Total synthesis of (±)-deoxypenostatin A. Approaches to the syntheses of penostatins A and B

Article abstract of DOI:10.1021/jo000850x

A short synthesis of (±)-deoxypenostatin A (28) has been carried out using the convergent coupling of dienal 11, epoxide 13, and methylenetriphenylphosphorane (17) to prepare trienol 19 in only two steps. The key step is the Yb(OTf)₃-catalyzed intramolecular Diels-Alder reaction of hydrated trienyl glyoxylate 23, which gives lactone 24 stereoselectively. Elaboration of lactone 24 to enone 27 by an intramolecular Horner-Emmons Wittig reaction and epimerization completes the synthesis of 28. Modest yields of Diels-Alder adducts 45a and 46a could be prepared analogously from MEM ether 44c, but the sensitivity of several of the intermediates precluded the elaboration of 45a to penostatin A (1).

Full text of DOI:10.1021/jo000850x

8490
J . Org. Chem. 2000, 65, 8490-8498
Tota l Syn th esis of (()-Deoxyp en osta tin A. Ap p r oa ch es to th e
Syn th eses of P en osta tin s A a n d B
Barry B. Snider* and Tao Liu
Department of Chemistry, Brandeis University, Waltham, Massachusetts 02454-9110
Received J une 2, 2000
A short synthesis of (()-deoxypenostatin A (28) has been carried out using the convergent coupling
of dienal 11, epoxide 13, and methylenetriphenylphosphorane (17) to prepare trienol 19 in only
two steps. The key step is the Yb(OTf)3-catalyzed intramolecular Diels-Alder reaction of hydrated
trienyl glyoxylate 23, which gives lactone 24 stereoselectively. Elaboration of lactone 24 to enone
27 by an intramolecular Horner-Emmons Wittig reaction and epimerization completes the synthesis
of 28. Modest yields of Diels-Alder adducts 45a and 46a could be prepared analogously from MEM
ether 44c, but the sensitivity of several of the intermediates precluded the elaboration of 45a to
penostatin A (1).
In tr od u ction
to 1 and 2 and might be available by an intramolecular
Diels-Alder reaction of trienyl glyoxylate 8. Intramo-
lecular Diels-Alder reactions with aldehydes are known
but little studied.4,5 Intermolecular Diels-Alder reactions
of glyoxylate esters have been extensively studied,5 and
intramolecular ene reactions of glyoxylates are known.6
However, to the best of our knowledge, the only example
of an intramolecular Diels-Alder reaction of a glyoxylate
ester was reported after this work was completed.7
Furthermore, the Diels-Alder reaction of 8 can give four
stereoisomers. Despite these concerns, this approach is
still attractive, since glyoxylate ester 8 should be acces-
sible by the Kornblum procedure8 from trienol 9, which
should be available in a single step by addition of
methylenetriphenylphosphorane to epoxide 10,9 depro-
tonation of the resulting betaine to give a γ-oxido ylide,
and addition of dienal 11.10
Penostatins A (ent-1), B (2), C (ent-3), D (4), and E (ent-
5) were isolated from a Penicillium sp. separated from
the green alga Enteromorpha intestinalis by Numata and
co-workers.1-3 All the penostatins except for penostatin
D exhibited significant cytotoxic activity against P388
cells. Penostatins A and B have the same stereochemistry
at C-5 and the opposite stereochemistry at the other four
carbons. The novel ring systems and functionality of the
penostatins and their cytotoxicity prompted us to under-
take their syntheses.
Resu lts a n d Discu ssion
Syn th esis of Deoxyp en osta tin A. We chose to test
this scheme by synthesizing deoxypenostatin A (28)
starting with epoxycyclopentane (13) rather than 10.11
Dienal 11 was prepared by a Wittig aldol condensation.12
Addition of propanal to neat cyclohexylamine at -20 °C
followed by dehydration gave 76% of imine 12. Treatment
of 12 with LDA afforded the lithium enamide, which was
(4) Ciganek, E. In Organic Reactions; Wiley: New York, 1984;
Volume 32, Chapter 1.
(5) Weinreb, S. M. In Comprehensive Organic Synthesis; Perga-
mon: Oxford, 1991; Volume 5, Chapter 4.2.
The dihydropyran ring of penostatins A and B (ent-1,
2) could, in principle, be formed by an intramolecular
Diels-Alder reaction with an aldehyde as the dienophile.
The obvious precursor 6 was rejected because the tet-
raenonal functionality was expected to be both syntheti-
cally inaccessible and too unstable. Lactone 7 was a more
attractive precursor, since it should be easily convertible
(6) (a) Lindner, D. L.; Doherty, J . B.; Shoham, G.; Woodward, R. B.
Tetrahedron Lett. 1982, 23, 5111-5114. (b) Garigipati, R. S.; Tschaen,
D. M.; Weinreb, S. M. J . Am. Chem. Soc. 1985, 107, 7790-7792.
(7) Craig, D.; Gordon, R. S. Tetrahedron Lett. 1998, 39, 8337-8340.
(8) (a) Kornblum, N.; Frazier, H. W. J . Am. Chem. Soc. 1966, 88,
865-866. (b) Tschaen, D. M.; Whittle, R. R.; Weinreb, S. M. J . Org.
Chem. 1986, 51, 2604-2605.
* Corresponding author. Phone: 781-736-2550. Fax: 781-736-2516.
E-mail: snider@brandeis.edu.
(1) Takahashi, C.; Numata, A.; Yamada, T.; Minoura, K.; Enomoto,
S.; Konishi, K.; Nakai, M.; Matsuda, C.; Nomoto, K. Tetrahedron Lett.
1996, 37, 655-658.
(2) Iwamoto, C.; Minoura, K.; Hagishita, S.; Nomoto, K.; Numata,
A. J . Chem. Soc., Perkin Trans. 1 1998, 449-456.
(3) Iwamoto, C.; Minoura, K.; Oka, T.; Ohta, T.; Hagishita, S.;
Numata, A.; Tetrahedron 1999, 55, 14353-14368.
(9) Both isomers of 10, R ) TBDMS, are readily available: Asami,
M. Bull. Chem. Soc. J pn. 1990, 63, 1402-1408.
(10) (a) Corey, E. J .; Kang, J . J . Am. Chem. Soc. 1982, 104, 4724-
4725. (b) Corey, E. J .; Kang, J .; Kyler, K. Tetrahedron Lett. 1985, 26,
555-558.
(11) For a preliminary communication see: Snider, B. B.; Liu, T. J .
Org. Chem. 1999, 64, 1088-1089.
(12) Wittig, G.; Hesse, A. In Organic Syntheses; Wiley: New York,
1988; Collect. Vol. 6, pp 901-904.
10.1021/jo000850x CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/17/2000

Total Synthesis of (()-Deoxypenostatin A
J . Org. Chem., Vol. 65, No. 25, 2000 8491
s-BuLi afforded γ-oxido ylide 15, which reacted with
dienal 11 to give 51% of the required trienol 19 as a single
stereoisomer.
treated with 2-E-decenal to afford 81% of dienal 11
containing e5% of the 2Z-isomer.13
Conversion of 19 to crude partially hydrated glyoxylate
23 was accomplished by the Kornblum procedure.8 Treat-
ment of alcohol 19 with bromoacetyl bromide and pyri-
dine gave the unstable crude bromo ester 20, which was
directly converted to iodo ester 21 with sodium iodide in
acetone. Crude 21 was dissolved in the acetonitrile and
treated with silver nitrate to afford nitrate ester 22 in
80% yield for this three-step sequence. Treatment of 22
with sodium acetate in DMSO afforded a crude mixture
of glyoxylate ester 23 and its hydrate, which decomposed
on attempted dehydration. Acid-catalyzed esterification
of 19 with glyoxylic acid monohydrate was unsuccessful.17
Presumably the triene moiety of 23 makes it very
sensitive, restricting the methods that can be used to
introduce the glyoxylate ester or convert the hydrate to
the free aldehyde.
In 1982, Corey and Kang reported the preparation and
reactivity of R-lithio ylide 14 by treatment of methyl-
triphenylphosphonium bromide with 2 equiv of s-BuLi.10
Addition of 14 to epoxide 13 generated γ-oxido ylide 15,
which was treated with benzaldehyde to form 65% of the
homoallylic alcohol 16. Despite considerable controversy
as to whether the second deprotonation of methyltriphen-
ylphosphonium bromide occurs at the CH2 group or on
the phenyl ring,14 this route has been widely used to
synthesize homoallylic alcohols.15
Not surprisingly, Diels-Alder reactions of 23 with
strong Lewis acid catalysts or heating were unsuccessful.
We were intrigued by the report by Qian and Huang that
Yb(OTf)3 catalyzes the Diels-Alder reaction of glyoxylate
esters with isoprene, since lanthanide triflates are mild
Lewis acids and compatible with water liberated from
the hydrate of 23.18
Treatment of crude, partially hydrated 23 with 0.2
equiv of Yb(OTf)3 in 20:1 CH3CN/CH2Cl2 for 2 d afforded
a 3:5 mixture of Diels-Alder adducts 24 and 25 in 24%
yield from trienol 19, whose structures were established
by NOE experiments. We observed strong NOE cross-
peaks in the minor product 24 between H-3 and H-8, H-3
and H-9, H-8 and H-9, and H-9 and H-12. These results
established that these four hydrogens are on the same
side of the molecule, so the adduct must be 24. We
observed strong NOE cross-peaks in the major product
25 between H-3 and H-8, H-3 and H-9, and H-8 and H-9,
but no cross-peaks from either H-8 or H-9 to H-12. This
We obtained 19 in <20% yield from 11, 13, and 14
prepared either as described above10a or from bromometh-
yltriphenylphosphonium bromide.10b Fortunately, a slight
variation of the reaction conditions led to 51% of 19.
