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

166 Consecutive Radical Reactions
O
O
N
Ph
O
I
I
anti-(s)-Z-49 syn-(s)-Z-49
O
O
N
(TMS) 3SiH, AIBN, 80 �C
(TMS) 3 SiH, hν, r.t.
O
(TMS) 3 SiH, Bu 3 SnCl, AIBN, 80 �C
(TMS) 3SiH, Bu 3SnCl, hν, r.t.
Ph
O
O N
50 (E:Z)
Ph
O
78% (100:0)
72% (71:29)
0%
60% (60:40)
+
O
O N
Scheme 7.6 Manipulation of rotamer population in N-enoyloxazolidinones
H
O
51 (de %)
13% (87)
0%
87% (>97)
14% (86)
An example of the influence of structure and geometry on radical cyclization is
shown by the stereoselectivity of the 5-endo-trig cyclization in compound 52 and 5exo-trig
cyclizationin53using (TMS) 3SiH as radical mediators [66]. Insystem 52 a
mixture of cis-fused and trans-fused rings are obtained, whereas in system 53 the
reaction proceeds in a stereoselective manner to give only the cis-fused product.
An extensive use of (TMS) 3SiH can be found in the key steps of alkaloid
syntheses. The synthesis of derivative 55, as the key intermediate for the preparation
of alkaloid ( )-pancracine [67], has been obtained from the reaction of 54
under normal conditions, in an 84 % yield as a single stereoisomer (Reaction 7.56).
O
O
O
54
SPh
N
PMB
Ar X
O
N
PMB
Ar X
O
N
PMB
52 X = SePh 53 X = Cl, SPh
PMB = p-MeOC6H4CH2 OAc (TMS) 3 SiH
AIBN, 80 �C
PMB = p-MeOC 6H 4CH 2
O
O
H
H
O N
H
PMB
55, 84%
OAc
Ph
(7.56)