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OH
OH
C
3
H
7
COCH
2
COOEt
con. H
2
SO
4
HO
O
O
HO
OH
1
(C
2
H
5
CO)
2
O
AlCl
3
O
OH
OH
O
OH
+
+
HO
O
O
HO
O
O
HO
O
O
O
O
7 8 9
OMe
HO
OMe
O
O
CH
3
CH(OEt)
2
NaBH
4
(±)-calanolide A
HO
OO
CF
3
COOH,Py
O
CeCl
3
7H
2
O
O
O
O
O
10 3
Scheme 8-2
This novel approach for synthesis of calanolide A (1) was somewhat different
from the first one reported.
42
Use of 8-propionyl-5,7-dihydroxy-4-propylcoumarin
(7) constructed the chromene ring (ring D) with 4,4-dimethoxy-2-methyl butan-
2-ol. The chromanone ring (ring C) was built last by treatment of chromene (10)with
acetoaldehyde diethyl acetal in the presence of trifluoroacetic acid and pyridine to
give racemic chromanone (3) with the desired stereochemical arrangement. Luche
reduction of the chromanone (3) with NaBH
4
/CeCl
3
at
10
C was highly selective
in giving (
)-calanolide A in 90% yield with 90% purity, which was purified by
preparative high-performance liquid chromatography (HPLC). However, the total
yield of (
þ
)-calanolide A was around only 5-6.5%, which was less than other syn-
thetic approaches. Because propionation of 5,7-dihydroxy-4-propyl coumarin with
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