Treatment of methyltriphenylphosphonium bromide with
1 equiv of s-BuLi in THF generated ylide 17 and LiBr,
which reacted with epoxycyclopentane (13) to give be-
taine 18.16 Treatment of 18 with a second equivalent of
(13) Meyers, A. I.; Brinkmeyer, R. S. Tetrahedron Lett. 1975, 1749-
1752.
(14) (a) Schaub, B.; J enny, T.; Schlosser, M. Tetrahedron Lett. 1984,
25, 4097-4100. (b) Schaub, B.; Schlosser, M. Tetrahedron Lett. 1985,
26, 1623-1626. (c) Cristau, H. J . Chem. Rev. 1994, 94, 1299-1313.
(15) (a) Becker, D.; Sahali, Y. Tetrahedron 1988, 44, 4541-4546.
(b) Enholm, E. J .; Satici, H.; Prasad, G. J . Org. Chem. 1990, 55, 324-
329. (c) Clive, D. L. J .; Murthy, K. S. K.; Wee, A. G. H.; Prasad, J . S.;
da Silva, G. V. J .; Majewski, M.; Anderson, P. C.; Evans, C. F.; Haugen,
R. D.; Heerze, L. D.; Barrie, J . R. J . Am. Chem. Soc. 1990, 112, 3018-
3028. (d) Okabe, M.; Sun, R. C. Tetrahedron Lett. 1993, 34, 6533-
6566. (e) Okuma, K.; Tanaka, Y.-i.; Hirabayashi, S.-i.; Shioji, K.;
Matsuyama, H. Heterocycles 1997, 45, 1385-1390.
(17) Whitesell, J . K.; Lawrence, R. M.; Chen, H. J . Org. Chem. 1986,
51, 4779-4784.
(18) Qian, C.; Huang, T. Tetrahedron Lett. 1997, 38, 6721-6724.
(16) Schlosser, M.; Tuong, H. B.; Respondek, J .; Schaub, B. Chimia
1983, 37, 10-11.

8492 J . Org. Chem., Vol. 65, No. 25, 2000
Snider and Liu
suggests that the major product 25 differs from the minor
product 24 only in the stereochemistry at C-12.
LiCH2PO(OMe)2 gave 93% of the hydroxy keto phospho-
nate as hemiacetal 29. Oxidation of 29 with the Dess-
Martin reagent in CH2Cl2 provided the unstable diketo-
phosphonate, which was treated with NaH in THF at
-30 °C overnight to give cyclohexenone 30 in 70% yield.
Isomerization of 30 with K2CO3 in MeOH for 12 h at
25 °C afforded 73% of epi-deoxypenostatin A (31) as the
only product.
Lactone 25 cannot be a primary adduct, since Diels-
Alder addition to an E,E-diene must give products with
H-8 and H-12 cis. It is probably formed by isomerization
at the doubly allylic C-12 in 24. This was established by
treatment of 24 with Yb(OTf)3 in CH3CN for 1 d to afford
a 1:1 mixture of 24 and 25, indicating that the pyran of
24 opens to the pentadienyl cation in the presence of
Lewis or Brønsted acids. No isomerization occurs on
treatment of 25 with Yb(OTf)3 in CH3CN for 3 d,
suggesting that 25 is more thermodynamically stable
than 24. The recent isolation of penostatin E (ent-5)3
suggests that this process is equally facile in nature. Acid-
catalyzed ring opening of the pyran of penostatin C (ent-
3) and reaction of the pentadienyl cation with water at
the distal position will give ent-5.
Fortunately, this isomerization can be suppressed by
carrying out the Diels-Alder reaction in the presence of
2,6-di-tert-butylpyridine. Treatment of a 0.01 M solution
of crude 23 in 15:1 CH3CN/CH2Cl2 with 0.2 equiv of Yb-
(OTf)3 and 0.15 equiv of 2,6-di-tert-butylpyridine for 18
h provided 24 as a single stereoisomer in 21% overall
yield from alcohol 19. Diels-Alder adduct 24 is one of
two possible endo products and has the same stereo-
chemistry as the penostatins at all centers except C-9,
which should be readily epimerizable since it is adjacent
to the carbonyl group.
Ap p r oa ch es to th e Syn th eses of P en osta tin s A
a n d B. We have developed a very short, convergent
synthesis of deoxypenostatin A (28) that features a novel,
stereoselective, intramolecular Diels-Alder reaction of
hydrated glyoxylate ester 23 catalyzed by Yb(OTf)3. We
now turned our attention to the application of this
sequence to the synthesis of penostatin B (2). trans-
Epoxide 32a 9 was converted to trienol 33a in 42% yield
analogously to the preparation of 19. The Kornblum
sequence afforded a 68% yield of the nitrate ester, which
was converted to crude partially hydrated glyoxylate 34a
with NaOAc in DMSO. To our surprise, treatment of
crude 34a with 0.35 equiv of Yb(OTf)3 and 0.25 equiv of
2,6-di-tert-butylpyridine in 20:1 of CH3CN and CH2Cl2
for more than 2 d afforded polymer but no desired Diels-
Alder product. No adduct was obtained with excess
Yb(OTf)3 with or without di-tert-butylpyridine. Heating
34a in toluene at 160 °C in a resealable tube led to
extensive decomposition. We investigated other Lewis
acids that have been used to catalyze Diels-Alder
reactions of aldehydes. Unfortunately, treatment of 34a
with BiCl320 or scandium(III) perfluorooctanesulfonate21
with or without di-tert-butylpyridine was also unsuccess-
ful.
Treatment of lactone 24 with LiCH2PO(OMe)2 gave
86% of the hydroxy keto phosphonate as hemiacetal 26.
Oxidation of 26 with the Dess-Martin reagent in CH2Cl2
provided the unstable diketophosphonate, which was
treated with NaH in THF19 at -30 °C overnight to give
cyclohexenone 27 in 52% (64% based on recovered 26)
yield. Although 27 is somewhat unstable to base, isomer-
ization with K2CO3 in MeOH for 12 h at 25 °C afforded
28% of deoxypenostatin A (28) and 40% of recovered 27.
This is not an equilibrium mixture, since 28 was not
isomerized back to 27 under these conditions. Unfortu-
nately, isomerization of 27 for longer periods of time
resulted in extensive decomposition, which precluded the
determination of the equilibrium ratio under these
1
isomerization conditions. The H and 13C NMR spectral
data of 28 are identical to those of penostatins A and B,
except for the expected differences in the cyclopentane
ring.
Since the Diels-Alder reaction was not compatible
with the TBDMS ether, we prepared the more hindered
(19) Aristoff, P. A. J . Org. Chem. 1985, 50, 1765-1766.
(20) Robert, H.; Garrigues, B.; Dubac, J . Tetrahedron Lett. 1998,
39, 1161-1164.
(21) Hanamoto, T.; Sugimoto, Y.; J in, Y.; Inanaga, J . Bull. Chem.
Soc. J pn. 1997, 70, 1421-1426.
An analogous series of reactions converted lactone 25
to epi-deoxypenostatin A (31). Treatment of 25 with

Total Synthesis of (()-Deoxypenostatin A
J . Org. Chem., Vol. 65, No. 25, 2000 8493
TBDPS ether 32b in 65% yield22 by silylation of 3-cyclo-
pentenol23 and epoxidation.9 The Wittig sequence af-
forded 36% of 33b, which was elaborated to 34b. Unfor-
tunately, the attempted Diels-Alder reaction of 34b
resulted in polymerization. Since we thought that the
silyl ether might be incompatible with the Diels-Alder
reaction, we prepared 34c from THP epoxide 32c9
analogously, which also did not undergo the Diels-Alder
reaction. Finally we prepared tert-butyl ether epoxide 32d
from 3-cyclopentenol by protection27 and epoxidation in
72% yield.22 The Wittig sequence afforded 32% of 33d ,
which was elaborated to 34d . Attempted Diels-Alder
reaction with Yb(OTf)3 and di-tert-butylpyridine in CH3CN
with or without added water,25 tris(pentafluorophenyl)-
borane,26 La(OTf)3, Sm(OTf)3, or Sc(OTf)3 was unsuccess-
ful. Heating 34d with Yb(OTf)3 and di-tert-butylpyri-
dine in acetonitrile hydrolyzed the ester to give 33d .
We next prepared ketal protected glyoxylate ester 37
by carrying out the Wittig sequence to give 37% of 36
from epoxy ketal 35.27 The Kornblum sequence on 36
gave crude partially hydrated glyoxylate 37. Treatment
of crude 37 with 2 equiv of Yb(OTf)3 and 1 equiv of 2,6-
di-tert-butylpyridine in 1:10 CH2Cl2 and CH3CN at 70 °C
for 6 h afforded 41 in 28% overall yield from alcohol 36.
The initial reaction is hydrolysis of the ketal, since ketone
38 can be isolated after reaction for 0.5 h. The desired
Diels-Alder reaction then occurs to give 39. Unfortu-
nately, the pyran ring is cleaved by acid to form δ-lactone
diol 40, which isomerizes to the more stable γ-lactone 41.
