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

Intramolecular Formation of Carbon–Carbon Bonds (Cyclizations) 165
and leads to the formation of cyclic pyroglutamate derivatives in excellent
yields. Indeed, the products derived from the cyclization of a-silylthio imidoyl
radical (Reaction 7.55) hydrolysed spontaneously during chromatography
[64].
O
N
46
R 1 R 2
N
OBu −t
C
S
47, R1 = R2 = H
48, R1 = R2 = Me
(TMS) 3 SiH
AIBN, 80 �C
(TMS) 3 SiH
AIBN, 110 �C
O
O
R 1 R 2
OBu −t
R 1
N
OBu −t
93-96%
HN Si(TMS) 3
H 2 O, SiO 2
R 2
S
NH
SSi(TMS) 3
(7.54)
(7.55)
From all the data available so far, it is well estabilished that the geometries of
the reactants play an important role in the regio- and stereochemical outcome
of radical reactions since they are commonly involved in the early transition
states. Previous attempts to affect rotamer populations during the reaction
included, among others, control of temperature and addition of a Lewis acid.
It was reported [65] that organotin halides, common by-products of radical
reactions, act as Lewis acids and control the course of such reactions. An
indicative example of this control is given in Scheme 7.6; the 5-exo-trig radical
cyclization of N-enoyloxazolidinone 49, mediated by (TMS) 3SiH has been
performed in the absence and in the presence of organotin halides. The results
demonstrated that higher temperatures and weak Lewis acids were necessary,
not only for inducing the conformational change from the stable anti-(s)-Z to
the syn-(s)-Z required for the radical cyclization reaction, but also for obtaining
high diastereoselectivity.