Structure revision and syntheses of epohelmins A and B

Article abstract of DOI:10.1021/ol0516061

(Chemical Equation Presented) Epohelmins A (24) and B (26) have been reassigned as pyrrolizidin-1-ols, rather than the proposed 9-oxa-4-azabicyclo[6. 1.0]nonane structures 1 and 2, respectively. Syntheses of epohelmin A (24) (eight steps, 52% overall yiel

Full text of DOI:10.1021/ol0516061

ORGANIC
LETTERS
2005
Vol. 7, No. 20
4419-4422
Structure Revision and Syntheses of
Epohelmins A and B
Barry B. Snider* and Xiaolei Gao
Department of Chemistry, MS 015, Brandeis UniVersity,
Waltham, Massachusetts 02454-9110
snider@brandeis.edu
Received July 8, 2005
ABSTRACT
Epohelmins A (24) and B (26) have been reassigned as pyrrolizidin-1-ols, rather than the proposed 9-oxa-4-azabicyclo[6.1.0]nonane structures
1 and 2, respectively. Syntheses of epohelmin A (24) (eight steps, 52% overall yield) and epohelmin B (26) (11 steps, 43% overall yield) have
been achieved starting from N-Cbz-(S)-prolinal (9) and ortho ester ketone 17 using a stereoselective aldol reaction and a stereoselective
reductive cyclization as the key steps.
Shibuya, Ebizuka, and co-workers recently reported the
isolation of the novel lanosterol synthase inhibitors epo-
helmins A (1) and B (2) from a fungal strain FKI-0929.1
The structures were determined by detailed spectroscopic
analysis and proposed to be novel 9-oxa-4-azabicyclo[6.1.0]-
nonanes (see Figure 1).
the epoxy groups of epohelmins A and B absorb in the range
δ 4-4.5 and the carbons in the range δ 70-74 (see Figure
1). The analogous protons and carbons of trans- and cis-
epoxy cyclooctanes (5 and 6) absorb at much higher field
than those of the epohelmins with the protons at δ 2.8 and
2.9 and the carbons at δ 59.6 and 55.6, respectively (see
Figure 2).4
On the other hand, the methine hydrogens and carbons of
trans- and cis-pyrrolizidin-1-ols (7 and 8) have similar
chemical shifts to the epohelmins as shown.5 Although
Figure 1. Proposed structures of epohelmins A (1) and B (2).
(1) (a) Sakano, Y.; Shibuya, M.; Yamaguchi, Y.; Masuma, R.; Tomada,
H.; Oˆ mura, S.; Ebizuka, Y. J. Antibiot. 2004, 57, 564-568. (b) Oˆ mura, S.;
Kyoda, H.; Masuma, R.; Ebizuka, Y.; Shibuya, M. Japan Kokai Tokkyo
Koho JP 2004196680, 2004; Chem. Abstr. 2004, 141, 122412m.
(2) Glass, R. S.; Deardorff, D. R.; Gains, L. H. Tetrahedron Lett. 1978,
There were both chemical and spectroscopic grounds to
question these structure assignments. 9-Oxa-4-azabicyclo-
[6.1.0]nonanes cyclize readily to give pyrrolizidin-1-ols.2,3
Such a cyclization of any of the four isomers of 3 would
give pyrrolizidin-1-ols 4 (see eq 1). The protons assigned to
2965-2968.
(3) Subramanian, T.; Lin, C.-C.; Lin, C.-C. Tetrahedron Lett. 2001, 42,
4079-4082.
(4) Poon, T. H. W.; Pringle, K.; Foote, C. S. J. Am. Chem. Soc. 1995,
117, 7611-7618.
10.1021/ol0516061 CCC: $30.25
© 2005 American Chemical Society
Published on Web 09/07/2005

