Total synthesis and configurational validation of (+)-phorbaside A

Article abstract of DOI:10.1021/ol100693c

Figure presented The configurational assignment of the cytotoxic marine macrolide phorbaside A has been verified by the stereodefined synthesis of the proposed structure 1 and its (18S,19R)-diastereomer 3, followed by correlation using circular dichroism spectroscopy. This first total synthesis, which proceeds in 8.2% yield over 23 steps, features two 1,4-syn boron aldol reactions, a Sonogashira coupling, and an α-glycosylation to append the l-evalose sugar moiety.

Full text of DOI:10.1021/ol100693c

ORGANIC
LETTERS
2010
Vol. 12, No. 9
2158-2161
Total Synthesis and Configurational
Validation of (+)-Phorbaside A
Ian Paterson* and Tanya Paquet
UniVersity Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, U.K.
ip100@cam.ac.uk
Received March 24, 2010
ABSTRACT
The configurational assignment of the cytotoxic marine macrolide phorbaside A has been verified by the stereodefined synthesis of the
proposed structure 1 and its (18S,19R)-diastereomer 3, followed by correlation using circular dichroism spectroscopy. This first total synthesis,
which proceeds in 8.2% yield over 23 steps, features two 1,4-syn boron aldol reactions, a Sonogashira coupling, and an r-glycosylation to
append the L-evalose sugar moiety.
In recent years, marine macrolides have gained prominence
as potent disrupters of cell cycle events and promising lead
structures for the development of anticancer agents, provided
the supply issue can be resolved.1 Phorbaside A is a cytotoxic
marine macrolide, isolated in 2007 by Molinski and co-
workers from the sponge Phorbas sp. collected off the
Western Australian coastline.2 Together with the congeneric
phorbasides B-E, this rare series of glycosylated macrolides,
differing only in their respective sugar moieties, exhibited
significant levels of cytotoxicity (IC50 ) 2-62 µM) against
the HCT 116 (human colon cancer) cell line. Such sensitivity
to relatively minor structural variation renders the phorba-
sides prime candidates for further mode of action and
structure-activity relationship studies.
glycosylated macrolides contain a six-membered cyclic
hemiacetal, an (E)-trisubstituted alkene, and a distinctive
unsaturated side chain terminating in a trans-chlorocyclo-
propane ring. Within the macrolide ring, a high degree of
structural homology is apparent between the phorbasides and
the callipeltosides, which also extends to the aurisides5 and
dolastatin 19,6 related cytotoxic polyketides isolated from
the sea hare Dolabella auricularia, suggesting a common
bacterial biogenesis.4 Despite these compelling similarities,
the assigned (18R,19S) configuration of the cyclopropane ring
of the phorbasides, as defined by elegant semiquantitative
circular dichroism (CD) studies using model fragments,
surprisingly was determined to be opposite to that found in
the callipeltosides and, more recently, muironolide A.7
Extensive spectroscopic analysis of phorbaside A (1,
Figure 1) highlighted its structural relationship to callipel-
toside A (2),3,4 previously isolated from a lithistid sponge,
Callipelta sp., found in New Caledonia. Both 14-membered
(3) (a) Zampella, A.; D’Auria, M. V.; Minale, L.; Debitus, C.; Roussakis,
C. J. Am. Chem. Soc. 1996, 118, 11085. (b) Zampella, A.; D’Auria, M. V.;
Minale, L.; Debitus, C. Tetrahedron 1997, 53, 3243.
(4) Paterson, I.; Findlay, A. D. Pure Appl. Chem. 2008, 80, 1773.
(5) Sone, H.; Kigoshi, H.; Yamada, K. J. Org. Chem. 1996, 61, 8956.
(6) (a) Pettit, G. R.; Xu, J.-P.; Doubek, D. L.; Chapuis, J.-C.; Schmidt,
J. M. J. Nat. Prod. 2004, 67, 1252. For the stereochemical reassignment of
dolastatin 19, see: (b) Paterson, I.; Findlay, A. D.; Florence, G. J. Org.
Lett. 2006, 8, 2131.
(1) (a) Molinski, T. F.; Dalisay, D. S.; Lievens, S. L.; Saludes, J. P.
Nat. ReV. Drug DiscoVery 2009, 8, 69. (b) Yeung, K.-S.; Paterson, I. Chem.
ReV. 2005, 105, 4237. (c) Yeung, K.-S.; Paterson, I. Angew. Chem., Int.
Ed. 2002, 41, 4632.
(2) (a) Skepper, C. K.; MacMillan, J. B.; Zhou, G.-X.; Masuno, M. N.;
Molinski, T. F. J. Am. Chem. Soc. 2007, 129, 4150. (b) MacMillan, J. B.;
Zhou, G.-X.; Skepper, C. K.; Molinski, T. F. J. Org. Chem. 2008, 73, 3699.
(7) Dalisay, D. S.; Morinaka, B. I.; Skepper, C. K.; Molinski, T. F. J. Am.
Chem. Soc. 2009, 131, 7552.
10.1021/ol100693c  2010 American Chemical Society
Published on Web 04/13/2010

