Introduction of the (-)-berkelic acid side chain and assignment of the C-22 stereochemistry

Article abstract of DOI:10.1021/jo901221a

(Chemical Equation Presented) A Kiyooka aldol condensation of an aldehyde with a trimethylsilyl ketene acetal and the oxazaborolidinone prepared from N-Ts-(S)-valine gives two of the four possible aldol adducts, which were oxidized and deprotected to comp

Full text of DOI:10.1021/jo901221a

pubs.acs.org/joc
Introduction of the (-)-Berkelic Acid Side Chain and Assignment of the
C-22 Stereochemistry
Xiaoxing Wu, Jingye Zhou, and Barry B. Snider*
Department of Chemistry MS 015, Brandeis University, Waltham, Massachusetts 02454-9110
snider@brandeis.edu
Received June 9, 2009
A Kiyooka aldol condensation of an aldehyde with a trimethylsilyl ketene acetal and the oxaza-
borolidinone prepared from N-Ts-(S)-valine gives two of the four possible aldol adducts, which were
oxidized and deprotected to complete the synthesis of (-)-berkelic acid and (-)-22-epi-berkelic acid.
This synthesis establishes the absolute stereochemistry and assigns the stereochemistry at C-22. A
biosynthetic pathway is proposed that is consistent with the known absolute stereochemistry at the
quaternary carbon of spiciferone A, spicifernin, and berkelic acid and provides a simple explanation
for the differing stereochemistry at C-18 and C-19 of spicifernin and berkelic acid.
Introduction
We hypothesized that 1 could be prepared by a short
sequence starting from ketal aldehyde 4 and 2,6-dihydroxy-
benzoic acid 5 (see Scheme 1). Benzoic acid 5, a synthetic, and
possibly biosynthetic, precursor to pulvilloric acid (6), has
been synthesized in both racemic2 and optically pure form.3
Berkelic acid (1) might be biosynthesized by an analogous
coupling of 5 and a ketal aldehyde related to 4. Alternatively,
it is possible that berkelic acid (1) is biosynthesized by the
nucleophilic addition of ketone 7 to pulvilloric acid (6). Ketal
aldehyde 4 and ketone 7 are related to spicifernin (3), which
occurs with spiciferone A (2) in Cochliobolus spicifier.4
In a model study,5,6 we condensed 4a with (()-5 in MeOH
containing Dowex 50WX8-400-Hþ for 12 h at 25 °C to
provide a mixture of the tetracyclic acids that was treated
with diazomethane to give 8a (24%) and 9a (24%) and minor
Berkelic acid (1) and its congener, spiciferone A (2),
were recently isolated by Stierle and co-workers from a
Penicillium species obtained from the surface water of
Berkeley Pit Lake, an abandoned open-pit copper mine
(see Chart 1).1 The acidic water (pH 2.5) is contaminated
with metal sulfates at high concentrations so that only
extremophiles, which may produce novel natural products,
can survive. The structure of berkelic acid (1) was determined
to be a novel spiroketal from the analysis of the NMR and
mass spectral data of 1 and the analogous methyl ester. The
relative stereochemistry of the side chain stereocenter (C-22)
and the absolute stereochemistry were not assigned.
Berkelic acid (1) inhibits MMP-3 in the micromolar range
(GI50 = 1.87 μM) and caspase-1 in the millimolar range
(GI50 = 0.098 mM). Inhibitors that act by blocking the
activity of proteolytic enzymes (MMPs) used by tumor cells
to promote metastatic spread provide a new approach for
cancer treatment. Berkelic acid was tested in the National
Cancer Institute antitumor screen against 60 human cell
lines and showed selective activity toward ovarian cancer
OVCAR-3 with a GI50 of 91 nM.1
(2) Bullimore, B. K.; McOmie, J. F. W.; Turner, A. B.; Galbraith, M. N.;
Whalley, W. B. J. Chem. Soc. C 1967, 1289–1293.
€
(3) Rodel, T.; Gerlach, H. Liebigs Ann. Chem. 1997, 213–216.
(4) (a) Nakajima, H.; Hamasaki, T.; Maeta, S.; Kimura, Y.; Takeuchi, Y.
Phytochemistry 1990, 29, 1739–1743. (b) Nakajima, H.; Matsumoto, R.;
Kimura, Y.; Hamasaki, T. J. Chem. Soc., Chem. Commun. 1992, 1654–1656.
(c) Nakajima, H.; Fujimoto, H.; Matsumoto, R.; Hamasaki, T. J. Org. Chem.
1993, 58, 4526–4528. (d) Nakajima, H.; Fukuyama, K.; Fujimoto, H.; Baba,
T.; Hamasaki, T. J. Chem. Soc., Perkin Trans. 1 1994, 1865–1869.
(5) Zhou, J.; Snider, B. B. Org. Lett. 2007, 9, 2071–2074.
(6) For another model study, see: (a) Marsini, M. A.; Huang, Y.; Lindsey,
C. C.; Wu, K.-L.; Pettus, T. R. R. Org. Lett. 2008, 10, 1477–1480. (b) Huang,
Y.; Pettus, T. R. R. Synlett 2008, 1353–1356.
(1) Stierle, A. A.; Stierle, D. B.; Kelly, K. J. Org. Chem. 2006, 71, 5357–
5360.
DOI: 10.1021/jo901221a
r
Published on Web 07/24/2009
J. Org. Chem. 2009, 74, 6245–6252 6245
2009 American Chemical Society

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CHART 1. Structures of Berkelic Acid (1), Spiciferone A (2),
and Spicifernin (3)
FIGURE 1. Proposed and revised structures of berkelic acid show-
ing NOE correlations to the C-25 methyl group.
SCHEME 1. Retrosynthesis and Possible Biosynthesis of
(-)-Berkelic Acid (1)
SCHEME 2. Preparation of Tetracycles 8a and 8b
amounts of the other two diastereomers (see Scheme 2). The
desired isomer 8a was calculated to be 2.4 kcal/mol more
stable than 9a using MMX with conformational searching.7
As expected, treatment of the mixture of four methyl esters
with TFA in CDCl3 afforded a mixture containing mainly
8a, which was isolated in 50% yield from 5.