Scheme 1. Synthesis of Epohelmin A Model 11
Figure 2. NMR data of epoxycyclooctanes (5 and 6) and
pyrrolizidin-1-ols (7 and 8).
pyrrolizidin-1-ols 4 look quite different than 9-oxa-4-
azabicyclo[6.1.0]nonanes 3, the only difference is the forma-
tion of a C-N bond and cleavage of a C-O bond so that
the 2D NMR correlations used to assign the structure of
epohelmins A and B as 1 and 2 will be equally applicable
to the stereoisomers of 4.
This analysis suggests that epohelmins A and B are two
of the four stereoisomers of 4. If this is the case, the
stereochemical analysis of 1 and 2 is based on the wrong
skeleton and may not be applicable to the stereochemistry
of 4. We decided that the stereochemistry was best deter-
mined by the synthesis of model 3-alkylpyrrolizidin-1-ols
such as 11. Aldol addition of the kinetic enolate of 2-unde-
canone to N-Cbz-phenylalaninal followed by reductive
pyrrolidine formation and methylation led to all four isomers
of preussin (2-benzyl-1-methyl-5-(nonyl)pyrrolidin-3-ol).6
This short sequence seemed ideal since at this time we did
not know the stereochemistry of the epohelmins.
of lithium enolates to N-Cbz-phenylalaninal; the zinc enolate
added stereoselectively.6 Addition of zinc and lithium
enolates of ethyl acetate to N-Boc-prolinal afforded 2:1 to
4:1 mixtures of isomers favoring the Felkin-Anh product.9
We decided to prepare oxazolidinone 14 to prove the
stereochemistry of aldol product 10. Treating 10 with K2-
CO3 did not form the expected oxazolidinone but instead
resulted in hydrolysis and double dehydration to give pyrrole
12.10 Reduction of the ketone with NaBH4 in MeOH followed
by treatment with K2CO3 in 1:1 2-propanol/water for 18 h
at 70 °C provided oxazolidinone 13 in 71% overall yield as
a mixture of stereoisomers. Dess-Martin oxidation afforded
the desired oxazolidinone ketone 14 in 99% yield. The
coupling constant between the methine hydrogens of 14 is
7.3 Hz as in related compounds indicating that the hydrogens
are cis; in similar compounds with anti hydrogens the
coupling constant is 4.0 Hz.11 This established that the enolate
of 2-heptanone added to 9 with high Felkin-Anh selectivity
to give 10.
The stereochemistry at C3 of epohelmin A model 11 was
established by the NOE between H1 and H3. We had not
expected the reductive cyclization to be stereoselective
because reductive cyclization of related pyrrolidine ketones
lacking the hydroxyl group gave 1:1 mixtures of 3-alkylpyr-
rolizidines.12 Presumably, the hydroxyl group blocks the
bottom face so that hydrogenation occurs preferentially from
the top face to give 11.
Treatment of 2-heptanone with LDA in THF at -78 °C
provided the kinetic enolate, which was treated with N-Cbz-
(S)-prolinal (9)7 to afford 10 with excellent stereoselectivity
in 95% yield (see Scheme 1). Hydrogenolysis (1 atm) of 10
over Pd(OH)2 in MeOH for 12 h liberated the secondary
amine, which reacted with the ketone to form an iminium
salt or enamine, which was reduced to give pyrrolizidinol
11 in 71% yield. Several minor byproducts were formed that
may include diastereomers of 11 other than epohelmin B
model 16. Similar results were obtained with other Pd
1
catalysts. As expected, the H and 13C NMR spectra of 11
vary as a function of pH. The epohelmins were isolated using
an eluent containing HOAc, suggesting that the natural
products were isolated as acetate salts. We were pleased to
1
find that the H and 13C NMR spectra of a CDCl3 solution
We now turned our attention to the preparation of an
epohelmin B model. As suggested by Ebizuka and Shibuya,
we suspected that epohelmins A and B differed in the
alcohol, not alkyl, stereochemistry. Changing the stereo-
chemistry at the alkyl group should change the 13C NMR
of 11 containing 0.85-0.90 equiv of HOAc corresponded
precisely with those of the ring portion of epohelmin A.8
We had not expected either step in the formation of 11 to
be stereoselective. Kitahara obtained mixtures in the addition
(5) Christine, C.; Ikhiri, K.; Ahond, A.; Mourabit, A. A.; Poupat, C.;
Potier, P. Tetrahedron 2000, 56, 1837-1850.
(6) (a) Okue, M.; Watanabe, H.; Kitahara, T. Tetrahedron 2001, 57,
4107-4110. (b) Okue, M.; Watanabe, H.; Kasahara, K.; Yoshida, M.;
Horinouchi, S.; Kitahara, T. Biosci. Biotechnol. Biochem. 2002, 66, 1093-
1096.
(9) (a) Andre´s, J. M.; Pedrosa, R.; Pe´rez, A.; Pe´rez-Encabo, A.
Tetrahedron 2001, 57, 8521-8530. (b) Hanson, G. J.; Baran, J. S.; Lindberg,
T. Tetrahedron Lett. 1986, 27, 3577-3580.
(10) For a similar compound see: Pommelet, J. C.; Jourdain, F.;
Dhimane, H. Molecules 2000, 5, 1130-1138.
(7) (a) Langley, D. R.; Thurston, D. E. J. Org. Chem. 1987, 52, 91-97.
(b) Corey, E. J.; Shibata, S.; Bakshi, R. K. J. Org. Chem. 1988, 53, 2861-
2863. (c) Dolbeare, K.; Pontoriero, G. F.; Gupta, S. K.; Mishra, R. K.;
Johnson, R. L. Bioorg. Med. Chem. 2003, 11, 4103-4112.
(8) We thank Prof. M. Shibuya for a complete set of 1D and 2D NMR
spectra of epohelmins A and B.
(11) (a) Parker, K. A.; O’Fee, R. J. Am. Chem. Soc. 1983, 105, 654-
655. (b) Kiyooka, S.-i.; Nakano, M.; Shiota, F.; Fujiyama, R. J. Org. Chem.
1989, 54, 5409-5411. (c) St-Denis, Y.; Chan, T.-H. J. Org. Chem. 1992,
57, 3078-3085.
(12) Provot, O.; Ce´le´rier, J.-P.; Lhommet, G. J. Heterocycl. Chem. 1998,
35, 371-376.
4420
Org. Lett., Vol. 7, No. 20, 2005