1D NOESY experiments. TLC analysis showed that 41
was very polar (Rf ) 0.20; hexane:EtOAc 1/9), which
suggested that it contained free hydroxyl groups. The
TLC spot absorbed at 254 nm, suggesting that it con-
tained a conjugated diene. The presence of a conjugated
diene was confirmed by the absorptions for the central
hydrogens of the conjugated diene at δ 6.20 and 6.12. The
carbonyl groups of 41 absorb at 1775 and 1744 cm-1, as
expected for a γ-butyrolactone and a cyclopentanone,
respectively. The absorption at 1775 cm-1 is not consis-
tent with that expected for the δ-valerolactone of 40,
which should come at 1748 cm-1 as in 24. Furthermore,
the absorption for H-9 at δ 4.52 (d, J ) 10.4) is not
consistent with that expected for the cis coupling between
H-8 and H-9 in 40, which should be 2-5 Hz.
1
The most characteristic H NMR peak of 41 is that of
H-8 at δ 2.41 (ddd, 1, J ) 10.4, 10.1, 10.1). The large
vicinal coupling constants between H-8 and H-9 and H-8
and H-10 establish that H-8 is trans to both H-9 and H-10
on the lactone ring. Theoretical and experimental studies
indicate that one or both coupling constants are smaller
in the other three lactone stereoisomers.28 The NOE
between H-8 and the allylic methyl group confirms that
H-8 and the conjugated diene side chain are cis on the
lactone. Since H-3 and H-7 are trans in the starting
material, there are only two possible structures that have
the three substituents on the γ-butyrolactone in a trans,
trans relationship. The large coupling constant between
H-7 and H-8 establishes that H-7 is antiperiplanar to H-8
in the predominant conformation of the product. The
structure was assigned as 41 on the basis of the NOE
between H-3 and C-9-OH, which should be observed only
in 41.
The formation of 41 was encouraging, since it indicated
that the Diels-Alder reaction could occur with an
oxygenated cyclopentane. However, we needed to develop
milder conditions so that Diels-Alder adduct 39 would
not react further. Treatment of 37 with 1.6 equiv of In-
29
(OTf)3 and 1 equiv of 2,6-di-tert-butylpyridine in 1:4
CH2Cl2 and CH3CN at 25 °C for 12 h also gave 41 in 19%
yield from 36. Fortunately, reaction of crude 37 with only
0.6 equiv of Yb(OTf)3 in 1:5 CH2Cl2/CH3CN at 65 °C for
3 h afforded the desired Diels-Alder product 39 in 14%
overall yield from alcohol 36. At longer reaction times,
rearrangement occurs to give 41. The characteristic
1
downfield H NMR absorptions of 39 at δ 5.78 (dt, 1, J )
15.3, 6.7, H-14), 5.59 (br d, 1, J ) 6.1, H-10), 5.27 (dd, 1,
J ) 15.3, 8.5, H-13), 4.45 (br d, 1, J ) 8.5, H-12), and
4.40 (d, 1, J ) 3.7, H-9) are comparable to those of 24 at
δ 5.78 (dt, 1, J ) 15.3, 7.2), 5.52 (br d, 1, J ) 5.5), 5.31
(dd, 1, J ) 15.3, 8.6), 4.51 (br d, 1, J ) 8.6), and 4.26 (d,
1, J ) 4.6).
The success of this Diels-Alder reaction was quite
encouraging. However, the loss of the ketone protecting
group complicates the conversion of the lactone to the
cyclohexenone, so we decided to examine Diels-Alder
reactions of a cyclopentanol protected as the methyl
ether, the smallest and most stable hydroxy-protect-
(26) Ishihara, K.; Hanaki, N.; Funahashi, M.; Miyata, M.; Yama-
moto, H. Bull. Chem. Soc. J pn. 1995, 68, 1721-1730.
(27) Lai, S.; Mendoza, J .; Hubbard, F.; Kalter, K. Bioorg. Med. Chem.
Lett. 1995, 5, 2155-2160.
The structure of 41 was determined by analysis of the
1
1H and 13C NMR and IR spectra and H-1H COSY and
(22) 13-18% of the cis epoxide was also formed.
(28) (a) J aime, C.; Segura, C.; Dinares, I.; Font, J . J . Org. Chem.
1993, 58, 154-158. (b) Stork, G.; Rychnovsky, S. D. J . Am. Chem. Soc.
1987, 109, 1564-1565.
(23) Hess, H. M.; Brown, H. C. J . Org. Chem. 1967, 32, 4138-4139.
(24) Wright, S. W.; Hageman, D. L.; Wright, A. S.; McClure, L. D.
Tetrahedron Lett. 1997, 38, 7345-7348.
(29) Ali, T.; Chauhan, K. K.; Frost, C. G. Tetrahedron Lett. 1999,
40, 5621-5624.
(25) Kobayashi, S.; Hachiya, I. J . Org. Chem. 1994, 59, 3590-3596.

8494 J . Org. Chem., Vol. 65, No. 25, 2000
Snider and Liu
ing group. Epoxidation of 3-methoxycyclopentene with
m-CPBA gave an inseparable mixture of volatile ep-
oxides. Epoxidation of 3-cyclopentenol with tert-butyl
hydroperoxide and a catalytic amount of VO(acac)29 gave
80% of cis-3,4-epoxycyclopentanol (42a), which was treated
with MeI and sodium hydride to afford 90% of cis-4-
methoxy-1,2-epoxycyclopentane (42b). The Wittig se-
quence afforded 20% of trienol 43b, which was converted
to partially hydroxylated glyxoylate ester 44b by the
Kornblum sequence.
CH3CN at 65 °C for 2.5 h afforded a 3:2 inseparable
mixture of Diels-Alder adducts 45a and 46a in 16% yield
1
from 43c. The most characteristic peaks in the H NMR
spectrum are at δ 4.52 (br d, 1, J ) 7.9, H-12), and 4.27
(d, 1, J ) 4.9, H-9) for 45a , which are comparable to those
at δ 4.51 (br d, 1, J ) 8.6), and 4.26 (d, 1, J ) 4.6) in 24,
and at δ 4.77 (br d, 1, J ) 6.7, H-12), and 4.64 (d, 1, J )
7.9, H-9) for 46a , which are comparable to those at δ 4.75
(br d, 1, J ) 8.0), and 4.63 (d, 1, J ) 8.6) in 25. Quenching
the reaction after 30 min afforded mainly 44a , indicating
that hydrolysis of the MEM ether precedes the Diels-
Alder reaction. Unfortunately the isomerization of 45a
to 46a could not be prevented by addition of 2,6-di-tert-
butylpyridine to the reaction mixture, since neither
hydrolysis of the MEM ether nor Diels-Alder reaction
occur after 5 h at 80 °C.
Treatment of the mixture of 45a and 46a with imida-
zole and TBSCl gave the crude silyl ethers 45d and 46d .
Unfortunately, 45d easily isomerized to 46d on silica gel.
Therefore, the crude mixture of 45d and 46d was treated
with excess LiCH2PO(OMe)2 to give a chromatographi-
cally separable mixture of hydroxy keto phosphonate
hemiacetals 47 (48% from 45a and 46a mixture) and 48
(29% from 45a and 46a mixture).
The structures of 47 and 48 were determined by
comparison of the NMR spectra with those of 26 and 29.
1
The H NMR spectrum of 47 is very similar to that of
26, except for the expected differences in the cyclopentane
ring. The most characteristic peaks in 26 are at δ 5.64
(br s, 1), 4.35 (br d, 1, J ) 8.5) and 3.35 (d, 1, J ) 1.8),
which are comparable to those at δ 5.61 (br s, 1), 4.35
1
(br d, 1, J ) 8.6), and 3.35 (d, 1, J ) 2.2) in 26. The H
NMR spectrum of 48 is very similar to that of 29, except
for the expected differences in the cyclopentane ring. The
most characteristic peaks in 48 are at δ 4.42 (br d, 1,
J ) 6.7) and 3.47 (d, 1, J ) 2.4), which are comparable
to those at δ 4.40 (br d, 1, J ) 6.7) and 3.48 (d, 1, J )
2.5) in 29.
Oxidation of 47 with the Dess-Martin reagent in
CH2Cl2 provided the unstable diketophosphonate, which
was treated with NaH in THF at -30 °C overnight.
Unfortunately, the desired enone 49 was not formed.
Decomposition occurred at 0 °C. Use of DBU and LiCl
as the base also failed.30
Oxidation of 48 with the Dess-Martin reagent in
CH2Cl2 provided the unstable diketophosphonate, which
was treated with NaH in THF at -30 °C overnight to
give 24% of protected bis-epi-penostatin A (50). The
proton spectrum of 50 is very similar to that of 30, except
for the expected differences in the cyclopentane ring. The
most characteristic peaks in 50 are at δ 5.98 (br s, 1),
4.51 (d, 1, J ) 6.1), and 3.93 (d, 1, J ) 3.7), as compared
to δ 5.99 (br s, 1), 4.46 (d, 1, J ) 6.7), and 3.90 (d, 1, J )
3.4) in 30.