For the synthesis of (-)-berkelic acid,8 we prepared
(R)-(-)-5 and 4b and condensed them in MeOH containing
Dowex 50WX8-400-Hþ for 12 h at 25 °C to provide a
mixture of the tetracyclic acids that was treated with diazo-
methane to give 8b (29%) and 9b (22%) and minor amounts
of a third diastereomer. Equilibration with TFA in CDCl3
had little effect, giving 8b (39%) and 9b (19%). As a result of
the substituents on the tetrahydrofuran, the desired isomer
8b was calculated7,9 to be only 0.8 kcal/mol more stable than
9b, which is consistent with the formation of a 2:1 mixture of
8b and 9b after equilibration.
to be correctly assigned. The relative stereochemistry of these
two centers relative to the anomeric center C-17 was assigned
solely on the basis of an NOE from the methyl group (C-25)
to H-16β and an H-20 (see Figure 1).1 However, our MMX
calculations indicated that this NOE is more consistent with
structure 10, which has the opposite stereochemistry to 1 at
both C-18 and C-19, because structure 1 should have NOEs
to both H-16β and H-16R. As this work was being com-
€
pleted, Furstner reported a synthesis of the enantiomer of
berkelic acid methyl ester and reassigned the stereochemistry
at C-18 and C-19 as shown in 10.10
1
At first, this was of great concern. However, the H NMR
We therefore prepared ent-4b and condensed it with
(R)-(-)-5 in MeOH containing Dowex 50WX8-400-Hþ for
60 h at 25 °C to provide a mixture of the tetracyclic acids
corresponding to esters 11 and 12 (see Scheme 3).8 Partial
cleavage of the TBDPS group occurred because this reaction
was slower than that with 4, requiring 60 h for complete
reaction. Esterification with diazomethane provided silyl
ether 11 (30%) and alcohol 12 (22%) with no diastereomers
formed at C-15 and C-17 before or after TFA equilibration.
The selectivity is consistent with the calculated strain
spectral data of the desired product 8b did not fit well with
the data for berkelic acid methyl ester, suggesting that the
stereochemistry of berkelic acid had been assigned incor-
rectly. The trans stereochemistry at C-18 and C-19 appeared
(7) PCMODEL version 8.0 from Serena Software was used with MMX.
Calculations were carried out on analogues with the pentyl side chain
replaced with a methyl group to minimize irrelevant conformational com-
plexity.
(8) Wu, X.; Zhou, J.; Snider, B. B. Angew. Chem., Int. Ed. 2009, 48, 1283–
1286.
(9) Calculations were carried out on analogues with both the pentyl and
TBDPSOCH2CH2 side chains replaced by methyl groups to minimize
irrelevant conformational complexity.
(10) Buchgraber, P.; Snaddon, T. N.; Wirtz, C.; Mynott, R.; Goddard, T.;
Furstner, A. Angew. Chem., Int. Ed. 2008, 47, 8450–8454.
€
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SCHEME 3. Synthesis of Tetracyclic Acetoxy Aldehyde 15
SCHEME 4. Synthesis of a Mixture of Berkelic Acid Methyl
Ester and the C-22 Epimer (21)
energies, which indicated that 11 is 3.9 kcal/mol more stable
than the isomer analogous to 9b. As we expected, the spectral
data of 11 fit well with those of berkelic acid methyl ester,
suggesting that 10 is the structure of berkelic acid with the
opposite stereochemistry at C-18 and C-19 to that originally
proposed in 1 and that the stereochemistry at both C-15
and C-17 in the natural product is thermodynamically
controlled.
Although the structure of 12 is very close to the revised
structure of berkelic acid (10), the introduction of the five-
carbon side chain, the assignment of the stereochemistry at
C-22, and the choice of protecting group for the aromatic
acid and phenol proved to be very challenging. We describe
here the full details of the completion of the synthesis,8 the
assignment of stereochemistry at C-22, and an analysis of the
biosynthesis of berkelic acid.
acetal 1912 (see Scheme 4) prepared from methyl 2-methyl-
butanoate, although aldol adducts (74%) were obtained
from 3-phenylbutanal under these conditions. We needed
to find a protecting group for the phenol that was compatible
with the aldol reaction and to develop a procedure that
utilized both TBDPS ether phenol 11 and hydroxy phenol 12.
Desilylation of 11 with TBAF/AcOH in THF afforded
alcohol 12 in 91% yield. Protection of both hydroxy groups
in 12 with TBSOTf and 2,6-lutidine gave bis-TBS ether 16,
whereas protection of 12 with TBSCl and imidazole in DMF
selectively protected the primary alcohol. Treatment of bis-
Results and Discussion
Protection of 11 with Ac2O and pyridine afforded 13,
which was desilylated with TBAF/HOAc to give alcohol 14
in 74% overall yield from 11. Dess-Martin oxidation of 14
gave aldehyde 15 in 87% yield. Attempted oxidation of
phenol alcohol 12 with Dess-Martin periodinane resulted
in decomposition, establishing that protection of the phenol
is necessary.11
Unfortunately, addition of 15 to the enolate prepared by
treatment of methyl 2-methylbutanoate with LDA at -78 °C
destroyed the aldehyde and cleaved the acetate group, rather
than forming the desired aldol adducts. These aldol condi-
tions are successful with 3-phenylbutanal (83%) and citro-
nellal (70%). Acidic Mukaiyama aldol conditions were
equally unsuccessful: no aldol products were obtained from
the TiCl4-catalyzed reaction of 15 and trimethylsilyl ketene
TBS ether 16 with Ce(OTf)n xH2O (19-23% Ce) selectively
3
cleaved the alkyl TBS ether rather than the phenol TBS
group to give alcohol 17 in 81% yield.13 Oxidation of 17 with
Dess-Martin periodinane afforded aldehyde 18.
Unfortunately, addition of 18 to the enolate prepared by
treatment of methyl 2-methylbutanoate with LDA at -78 °C
gave no aldol products. The N-methylimidazole and LiCl-
catalyzed Mukaiyama aldol reaction14 proceeds under very
mild conditions and appeared to be compatible with the
functionality in 18. We were pleased to find that reaction of
18 with trimethylsilyl ketene acetal 19 (10 equiv), LiCl (0.5
˚
equiv), N-methylimidazole (0.25 equiv), and 4 A molecular
sieves in DMF afforded an inseparable mixture of all
four aldol products, which was subjected to Dess-Martin
(12) Iida, A.; Nakazawa, S.; Okabayashi, T.; Horii, A.; Misaki, T.;
Tanabe, Y. Org. Lett. 2006, 8, 5215–5218.
(13) (a) Crouch, R. D. Tetrahedron 2004, 60, 5833–5871. (b) Bartoli, G.;
Cupone, G.; Dalpozzo, R.; De Nino, A.; Maiuolo, L.; Procopio, A.; Sambri,
L; Tagarelli, A. Tetrahedron Lett. 2002, 43, 5945–5947.
