First asymmetric synthesis of piperidine alkaloid (-)-morusimic acid D

Article abstract of DOI:10.1055/s-2008-1072737

The first asymmetric synthesis of (-)-morusimic acid D, a 2,3-trans-2,6-cis-2-methyl-6-substituted piperidin-3-ol containing alkaloid is reported. The key steps are the reductive alkylation of N,O-diprotected 3-hydroxyglutarimide, a stepwise reductive alkylation, and an asymmetric aldol-type reaction using a modified Evans chiral auxiliary. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1072737

LETTER
1189
First Asymmetric Synthesis of Piperidine Alkaloid (–)-Morusimic Acid D
A
symmetric Synth
e
esisof (–)-
-
M
orusim
S
ic
A
cid
D
heng Yu,a Wei-Xuan Xu,a Liang-Xian Liu,a Pei-Qiang Huang*a,b
a
Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen, Fujian 361005, P. R. of China
Fax +86(592)2186400; E-mail: pqhuang@xmu.edu.cn
The State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, P. R. of China
b
Received 4 January 2008
OH
Me
OCOR
Me
Abstract: The first asymmetric synthesis of (–)-morusimic acid D,
a 2,3-trans-2,6-cis-2-methyl-6-substituted piperidin-3-ol contain-
ing alkaloid is reported. The key steps are the reductive alkylation
of N,O-diprotected 3-hydroxyglutarimide, a stepwise reductive
alkylation, and an asymmetric aldol-type reaction using a modified
Evans chiral auxiliary.
R(CH2)n
N
MeC(O)(CH2)10
N
H
H
B
A
azimic acid, n = 5, R = COOH
carpamic acid, n = 7, R = COOH
spectaline, n = 12, R = COMe
spectalinine, n = 12, R = CH(OH)Me
leptophyllin, n =10, R = CH(OH)CH2OH
spectamine A, R = Ph
spectamine B, R = Me
iso-6-cassine, R = H
Key words: asymmetric synthesis, N-acyliminium ions, imides, al-
dol reaction, piperidines, alkaloids, morusimic acid D
OH
O
OX
Substituted, hydroxylated piperidines constitute a class of
natural products exhibiting important bioactivities.1 As a
key structural feature shared by many piperidine alka-
loids, 2-methyl-6-substituted piperidin-3-ols with differ-
ent stereochemical patterns have been the targets of
numerous synthetic efforts, which have culminated in a
number of methods for the synthesis of these molecules.2–
HO
(CH2)9
N
Me
H
morusimic acid C, X = Glu (1)
morusimic acid D, X = H (2)
Figure 1 Some alkaloids containing a 2-methyl-6-substituted pipe-
ridin-3-ol moiety
8
Because most of this class of piperidine alkaloids pos-
sess a 2,3-cis stereochemistry with either a 2,6-cis-stere-
Me
O
O
OH
O
morusimic
acid D
(2)
BocHN
OH
ochemical pattern (A, Figure 1), or
a 2,6-trans
9
OBn
stereochemistry pattern (B), much attention has been de-
voted to the construction of cis-2-methylpiperidin-3-ol
skeleton.3 Only a few methods are available for the syn-
thesis of trans-2-methylpiperidin-3-ols,4 although several
methods have been developed for the syntheses of struc-
turally related piperidine alkaloids prosophylline5 and mi-
cropine,6 quinolizidine alkaloids clavepictines7 and
pictamine,7a as well as decahydroquinoline alkaloid lepa-
din D.8 In 2002, two 2,3-trans-2,6-cis-2-methyl-6-substi-
tuted piperidin-3-ol containing alkaloids, morusimic acids
C (1) and D (2) were isolated from white ripened fruit of
M. alba grown in Turkey.9
3
Me
H
BocHN
O
9
OBn
4
OBn
OBn
O
N
Me
O
N
O
Boc
PMB
(R)-6
(5R,6S)-5
Scheme 1 Retrosynthetic analysis of (–)-morusimic acid D
In continuation of our studies on the development of pro-
tected 3-hydroxyglutarimide-based synthetic methodolo-
gy,10–12 we now report an application of this methodology
to the first asymmetric synthesis of morusimic acid D (2),
an alkaloid having a 2,3,6-trans,trans stereochemistry.
was considered to be accessible by another diastereoselec-
tive reductive alkylation method.13
The synthesis commenced with (R)-3-benzyloxy-1-(4-
methoxybenzyl)glutarimide [(R)-6],12 which was pre-
pared from D-glutamic acid following essentially the pro-
cedure described for its enantiomer (Scheme 2).12
As displayed retrosynthetically in Scheme 1, both the C2
methyl group and the 2,3-trans stereochemistry were en-
visioned to be introduced by the reductive alkylation
method10,11 starting from the protected 3-hydroxyglutar-
imide 6.12 The C6 side chain with 2,6-cis stereochemistry
For the reductive methylation, although our previous
study showed that the addition of methyl magnesium io-
dide (3 molar equiv) to (S)-3-hydroxyglutarimide deriva-
tive (S)-6 in THF at –78 °C yielded a diastereomeric
mixture of N,O-acetal 10 and its C6 adduct in 86:14 C2/
C6 regioselectivity, it was found that almost only the C2
addition product was obtained when undertaking the reac-
SYNLETT 2008, No. 8, pp 1189–1192
x
x.
x
x.
2
0
0
8
Advanced online publication: 16.04.2008
DOI: 10.1055/s-2008-1072737; Art ID: W00408ST
© Georg Thieme Verlag Stuttgart · New York