Scheme 1
Figure 1. Phorbaside and callipeltoside marine macrolides.
As part of our ongoing interest in these structurally
intriguing marine macrolides,4,8 we sought to apply our
versatile aldol-based strategy to achieving an efficient
synthesis of the phorbasides. We now report the first total
synthesis of (+)-phorbaside A (1), together with its (18S,19R)-
diastereomer 3 having the antipodal cyclopropane ring as
found in the callipeltosides, thus validating the Molinski
configurational assignment for the phorbasides by CD
spectroscopic correlation.
adduct 12 (87%, >95% ee, Scheme 2). TBS protection, ester
reduction, and subsequent allylic oxidation using MnO2
proceeded smoothly to form aldehyde 9 (97% over 3 steps),
primed for further chain elongation using the first boron-
mediated aldol addition.
In the event, treatment of the (E)-boron enolate of ethyl
ketone 78b with aldehyde 9 at -78 °C gave the expected
1,2-anti,1,4-syn adduct 13 in excellent yield and diastereo-
selectivity (98%, >95:5 dr).11 The newly formed hydroxyl
stereocenter was then relayed to set the C7 configuration
through Evans-Tishchenko reduction,12 which also allowed
for diol differentiation. TES protection followed by ester
cleavage then provided 14 in 87% yield. Methylation of the
C9 hydroxyl was then achieved using Meerwein’s salt.
Following conditions developed previously,8b treatment with
DDQ in CH2Cl2/pH 7 buffer at 60 °C led to rapid (10 min)
and clean removal of the DMB ether. Dess-Martin oxidation
then afforded aldehyde 15 (95% over 3 steps), set for the
second boron aldol reaction. In this case, the dicyclohexy-
lboron enolate of methyl ketone 88d was treated with 15 at
-78 °C to afford adduct 6 (97%, >95:5 dr). Here the
dominant 1,4-syn influence from ketone 8 is matched with
the Felkin-Anh bias of aldehyde 15.8d
To allow for appropriate late-stage diversification to
assemble 1 and 3, our strategy involved the installation of
the trans-chlorocyclopropyl enyne side chain through So-
nogashira couplings of alkynes 4 and ent-4, respectively, with
a preformed macrolactone, followed by R-selective glyco-
sylation at the C5 hydroxyl using thioglycoside 5 (Scheme
1). This pivotal macrocyclic intermediate would be obtained
by elaboration of linear precursor 6 containing a terminal
vinyl iodide. Recognition of the indicated 1,4-syn relation-
ships in 6 guided the use of two boron-mediated aldol
couplings using ketones 7 and 8 to construct the carbon
backbone of the macrocycle. An asymmetric vinylogous
Mukaiyama aldol reaction would be used to set the (13R)-
hydroxyl configuration and simultaneously install the (E)-
geometry in enal 9.8b
Accordingly, treatment of 109 with (R)-BINOL-Ti(Oi-
Pr)4,8b followed by addition of 1110 at -78 °C, afforded aldol
(9) Savard, J.; Brassard, P. Tetrahedron 1984, 40, 3455.
(10) (a) Marek, I.; Meyer, C.; Normant, J.-F. Org. Synth. 1997, 74, 194.
(b) Trost, B. M.; Frederiksen, M. U.; Papillon, J. P. N.; Harrington, P. E.;
Shin, S.; Shireman, B. T. J. Am. Chem. Soc. 2005, 127, 3666.
(11) (a) Paterson, I.; Goodman, J. M.; Isaka, M. Tetrahedron Lett. 1989,
30, 7121. (b) Paterson, I.; Norcross, R. D.; Ward, R. A.; Romea, P.; Lister,
M. A. J. Am. Chem. Soc. 1994, 116, 11287.
(8) (a) Paterson, I.; Davies, R. D. M.; Marquez, R. Angew. Chem., Int.
Ed. 2001, 40, 603. (b) Paterson, I.; Davies, R. D. M.; Heimann, A. C.;
Marquez, R.; Meyer, A. Org. Lett. 2003, 5, 4477. (c) Paterson, I.; Florence,
G. J.; Heimann, A. C.; Mackay, A. C. Angew. Chem., Int. Ed. 2005, 44,
1130. (d) Paterson, I.; Findlay, A. D.; Florence, G. J. Tetrahedron 2007,
63, 5806
.
(12) Evans, D. A.; Hoveyda, A. H. J. Am. Chem. Soc. 1990, 112, 6447.
Org. Lett., Vol. 12, No. 9, 2010
2159