These results indicate that the intramolecular glyox-
ylate Diels-Alder reaction is very sensitive to the oxygen
functionality on the cyclopentane ring. The reaction
proceeds in modest yield with some scrambling at C12
with cyclopentanone 38 and 44b and 44a , with methoxy
and hydroxy groups cis to the glyoxylate ester. We were
not able to determine whether this stereochemical rela-
tionship was important, because substrates with methoxy
Treatment of crude 44b with 0.6 equiv of Yb(OTf)3 and
0.7 equiv of 2,6-di-tert-butylpyridine in 1:4 CH2Cl2 and
CH3CN at 25 °C overnight afforded a 1:1 inseparable
mixture of Diels-Alder product 45b and isomer 46b in
10% yield from 43b. The most characteristic peaks in the
1H NMR spectrum are at δ 4.52 (br d, 1, J ) 8.4, H-12),
and 4.27 (d, 1, J ) 4.3, H-9) for 45b, which are
comparable to those at δ 4.51 (br d, 1, J ) 8.6), 4.26 (d,
1, J ) 4.6) in 24, and at δ 4.76 (br d, 1, J ) 7.3, H-12),
and 4.64 (d, 1, J ) 8.5, H-9) in 46b, which are comparable
to those at δ 4.75 (br d, 1, J ) 8.0), 4.63 (d, 1, J ) 8.6) in
25. The isolation of the Diels-Alder adducts with the
methoxy protecting group was encouraging, but the pyran
will not withstand the conditions required for deprotec-
tion.
cis-4-MEMO-1,2-epoxycyclopentane (42c) was pre-
pared in 80% yield by treatment of 42a with MEMCl and
N,N-diisopropylethylamine. The Wittig sequence afforded
37% of trienol 43c. The Kornblum sequence afforded
partially hydrated glyoxylate 44c. Treatment of crude
44c with only 0.6 equiv of Yb(OTf)3 in 1:10 CH2Cl2 and
(30) Blanchette, M. A.; Choy, W.; Davis, J . T.; Essenfeld, A. P.;
Masamune, S.; Roush, W. R.; Sakai, T. Tetrahedron Lett. 1984, 25,
2183-2186.

Total Synthesis of (()-Deoxypenostatin A
J . Org. Chem., Vol. 65, No. 25, 2000 8495
hexane/EtOAc) gave e5% of the 2(Z)-isomer of 11 followed by
7.82 g (81%) of pure 11.
and MEM ethers trans to the hydroxy group could not
be prepared. However, we prepared 44e with a hindered
TIPS ether, which is known to coordinate poorly to Lewis
acids31 to test whether 44e with a silyl ether cis to the
glyxoylate would undergo the Diels-Alder reaction better
than 34a and 34b with the silyl ether trans to the
glyxoylate.
Data for the 2(Z)-isomer: 1H NMR 10.27 (s, 1), 7.00 (dd, 1,
J ) 14.1, 11.6), 6.91 (d, 1, J ) 11.6), 6.03 (dt, 1, J ) 14.1, 7.0),
2.21 (br dt, 2, J ) 7.0, 7.3), 1.82 (s, 3), 1.5-1.2 (m, 10), 0.88 (t,
3, J ) 6.7). These data correspond closely to those reported
for 2-methyl-2Z,4E-hexadienal.32
Data for 11: 1H NMR 9.41 (s, 1), 6.81 (d, 1, J ) 11.6), 6.51
(ddt, 1, J ) 15.2, 11.6, 1.5), 6.24 (dt, 1, J ) 15.2, 7.3), 2.24 (br
dt, 2, J ) 7.3, 7.3), 1.83 (s, 3), 1.5-1.2 (m, 10), 0.88 (t, 3, J )
6.7); 13C NMR 195.1, 149.4, 146.0, 135.9, 125.8, 33.5, 31.7, 29.2,
29.1, 28.8, 22.6, 14.1, 9.4; IR (neat) 1683, 1635; HRMS (DCI/
NH3) calcd for C13H23O (MH+) 195.1748, found 195.1746.
tr a n s-2-(3-Met h yl-1E,3E,5E-t r id eca t r ien yl)cyclop en -
ta n ol (19). s-BuLi (1.3 M, 13 mL, 16.9 mmol) was added
dropwise over 20 min to a solution of methyltriphenylphos-
phonium bromide (5 g, 14 mmol) in 50 mL of anhydrous ether
at -78 °C under N2. The solution was stirred for 20 min at
-78 °C, 1.5 h at -40 °C, and 3 h at 25 °C. The resulting
orange-red suspension was cooled to 0 °C, and a solution of
cyclopentene oxide (13) (1.34 g, 16 mmol) in 15 mL of dry THF
was added. Stirring for 16 h at 25 °C furnished a brown
suspension which was cooled to -78 °C and treated with
s-BuLi (1.3 M, 13 mL, 16.9 mmol) dropwise over 20 min. After
stirring for 20 min at -78 °C, 1 h at -40 °C, and 3 h at 25 °C,
the resulting brown solution was cooled to -78 °C and treated
with a solution of dienal 11 (2.3 g, 12 mmol) in 30 mL of THF.
After stirring for 6 h at 25 °C, the reaction mixture was
quenched by the addition of water (50 mL). The solution was
extracted with ether (3 × 80 mL), which was dried (Na2SO4)
and concentrated. Flash chromatography of the residue on
silica gel (95:5 hexane/EtOAc) gave 0.72 g (32%) of 3-methyl-
1,3E,5E-tridecatriene followed by 1.69 g (51%) of pure 19 as a
colorless liquid.
Treatment of 42a with TIPSCl and imidazole gave 80%
of the required epoxide 42e. The Wittig sequence gave
41% of triene alcohol 43e, which was converted to crude
partially hydrated glyoxylate ester 44e by the Kornblum
procedure. Treatment of 44e with Yb(OTf)3 with or
without 2,6-di-tert-butylpyridine gave no Diels-Alder
product, suggesting that the protecting group is more
important than the stereochemistry for the success of the
intramolecular Diels-Alder reaction.
In conclusion, we have developed an eleven-step,
convergent synthesis of deoxypenostatin A (28) that
features a novel, stereoselective, intramolecular Diels-
Alder reaction of a hydrated glyoxylate ester catalyzed
by Yb(OTf)3 and a convergent Wittig sequence that
formed triene alcohol 19 in only two steps. Diels-Alder
adducts 45a and 46a could be prepared in modest yield
analogously from MEM ether 44d , but the sensitivity of
several of the intermediates precluded the elaboration
of 45a to penostatin A (1).
Exp er im en ta l Section
Gen er a l. NMR spectra were recorded at 400 MHz in CDCl3.
Chemical shifts are reported in δ and coupling constants in
Data for 3-methyl-1,3E,5E-tridecatriene: 1H NMR 6.39 (dd,
1, J ) 10.5, 17.5), 6.36 (ddt, 1, J ) 10.9, 14.9, 1.3), 6.04 (d, 1,
J ) 10.9), 5.76 (dt, 1, J ) 14.9, 7.1), 5.14 (d, 1, J ) 17.5), 5.00
(d, 1, J ) 10.5), 2.15 (dt, 2, J ) 7.1, 8.5), 1.85 (s, 3), 1.5-1.1
(m, 10), 0.88 (t, 3, J ) 6.8).
hertz. IR spectra are reported in cm-1
.
2-Meth yl-2E,4E-d od eca d ien a l (11). Cyclohexanamine (10
g, 100 mmol) was added to a dry, 250-mL, round-bottomed
flask fitted with a magnetic stir bar. The flask was flushed
with N2 and cooled to -20 °C. Propionaldehyde (7 g, 120 mmol)
was added dropwise via a cannula over 10 min. During the
initial phase of this addition a large amount of white solid
separated, which redissolved as the addition was continued.
The resulting cold solution was stirred for 1 h, at which time
a large amount of white solid separated and further stirring
was impractical. The resulting mixture was allowed to stand
for 20 min and 5 g of anhydrous Na2SO4 was added. The whole
mixture was allowed to melt and warm to 25 °C under N2 and
then was gravity filtered. The residue was washed with ether.
The combined filtrates were dried (Na2SO4) and concentrated
to give 19.6 g of crude imine, which was distilled under reduced
pressure to provide 10.5 g (76%) of N-propylidenecyclohexan-
amine (12) as a colorless liquid: bp 42-45 °C (3 Torr); 1NMR
7.67 (t, 1, J ) 4.9), 2.90 (m, 1), 2.22 (qd, 2, J ) 7.3, 4.9), 1.1-
1.8 (m, 10), 1.07 (t, 3, J ) 7.3).
A solution of diisopropylamine (5.5 g, 54 mmol) in 50 mL of
dry THF was treated dropwise with n-BuLi (21 mL, 2.5 M,
52.5 mmol) under N2 at 0 °C. The solution was stirred for 20
min, a solution of 7 g (50 mmol) of 12 in 40 mL of dry THF
was added dropwise via a cannula to the cold (0 °C) solution
of lithium diisopropylamide, and the resulting solution was
stirred for 30 min. This solution was then cooled to -78 °C
and a solution of 7.5 g of trans-2-decenal (49 mmol) was added
over 20 min. The mixture was stirred at -78 °C for 6 h and
quenched with saturated aqueous oxalic acid (100 mL) at
0 °C. The mixture was stirred at 0 °C for 6 h and extracted
with ether (4 × 100 mL). The combined organic layers were
washed with saturated sodium bicarbonate solution, dried
(Na2SO4), and concentrated under reduced pressure to give
12.5 g of crude 11. Flash chromatography on silica gel (97:3
Data for 19: 1H NMR 6.34 (ddt, 1, J ) 15.0, 11.0, 1.8), 6.17
(d, 1, J ) 15.9), 5.98 (d, 1, J ) 11.0), 5.71 (dt, 1, J ) 15.0, 6.7),
5.54 (dd, 1, J ) 15.9, 8.3), 3.86 (dddd, 1, J ) 7.0, 7.0, 7.0, 3.4),
2.38 (dddd, 1, J ) 8.3, 8.4, 8.4, 7.0), 2.12 (br dt, 2, J ) 6.7,
7.3), 2.05-1.90, (m, 2), 1.82 (s, 3), 1.80-1.70 (m, 1), 1.7-1.5
(m, 2), 1.63 (d, 1, J ) 3.4, OH), 1.5-1.35 (m, 3), 1.2-1.35 (m,
8), 0.88 (t, 3, J ) 6.8); 13C NMR 135.7, 135.6, 132.4, 130.2 (2),
126.6, 78.9, 52.4, 33.4, 33.2, 31.8, 30.3, 29.5, 29.2 (2), 22.6, 21.2,
14.1, 12.7; IR (neat) 3344, 965; HRMS (DEI) calcd for C19H32
O
(M+) 276.2453, found 276.2448.
