expansion leading to a new benzylic radical intermediate. A concomitant oxidative
decarboxylation/elimination promoted by a second equivalent of manganese salt
could then deliver dibenzooxepin 150 (Scheme 57) [81].
O
O
OMOM
OBn
O
OBn
OBn
SPh
O
H
H
H
O
OBn
OBn
H
O
CO 2 Me
O
O
MOMO
H
H
OBn
H
n-Bu 3 SnH
(10 equiv)
Ph-Me
(c = 10
-3
M)
80 °C
AIBN (cat.)
CO 2 Me
143
85%
O
OBn
OBn
H
O
CO 2 Me
O
O
MOMO
H
H
OBn
H
145
G
144
O
O
O
O
O
O
H
O
OH
OH
H
H
H
H
HO
H
O
O
O
O
O
O
H
H OH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
HO
OH
Ciguatoxin
G
Scheme 55 Radical cyclisation used to generate the G ring of polyether ciguatoxin
n-Bu 3 SnH
AIBN (cat.)
Ph-H, 80 °C
N
O
Br
Me
O
N
Me
O
O
148
146
N
Me
O
O
86%
147
Scheme 56 7-Endo-trig cyclisation leading to the formation of a tetracyclic oxepin
308
O. Piva