(11) For the necessity of protection of the free phenols before Dess-
Martin oxidation, see: (a) Yang, Z.-Q.; Danishefsky, S. J. J. Am. Chem. Soc.
2003, 125, 9602–9603. (b) Yang, Z.-Q.; Geng, X.; Solit, D.; Pratilas, C. A;
Rosen, N.; Danishefsky, S. J. J. Am. Chem. Soc. 2004, 126, 7881–7889. (c)
Wagner, J.; Andres, H.; Rohrbach, S.; Wagner, D.; Oberer, L.; France, J. J.
Org. Chem. 2005, 70, 9588–9590.
(14) Hagiwara, H.; Inoguchi, H.; Fukushima, M.; Hoshi, T.; Suzuki, T.
Tetrahedron Lett. 2006, 47, 5371–5373.
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SCHEME 5. Synthesis of a Mixture of Berkelic Acid and the
C-22 Epimer (25)
SCHEME 6. Unsuccessful Aldol Reactions
of keto esters in 60% yield from 24. Removal of both allyl
groups15 with Pd(Ph3P)4, Et3N, and HCO2H afforded 25 (1:1
mixture of berkelic acid and 22-epi-berkelic acid) in 81%
yield. The 1H and 13C NMR spectral data of 25 match those
of natural berkelic acid in both CDCl3 and CD3OD, but
there are doubled peaks for the C-20 methylene group and
the methyl and ethyl groups attached to C-22 in the 1H NMR
spectrum. The optical rotation of synthetic 25, [R]22D -104.6
(c=0.07, MeOH), has the same sign as that of the natural
berkelic acid, [R]22D -83.5 (c=0.0113, MeOH),1 suggesting
that the absolute stereochemistry of natural berkelic acid is
most likely as shown in 25. The use of allyl protecting groups
for both the phenol and carboxylic acid provides a very efficient
solution of selectively protecting an acid and phenol without
protecting a primary alcohol. The allyl groups also protect the
phenol during the Dess-Martin oxidation of the C-21 alcohol
to a ketone. In his synthesis of the methyl ester of berkelic acid,
oxidation to give an inseparable 1:1 mixture of keto esters 20
in 44% overall yield. Removal of the TBS group using CsF in
MeOH afforded a 1:1 mixture of the methyl esters of berkelic
acid and 22-epi-berkelic acid (21) in 88% yield. The 1H NMR
spectral data of 21 match those of berkelic acid methyl
ester in CDCl3, but there are doubled peaks for the C-20
methylene group and the methyl and ethyl groups attached
to C-22 as expected since 21 is a mixture of isomers at C-22.
The correspondence of the 1H NMR spectral data of 21 and
natural berkelic acid methyl ester confirmed the reassign-
ment of the stereochemistry at C-18 and C-19.
€
Furstner reported that only the Swern oxidation (35%) could
be used to oxidize the C-21 alcohol with a free phenol because
other oxidants destroy the electron-rich aromatic ring.10
We still needed to develop a procedure to either control the
stereochemistry at the quaternary center16 C-22 or at least
allow us to separate the aldol products. Kobayashi reported
the diastereoselective alkylation of the enolate of 26 (see
Scheme 6).17 Unfortunately, no aldol products were ob-
tained from addition of the sodium enolate of 26 to the
model aldehyde heptanal. TBS ketene aminal 27 undergoes a
vinylogous Mukaiyama aldol reaction with hexanal at the
γ- not R-position, so this reagent cannot be used.18 We
prepared 27 and hydrogenated it with 5% Pd/C under
1 atm of H2 to give 28 stereospecifically as determined by
an NOE between the methyl and TBS groups. We hoped that
28 would undergo a Mukaiyama aldol reaction now that a
vinylogous Mukaiyama reaction was no longer possible. To
our disappointment, attempted Mukaiyama aldol reactions
of 28 with heptanal using either TiCl4 at various tempera-
tures, LiCl and N-methylimidazole, or TMSOTf and
i-Pr2EtN were unsuccessful.
Selective cleavage of the methyl benzoate of 21 in the
presence of the β-keto ester group is not possible, as was also
10
observed by Furstner, so we needed to use a different
€
protecting group for the carboxylic acid. The protection of
the phenol by silylating both the alcohol and phenol and
selective liberation of the alcohol is also cumbersome. We
therefore decided to protect both the phenol and carboxylic
acid with allyl groups as reported by Oikawa in his synthesis
of prelasalocid.15 Condensation of ent-4b and (R)-(-)-5 with
Dowex 50WX8-400-Hþ in MeOH for 60 h gave crude
tetracyclic salicylic acids with a partially deprotected side
chain. Protection of this mixture with allyl bromide and
K2CO3 in DMF gave silyl ether 22 (32%) and alcohol 23
(20%) without allylation of the primary alcohol of 23 (see
Scheme 5). Additional alcohol 23 was prepared by desilyla-
tion of 22 with TBAF/HOAc (86%) so that the overall yield
of 23 from ent-4b is 48%. Aldehyde 24 was prepared in 88%
yield by Dess-Martin oxidation of 23.
Treatment of aldehyde 24 with trimethylsilyl ketene acetal
19, LiCl, and N-methylimidazole afforded an inseparable
mixture of all four possible aldol products, which was
subjected to Dess-Martin oxidation to give a 1:1 mixture
(16) For a review of stereoselective formation of quaternary carbon
centers, see: Denissova, I.; Barriault, L. Tetrahedron 2003, 59, 10105–10146.
(17) (a) Hosokawa, S.; Sekiguchi, K.; Enemoto, M.; Kobayashi, S.
Tetrahedron Lett. 2000, 41, 6429–6433. (b) Abe, T.; Suzuki, T.; Sekiguchi,
K.; Hosokawa, S.; Kobayashi, S. Tetrahedron Lett. 2003, 44, 9303–9305.
(18) Shirokawa, S.-i.; Kamiyama, M.; Nakamura, T.; Okada, M.;
Nakazaki, A.; Hosokawa, S.; Kobayashi, S. J. Am. Chem. Soc. 2004, 126,
13604–13605.
