A synthesis of cyclohexanoid butenolides isolated from Sinomenium acutum

Article abstract of DOI:10.1016/S0040-4039(01)02319-X

Starting from an enantiopure building-block serving as the chiral equivalent of 4-hydroxycyclohexa-2,5-dien-1-one, a diastereocontrolled synthesis of four cyclohexanoid butenolides has been investigated. The inherent convex-face selectivity exerted by the chiral building block was a key aspect of this method which afforded three of the five known butenolides, with only one losing its original chiral integrity during the conversion.

Full text of DOI:10.1016/S0040-4039(01)02319-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1047–1049
A synthesis of cyclohexanoid butenolides isolated from
Sinomenium acutum
Masatoshi Honzumi and Kunio Ogasawara*
Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980-8578, Japan
Received 18 November 2001; revised 28 November 2001; accepted 6 December 2001
Abstract—Starting from an enantiopure building-block serving as the chiral equivalent of 4-hydroxycyclohexa-2,5-dien-1-one, a
diastereocontrolled synthesis of four cyclohexanoid butenolides has been investigated. The inherent convex-face selectivity exerted
by the chiral building block was a key aspect of this method which afforded three of the five known butenolides, with only one
losing its original chiral integrity during the conversion. © 2002 Elsevier Science Ltd. All rights reserved.
(−)-Phyllanthurinolactone1 1, the leaf-closing factor of
a large tamarind tree (Tamarindus indica L.), has as its
aglycone (−)-menisdaurilide2,3 2, which is one of several
cyclohexanoids isolated from Sinomenium acutum con-
we had already developed a versatile chiral building-
block10 6 serving as the synthetic equivalent of chiral
4-hydroxycyclohexa-2,5-dien-1-one, we decided to
extend its utilization by exploring an enantiocontrolled
route to these natural butenolides in enantiomerically
pure form. The high functionality and inherent convex-
face selectivity of the chiral building block 6 would play
a pivotal role in this approach. We report here our
results leading to the formation of the enantiomerically
pure natural (+)-dihydroaquilegiolide 5 and unnatural
(+)-dihydromenisdaurilide ent-4, along with the unex-
pected generation of racemic menisdaurilide ( )-2.
taining
a butenolide moiety; others include (−)-
aquilegiolide3,4 3, (−)-dihydromenisdaurilide4 4 and
(+)-dihydroaquilegiolide4 5 (Fig. 1). Otsuka and co-
workers4 established the absolute structure of natural
(−)-menisdaurilide 2 by X-ray analysis. More recently,
Mori and co-workers5 proved that the aglycone menis-
daurilide 2 has the same absolute structure to that of
natural (−)-menisdaurilide 2 by synthesis of (−)-phyl-
lanthurinolactone 1 from D-glucose and racemic menis-
daurilide ( )-2. However, the enantiocontrolled
syntheses of both can still be improved upon. Until
now, only (−)-dihydromenisdaurilide 4 and (+)-dihy-
droaquilegiolide 5 have been obtained in enantio-
enriched forms by Majewski and co-workers.6,7 Since,
by use of either a catalytic8 or an enzymatic9 procedure,
Thus, the enantiopure (+)-6 was transformed into the
exo-epoxide 7, mp 65–66°C, [h]3D1 +56.2 (c 1.2, CHCl3),
diastereoselectively, using tert-butyl hydroperoxide in
the presence of Triton B.11 Reaction of 7 with the
carbanion generated from ethyl acetate with LDA pro-
ceeded without reaction with the epoxy functionality to
Figure 1.
* Corresponding author. E-mail: konol@mail.cc.tohoku.ac.jp
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02319-X

