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O
Ph
OH
N
OH
H
CH
3
ZnCl
2
/DMF/Et
2
O/K
2
CO
3
/
2
-butanone
POCl
3
, ClCH
2
CH
2
Cl
HO
O
O
HO
O
O
Cl
1
O
H
26
O
O
B
THF
28
2
HO
O
O
HO
O
O
NaBO
3
/H
2
O
OR
O
H
27
29
R=H
30
R=TBDMS
TBDMSCl/CH
2
Cl
2
/DMAP/Py
O
PMe
3
/DEAD/ClCH
2
COOH
Hg(OAc)
2
/THF
TBAF/TH
F
(+)-calanolide B
(II)
NaBH
4
/THF
NH
4
OH/MeOH
O
O
O
OTBDMS
(+)-calanolide A
31
Scheme 8-7
reagent (
þ
)-(E)-crotyldiisopinocampheylborane (28), which was prepared accord-
ing to the procedure of Brown and Bhat. Treatment of the aldehyde (27) with
the organoborone reagent (28) gave threo b-methyl homoallylic alcohol 29,
½
a
2
D
¼þ
78
with 66% yield. Acylation of 29 with TBDMSCl to give the monosil-
lyated product 30
ð½
a
2
D
¼þ
26
Þ
with 95% yield caused by the phenolic hydroxyl
group was less active than the secondary homoallylic alcohol, which lead to pre-
ferred monosillylation. Cyclization of the compound 30 was carried out with mer-
cury (II) acetate in THF, and the resulting intermediate was reduced with an excess
of sodium borohydride to give silyl-protected (
þ
)-calanolide B (2) in 85% yield
ð½
a
2
D
¼
41
:
5
Þ
. Finally, deprotection of the silyl group with tetra-butylammo-
nium fluoride gave (
þ
)-calanolide B (2) in 86% yield
½
a
2
D
¼þ
44
. The
1
H and
13
C NMR spectra were identical with those reported for the corresponding natural
products. The conversion of (
þ
)-calanolide B into (
þ
)-calanolide A in 80% yield
ð½
a
2
D
¼þ
66
Þ
was accomplished with a modified Mitsunobo reaction. The total
yields of this seven-step process for synthesizing (
þ
)-calanolide B and eight-step
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