(15) For the protection of a salicylic acid with two allyl groups, see:
Migita, A.; Shichijo, Y.; Oguri, H.; Watanabe, M.; Tokiwano, T.; Oikawa,
H. Tetrahedron Lett. 2008, 49, 1021–1025.
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Koga reported the highly enantioselective alkylation of
SCHEME 7. Synthesis of Berkelic Acid (35) and 22-epi-Ber-
kelic Acid (36)
R-alkyl β-keto esters to give R,R-dialkyl β-keto esters.19,20
Chiral enamines were prepared from the R-alkyl β-keto
esters and (S)-valine tert-butyl ester (30)21 in the presence
of a catalytic amount of BF3 OEt2. Metalation of the
3
enamines with LDA, coordination of the lithium cations
with an additive (HMPA or THF), alkylation of the
enamides, and hydrolysis of the resulting imines afford the
R,R-dialkyl β-keto esters with >90% ee. β-Keto ester 29 was
synthesized by Dess-Martin oxidation of 39 (see below)
to test this approach. To our surprise, 29 did not react with
(S)-valine tert-butyl ester (30) to give enamine 31 under a
variety of conditions that were successful in our hands with
ethyl 2-methylacetoacetate and ethyl 2-oxocyclohexane-
carboxylate as reported.19,20 All of the reported examples
of enamine formation from 30 use either cyclic β-keto esters
or ethyl 2-methylacetoacetate, suggesting that our failure
with 29 results from steric interactions between the bulky
substituents of 29 and the valine moiety. This approach was
therefore not pursued.
Kiyooka reported that stoichiometric oxazaborolidinone
(S)-32, which is prepared in situ from N-Ts-(S)-valine22 and
BH3 THF, induces aldol reactions between aldehydes and
3
trimethylsilyl ketene acetals. Reaction occurs selectively at
the Si face of the aldehyde with excellent control of the
stereochemistry at the β-carbon with the hydroxy group.
Mixtures of isomers are formed at the R-carbon center.23 We
needed to control the stereochemistry at the R-carbon; the
stereochemistry of the β-carbon with the hydroxy group
is irrelevant since it is oxidized to a ketone. Although
Kiyooka’s protocol will not solve our stereoselectivity pro-
blem, the mildly acidic conditions should be compatible with
our substrate and we may be able to separate the aldol
products if only two, rather than four, isomers are formed.
As expected, reaction of 24 and 19 as described by
Kiyooka using (S)-32 afforded only 33 (40%) and 34
(40%), which were fortunately separable by preparative
TLC, although at this time we did not know which isomer
was which (see Scheme 7). To our surprise, the analogous
reaction using (R)-32 resulted in reduction of 24 to give
primary alcohol 23. This suggests that the stereochemical
preferences of 24 and (S)-32 are matched, while those of 24
and (R)-32 are mismatched, leading to reduction of the
aldehyde instead of the desired aldol reaction. Very different
yields have been previously observed with matched and
mismatched aldehydes.24
Dess-Martin oxidation of the less polar isomer 33
afforded the keto ester in 85% yield, which was deprotected
with Pd(Ph3P)4, Et3N, and HCO2H to afford berkelic acid
(35), although the stereochemistry at C-22 was still un-
assigned. The 1H and 13C NMR spectral data of 35 in both
CDCl3 and CD3OD are identical to those of natural berkelic
acid.8 The optical rotation of synthetic berkelic acid (35),
[R]22D -115.5 (c=0.55, MeOH), has the same sign as that of
the natural product, [R]22 -83.5 (c = 0.0113, MeOH),
D
indicating that the absolute stereochemistry is as shown.
The differing numerical values may result from the
low sample concentration used for the natural product. A
similar sequence from the more polar isomer 34 afforded
22-epi-berkelic acid (36). The spectral data of 36 are similar
to those of berkelic acid, but there are significant differences,
most notably in the 1H NMR spectra for the C-20 methylene
group and the methyl and ethyl groups attached to C-22. In
CDCl3, the chemical shifts of the two protons on C-20 of 36
(δ 2.90, dd, J=17.6, 2.9 Hz; δ 2.38, dd, J=17.6, 10.0 Hz) are
further apart from each other than those of 35 (δ 2.85, dd, J=
17.0, 2.4 Hz; δ 2.42 dd, J=17.0, 9.8 Hz). The optical rotation
of synthetic 22-epi-berkelic acid (36), [R]22D -107.0 (c=0.43,
MeOH), is very similar to that of berkelic acid, indicating
that the C-22 stereochemistry has a minor effect on the
rotation as expected.
(19) (a) Ando, K.; Takemasa, Y.; Tomioka, K.; Koga, K. Tetrahedron
1993, 49, 1579–1588. (b) Tomioka, K.; Ando, K.; Takemasa, Y.; Koga, K. J.
Am. Chem. Soc. 1984, 106, 2718–2719.
(20) For the application of Koga’s chemistry, see: (a) Cristau, H.-J.;
Marat, X.; Vors, J.-P.; Virieux, D.; Pirat, J.-L. Curr. Org. Synth. 2005, 2, 113–
119. (b) Peese, K. M.; Gin, D. Y. Org. Lett. 2005, 7, 3323–3325.
(21) Chen, H.; Feng, Y.; Xu, Z.; Ye, T. Tetrahedron 2005, 61, 11132–
11140.
€
(22) (a) Berry, M. B.; Craig, D. Synlett 1992, 41–44. (b) Vastila, P.; Pastor,
€
I. M.; Adolfsson, H. J. Org. Chem. 2005, 70, 2921–2929.
(23) (a) Kiyooka, S.-i.; Kaneko, Y.; Komura, M.; Matsuo, H.; Nakano,
M. J. Org. Chem. 1991, 56, 2276–2278. (b) Kiyooka, S.-i.; Shahid, K. A.
Tetrahedron: Asymmetry 2000, 11, 1537–1542. (c) Kiyooka, S.-i.; Shahid, K.
A.; Goto, F.; Okazaki, M.; Shuto, Y. J. Org. Chem. 2003, 68, 7967–7978. (d)
Kiyooka, S.-i.; Hena, M. A. J. Org. Chem. 1999, 64, 5511–5523. (e) Fujiyama,
R.; Goh, K.; Kiyooka, S.-i. Tetrahedron Lett. 2005, 46, 1211–1215.(f)
Ishiwara, K.; Yamamoto, H. in Modern Aldol Reactions: Metal Catalysis;
Mahrwald, R., Ed.; Wiley-VCH: Weinheim, Germany, 2004; Vol. 2, pp 25-68.
(24) Kiyooka, S.-i.; Maeda, H.; Hena, M. A.; Uchida, M.; Kim, C.-S.;
Horiike, M. Tetrahedron Lett. 1998, 39, 8287–8290.