Scheme 2
With the linear precursor 6 in hand, attention was directed
duced the appropriate enyne trans-chlorocyclopropane side
chain. Treatment with TFA in wet THF then hydrolyzed the
methyl acetal and cleaved the TBS group at C5 to provide
the diastereomeric phorbaside aglycons 19 and 20, in
readiness for sugar attachment.
toward formation of the macrolactone. Selective cleavage
of the TES ether in 6 occurred with cyclization to the six-
membered methyl acetal upon treatment with catalytic PPTS
and (MeO)3CH in MeOH. The remaining hydroxyl at C5
was then converted into the TBS ether, after which treat-
ment13 with DDQ in MeOH/CH2Cl2 removed the DMB
group cleanly to give 16 (77% over 3 steps). Sequential
oxidation then provided the corresponding carboxylic acid,
which functioned as an internal buffer during the selective
removal of the allylic C13 silyl ether using TBAF. This
sequence of reactions afforded seco-acid 17 in excellent yield
(93%).
In contrast to our previous experience with this class of
14-membered macrolides,4,8 the crucial macrolactonization
of 17 proved difficult to achieve. While mixed anhydride
formation proceeded cleanly, commonly this intermediate
could not be coaxed into undergoing a productive cyclization.
Presumably the additional methyl substituent present at C2
in the phorbaside system, relative to that in the callipeltosides,
sterically hinders lactonization. After extensive investigation,
this challenging macrocyclization was performed under
modified Yamaguchi conditions14 to provide 18 in 42% yield
(Scheme 3). At this stage, the route diverged to target 1,
having the proposed (18R,19S) stereostructure of phorbaside
A, as well as the diastereomeric side-chain analogue 3.
Sonogashira sp-sp2 coupling15 of iodide 18 with (18R,19S)-
alkyne 48a or ent-4,8a,b using (Ph3P)2PdCl2 and CuI, intro-
Synthesis of the appropriate thioglycoside 5 commenced
from the known L-evalose scaffold 212b (Scheme 4). After
silylation of the C4′ hydroxyl group with TIPSOTf, the
acetonide was cleaved using aqueous AcOH to yield diol
22. Selective methylation of the secondary alcohol was
achieved using Meerwein’s salt to provide 23 (84%), from
which thioglycoside 5 was obtained on treatment with
TMSSPh in the presence of ZnI2 and Bu4NI.16 This latter
transformation also resulted in silylation of the C3′ tertiary
hydroxyl group.
Completion of phorbasides 1 and 3 relied on the R-selec-
tive glycosylation of aglycons 19 and 20 with thioglycoside
5. This was achieved using NIS in combination with catalytic
TfOH and 2,6-di-tert-butyl-4-methylpyridine in CH2Cl2.17
Finally, cleavage of the silylethers within the sugar moiety
with TBAF in THF gave the targeted phorbasides 1 and 3,
obtained in 23 steps and 8.2% and 5.4% overall yield,
respectively, from 10.
With synthetic samples of 1 and 3 now in hand, it proved
possible to verify the structure of phorbaside A. As expected
from the callipeltosides,8a,17c,18 these diastereomers proved
1
to be indistinguishable by H and 13C NMR spectroscopy,
although both correlated with the NMR data reported19 for
(13) Jackson, K. L.; Henderson, J. A.; Motoyoshi, H.; Phillips, A. J.
Angew. Chem., Int. Ed. 2009, 48, 2346.
(15) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 16, 4467. (b) Andrus, M. B.; Lepore, S. D.; Turner, T. M. J. Am.
Chem. Soc. 1997, 119, 12159.
(14) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989.
2160
Org. Lett., Vol. 12, No. 9, 2010