A similar reaction using crude 11 which contained 3-5% of
the 2(Z)-isomer gave 3-5% of trans-2-(3-Methyl-1E,3Z,5E-
tridecatrienyl)cyclopentanol, which eluted faster than 11: 1H
NMR 6.70 (d, 1, J ) 16), 6.48 (dd, 1, J ) 15, 11), 5.88 (d, 1,
J ) 11), 5.64 (dt, 1, J ) 15, 7), 5.57 (dd, 1, J ) 16, 9), 3.9-3.8
(m, 1), 2.45-2.35 (m, 1), 2.2-2.0 (m, 2), 2.0-1.8 (m, 2), 1.8 (s,
3), 1.8-1.2 (m, 15), 0.9 (t, 3, J ) 6.8). These data correspond
to those expected for a triene with a central Z-trisubstituted
double bond.33
tr a n s-2-(3-Met h yl-1E,3E,5E-t r id eca t r ien yl)cyclop en -
tyl Br om oa ceta te (20). A solution of 1.05 g (3.8 mmol) of 19
and 0.8 mL of pyridine in 15 mL of CH2Cl2 was cooled to 0 °C.
Bromoacetyl bromide (0.83 g, 4.2 mmol) was added to the
solution dropwise with a syringe. The mixture was stirred for
an additional 15 min and was poured into 15 mL of ice water,
which was extracted with CH2Cl2. The organic layers were
washed with 1 N HCl and water, dried (Na2SO4), and concen-
trated to give 1.58 g of crude 20, which decomposed on
chromatography: 1H NMR 6.34 (ddt, 1, J ) 15.0, 11.0, 1.3),
6.12 (d, 1, J ) 15.6), 5.98 (d, 1, J ) 11.0), 5.71 (dt, 1, J ) 15.0,
7.1), 5.54 (dd, 1, J ) 7.7, 15.6), 4.94 (ddd, 1, J ) 7.1, 5.2, 5.2),
(31) (a) Nelson, T. D.; Crouch, R. D. Synthesis 1996, 1031-1069.
(b) Shimizu, N.; Takesue, N.; Yasuhara, S.; Inazu, T. Chem. Lett. 1993,
1807-1812. (c) Shimizu, N.; Takesue, N.; Yamamoto, A.; Tsutsumi,
T.; Yasuhara, S.; Tsuno, Y. Chem. Lett. 1992, 1263-1266.
(32) Lee, S. Y.; Kulkarni, Y. S.; Burbaum, B. W.; J ohnston, M. I.;
Snider, B. B. J . Org. Chem. 1988, 53, 1848-1855.
(33) Brouwer, A. M.; Bezemer, L.; J acobs, H. J . C. Recl. Trav. Chim.
1992, 111, 138-143.

8496 J . Org. Chem., Vol. 65, No. 25, 2000
Snider and Liu
3.80 (s, 2), 2.66 (dddd, 1, J ) 7.1, 7.3, 7.3, 7.7), 2.10 (br dt, 2,
J ) 7.1, 7.1), 2.08-1.88 (m, 2), 1.80 (s, 3), 1.75-1.59 (m, 3),
1.52-1.40 (m, 1), 1.40-1.30 (m, 2), 1.30-1.18 (m, 8), 0.85 (t,
3, J ) 6.6); 13C NMR 167.1, 135.8, 135.5, 132.4, 130.4, 128.6,
126.6, 83.0, 82.9, 48.6, 33.1, 31.8, 31.1, 30.4, 29.4, 29.1 (2), 26.2,
22.6, 14.1, 12.6.
1), 1.3-1.2 (m, 9), 0.86 (t, 3, J ) 7.3); 13C NMR 172.2, 137.1,
134.3, 127.4, 120.5, 80.3, 74.8, 68.7, 47.7, 35.7, 32.3, 31.8, 29.3,
29.1, 29.0, 28.6, 25.6, 22.6, 20.4, 19.8, 14.1; IR (neat) 1757;
HRMS (DEI) calcd for C21H32O3 (M+) 332.2351, found 332.2358.
The chemical shifts of the methine protons were assigned by
COSY experiments: H-12, δ 4.75; H-9, δ 4.63; H-3, δ 4.19; H-7,
δ 1.6; H-8, δ 2.61.
tr a n s-2-(3-Met h yl-1E,3E,5E-t r id eca t r ien yl)cyclop en -
tyl Nitr ooxya ceta te (22). A solution of NaI (0.6 g) and crude
20 (1.30 g of the above 1.58 g) in 10 mL of acetone was stirred
at 25 °C for 6 h. The resulting precipitate was filtered off and
the solid was washed with CH2Cl2. The filtrate was concen-
trated to give a brown residue that was dissolved in CH2Cl2.
The solution was washed with 10% NaHSO3 and water, dried
(Na2SO4), and concentrated in vacuo to yield 1.5 g of crude
iodoacetate 21 as a yellow liquid, which was dissolved in
15 mL of acetonitrile. Silver nitrate (0.8 g, 4.5 mmol) was
added and the mixture was stirred in the dark overnight at
25 °C. The solution was filtered through Celite, which was
washed with ether. The combined organic filtrates were
washed with water, dried (Na2SO4), and concentrated to afford
1.25 g of crude 22. Flash chromatography on silica gel (97:3
hexane/EtOAc) gave 0.91 g of 22 (90% pure as determined by
analysis of the 1H NMR spectrum), which was used for the
next step: 1H NMR 6.33 (dd, 1, J ) 15.0, 11.0), 6.11 (d, 1, J )
15.6), 5.99 (d, 1, J ) 11.0), 5.73 (dt, 1, J ) 15.0, 7.0), 5.52 (dd,
1, J ) 15.6, 7.7), 5.01 (ddd, 1, J ) 5.0, 6.0, 6.0), 4.88 (s, 2),
2.68 (dddd, 1, J ) 5.0, 7.7, 7.0, 7.0), 2.16-2.02 (m, 3), 2.0-1.9
(m, 1), 1.81 (s, 3), 1.80-1.62 (m, 3), 1.54-1.46 (m, 1), 1.44-
1.32 (m, 2), 1.32-1.20 (m, 8), 0.88 (t, 3, J ) 6.6); 13C NMR
165.6, 135.9, 135.7, 132.3, 130.6, 128.2, 126.6, 83.0, 67.3, 48.7,
33.1, 31.8, 31.2, 30.3, 29.4, 29.2 (2), 22.6, 22.2, 14.1, 12.6; IR
(neat) 1755, 1655.
H yd r oxyk et op h osp h on a t e 26. A solution of 35 mg
(0.28 mmol) of dimethyl methylphosphonate in 4 mL of THF
at -78 °C was treated dropwise with 120 mL (0.3 mmol) of
2.5 M n-butyllithium in hexane. The resulting solution was
stirred for 1 h at -78 °C and then treated dropwise with the
solution of 45 mg (0.14 mmol) of 24 in 1 mL of THF. The
resulting solution was stirred at -78 °C for 1 h and quenched
with 0.5 mL of saturated NH4Cl solution and 2 mL of water.
The mixture was extracted with ether (4 × 5 mL). The
combined organic layers were dried (Na2SO4) and concentrated
to give 76 mg of crude 26. Flash chromatography on silica gel
(85:15 hexane/EtOAc) gave 53 mg (86%) of pure 26: 1H NMR
5.68 (dt, 1, J ) 15.3, 6.8), 5.63 (br d, 1, J ) 6.4), 5.61 (br s, 1),
5.32 (dd, 1, J ) 15.3, 8.6), 4.35 (br d, 1, J ) 8.6), 3.88 (ddd, 1,
J ) 7.0, 10.7, 10.7), 3.79 (br d, 3, J H-P ) 11.0), 3.70 (br d, 3,
J H-P ) 11.0), 3.35 (d, 1, J ) 2.2), 2.71 (dd, 1, J H-P ) 17.4,
J H-H ) 15.3), 2.22-2.14 (m, 1), 2.07 (dd, 1, J H-P ) 17.4,
J H-H ) 15.3), 2.06-2.0 (m, 2), 1.86-1.74 (m, 2), 1.68-1.54 (m,
2), 1.52 (s, 3), 1.51-1.46 (m, 1), 1.42-1.34 (m, 2), 1.32-1.22
(m, 9), 1.22-1.1 (m, 1), 0.87 (t, 3, J ) 6.8); 13C NMR 135.3,
134.7, 129.4, 123.6, 97.7 (d, 1, J C-P ) 7.6), 81.2, 75.5, 74.0 (d,
1, J C-P ) 12.2), 53.7 (d, 1, J C-P ) 5.3), 51.5 (d, 1, J C-P ) 6.9),
44.7, 37.1 (d, 1, J C-P ) 1.6), 33.4 (d, 1, J C-P ) 137), 32.2, 31.8,
29.2, 29.1, 29.1, 27.6, 24.1, 22.6, 19.3, 19.2, 14.1; IR (neat) 3345,
1454.
tr a n s-2-(3-Met h yl-1E,3E,5E-t r id eca t r ien yl)cyclop en -
tyl Oxoa ceta te (23). To a solution of 0.43 g of the above 90%
pure 22 in 8 mL of DMSO was added 110 mg of sodium acetate.
The solution was stirred vigorously at 25 °C for 25 min and
poured into 40 mL of ice-cold brine, which was extracted with
ether (6 × 50 mL). The organic layers were washed with
saturated sodium bicarbonate solution and water, dried
(Na2SO4), and concentrated to give 0.42 g of crude 23 which
was used immediately in the next step.
