Periodinane-Mediated Cyclization of Anilides 267
A An ns sw we er r t to o t th he e Q Qu ue es st ti io on n
To explain the transformation of anilide 14 into tetracycle 15 we should follow the
different steps of the mechanism previously discussed. SET from the anilide to the
IBX-THF oxidant reagent leads to radical cation 16 that after loss of a proton will
be transformed into radical 17. Cyclization into the C=C double bond will lead to
cyclic radical 18. At this point, the reaction could end by capture of a hydrogen
atom. However, in this case, vinylcyclopropane radical 18 is transformed into
benzylic radical 19 by rupture of the adjacent cyclopropyl ring. Oxidation of this
radical in the reaction medium (IBX-THF) leads to cation
m
20 which cyclizes to 22
(the cyclization is better understood via canonical form 21). Finally, rearomatization and loss of a proton in 22 yields the observed product 15. The overall reaction
is a cascade radical cyclization (Scheme 39.8).
IBX-THF
N
O
-H
Ph
O
H
N
N
O
N
O
IBX-THF
N
O
Ph
HN
O
O
N
-H
N
O
Ph
N
O
(SET)
15
14
20
22
21
SET [O]
16
1 7
18
19
Scheme 39.8