Page 1011 - Advanced Organic Chemistry Part B - Reactions & Synthesis

R 987
R CO CH 3 Ph SnH
2
3
CO CH SECTION 10.3
CO CH 3 AIBN 2 3
2
CO CH 3 Reactions Involving Free
2
Ref. 358 Radical Intermediates
These reactions presumably involve terminal addition of the chain-carrying radical,
followed by fragmentation and recyclization.

R CO CH 3 R CO CH 3 R CO CH 3
2
2
2
.
X . CO CH 3 X . CO CH 3 X CO CH 3
2
2
2
R
R
X .
CH O C CO CH 3 CH 3 O C CO CH 3
3
2
2
2
2
Other intramolecular cyclizations can follow generation and fragmentation of cyclo-
propylcarbinyl radicals. In the example below, the fragmented radical adds to the
alkyne.
S
N
OC N
CH CH .
CH 3 Bu SnH 3 2 CHSi(CH )
3 3
3
CH CH 3
CH 3 AIBN 3 CH 3
CH 3
CH 3 CH 3 81%
Si(CH )
3 3
Si(CH )
3 3
Ref. 359
Cyclic -halomethyl or -phenylselenenylmethyl -ketoesters undergo one-
carbon ring expansion via transient cyclopropylalkoxy radicals. 360
O .
CH 2 X O O
Bu SnH O
3
C H
CO 2 2 5
(CH ) AIBN CO C H . (CH )
2 n
(CH ) 2 2 5 (CH ) CO C H 2 n CO C H
2 n
2 n
2 2 5
X = Br, I, SePh; n = 1–3 2 2 5
Comparable cyclization-fragmentation sequences have been developed for acyclic and
heterocyclic systems.
358 K. Miura, K. Fagami, K. Oshima, and K. Utimoto, Tetrahedron Lett., 29, 1543 (1988).
359 R. A. Batey, J. D. Harling, and W. R. Motherwell, Tetrahedron, 46, 8031 (1992).
360
P. Dowd and S.-C. Choi, Tetrahedron, 45, 77 (1989); A. L. J. Beckwith, D. M. O’Shea, and
S. W. Westwood, J. Am. Chem. Soc., 110, 2565 (1988), P. Dowd and S.-C. Choi, Tetrahedron, 48,
4773 (1992).