It still remained to assign the stereochemistry of 33-36 at
ꢀ
C-22. Frater reported the stereoselective R-alkylation of
chiral β-hydroxy esters in excellent diastereoselectivity using
chelation to control the stereochemistry of the alkylation.25
ꢀ
€
€
(25) (a) Frater, G.; Muller, U.; Gunther, W. Tetrahedron 1984, 40, 1269–
ꢀ
1277. (b) Frater, G. Helv. Chim. Acta 1979, 62, 2825–2828.
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SCHEME 8. Synthesis of Aldol Adducts 42-45
SCHEME 9. Structure Assignments of Aldol Adducts 42-45
at C-3, must be 42. Therefore, the other isomer produced
from Kiyooka’s procedure using N-Ts-(S)-valine must be 44.
ꢀ
The compound produced by both Frater’s procedure and
Kiyooka’s procedure using N-Ts-(R)-valine must be 43.
Therefore, the other isomer produced from Kiyooka’s pro-
cedure using N-Ts-(R)-valine must be 45.
The structure assignments of 42-45 were confirmed by
Dess-Martin oxidation of 45 and 42 to keto ester 46 and of
43 and 44 to keto ester 47, and by conversion of 42 and 44 to
ketals 48 and 49, respectively, whose stereochemistry was
established by NOE studies (see Scheme 9).27 At this point,
the structure assignments of 42-45 were secure, although
they are based on the known selectivity of the Kiyooka aldol
reaction for the Si face with N-Ts-(S)-valine, which has been
confirmed in numerous examples.23
We then compared the spectral data of 42-45 with those
of 33 and 34. The subtle differences in the 1H and 13C NMR
spectra of (21R,22S)-33, (2S,3R)-42, and (2R,3S)-43 on the
one hand and (21R,22R)-34, (2R,3R)-44, and (2S,3S)-45 on
the other hand are identical in all 10 features that we can
distinguish as shown in Tables S1 and S2 (see Supporting
Information), suggesting that the stereochemistry of 33 and
34 can be assigned as shown by analogy to that of 42-45. The
chromatographic properties also support this assignment:
33, 42, and 43 are less polar than 34, 44, and 45, respectively.
Heathcock assigned the stereochemistry of aldol products by
analysis of the 13C NMR spectral data.28 However, this
approach was not extended to R,R-dialkyl aldol products
and is not useful with our compounds.
ꢀ
Frater’s alkylation might allow us to use the hydroxy group
stereochemistry introduced at C-21 in the Kiyooka aldol
reaction to control the stereochemistry at the quaternary
center C-22. We decided to explore this approach first in a
model system. Addition of (R)-citronellal (37) to the lithium
enolate of methyl butyrate (38) afforded 39 as a mixture of
four isomers (see Scheme 8). Treatment of 39 with 2.2 equiv
of LDA gave a mixture of chelated alkoxy enolates 40 and 41,
which was treated with MeI and HMPA as described by
ꢀ
Frater to give 16% of an inseparable 1:1 mixture of 42 and 43
containing a little 44 and 45. This sequence typically pro-
ceeds with excellent relative stereocontrol but in poor
yield.26 Therefore, it is not practical to use it with aldehyde
24 to prepare berkelic acid. However, it might be pos-
sible to prepare 42-45 with known stereochemistry and
to assign the stereochemistry of 33-36 by comparison of
the spectral data of hydroxy esters 33 and 34 with hydroxy
esters 42-45.
Reaction of (R)-citronellal (37) and 19 using (S)-32 gave 42
(38%) and 44 (40%). A similar sequence with the mis-
matched (R)-32 provided 43 (21%) and 45 (18%). Isomers
42 and 43 are less polar than 44 and 45, so that these
compounds can be easily separated. The stereochemistry
can now be easily assigned. The compound produced by
Because berkelic acid was reported to show selective
activity toward ovarian cancer OVCAR-3 with a GI50
of 91 nM,1 we submitted synthetic berkelic acid (35) and
22-epi-berkelic acid (36) to the NCI human disease-oriented
60-cell line in vitro antitumor screening protocol. Unfortu-
nately, neither of them showed significant activity against
ꢀ
both Frater’s procedure, which controls the relative stereo-
chemistry at C-2 and C-3, and Kiyooka’s procedure using
N-Ts-(S)-valine, which controls the absolute stereochemistry
(27) Crump, R. A. N. C.; Fleming, I.; Hill, J. H. M.; Parker, D.; Reddy
N. L.; Waterson, D. J. Chem. Soc., Perkin Trans. 1 1992, 3277–3294.
(28) Heathcock, C. H.; Pirrung, M. C.; Sohn, J. E. J. Org. Chem. 1979, 44,
4294–4299.
(26) (a) Tayama, E.; Hashimoto, R. Tetrahedron Lett. 2007, 48, 7950–
7952. (b) Akeboshi, T.; Ohtsuka, Y.; Sugai, T.; Ohta, H. Tetrahedron 1998,
54, 7387–7394.
6250 J. Org. Chem. Vol. 74, No. 16, 2009

Wu et al.
JOCArticle
SCHEME 10. Proposed Biosynthesis of Spiciferone A (2), Spi-
cifernin (3), Spiciferinone (54) and Possible Berkelic Acid Pre-
cursor (58)
and spicifernin (3), he suggested 51 was formed by a retro-
aldol reaction of a hydroxycyclodecatetraone.4 However,
deuterium washout from the starter acetate has been ob-
served in the biosynthesis of monensin A and other natural
products, so aldehyde 51 might be derived from the reduc-
tion of polyketide 50.29 An aldol reaction of 51 through
pathway A should afford 52, which could undergo dehydra-
tion to give spiciferone A (2). An aldol reaction of 51 through
pathway B should afford 53, which would undergo dehydra-
tion to give spiciferinone (54). Oxidative cleavage of 54 at the
indicated bond will give aldehyde ester 55. Hydrolysis of the
pyran will give β-keto aldehyde 56, which can easily lose
formic acid to give aldehyde 57. This could also occur by
oxidation of the aldehyde to an acid and decarboxylation of
the β-keto acid. Oxidation of 57 will give spicifernin (3), and
reduction will give the possible berkelic acid precursor 58.
The stereochemistry at the quaternary carbon center (C-22)
was already set in 51. However, the stereochemistry at C-18
and C-19 is set in the further elaboration of spiciferinone
(54), which proceeds through easily epimerizable intermedi-
ates. Four isomers are possible, but equilibration can occur
at either C-18 or C-19 before reduction or oxidation of
aldehyde 57. This scheme provides a simple biosynthetic
pathway from the common intermediate 51 to spiciferone A
(2), spicifernin (3), spiciferinone (54), and berkelic acid
precursor 58. It is consistent with the known absolute
stereochemistry at the quaternary carbon of the spiciferone
A (2), spicifernin (3), and berkelic acid (35) and provides a
simple explanation for the differing stereochemistry at C-18
and C-19 of spicifernin (3) and berkelic acid (35).