Hyd r oxyk etop h osp h on a te 29 (34 mg, 93%) was prepared
analogously from 27 mg of 25: 1H NMR 5.68 (s, 1), 5.65 (br d,
1, J ) 5.5), 5.61 (dt, 1, J ) 15.3, 6.7), 5.49 (dd, 1, J ) 15.6,
6.7), 4.40 (br d, 1, J ) 6.7), 3.92 (ddd, 1, J ) 10.8, 10.8, 7.0),
3.77 (d, 3, J H-P ) 10.8), 3.68 (d, 3, J H-P ) 12.0), 3.48 (d, 1,
J ) 2.5), 2.71 (dd, 1, J H-P ) 17.9, J H-H ) 15.6), 2.22-2.12 (m,
1), 2.05 (dt, 2, J ) 6.7, 7.3), 1.97 (dd, 1, J H-P ) 17.1, J H-H
)
15.6), 1.84-1.76 (m, 2), 1.72-1.68 (m, 2), 1.56 (s, 3), 1.54-
1.42 (m, 2), 1.38-1.30 (m, 2), 1.30-1.20 (m, 8), 1.18-1.08 (m,
1), 0.86 (t, 3, J ) 7.0); 13C NMR 135.8, 133.6, 126.9, 124.0,
98.0 (d, 1, J C-P ) 10.4), 77.7, 75.1, 68.0 (d, 1, J C-P ) 13.0),
53.8 (d, 1, J C-P ) 5.3), 51.3 (d, 1, J C-P ) 6.9), 44.3, 36.9, 33.6
(d, 1, J C-P ) 137), 32.3, 31.8, 29.1 (2), 29.1, 27.6, 24.0, 22.7,
19.9, 19.3, 14.1; IR (neat) 3348, 1451.
(3a [E],4a â,9a r,9bâ)-4a ,7,8,9,9a ,9b-Hexa h yd r o-2-m eth yl-
3-(1-n on en yl)cyclop en ta [f][1]ben zop yr a n -5(3H)-on e (27).
Dess-Martin reagent (50 mg, 0.12 mmol) was added to a
solution of 26 (35 mg, 0.08 mmol) in 0.8 mL of dry CH2Cl2.
The mixture was stirred for 1 h at 25 °C. An additional 30 mg
of Dess-Martin reagent was added and the reaction was
stirred for another 1 h. Saturated Na2S2O3 solution (0.5 mL)
was added dropwise and the mixture turned clear after 5 min.
Saturated NaHCO3 solution (0.5 mL) was added dropwise, and
the solution was extracted with ether (3 × 5 mL). The organic
layers were dried (Na2SO4) and concentrated to give 33 mg of
crude diketophosphonate, which decomposed on chromatog-
raphy.
The crude diketophosphonate was dissolved in 4 mL of dry
THF. The resulting solution was added dropwise to a suspen-
sion of NaH (4.5 mg, 60% dispersion in mineral oil, 0.11 mmol)
in 4 mL of dry THF at -78 °C under N2. The mixture was
stirred at -30 °C for overnight and quenched with 3 mL of
water. The mixture was extracted with ether (4 × 5 mL), which
was dried (Na2SO4) and concentrated to give 28 mg of crude
27. Flash chromatography on silica gel (95:5 hexane/EtOAc)
gave 13.6 mg (52%, 64% based on recovered 26) of pure 27
followed by 6.5 mg (19%) of unreacted 26: 1H NMR 5.98 (d, 1,
J ) 1.9), 5.79 (dt, 1, J ) 15.3, 6.7), 5.75 (br d, 1, J ) 5.8), 5.29
(dd, 1, J ) 15.3, 8.9), 4.56 (br d, 1, J ) 8.9), 3.81 (d, 1, J )
3.1), 2.68 (br dd, 1, J ) 19.4, 9.5), 2.64 (m, 1), 2.50 (br ddd, 1,
J ) 19.4, 9.0, 9.5), 2.22 (ddd, 1, J ) 11.6, 6.9, 6.9), 2.16-2.08
(m, 1), 2.08-1.99 (m, 2), 1.99-1.90 (m, 1), 1.76-1.64 (m, 1),
1.60 (s, 3), 1.42-1.30 (m, 2), 1.30-1.20 (m, 9), 0.87 (t, 3, J )
(3r[E],4a â,6a â,9a r,9b â)-4a ,6,6a ,7,8,9,9a ,9b-oct a h yd r o-
2-m eth yl-3-(1-n on en yl)cyclop en ta [f]p yr a n o[3,4-n ]p yr a n -
5(3H)-on e (24). A solution of crude 23 (0.42 g) in 20 mL of
CH2Cl2 was added dropwise to a solution of Yb(OTf)3 (150 mg,
0.25 mmol) and 2,6-di-tert-butylpyridine (36 mg, 0.19 mmol)
in 300 mL of CH3CN. The mixture was stirred at 25 °C
overnight and concentrated. The residue was taken up in ether
and filtered through a short silica gel column. The eluent was
concentrated to give 0.55 g of crude 24. Flash chromatography
on silica gel (93:7 hexane/EtOAc) gave 106 mg (21% from 19)
of 24: 1H NMR 5.78 (dt, 1, J ) 15.3, 7.2), 5.52 (br d, 1, J )
5.5), 5.31 (dd, 1, J ) 8.6, 15.3), 4.51 (br d, 1, J ) 8.6), 4.26 (d,
1, J ) 4.6), 4.03 (ddd, 1, J ) 11.1, 8.6, 8.6), 2.20-1.62 (m, 9),
1.59 (s, 3), 1.44-1.32 (m, 2), 1.32-1.18 (m, 9), 0.87 (t, 3, J )
6.7); 13C NMR 170.0, 137.0, 136.5, 127.8, 119.6, 84.5, 80.2, 71.5,
43.9, 38.9, 32.3, 31.8, 29.2, 29.1, 28.9, 28.6, 23.7, 22.6, 19.5,
19.3, 14.1; IR (neat) 1748. Anal. Calcd for C21H32O3: C, 75.86;
H, 9.70. Found: C, 75.74, H, 9.41. The chemical shifts of the
methine protons were assigned by COSY experiments: H-12,
δ 4.51; H-9, δ 4.26; H-3, δ 4.03; H-7, δ 1.90-2.1; H-8, δ 2.15.
24 a n d (3r[E],4a r,6a r,9a â,9br)-4a ,6,6a ,7,8,9,9a ,9b-oc-
ta h yd r o-2-m eth yl-3-(1-n on en yl)cyclop en ta [f]p yr a n o[3,4-
n ]p yr a n -5(3H)-on e (25). A solution of crude 23 (0.27 g) in
15 mL of CH2Cl2 was added dropwise to a solution of Yb(OTf)3
(90 mg, 0.14 mmol) and 200 mL of CH3CN. The mixture was
stirred at 25 °C for 2 d. Workup as described above gave
0.28 g of a crude 2:3 mixture of 24 and 25. Flash chromatog-
raphy on silica gel (93:7 hexane/EtOAc) gave 45 mg (15% from
19) of 25 followed by 28 mg (9% from 19) of 24. Data for 25:
1H NMR 5.82 (dt, 1, J ) 15.3, 6.7), 5.41 (q, 1, J ) 1.3), 5.35
(ddt, 1, J ) 8.0, 15.3, 1.2), 4.75 (br d, 1, J ) 8.0), 4.63 (d, 1,
J ) 8.6), 4.19 (ddd, 1, J ) 11.0, 8.6, 8.6), 2.61 (br dd, 1, J )
10.3, 8.5), 2.14-2.0 (m, 3), 1.94-1.72 (m, 2), 1.69-1.58 (m, 1),
1.57 (s, 3), 1.58-1.56 (m, 1), 1.42-1.32 (m, 2), 1.32-1.31 (m,

Total Synthesis of (()-Deoxypenostatin A
J . Org. Chem., Vol. 65, No. 25, 2000 8497
6.8); 13C NMR 195.2, 176.0, 137.1, 136.4, 127.9, 122.2, 121.2,
81.5, 75.1, 46.0, 41.4, 32.3, 32.0, 31.8, 30.7, 29.2, 29.1, 28.9,
23.8, 22.6, 19.4, 14.1; IR (neat) 1673; HRMS (GC/MS, EI 20
eV) calcd for C22H32O2 (M+) 328.2402, found 328.2410. The
chemical shifts of the methine protons were assigned by COSY
experiments: H-12, δ 4.56; H-9, δ 3.81; H-7, δ 2.64; H-8, δ
2.12. NOESY experiments showed cross-peaks between H-9
and H-12, and H-8 and H-9.
(3r[E],4ar,9aâ,9br)-4a ,7,8,9,9a ,9b-Hexa h yd r o-2-m eth yl-
3-(1-n on en yl)cyclop en ta [f][1]ben zop yr a n -5(3H)-on e (30).