Scheme 2. Conversion of Epohelmin A Model 11 to
Epohelmin B Model 16
Scheme 3. Preparation of Epohelmins A (24) and B (26)
spectrum of the lower portion of the molecule,13 whereas
changing the stereochemistry at the alcohol should change
the 13C NMR spectrum of the top portion of the molecule
as is observed. Inversion of trans-pyrrolizidin-1-ol stereo-
chemistry has been addressed by Chamberlin, who found
that this was best accomplished by oxidation to the ketone
and reduction with L-Selectride.14 Swern oxidation of 11
afforded the unstable ketone 15 in 97% yield. Reduction with
L-Selectride in THF provided epohelmin B model 16 in 69%
yield. The 1H and 13C NMR spectra of the acetate salt of 16
corresponded precisely with those of the ring portion of
epohelmin B. The stereochemistry of the pentyl side chain
was confirmed by the NOE between H3 and H4, which
established that H3 is on the concave face and the pentyl
group is on the convex face. Reduction of 15 with NaBH4
in MeOH gave a 25:1 mixture of 11 and 16 analogous to
that observed by Chamberlin in related systems.14 Although
reduction of pyrrolizidin-1-ones with NaBH(OMe)3 in ben-
zene has been reported to give cis-pyrrolizidin-1-ols,15 no
reaction occurred with 15 under these conditions.
We now turned our attention to adapting this synthesis to
accommodate an enone in the side chain. Ortho ester ketone
17 was prepared from 4-acetylbutyric acid by the literature
procedure.16 Conversion of 17 to the kinetic enolate with
LDA and addition of prolinal 9 afforded aldol product 18
(see Scheme 3). Hydrolysis of the ortho ester in 4:2:1 AcOH/
THF/H2O and transesterification with K2CO3 in MeOH
afforded methyl ester 19 in 75% overall yield from 17.17
Protection of the hydroxyl group facilitated purification, the
preparation of the keto phosphonate, and the Wittig reaction.
Treatment of 19 with excess ethyl vinyl ether and catalytic
PPTS in CH2Cl2 provided 97% of 20. Hydrogenolysis and
reductive cyclization with H2 (1 atm) over Pd(OH)2 in MeOH
for 12 h afforded 93% of the crude protected pyrrolizidine
methyl ester 21.
°C gave keto phosphonate 22 in 81% yield after chroma-
tography. Treatment of 22 with NaH in THF followed by
addition of hexanal and stirring for 3 h afforded enone 23
in 95% yield. Deprotection by stirring in 10% aqueous HOAc
for 4 h at 25 °C gave epohelmin A (24) quantitatively. The
1H and 13C NMR spectra of 24 that is 85-90% protonated
with HOAc are identical to those reported for epohelmin A,
thereby unambiguously establishing the revised structure of
epohelmin A. The optical rotation of 24 that is 85-90%
Addition of crude 21 to a large excess of LiCH2PO(OMe)2
in THF at -78 °C and stirring for 12 h with warming to 25
(13) For the assigned spectra of cis- and trans-3-methylpyrrolizidine,
see: Skvortsov, I. M.; Antipova, I. V. J. Org. Chem. USSR 1979, 15, 777-
783; Zh. Org. Khim. 1979, 15, 868-875.
(14) Chamberlin, A. R.; Chung, J. Y. L. J. Org. Chem. 1985, 50, 4425-
4431.
(15) Gundermann, H.; Schnekenburger, J. Arch. Pharm. (Weinheim)
1998, 321, 925-929.
(16) (a) Corey, E. J.; Raju, N. Tetrahedron Lett. 1983, 24, 5571-5574.
(b) Oku, A.; Numata, M. J. Org. Chem. 2000, 65, 1899-1906. (c)
Mohapatra, S.; Capdevila, J. H.; Murphy, R. C.; Hevko, J. M.; Falck, J. R.
Tetrahedron Lett. 2001, 42, 4109-4110.
(17) Kwok, P.-Y.; Muellner, F. W.; Chen, C.-K.; Fried, J. J. Am. Chem.
Soc. 1987, 109, 3684-3692.
protonated with HOAc, [R]22 +22, is identical to that of
D
the natural product, indicating that the absolute configuration
is as shown since 9 is prepared from (S)-proline.
Epohelmin B (26) was prepared from 24 by Swern
oxidation to give the unstable dione followed by reduction
with L-Selectride in THF to give diol 25 in 82% yield as a
Org. Lett., Vol. 7, No. 20, 2005
4421