In conclusion, we have completed the first synthesis
of (-)-berkelic acid (35), established the absolute stereo-
chemistry, and assigned the stereochemistry at C-22.
Although the aldol reaction to introduce the side chain is
challenging, it proceeds effectively under Kiyooka’s condi-
tions using (S)-32 to give only two of the four possible aldol
products.
any cell lines at 10-5 M (see Supporting Information). The
racemic model acid prepared by hydrolysis of 8a inhibited lung
cancer cell line HOP-92 and ovarian cancer IGROV1 very
effectively at 10-5 M but had little effect on other cell lines.
The proposed biosynthesis in Scheme 1 needs to be
reconsidered in light of the revision of the stereochemistry
of berkelic acid (35) at C-18 and C-19 and the assignment of
the stereochemistry at C-22 and the absolute stereochemi-
stry. The stereochemistry of berkelic acid (35) at C-9 is the
same as that of pulvilloric acid (6). Berkelic acid co-occurs
with spiciferone A (2), which makes it plausible that a
spicifernin-like compound might be an intermediate in its
biosynthesis. However, spicifernin (3) has the same stereo-
chemistry as berkelic acid at C-22 but the opposite stereo-
chemistry at C-18 and C-19. It is not immediately obvious
how to relate the stereochemistry at C-22 of berkelic acid to
that of the quaternary center of the congener spiciferone A
(2). We therefore considered the biogenetic relationship
between spiciferone A (2), spicifernin (3), and spiciferinone
(54), which were isolated from same strain of Cochliobolus
spicifer.4
Experimental Section
Methyl (rS,βR,2S,3S,30aS,4S,50R)- and (rR,βR,2S,3S,30aS,
4S,50R)- r,3-Dimethyl-r-ethyl-30,30a,4,5,50,60-hexahydro-β-hy-
droxy-3-methyl-50-pentyl-80-(2-propenyloxy)-90-(2-propenyloxy-
carbonyl)spiro[furan-2(3H),20-[2H]pyrano[2,3,4-de][1]benzopyran]-
4-butanoate (33 and 34). To a solution of N-Ts-(S)-valine22,23
(84 mg, 0.31 mmol) in dry CH2Cl2 (2 mL) was added BH3 THF
3
(0.31 mL, 1 M solution in THF, 0.31 mmol) by syringe over 3
min under N2 at 0 °C. The solution was stirred for 30 min at 0 °C
and additionally for 30 min at 25 °C. The solution was cooled to
-78 °C, and a solution of aldehyde 24 (44.0 mg, 88.4 μmol) in
CH2Cl2 (0.5 mL) was added dropwise over 3 min. After stirring
for 5 min, silyl ketene acetal 19 (49 mg, 0.31 mmol) in CH2Cl2
(0.5 mL) was added dropwise over 3 min. The reaction mixture
was stirred for 4 h at -78 °C and quenched with 1 M aqueous
HCl (4 mL). The mixture was allowed to warm to 25 °C and the
aqueous layer was extracted with CH2Cl2 (4 ꢀ 5 mL). The
combined organic layers were washed with saturated NaHCO3
(2 ꢀ 10 mL), dried (MgSO4), and concentrated to yield 71.6 mg
of crude aldol product. Preparative TLC (4:1 hexanes/EtOAc,
developed four times) gave 21.5 mg (40%) of 33 followed by 21.9
mg (40%) of 34.
The relative and absolute stereochemistries of spiciferone
A (2) and spicifernin (3) have been assigned as shown.4 From
feeding studies with [1,2-13C]- and [1-13C,2H3]-acetate,
Nakajima established that they are biosynthesized from
acetate and (S)-adenosylmethionine and proposed that they
are biosynthesized from 51 (see Scheme 10). Because CD3
was incorporated only in the ethyl groups of spiciferone A (2)
(29) (a) Sood, G. R.; Ashworth, D. M.; Ajaz, A. A.; Robinson, J. A. J.
Chem. Soc., Perkin Trans. 1 1988, 3183–3193. (b) Florova, G.; Kazanina, G.;
Reynolds, K. A. Biochemistry 2002, 41, 10462–10471.
J. Org. Chem. Vol. 74, No. 16, 2009 6251

Wu et al.
JOCArticle
Data for 33: [R]22D -95.0 (c 0.22, CHCl3); 1H NMR δ 6.23 (s,
1, H-4), 6.07-5.92 (m, 2), 5.40 (br d, 1, J=17.1 Hz), 5.36 (br d, 1,
J=16.5 Hz), 5.25 (br d, 1, J=9.8 Hz), 5.22 (br d, 1, J=9.2 Hz),
4.86-4.72 (m, 3, H-15), 4.51 (d, 2, J=4.9 Hz), 4.28 (dd, 1, J=8.5,
8.5 Hz, H-26), 3.86-3.78 (m, 1, H-9), 3.77-3.69 (m, 1, H-21),
3.709 (s, 3, OMe), 3.65 (dd, 1, J=8.5, 8.5 Hz, H-26), 2.75 (dd, 1,
J=17.1, 3.4 Hz, H-8), 2.60 (dd, 1, J=17.1, 11.0 Hz, H-8), 2.27-
2.20 (m, 1, H-19), 2.22 (d, 1, J=6.1 Hz, OH), 2.14 (dd, 1, J=12.2,
5.5 Hz, H-16), 1.96 (dd, 1, J=12.2, 12.2 Hz, H-16), 1.80 (dq, 1,
J=13.4, 7.3 Hz), 1.71-1.60 (m, 3), 1.59-1.46 (m, 3), 1.45-1.24
(m, 6), 1.14 (s, 3, H-27), 1.03 (d, 3, J=6.7 Hz, H-25), 0.90 (t, 3, J=
6.7 Hz), 0.87 (t, 3, J = 7.3 Hz); 13C NMR δ 176.94, 165.66,
155.55, 149.09, 135.89, 132.96, 132.40, 118.17, 117.13, 114.65,
109.52, 108.52, 104.23, 76.22, 75.31, 73.87, 69.38, 68.11, 65.64,
51.79, 51.23, 49.04, 42.48, 36.32, 35.31, 34.55, 34.41, 31.79,
28.38, 25.14, 22.61, 16.75, 14.04, 11.87, 8.99; IR (neat) 3508,
2953, 2933, 2860, 1739, 1732, 1715; HRMS (EI) calcd for
C35H50O9 (Mþ) 614.3455, found 614.3451.
