"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Intramolecular Formation of Carbon–Carbon Bonds (Cyclizations) 167
The synthesis of tacamonine, an indole alkaloid of the Iboga type, was
accomplished in both racemic and homochiral forms, by incorporating a classical
6-exo-trig radical cyclization in the key step of the synthesis (Reaction
7.57) [52]. The cyclization produced piperidinone in a 72 % yield as a diastereomeric
mixture.
HN
O
EtO 2 C
N
Br
H
(TMS) 3 SiH
AIBN, 80 �C
HN
O
H
EtO2C N
72%
H
(7.57)
The reduction of the trichloroacetamide derivative 56 with (TMS) 3SiH
afforded 57 in a 60 % yield (Reaction 7.58) [68]. The final product is the result
of cyclization and further removal of two chlorine atoms. In fact, 3.5 equiv of
silane and 1.1 equiv of AIBN are used.
N
OAc
O
N
CCl 3
OAc
(TMS) 3SiH AIBN, 80 �C
N
OAc
56 57, 60 %
O
H
N
H
OAc
(7.58)
For the synthesis of tricyclic isoindolinones, the 5-exo-trig cyclization of aryl
radicals has been applied [69]. Reaction (7.59) shows the example of a tricyclic
system obtained in a 71 % yield.
MeO
Br
O
N
O
O
(TMS) 3SiH N
AIBN, 80 �C
MeO
O
71%
O
O
(7.59)
In model studies directed towards the synthesis of ( )-gelsemine, 5-exo-trig
cyclization of an aryl radical, derived from the vinylogous uretane 58, onto a
methoxymethyl enolether resulted in partial fragmentation of the intermediate