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OR
a
(
−
)-
R
-Cembrene-A (
59
)
163
Reagents and Conditions: (a) SnCl
4
,CH
2
Cl
2
,
78
C, 60 h. R*
¼
(R)-BINOL.
Scheme 6-14
6.4.2.2 Total Synthesis of Cembrene C
Li et al. reported on the total synthesis of cembrene C from geraniol (137)ortrans,
trans-farnesol (91) by using the titantium-induced intramolecular coupling of the
dicarbonyl precursor keto enal 176 as the key macrocyclization step (Scheme 6-17).
From E,E-farnesol, cembrene C (27) was prepared via five steps in 21% overall
yield by tiantium-induced macrocyclization (Scheme 6-18).
97
O
O
O
a
b
c
O
O
O
O
O
I
P(O)(OEt)
2
164
165
166
167
CH(OMe)
2
d
O
168
169
O
CH(OMe)
2
CHO
O
e
f
g
O
(−)-
R
-Cembrene-A (
59
)
170 171
Reagents and Conditions: (a) 1) ethylene glycol, p-TsOH, C
6
H
6
,reflux, 100%; 2) O
3
,CH
2
Cl
2
,
78
C,
then rt, 78%; (b) 1) NaBH
4
, MeOH, 0
C, 8%; 2) Ph
3
P, imidazole, I
2
,0
C, 92%; (c) (EtO)
3
P, r e flux, 78%;
(d) O
3
, MeOH,
78
C, then, Me
2
S, p-TsOH,
78
C
rt, 73%; (e) n-BuLi, 167,
78
C, 0.5 h, then
169, THF, reflux overnight, 58%; (f) i) p-TsOH, acetone, felux, 2 h; ii) CF
3
CO
2
H, CHCl
3
,0
C, 96%; (g)
TiCl
4
, Zn, DME, reflux, 81%.
Scheme 6-15
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