H-16), 1.94 (dq, 1, J=14.2, 7.3 Hz, H-23), 1.91 (dd, 1, J=12.2, 12.2
Hz, H-16), 1.88-1.78 (m, 2, H-23, H-18), 1.63-1.48 (m, 3), 1.46-
1.27 (m, 5), 1.32 (s, 3, H-27), 1.08 (d, 3, J=6.3 Hz, H-25), 0.92 (t, 3,
J=6.8 Hz), 0.83 (t, 3, J=7.6 Hz); 13C NMR (CDCl3) δ 206.0,
173.4, 170.5, 162.5, 149.8, 142.2, 112.2 (2 C), 110.5, 98.6, 75.2, 73.5,
67.2, 59.7, 52.5, 48.2, 41.6, 39.4, 36.2, 34.30, 34.29, 31.8, 27.9, 25.0,
22.6, 18.4, 14.0, 12.0, 8.7; 13C NMR (CD3OD, 400 MHz) δ 208.8,
174.8, 173.7, 163.4, 153.1, 142.3, 113.8, 110.7, 109.4, 101.1, 76.6,
74.2, 69.5, 61.0, 52.9, 42.6, 40.5, 37.4, 35.4, 35.0, 33.0, 28.9, 26.2,
23.7, 18.9, 14.4, 11.9, 9.0 (a peak near δ 49.2 is obscured by the
solvent peak); IR (neat) 3238, 2957, 2933, 2860, 1713, 1694; HRMS
(EI) calcd for C29H40O9 (Mþ) 532.2672, found 532.2659. The 1H
and 13C NMR spectral data in both CDCl3 and CD3OD are
identical to those of the natural product (see Tables S2-S5 in
Supporting Information).1
Methyl (rR,2S,3S,30aS,4S,50R)-90-Carboxy-r,3-dimethyl-r-
ethyl-30,30a,4,5,50,60-hexahydro-80-hydroxy-3-methyl-β-oxo-50-
pentylspiro[furan-2(3H),20-[2H]pyrano[2,3,4-de][1]benzopyran]-
4-butanoate (36, 22-epi-Berkelic Acid). An identical reaction
with the allyl ether and allyl ester prepared from 34 (14.0 mg,
22.8 μmol) afforded 8.7 mg (72%) of 22-epi-berkelic acid (36):
Data for 34: [R]22D -96.3 (c 0.22, CHCl3); 1H NMR δ 6.23 (s,
1, H-4), 6.07-5.92 (m, 2), 5.40 (br d, 1, J=17.1 Hz), 5.36 (br d, 1,
J=16.5 Hz), 5.25 (br d, 1, J=9.8 Hz), 5.22 (br d, 1, J=9.2 Hz),
4.86-4.72 (m, 3, H-15), 4.51 (d, 2, J=4.9 Hz), 4.31 (dd, 1, J=8.5,
8.5 Hz, H-26), 3.86-3.77 (m, 1, H-9), 3.726 (s, 3, OMe), 3.74-
3.68 (m, 1, H-21), 3.66 (dd, 1, J=8.5, 8.5 Hz, H-26), 2.75 (dd, 1,
J=17.1, 3.4 Hz, H-8), 2.60 (dd, 1, J=17.1, 11.0 Hz, H-8), 2.51 (d,
1, J=7.3 Hz, OH), 2.30-2.21 (m, 1, H-19), 2.14 (dd, 1, J=12.2,
4.9 Hz, H-16), 1.97 (dd, 1, J=12.2, 12.2 Hz, H-16), 1.81-1.44
(m, 7), 1.43-1.17 (m, 6), 1.13 (s, 3, H-27), 1.04 (d, 3, J=6.1 Hz,
H-25), 0.90 (t, 3, J=6.4 Hz), 0.85 (t, 3, J=7.3 Hz); 13C NMR δ
177.58, 165.63, 155.56, 149.10, 135.87, 132.96, 132.39, 118.11,
117.12, 114.64, 109.51, 108.43, 104.22, 75.49, 75.30, 74.00, 69.37,
68.11, 65.62, 51.86, 51.45, 49.07, 42.57, 36.31, 34.57, 34.49,
34.41, 31.78, 29.60, 25.13, 22.60, 16.94, 14.03, 11.87, 8.80; IR
(neat) 3522, 2955, 2933, 2860, 1737, 1732, 1715; HRMS (EI)
calcd for C35H50O9 (Mþ) 614.3455, found 614.3458.
[R]22 -107.0 (c 0.43, MeOH); 1H NMR (CDCl3, the residual
D
peak of solvent is referenced as δ 7.24 rather than 7.27 to
facilitate comparison with the literature data1) δ 11.83 (s, 1,
OH), 11.08-10.97 (br, 1, OH), 6.42 (s, 1, H-4), 4.77 (dd, 1, J=
12.2, 5.4 Hz, H-15), 4.45 (dd, 1, J=8.5, 8.5 Hz, H-26), 3.84-3.76
(m, 1, H-9), 3.73 (s, 3, OMe), 3.59 (dd, 1, J=8.5, 8.5 Hz, H-26),
2.90 (dd, 1, J=17.6, 2.9 Hz, H-20), 2.79 (dd, 1, J=17.6, 3.9 Hz,
H-8), 2.60 (dd, 1, J=17.6, 11.2 Hz, H-8), 2.54-2.44 (m, 1, H-19),
2.38 (dd, 1, J=17.6, 10.0 Hz, H-20), 2.21 (dd, 1, J=12.2, 5.4 Hz,
H-16), 2.06 (dd, 1, J=12.2, 12.2 Hz, H-16), 1.95 (dq, 1, J=14.2,
7.3 Hz, H-23), 1.87 (dq, 1, J=11.3, 6.8 Hz, H-18), 1.80 (dq, 1, J=
14.2, 7.3 Hz, H-23), 1.68-1.57 (m, 1), 1.58-1.43 (m, 2), 1.43-
1.20 (m, 5), 1.33 (s, 3, H-27), 1.09 (d, 3, J=6.8 Hz, H-25), 0.88 (t,
1
3, J=6.6 Hz), 0.81 (t, 3, J=7.3 Hz); H NMR (CD3OD, 500
Methyl (rS,2S,3S,30aS,4S,50R)-90-Carboxy-r,3-dimethyl-r-eth-
yl-30,30a,4,5,50,60-hexahydro-80-hydroxy-3-methyl-β-oxo-50-pentyl-
spiro[furan-2(3H),20-[2H]pyrano[2,3,4-de][1]benzopyran]-4-butano-
ate (35, Berkelic Acid). To a solution of Pd(PPh3)4 (6.1 mg,
