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propionic anhydride offered a mixture of an 8-position propionated product (7)
along with 6-acylated (8) and bis-acylated compound (9), which was separated
from 8 and 9 by silica gel chromatography, and the yield of the 8-acylated
compound 7 was 30%.
In our research,
44,45
we prepared racemic calanolide A under the same strategy.
calanolide A has three different heterocyclic rings, B, C, and D, which are con-
structed from the phloroglucinol core (A). We used phloroglucinol as the starting
material and then constructed the coumarin scaffold, followed by the chromanone
ring (ring C) as a key intermediate (2). The chromene ring was built last (Scheme 8-3).
This method is different from the case of chromene reported before.
42
The Friedel-
Crafts acylation and the ring closure of coumarin in a one-step reaction using
tigloyl chloride in the presence of AlCl
3
formed a key intermediate (2) directly
with 66% yield. The chromene ring was then introduced using pyridine-catalyzed
condensation of 1,1-diethoxy-3-methyl-2-butene with the compound (2) in the pre-
sence of toluene. The reaction proceeded readily to give chromene (
)-3 and (
)-4
in the ratio of 1.75 : 1 in 68% yield. Luche reduction of the ketone (
)-3 using
NaBH
4
/CeCl
3
at 0
5
C produced (
)-calanolide A in 63% yield. Its spectral
O
OH
OH
Cl
C
3
H
7
COCH
2
COOEt
con. H
2
SO
4
AlCl
3
, CS
2
, PhNO
2
HO
O
O
HO
OH
1
OH
O
O
O
H
NaBH
4
(±)-Calanolide A
O
O
O
O
O
O
CeCl
3
7H
2
O
pyridine,toluene
O
O
2 3
O
O
O
O
O
4
Scheme 8-3
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