matched the degradation product
(2, À9.0) obtained from ozonolysis of palmerolide,
establishing the configuration of palmerolide A’s C-7
8. Diyabalanage, T. Ph.D. Dissertation, University of South
Florida, 2006.