mixture of diastereomers at the side chain alcohol. Oxidation
of 25 with MnO2 proceeded slowly and in modest yield. We
had previously observed that the Dess-Martin periodinane
was not effective for the oxidation of model 11 or epohelmin
A (24). As expected from this observation, selective oxidation
of diol 25 with Dess-Martin periodinane afforded epohelmin
Scheme 4. Possible Biosynthesis of Epohelmins A and B
1
B (26) in 98% yield. The H and 13C NMR spectra of the
HOAc salt of 26 are identical to those reported for epohelmin
B, thereby unambiguously establishing the revised structure
of epohelmin B. The optical rotation of the acetate salt of
26, [R]22D +29, is similar to that of the natural product, [R]22
D
+25, indicating that the absolute configuration is as shown.
A plausible biosynthesis of epohelmins A (24) and B (26)
proceeds through the Claisen condensation of heptaketide
28 with activated proline 27 to give adduct 29. Hydrolysis
and decarboxylation will give trione 30. Reductive cycliza-
tion and reduction of the ring ketone will lead to epohelmins
A and B. If this is the biosynthetic pathway, the very efficient
key reductive cyclization step leading to 11 and 21 is
biomimetic.
Acknowledgment. We thank the NIH (GM50151) for
generous financial support. We thank Prof. M. Shibuya for
a complete set of the NMR spectra of epohelmins A and B.
In conclusion, we have reassigned the proposed 9-oxa-4-
azabicyclo[6.1.0]nonane structures of epohelmins A (1) and
B (2) as pyrrolizidin-1-ols 24 and 26, respectively. We have
developed syntheses from N-Cbz-(S)-prolinal (9) and ortho
ester ketone 17 that provide epohelmin A (24) in eight steps
and 52% overall yield and epohelmin B (26) in 11 steps and
43% overall yield using a stereoselective aldol reaction and
a stereoselective reductive cyclization as the key steps.
Supporting Information Available: Full experimental
details and copies of 1H and 13C NMR spectra. Comparison
of NMR data of epohelmin A and 11. NOE studies of 11
and 16. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL0516061
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Org. Lett., Vol. 7, No. 20, 2005