Scheme 3
Figure 2. CD spectra in MeOH of synthetic compounds 1, [R]D )
+53.5 (c ) 0.2, MeOH) cf. lit. +38 (c ) 0.06, MeOH), and 3,
with peak maxima compared to that of natural (+)-phorbaside A.
spectra provide convincing evidence that the structure of (+)-
phorbaside A is indeed as depicted in 1.
In conclusion, we have completed the first total synthesis
of the rare marine macrolide (+)-phorbaside A, thereby
verifying its full configurational assignment and providing
further material for evaluation of its anticancer properties.
Notably, the cyclopropane configuration found in the phor-
basides is established by CD correlation to be opposite to
that found in the callipeltosides, suggesting antipodal stere-
opreferences in the biosynthesis of the starter unit required
for chain extension by the polyketide synthase in these
otherwise analogous natural products.2a Finally, the present
work further demonstrates the effectiveness of our boron
aldol methodology to efficiently construct such complex
marine macrolides.
Acknowledgment. We thank the EPSRC (EP/F025734/
1), the Commonwealth Scholarship Commission, and St
John’s College, Cambridge for support.
Note Added after ASAP Publication. The Abstract text
read “3 steps” instead of “23 steps” in the version published
ASAP April 13, 2010; the corrected version reposted April
15, 2010.
natural phorbaside A. Ultimately, CD studies confirmed
Molinski’s assignment of the (18R,19S) configuration of the
cyclopropane ring in phorbaside A (Figure 2).2 The peak
shape and intensity of the CD spectrum of 1 correlates with
that reported for the natural product, whereas that of 3 shows
a drop in intensity and reaches a maximum at a lower
wavelength. Together with the respective NMR data, the CD
Supporting Information Available: Experimental details
1
and spectroscopic data for new compounds, copies of H
and 13C NMR spectra for synthetic phorbasides 1 and 3, and
comparison with natural phorbaside A. This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 4
OL100693C
(16) Hanessian, S.; Guindon, Y. Carbohydr. Res. 1980, 86, C3.
(17) (a) Veeneman, G. H.; Van Leeuwen, S. H.; Van Boom, J. H.
Tetrahedron Lett. 1990, 31, 1331. (b) Konradsson, P.; Mootoo, D. R.;
McDevitt, R. E.; Fraser-Reid, B. J. Chem. Soc., Chem. Commun. 1990,
270. (c) Evans, D. A.; Burch, J. D.; Hu, E.; Jaeschke, G. Tetrahedron 2008,
64, 4671.
(18) Trost, B. M.; Gunzner, J. L.; Dirat, O.; Rhee, Y. H. J. Am. Chem.
Soc. 2002, 124, 10396.
(19) The chemical shift of the C19 chloromethine carbon in natural
phorbaside A appears to be incorrectly tabulated as 36.9 ppm in ref 2a.
Inspection of the HSQC spectrum shows that H19 correlates instead to a
signal at around 34.3 ppm, which is consistent with the C19 shifts measured
for both synthetic 1 and 3, as well as the analogous C21 chloromethine
carbon in the callipeltosides. We thank Professor Molinski for confirming
our proposed correction.
Org. Lett., Vol. 12, No. 9, 2010
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