1190
D.-S. Yu et al.
LETTER
yield.17 The resulting alcohol 15 was oxidized with Dess–
Martin periodinane (DMP)18 to give aldehyde 4 in quanti-
tative yield (Scheme 3).
H
ref. 12
KOt-Bu, THF
CONPMB
H
(R)-Glu
3 steps
75% overall
–78 °C to –62 °C
92%
O
O
(R)-8
For the asymmetric carboxymethylation, an enzymatic
method,19 Evans asymmetric aldol-type reaction,20 and
Brown’s asymmetric allylation reaction21 were all plausi-
ble. We sought to use Evans’ aldol chemistry in view of
its excellent enantio- and diastereoselectivities, as well as
its compatibility with different substituents that may be
useful for the synthesis of analogues of (–)-morusimic
acid D. To take advantages of the progress in Evans aldol
chemistry,22,23 oxazolidine-2-thione derivative 17 was se-
lected for the asymmetric aldol reaction, where the chloro
atom will serve as a stereodirecting group for the asym-
metric aldol-type reaction.20 The starting oxazolidine-2-
thione 16 was prepared in 91% yield by the method re-
ported by Wu.24 Successive treatment of oxazolidine-2-
thione 16 with n-butyllithium and chloroacetyl chloride
gave 17 in 85% yield. Asymmetric aldol reaction of 4 with
17 under Crimmins’ conditions [TiCl4, (i-Pr)2EtN, N-
methylpyrrolidin-2-one (NMP), CH2Cl2, 0 °C]23 provided
18 as the only isolable diastereomer in 70% yield
OH
OBn
O
MeMgI
Ag2O, BnBr
O
N
O
O
N
–20 °C to –15 °C
92%
Et2O, r.t., 7 d
PMB
(R)-9
PMB
(R)-6
98%
CAN
MeCN–H2O
(3:1), r.t.
OBn
Me
OBn
OH
Me
Et3SiH, BF3·OEt2
CH2Cl2
–78 °C to r.t.
85%
O
N
O
N
58%
PMB
(5R,6S)-11
PMB
10
OBn
OBn
Me
(Boc)2O, DMAP
O
N
Me
O
N
H
Et3N, CH2Cl2, 24 h
95%
Boc
(5R,6S)-5
(5R,6S)-12
Scheme 2
tion in CH2Cl2 and at –20 °C. The subsequent reductive (Scheme 4).
dehydroxylation under ionic hydrogenolytic conditions14
(Et3SiH/BF3·OEt2, CH2Cl2, –78 °C to r.t.) furnished pre-
dominantly trans-11 (trans/cis = 92:8). Cleavage of the
N-(4-methoxybenzyl) group by treating (5R,6S)-11 with
ceric ammonium nitrate (CAN) in MeCN–H2O (3:1)15 at
room temperature gave lactam (5R,6S)-12 in 58% yield
(Scheme 2).
O
S
S
n-BuLi, ClCH2COCl
Cl
HN
Bn
O
N
O
THF, –78 °C, 15 min
85%
Bn
17
16
17
Me
O
H
Next, we turned our attention to the introduction of the
chiral C6 side chain. To this end, another diastereoselec-
tive reductive alkylation method13 was adopted. Thus, lac-
tam 12 was first converted [(Boc)2O, Et3N, DMAP,
CH2Cl2, 24 h] to imide (5R,6S)-5, an activated form13b of
the former (Scheme 2). Reaction of Grignard reagent 1316
with imide (5R,6S)-5 in CH2Cl2 at –78 °C proceeded
smoothly to give the ring-opening product 14 in 68%
yield. To introduce the ethoxycarbonylmethyl group, the
TBS group in compound 14 was cleaved under acidic con-
ditions (BF3·OEt2, CH2Cl2, 0 °C) to give 15 in 92%
TiCl4, (i-Pr)2NEt
BocHN
BocHN
O
9
NMP, CH2Cl2
70%
OBn
OBn
4
S
Me
O
OH
O
N
O
9
Cl
18
Bn
Scheme 4
The subsequent steps required for the synthesis of moru-
simic acid D (2) included N-Boc deprotection, one-pot re-
ductive cyclization–debenzylation, dechlorination, and
cleavage of the chiral auxiliary. Attempts to cleave the
protecting group Boc by TFA, followed by Pd(OH)2 or
Pd/C-catalyzed hydrogenolysis to obtain 19 was unsuc-
cessful, and other attempts under the conditions shown in
Scheme 5 also failed.
BrMg
Me
O
OTBS
13
9
5
HN
Boc
OTBS
9
CH2Cl2, –78 °C
68%
OBn
14
Me
O
DMP
BF3·OEt2
HN
Boc
OH
After extensive trials, it was found that after the cleavage
of the chiral auxiliary under Evans’ conditions25 (LiOH,
H2O2, THF–H2O, 1 h) selective dechlorination could be
achieved by subjecting 20 (colorless oil, yield 92%) to
Abushanab’s hydrogenation conditions26 [H2 (l atm), 10%
Pd/C, Et3N, MeOH, 24 h, r.t.], which furnished the
dechloro product 3 in 85% yield along with 10% of the re-
covered starting material (Scheme 6).
CH2Cl2, 0 °C
92%
9
CH2Cl2, r.t.
quant.
OBn
15
Me
O
H
HN
O
9
OBn
Boc
4
Scheme 3
Synlett 2008, No. 8, 1189–1192 © Thieme Stuttgart · New York