(9.5 mg, 70%) was prepared analogously from 18 mg of 29:
1H NMR 5.99 (br s, 1), 5.72-5.65 (m, 2), 5.60 (dd, 1, J ) 15.6,
6.4), 4.46 (d, 1, J ) 6.7), 3.90 (d, 1, J ) 3.4), 2.66 (dd, 1, J )
8.3, 19.0), 2.62-2.51 (m, 1), 2.51-2.41 (m, 1), 2.21 (dt, 1, J )
11.9, 6.7), 2.1-2.0 (m, 2), 1.92 (dt, 1, J ) 13.1, 7.4), 1.66 (s, 3),
1.76-1.62 (m, 1), 1.42-1.32 (m, 2), 1.32-1.20 (m, 10), 0.86 (t,
3, J ) 6.8); 13C NMR 195.5, 175.6, 136.1, 135.5, 126.1, 122.3,
121.2, 78.1, 69.2, 45.2, 41.1, 32.5, 32.0, 31.8, 30.7, 29.2, 29.1
(2), 23.8, 22.6, 20.2, 14.1; IR (neat) 1673, 1641. The chemical
shifts of the methine protons were assigned by COSY experi-
ments: H-12, δ 4.46; H-9, δ 3.90; H-7, δ 2.40; H-8, δ 2.10; H-13,
δ 5.60; and H-14 δ 5.71-5.65. NOESY experiments showed
cross-peaks between H-9 and H-8, H-12 and H-7, and H-9 and
both H-13 and H-14.
(3r[E],4a r,9a r,9bâ)-4a ,7,8,9,9a ,9b-Hexa h yd r o-2-m eth -
yl-3-(1-n on en yl)cyclop en t a [f][1]b en zop yr a n -5(3H )-on e
(Deoxyp en ost a t in A) (28). K2CO3 (4 mg, 0.03 mmol) was
added to a solution of 27 (8 mg, 0.024 mmol) in 0.3 mL of dry
MeOH under N2. The mixture was stirred for 12 h, giving a
3:2 mixture of 27 and 28 as indicated by TLC analysis. Longer
reaction time led to decomposition. Concentration followed by
flash chromatography on silica gel (93:7 hexane/EtOAc) gave
3.2 mg (40%) of recovered 27, which can be recycled, followed
by 2.2 mg (28%, 46% based on recovered 27) of 28: 1H NMR
5.94 (q, 1, J ) 1.8), 5.67 (dt, 1, J ) 15.3, 6.7), 5.57 (br s, 1),
5.56 (dd, 1, J ) 15.3, 6.1), 4.59 (br d, 1, J ) 6.1), 4.02 (d, 1,
J ) 11.0), 2.7-2.6 (m, 1), 2.54-2.44 (m, 1), 2.44-2.30 (m, 1),
2.29-2.21 (m, 1), 2.05 (q, 2, J ) 7.9), 2.02-1.92 (m, 1), 1.8-
1.66 (m, 1), 1.66 (br s, 3), 1.42-1.32 (m, 2), 1.32-1.2 (m, 10),
0.87 (t, 3, J ) 6.4); 13C NMR 196.4, 172.5, 136.2, 136.1, 126.1,
122.1, 121.7, 73.8, 48.1, 44.8, 32.4, 31.8, 31.5, 30.1, 29.1, 29.1,
29.0, 23.8, 22.6, 20.0, 14.1 (one C obscured by CDCl3); IR (neat);
1693, 1633. The chemical shifts of the methine protons: H-12,
δ 4.59; H-9, δ 4.02; H-7, δ 2.40; H-14, δ 5.67; and H-13, 5.56.
NOESY experiments showed cross-peaks between H-9 and
H-7, and H-9 and both H-13 and H-14.
(3r[E],4a â,9a â,9br)-4a ,7,8,9,9a ,9b-Hexa h yd r o-2-m eth yl-
3-(1-n on en yl)cyclop en ta [f][1]ben zop yr a n -5(3H)-on e (31).
Similar treatment of 30 (7.6 mg, 0.23 mmol) with 4 mg of
K2CO3 in MeOH (0.5 mL) for 3 h provided 5.5 mg (73%) of 31
as the only product: 1H NMR 5.95 (q, 1, J ) 1.8), 5.79 (dt, 1,
J ) 15.3, 6.7), 5.55 (q, 1, J ) 1.8), 5.33 (ddt, 1, J ) 15.3, 9.2,
1.2), 4.53 (br d, 1, J ) 9.2), 3.89 (d, 1, J ) 11.0), 2.65 (dd, 1,
J ) 18.6, 10.1), 2.56-2.3 (m, 2), 2.26 (dt, 1, J ) 12.2, 6.4),
2.1-2.0 (m, 2), 2.0-1.92 (m, 1), 1.8-1.58 (m, 1), 1.60 (s, 3),
1.42-1.32 (m, 2), 1.32-1.2 (m, 10), 0.87 (t, 3, J ) 6.8); 13C
NMR 195.4, 172.6, 136.8 (2), 128.0, 121.7, 121.6, 81.0, 78.9,
47.9, 44.3, 32.3, 31.8, 31.6, 30.1, 29.2, 29.1, 28.9, 23.7, 22.6,
19.4, 14.1; IR (neat) 1685. The chemical shifts of the methine
protons were assigned by COSY experiments: H-12, δ 4.53;
H-9, δ 3.89; H-7, δ 2.35; H-8, δ 2.45. NOESY experiments
showed cross-peaks between H-12 and H-9, and H-7 and H-9.
La cton e 41. A solution of crude 37 (20 mg) in 1 mL of
CH2Cl2 was added dropwise to a solution of Yb(OTf)3 (30 mg,
0.048 mmol) and 2,6-di-tert-butylpyridine (8 mg, 0.04 mmol)
in 10 mL of CH3CN. The mixture was stirred overnight. An
additional 30 mg of Yb(OTf)3 was added, and the reaction
mixture was stirred at 42 °C for 16 h. The reaction tempera-
ture was then elevated to 70 °C for 6 h. Water (2 mL) was
added to the mixture and the organic layer was extracted with
ether (3 × 6 mL). The combined organic layers were dried (Na2-
SO4) and concentrated to give 23.6 mg of crude product. Flash
chromatography on silica gel (80:20 hexane/EtOAc, then 50:
50 hexane/EtOAc) gave 7.6 mg of partially pure hydrolyzed
product (the glyoxylate ester and the ketal protecting group
were gone) and 6.8 mg of 41 (41% from nitrooxyacetate): 1H
NMR 6.20 (dd, 1, J ) 14.7, 11.0), 6.12 (d, 1, J ) 11.0), 5.82
(dt, 1, J ) 14.7, 6.7), 4.52 (br d, 1, J ) 10.4, H-9), 4.50 (d, 1,
J ) 10.1, H-10), 4.31 (dddd, 1, J ) 7.3, 10.4, 8.5, 3.7, H-3),
3.83 (br d, 1, J ) 3.7, C-3-OH), 3.53 (br s, 1, C-9-OH), 2.71
(dd, 1, J ) 18.3, 7.3, H-4), 2.54 (dd, 1, J ) 6.7, 18.9, H-6), 2.41
(ddd, 1, J ) 10.4, 10.1, 10.1, H-8), 2.31 (ddd, 1, J ) 10.4, 18.3,
1.2, H-4), 2.2-2.14 (m, 1, H-7), 2.13 (br dt, 2, J ) 7.3, 7.8),
1.76 (dd, 1, J ) 12.2, 18.9, H-6), 1.73 (s, 3), 1.44-1.32 (m, 2),
1.32-1.20 (m, 8), 0.89 (t, 3, J ) 6.7); 13C NMR 212.0, 175.2,
139.9, 134.2, 127.3, 124.8, 87.0, 73.4, 72.5, 50.7, 46.9, 45.8, 43.3,
33.0, 31.8, 29.2, 29.1, 29.0, 22.6, 14.1, 11.2; IR (neat) 3262,
1775, 1744; HRMS (DEI) calcd for C21H32O5 (M+) 364.2250,
found 364.2266. The chemical shifts of the methine protons
were assigned by COSY experiments. NOESY experiments
showed cross-peaks between H-7 and H-4, H-7 and C-3-OH,
H-4 and H-12 (CH3 group).
(3r[E],4a â,6a â,9a r,9b â)-4a ,6,6a ,7,8,9,9a ,9b-oct a h yd r o-
2-m eth yl-3-(1-n on en yl)-8-oxocyclop en ta [f]p yr a n o[3,4-n ]-
p yr a n -5(3H)-on e (39). A solution of crude 37 (13 mg from
14 mg of nitrooxyacetate) in 1.2 mL of CH2Cl2 was added
dropwise to a solution of Yb(OTf)3 (10 mg, 0.6 equiv) in 6 mL
of CH3CN. The mixture was heated to 65 °C and stirred for 3
h. TLC analysis showed that the desired DA product 39 was
present and was isomerizing to 41. The reaction mixture was
cooled to room temperature and filtered through a short silica
gel column. The filtrate was concentrated to give 23 mg of
crude product containing some Yb(OTf)3. Flash chromatogra-
phy on silica gel (80:20 hexane/EtOAc, then 50:50 hexane/
EtOAc) gave 2.2 mg (∼ 20% from nitrooxyacetate) of 90% pure
39 as determined by analysis of the NMR spectrum: 1H NMR
5.78 (dt, 1, J ) 15.3, 6.7), 5.59 (br d, 1, J ) 6.1), 5.27 (dd, 1,
J ) 15.3, 8.5), 4.45 (br d, 1, J ) 8.5), 4.40 (d, 1, J ) 3.7), 4.26-
4.15 (m, 1), 3.0-2.92 (m, 1), 2.62 (dd, 1, J ) 7.3, 18.3), 2.57
(br dt, 1, J ) 19.5, 7.2), 2.20-2.00 (m, 4), 1.88-1.80 (m, 1),
1.62 (s, 3), 1.44-1.32 (m, 2), 1.32-1.20 (m, 8), 0.88 (t, 3, J )
6.7); HRMS (DEI) calcd for C21H30O4 (M+) 346.2144, found
346.2152.