5.3 μmol) and the above allyl ether and allyl ester prepared from
33 (16.2 mg, 26.4 μmol) in dry THF (2 mL) were added NEt3
(148 μL, 1.06 mmol) and HCO2H (40 μL, 1.06 mmol) under N2 at
25 °C. The yellow solution was stirred at 25 °C for 15 h and
quenched with saturated NaHCO3 (5 mL). The aqueous layer was
extracted with ether (4 ꢀ 5 mL). The combined organic layers were
dried (MgSO4) and concentrated. Flash chromatography on silica
gel (80:1:0 to 200:1:1 CH2Cl2/MeOH/AcOH) gave 11.1 mg (78%)
MHz) δ 6.27 (s, 1, H-4), 4.73 (dd, 1, J=12.2, 5.4 Hz, H-15), 4.30
(dd, 1, J=8.5, 8.5 Hz, H-26), 3.84-3.77 (m, 1, H-9), 3.73 (s, 3,
OMe), 3.50 (dd, 1, J=8.5, 8.5 Hz, H-26), 2.92 (dd, 1, J=17.6, 3.0
Hz, H-20), 2.79 (dd, 1, J=17.3, 3.6 Hz, H-8), 2.72-2.62 (m, 1, H-
19), 2.55 (dd, 1, J=17.3, 10.3 Hz, H-8), 2.49 (dd, 1, J=17.6, 10.5
Hz, H-20), 2.14 (dd, 1, J=12.2, 5.4 Hz, H-16), 1.94 (dq, 1, J=
14.2, 7.3 Hz, H-23), 1.91 (dd, 1, J=12.2, 12.2 Hz, H-16), 1.88-
1.78 (m, 2, H-23, H-18), 1.64-1.49 (m, 3), 1.47-1.30 (m, 5), 1.33
(s, 3, H-27), 1.08 (d, 3, J=6.3 Hz, H-25), 0.92 (t, 3, J=6.8 Hz),
0.82 (t, 3, J=7.3 Hz); 13C NMR (CDCl3) δ 206.0, 173.4 (tiny),
170.5, 162.5, 149.8, 142.2, 112.17, 112.15, 110.5, 98.6, 75.2, 73.5,
67.2, 59.7, 52.5, 48.2, 41.5, 39.3, 36.2, 34.31, 34.29, 31.8, 27.9,
25.0, 22.6, 18.3, 14.0, 12.0, 8.6; 13C NMR (CD3OD, 400 MHz) δ
208.7, 174.8, 173.6, 163.4, 153.1, 142.3, 113.8, 110.7, 109.4,
101.1, 76.6, 74.1, 69.5, 61.0, 52.9, 42.6, 40.5, 37.4, 35.4, 35.0,
33.0, 28.8, 26.2, 23.7, 18.8, 14.4, 11.9, 8.9 (a peak near δ 49.2 is
obscured by the solvent peak); IR (neat) 3233, 2957, 2933, 2860,
1713, 1694; HRMS (EI) calcd for C29H40O9 (Mþ) 532.2672,
found 532.2667.
of berkelic acid (35): [R]22D -115.5 (c 0.55, MeOH) {lit.1 [R]20
-
D
83.5 (c 0.0113, MeOH)}; 1H NMR (CDCl3, the residual peak of
solvent is referenced as δ 7.24 rather than 7.27 to facilitate
comparison with the literature data1) δ 11.82 (s, 1, OH), 11.13-
10.92 (br, 1, OH), 6.42 (s, 1, H-4), 4.76 (dd, 1, J=12.2, 5.4 Hz,
H-15), 4.44 (dd, 1, J=8.5, 8.5 Hz, H-26), 3.84-3.76 (m, 1, H-9),
3.73 (s, 3, OMe), 3.59 (dd, 1, J=8.5, 8.5 Hz, H-26), 2.85 (dd, 1, J=
17.0, 2.4 Hz, H-20), 2.78 (dd, 1, J=17.7, 3.7 Hz, H-8), 2.60 (dd, 1,
J=17.7, 11.0 Hz, H-8), 2.54-2.45 (m, 1, H-19), 2.42 (dd, 1, J=17.0,
9.8 Hz, H-20), 2.21 (dd, 1, J=12.2, 5.4 Hz, H-16), 2.05 (dd, 1, J=
12.2, 12.2 Hz, H-16), 1.95 (dq, 1, J=14.2, 7.3 Hz, H-23), 1.87 (dq, 1,
J=10.7, 6.8 Hz, H-18), 1.80 (dq, 1, J=14.2, 7.3 Hz, H-23), 1.68-
1.57 (m, 1), 1.58-1.43 (m, 2), 1.43-1.20 (m, 5), 1.32 (s, 3, H-27),
1.09 (d, 3, J=6.8 Hz, H-25), 0.88 (t, 3, J=6.6 Hz), 0.83 (t, 3, J=7.6
Hz); 1H NMR (CD3OD, 500 MHz) δ 6.27 (s, 1, H-4), 4.72 (dd, 1,
J=12.2, 5.4 Hz, H-15), 4.30 (dd, 1, J=8.5, 8.5 Hz, H-26), 3.84-
3.77 (m, 1, H-9), 3.73 (s, 3, OMe), 3.50 (dd, 1, J=8.5, 8.5 Hz, H-26),
2.88 (dd, 1, J=17.6, 3.1 Hz, H-20), 2.78 (dd, 1, J=17.3, 3.7 Hz,
H-8), 2.70-2.62 (m, 1, H-19), 2.54 (dd, 1, J=17.3, 11.2 Hz, H-8),
2.53 (dd, 1, J=17.6, 10.5 Hz, H-20), 2.14 (dd, 1, J=12.2, 5.4 Hz,
Acknowledgment. We are grateful to the National Insti-
tutes of Health (GM-50151) for support of this work. We
thank Prof. D. Stierle for copies of spectral data of berkelic
acid and berkelic acid methyl ester.
Supporting Information Available: Experimental proce-
dures for reactions not in the Experimental Section, comparison
1
Tables S1-S2 of NMR data, and H and 13C NMR spectral
data. Data for the acid prepared by hydrolysis of 8a, 35, and 36
in the NCI 60 cell screen. This material is available free of charge
via the Internet at http://pubs.acs.org.
6252 J. Org. Chem. Vol. 74, No. 16, 2009