LETTER
Asymmetric Synthesis of (–)-Morusimic Acid D
1191
S
Me
HN
O
OH
O
atm] for three hours, and then stirring with concentrated
hydrochloric acid for 24 hours (Scheme 7).
N
O
TFA
0 °C, 2 h;
Pd(OH)2/C, H2
9
In summary, by modifying the reaction conditions, the
C2/C6 regioselectivity of the addition of methyl magne-
sium iodide to N,O-diprotected 3-benzyloxyglutarimide 6
was improved from 86:14 to at least 95:5. On the basis of
this reaction, the first asymmetric synthesis of (–)-moru-
simic acid D is achieved in 11 steps with an overall yield
of 13% from (R)-6, with all the three stereocenters estab-
lished in excellent diastereoselectivities.
OBn
Cl
Boc
Bn
18
EtOH, 1 d
OH
LiOH, H2O2;
TFA;
Pd(OH)2/C, H2;
or Pd/C, H2;
O
OH
S
N
N
H
Me
9
O
Bn
2
19
Scheme 5
Acknowledgment
Me
O
OH
O
The authors are grateful to the NSFC (20572088), Qiu Shi Science
& Technologies Foundation, and the program for Innovative Rese-
arch Team in Science & Technology (University) in Fujian Pro-
vince for financial support. We thank Professor Y. F. Zhao for the
use of her Bruker Dalton Esquire 3000 plus LC-MS apparatus.
LiOH, H2O2
18
HN
OH
9
THF–H2O (4:1), 1 h
92%
OBn
Cl
Boc
20
Me
O
OH
O
References and Notes
Pd/C, H2
HN
Boc
OH
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9
Et3N, MeOH
24 h
85%
OBn
3
Scheme 6
Finally, treatment of 3 with trifluoroacetic acid in dichlo-
romethane at 0 °C for one hour, followed by subjecting
the resulting crude product to catalytic hydrogenolysis
(10% Pd/C, H2, MeOH, r.t., 24 h) led, in one pot, to the
formation of morusimic acid D (2) in 75% yield as a white
powder {[a]D20 –14.0 (c 0.25, MeOH), lit.9 [a]D20 –14.6 (c
0.25, MeOH)}. The 1H NMR and 13C NMR spectral data
of the synthetic material are identical with those reported
for the natural morusimic acid D (2).9
Alternatively, morusimic acid D methyl ester (22)27 could
be obtained in one pot by treatment of 3 with trifluoroace-
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O
OH
TFA
3
HO
N
Me
CH2Cl2
9
0 °C, 1 h
H
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21
OH
Me
O
O
OH
10% Pd/C, H2
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MeOH, r.t., 24 h
75% (from 3)
HO
N
9
H
2
or
20% Pd(OH)2/C
H2, 3 h
OH
Me
OH
MeOH–concd HCl
24 h
85% (from 3)
MeO
N
9
H
22
Scheme 7
Synlett 2008, No. 8, 1189–1192 © Thieme Stuttgart · New York

1192
D.-S. Yu et al.
LETTER
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(27) All new compounds gave satisfactory analytical and spectral
data. Selected physical and spectral data for morusimic acid
D methyl ester (22): pale yellow powder; mp 132–135 °C;
[a]D20 –11.3 (c 0.4, CHCl3). IR (film): 3387, 2929, 2854,
1731, 1438, 1380, 1305, 1165, 1061 cm–1. 1H NMR (400
MHz, CD3OD): d = 1.40 (s, 3 H), 1.22–1.68 (m, 20 H), 2.09
(m, 2 H), 2.30–2.50 (m, 2 H), 2.92 (m, 1 H), 3.08 (m, 1 H),
3.42 (s, 1 H), 3.64 (s, 3 H), 3.95 (m, 1 H). 13C NMR (100
MHz, CD3OD): d = 16.00, 26.52, 26.56, 28.33, 30.43, 30.48,
30.52, 30.56, 30.59, 32.92, 34.26, 38.05, 43.25, 52.15,
58.49, 59.24, 69.23, 70.70, 173.92. ESI-HRMS: m/z calcd
for C19H38NO4 [M + H]+: 344.2795; found: 344.2798.
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