Nitrile Anion Cyclization with Epoxysilanes Followed by Brook Rearrangement/Ring-Opening of Cyclopropane Nitriles/Alkylation

The reaction of δ-silyl-γ,δ-epoxypentanenitrile derivatives 9−12 with a base and an alkylating agent
affords (Z)-δ-siloxy-γ,δ-unsaturated pentanenitrile derivatives via a tandem process that involves
the formation of a cyclopropane derivative by epoxy nitrile cyclization followed by Brook
rearrangement and an anion-induced cleavage of the cyclopropane ring. Exclusive formation of a
(Z)-derivative from trans-epoxides is explained by the reaction pathway that involves a backside
displacement of the epoxide by the α-nitrile carbanion and the O−Si bond formation followed by
concerted processes involving Brook rearrangement and the anti-mode of eliminative ring fission
of the cyclopropane from the rotamer 19. The fact that (E)-isomers are exclusively obtained from
cis-epoxides and α-cyclopropyl-α-silylcarbinol derivative 26 provides experimental support for the
proposed pathway.