The embodiment of lactam rings within a wealth of physiologically active natural products
and pharmaceutical agents ensures that the development of synthetic methods, which
facilitate the preparation of these saturated N-heterocycles, is of critical importance. Herein the development of a versatile method
for the synthesis of 4 to 8-membered α-vinyl and α-(2-silylvinyl) lactams involving
the iodine(III)-mediated oxidative cyclization of unsaturated O-alkyl hydroxamates, which encompass an allylsilane, is reported. Importantly, the
outcome of this transformation can be effectively controlled through variation of
the substitution pattern at the silicon center. While allyltrimethylsilanes undergo
ring closure with desilylation to form α-vinyl lactams, the corresponding triisopropyl
and triphenylsilanes cyclize without loss of the larger silyl group to form E-vinylsilanes with excellent stereoselectivity. From a mechanistic standpoint, it is
proposed that this reaction proceeds via concerted alkene addition of a singlet nitrenium
ion (or its equivalent) to form a bicyclic N-acyl-N-alkoxyaziridinium ion, which undergoes eliminative ring opening.
Key words
nitrenium ion - allylsilane - hypervalent iodine -
O-alkyl hydroxamate - aziridinium
