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O
O
O
HO
CHO
CO
2
Me
a
b
Pseudoplexaurol (
182
)
Pseudoplexaural (
183
)
184
OMe
OH
O
O
O
O
3
c
H
inversion C-3
H
14-Deoxycrassin (
239
)
Reagents and conditions: (a) MnO
2
, 92%; (b) MnO
2
, NaCN, MeOH, HOAc, 58%; (c) ZnCl
2
,CH
2
Cl
2
,
0
Cto30
C, 71%.
Scheme 6-32
6.5 STUDIES ON NOVEL MACROCYCLIZATION METHODS
OF CEMBRANE-TYPE DITERPENOIDS
Although several synthetic strategies and methods for the construction of a
14-membered-ring system have appeared in literature over the past three decades
and notable progress has been made in this field, the lack of a general method for
the preparation of 14-membrened rings presents an ongoing challenge for total
synthesis. A novel general and diversified strategy for the stereocontrolled
total synthesis of cembrane-type diterpenoids is highly desirable.
6.5.1
A Stille Cyclization Approach to (
)-Isocembrene
Characteristic to some important cembrane diterpenoids, such as isocembrene,
cembrene, cembrene C, and sarcophytol A, is a 1,3-diene unit in the macrocyclic
skeleton. In continuation of our ongoing project on the total synthesis of cembrane-
type diterpenoids, we intended to explore a novel macrocyclization method to
O
O
O
O
O
O
H
+
or lewis acid
H
+
or lewis acid
cis
-Cembranolide (
256
)
Sarcophytonin (
67
)
trans
-Cembranolide (
257
)
Scheme 6-33
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