(3r[E],4aâ,6aâ,8r,9ar,9bâ)-4a,6,6a,7,8,9,9a,9b-octah ydr o-
8-h yd r oxy-2-m et h yl-3-(1-n on en yl)cyclop en t a [f]p yr a n o-
[3,4-n ]p yr a n -5(3H)-on e (45a ) a n d (3r[E],4a r,6a r,8â,9a â
,9br)-4a ,6,6a ,7,8,9,9a ,9b-octa h yd r o-8-h yd r oxy-2-m eth yl-
3-(1-n on e n yl)cyclop e n t a [f]p yr a n o[3,4-n ]p yr a n -5(3H )-
on e (46a ). A solution of crude 44c (400 mg) in 10 mL of CH2Cl2
was added dropwise to a solution of Yb(OTf)3 (320 mg, 0.6
equiv) in 100 mL of CH3CN. The reaction mixture was heated
to 65 °C and stirred for 2.5 h. To avoid epimerization at the
doubly allylic position, the reaction mixture was treated with
100 µL of 2,6-di-tert-butylpyridine, cooled to room temperature,
and filtered through a short silica gel column. The filtrate was
concentrated to give 484 mg of crude product containing some
Yb(OTf)3. Flash chromatography on silica gel (90:10 then 70:
30 hexane/EtOAc) afforded 71 mg (23% from nitrooxyacetate)
of a 3:2 inseparable mixture of 45a and 46a : HRMS (DCI/
NH3) calcd for C21H33O4 (MH+) 349.2378, found 349.2370.
Hyd r oxyk etop h osp h on a tes 47 a n d 48. A solution of 71
mg (0.2 mmol) of the mixture of 45a and 46a in 0.6 mL of
DMF was treated with 46 mg (0.8 mmol) of imidazole and 52
mg (0.34 mmol) of TBSCl. The mixture was stirred at room
temperature for 40 min, and hexane (40 mL) and water (10
mL) were added. The organic layer was separated and the
aqueous layer was extracted with Et2O (10 mL). The combined
organic layers were washed with saturated NaHCO3 and brine,
dried (Na2SO4), and concentrated to give 97 mg of crude silyl
ethers that were dissolved in 5 mL of dry THF.
A solution of 68 mg (0.54 mmol) of dimethyl methylphos-
phonate in 10 mL of THF at -78 °C was treated dropwise with
210 µL (0.53 mmol) of 2.5 M n-butyllithium in hexane. The
resulting solution was stirred for 1 h at -78 °C and then
treated dropwise with the silyl ether solution. The resulting
solution was stirred at -78 °C for 1 h and quenched with
4 mL of water. The mixture was extracted with ether
(3 × 8 mL). The combined organic layers were dried (Na2SO4)
and concentrated to give 125 mg of crude product. Flash
chromatography on silica gel (85:15 hexane/EtOAc) gave 57

8498 J . Org. Chem., Vol. 65, No. 25, 2000
Snider and Liu
mg (48%) of pure 47 followed by 34 mg (29%) of pure 48 as
colorless liquids.
was added dropwise and the mixture turned clear after 5 min.
Saturated NaHCO3 solution (0.5 mL) was added dropwise and
the solution was extracted with ether (3 × 5 mL). The organic
layers were dried (Na2SO4) and concentrated to give 24 mg of
crude diketophosphonate.
Data for 47: 1H NMR 5.69 (dt, 1, J ) 15.3, 6.7), 5.64 (br s,
1), 5.58 (br d, 1, J ) 5.5), 5.35 (dd, 1, J ) 15.3, 8.5), 4.35 (br
d, 1, J ) 8.5), 4.28 (ddd, 1, J ) 6.7, 6.7, 6.1), 3.82-3.70 (m, 1),
3.79 (br d, 3, J H-P ) 10.8), 3.74 (br d, 3, J H-P ) 10.8), 3.35 (d,
1, J ) 1.8), 2.72 (dd, 1, J H-P ) 17.7, J H-H ) 15.9), 2.24 (ddd,
1, J ) 7.0, 7.0, 11.9), 2.16-1.92 (m, 5), 1.72 (dd, 1, J ) 13.4,
7.3), 1.59 (dd, 1, J ) 11.6, 5.5), 1.53 (s, 3), 1.45 (dd, 1, J )
13.2, 7.6), 1.42-1.34 (m, 2), 1.34-1.2 (m, 8), 0.92-0.84 (m, 3),
0.87 (s, 9), 0.02 (d, 6, J ) 3.1); 13C NMR 135.3, 134.9, 129.3,
123.2, 97.7 (d, 1, J C-P ) 7.6), 81.3, 74.0 (d, 1, J C-P ) 12.2),
73.7, 69.5, 53.7 (d, 1, J C-P ) 5.3), 51.5 (d, 1, J C-P ) 6.9), 42.3,
40.1, 36.8, 36.1, 33.4 (d, 1, J C-P ) 136.6), 32.2, 31.8, 29.2, 29.1,
29.1, 25.9 (3), 22.6, 19.3, 18.1, 14.1, -4.7, -4.8; IR (neat) 3330,
1594, 1039.
The crude diketophosphonate was dissolved in 3 mL of dry
THF. The resulting solution was added dropwise to a suspen-
sion of NaH (5.4 mg, 60% dispersion in mineral oil, 0.1 mmol)
in 5 mL of dry THF at -78 °C under N2. The mixture was
stirred at -30 °C overnight and quenched with 3 mL of water.
The mixture was extracted with ether (4 × 5 mL), which was
dried (Na2SO4) and concentrated to give 20 mg of crude 50.
Flash chromatography on silica gel (95:5 hexane/EtOAc) gave
4 mg (24% from 48) of pure 50: 1H NMR 5.98 (br, s, 1), 5.76-
5.68 (m, 2), 5.62 (dd, 1, J ) 15.6, 6.1), 4.51 (d, 1, J ) 6.1, H-12),
4.46 (dd, 1, J ) 4.9, 4.9, H-5), 3.93 (d, 1, J ) 3.7, H-9), 3.06-
3.00 (m, 1, H-7), 2.76 (dt, 1, J ) 19.5, 2.4), 2.60 (d, 1, J ) 19.5),
2.22-2.0 (m, 3), 1.67 (s, 3), 1.48 (dd, 1, J ) 4.9, 12.2), 1.76-
1.62 (m, 1), 1.42-1.32 (m, 2), 1.32-1.20 (m, 8), 0.9-0.82 (m,
3), 0.87 (s, 9), 0.06 (d, 6, J ) 3.1); 13C NMR 195.4, 173.9, 136.2,
135.7, 126.1, 122.0, 121.7, 78.0, 71.3, 69.1, 42.8, 41.8, 41.2, 40.1,
32.5, 31.8, 29.2, 29.1 (2), 25.8 (3), 22.6, 20.2, 18.1, 14.1, -4.8,
-4.8; IR (neat) 1673, 1594; HRMS (DEI) calcd for C28H46O3Si
(M+) 458.3216, found 458.3210. The chemical shifts of the
methine protons were assigned by COSY experiments: H-12,
δ 4.51; H-5, δ 4.46; H-9, δ 3.93; H-7, δ 3.04; H-8, δ 2.05.
Data for 48: 1H NMR 5.69 (s, 1), 5.63-5.55 (m, 2), 5.48 (dd,
1, J ) 15.9, 6.7), 4.42 (br d, 1, J ) 6.7), 4.27 (ddd, 1, J ) 6.7,
6.7, 6.1), 3.85-3.78 (m, 1), 3.79 (d, 3, J H-P ) 11.2), 3.70 (d, 3,
J H-P ) 11.6), 3.47 (d, 1, J ) 2.4), 2.71 (dd, 1, J H-P ) 17.7,
J H-H ) 15.9), 2.25 (ddd, 1, J ) 7.0, 7.0, 12.2), 2.12-2.0 (m, 1),
2.05 (dt, 2, J ) 7.3, 6.7), 1.94 (dd, 1, J H-P ) 17.1, J H-H ) 15.9),
1.98-1.84 (m, 1), 1.73 (dd, 1, J ) 7.0, 12.8), 1.58 (s, 3), 1.54
(dd, 1, J ) 5.5, 11.6), 1.45 (dd, 1, J ) 7.3, 12.8), 1.40-1.30 (m,
2), 1.30-1.20 (m, 8), 0.92-0.84 (m, 3), 0.87 (s, 9), 0.01 (d, 6,
J
) 4.3); 13C NMR 135.7, 133.8, 126.9, 123.6, 97.9 (d, 1,
J C-P ) 8.4), 77.6, 73.4, 69.4, 68.0 (d, 1, J C-P ) 13.0), 53.8 (d,
1, J C-P ) 5.3), 51.3 (d, 1, J C-P ) 6.9), 41.8, 40.1, 36.6, 33.1,
34.5 (d, 1, J C-P ) 136.8), 32.3, 31.8, 29.1 (2), 29.0, 25.8 (3),
22.6, 20.0, 18.1, 14.1, -4.7, -4.9; IR (neat) 3342, 1592, 1039.
(3r[E],4ar,8â,9aâ,9br)-4a,7,8,9,9a,9b-Hexah ydr o-2-m eth -
yl-8-((d im eth yleth yl)d im eth ylsilyloxy)-3-(1-n on en yl)cy-
clop en ta [f][1]ben zop yr a n -5(3H)-on e (50). Dess-Martin
reagent (35 mg, 0.08 mmol) was added to a solution of 48 (21
mg, 0.04 mmol) in 0.5 mL of dry CH2Cl2. The mixture was
stirred for 2.5 h at 25 °C. Saturated Na2S2O3 solution (0.6 mL)
Ack n ow led gm en t. We are grateful to the NIH (GM-
50151) for generous support of this research.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails for preparation of 37 and 44c and 1H and 13C NMR
spectral data. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O000850X