1048
M. Honzumi, K. Ogasawara / Tetrahedron Letters 43 (2002) 1047–1049
give the tertiary alcohol 8 as a single diastereomer, [h]D22
+33.4 (c 1.2, CHCl3). In order to carry out regioselec-
tive cleavage of the epoxide functionality, the alcohol 8
was first deprotected to give the diol 9, mp 71–72°C,
[h]2D4 +32.8 (c 1.2, CHCl3), the secondary hydroxyl
functionality of which was oxidized under Dess–Martin
conditions12 to afford the keto-alcohol 10, [h]2D2 −11.7 (c
1.4, CHCl3). After some experimentation, it was found
that the regioselective cleavage was best carried out
using aluminum amalgam13 to give the keto-3,4-diol 11,
mp 71–72°C, [h]2D9 −104 (c 1.1, CHCl3), as a single
product. Unfortunately, complete convex-face selectiv-
ity was no longer exhibited by 11 on reaction with
sodium borohydride. An inseparable mixture (10:3) of
the endo- and exo-hydroxy-lactones 12 formed by in
situ lactonization, was obtained under these reaction
conditions. Since the intramolecular bromo-etherifica-
tion of 11 prior to the reduction failed, giving rise
instead to a complex mixture, we decided to use the
crude alcohol mixture 12. The retro-Diels–Alder reac-
tion of both 12 and its secondary monosilyl ether 13, in
refluxing diphenyl ether however resulted in decomposi-
tion. Moreover, attempted dehydration of the ether 13
into the corresponding butenolide under several condi-
tions also failed. Therefore, 13 was silylated to give the
disilyl ether 14, which was heated in refluxing diphenyl
ether to furnish the cyclohexene 15 as an inseparable
mixture in good yield (Scheme 1).
found to be completely racemic. As a result, menisdau-
rilide 2 obtained from 18 by diastereoselective reduction
was racemic. The resulting racemization may be due to
a facile enolization of the butenolide moiety at the
b-elimination step via the hydroxyfuran 19 or in the
oxidation step via the trienone 20. These concluded us
that the route involving the b-elimination of 15 was
inappropriate for the enantioselective synthesis of
menisdaurilide 2 (Scheme 2).
We next explored a route to the (+)-dihydroaquiledi-
olide 5 utilizing the same intermediate 15. Thus, 15 was
hydrogenated under catalytic conditions to afford the
separable 1,4-syn-ether 21 and the 1,4-anti-ether 23 in
yields of 78 and 21%, respectively. The major com-
pound 21 gave the tertiary alcohol 23 on exposure to
dilute hydrochloric acid, which was dehydrated with
methanesulfonyl chloride and triethylamine to give the
butenolide 25. Under these conditions, epimerization
did not take place with the dihydro-derivative. Desilyla-
tion of 25 with hydrofluoric acid in acetonitrile fur-
nished (+)-dihydroaquilediolide 5, mp 107–108°C, [h]D26
+125 (c 0.9, MeOH){lit.:6 [h]D24 +113 (c 1.0, MeOH)
(90% ee)}, the spectroscopic data of which were identi-
cal with those of the natural product. The overall yield
of 5 from 21 was 55%. On the other hand, the minor
isomer 22, under the same conditions, afforded (+)-
dihydromenisdaurilide ent-4, mp 79–80°C, [h]2D6 +123 (c
0.2, MeOH), the enantiomer of the natural (−)-dihy-
dromenisdaurilide 4, [h]2D6 −112 (c 2.0, MeOH) (90%
ee),6 in 35% overall yield from 22 without epimerization
(Scheme 3).
To obtain (−)-menisdaurilide 2, 15 was treated with
potassium carbonate in methanol to induce b-elimina-
tion. The expected reaction occurred to give the
butenolide 16 as an inseparable mixture. Exposure of
16 to hydrofluoric acid in acetonitrile afforded an
inseparable mixture 17 of menisdaurilide 2 and aquile-
giolide 3. Interestingly, the ratio changed from 10:3 to
Acknowledgements
1
2:3 during the elimination reaction as evidenced by H
NMR. A similar epimerization has previously been
reported3 during the isolation of (−)-aquileginolide 3.
The mixture was oxidized with pyridinium chlorochro-
mate (PCC) to give the single enone5 18, which was
We are grateful to the Ministry of Education, Culture,
Sports, Science and Technology, Japan for support of
this research.
CO2Et
HO
CO2Et
Dess-Martin
(98 %)
HO
O
O
O
30 % H2O2
Triton B
THF
LDA
AcOEt
THF
(93 %)
O
O
TBSO
RO
(93 %)
TBSO
O
7
6
8: R=TBS
9: R=H
TBAF
10
THF
(99 %)
O
OTMS
CO2Et
OH
R2O
HO
O
Ph2O
reflux
NaBH4
EtOH
Al-Hg
O
O
aq. EtOH
(88 %)
R1O
O
OTBS
15
(70 % from 11)
11
12: R1=R2=H
TBS-Cl
imidazole
TMS-Cl
13: R1=TBS, R2=H
14: R1=TBS, R2=TMS
imidazole
Scheme 1.

M. Honzumi, K. Ogasawara / Tetrahedron Letters 43 (2002) 1047–1049
1049
O
O
O
O
OTMS
NaBH4
CeCl3
O
O
O
O
PCC
K2CO3
MeOH
(78 %)
CH2Cl2
EtOH
OR
O
OTBS
15
OH
16:R=TBS
17:R=H
18
HF
MeCN
(97 %)
(±)-menisdaurilide 2
(64 % from 17)
O
HO
HO
O
O
OTBS
20
19
Scheme 2.
Scheme 3.
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