Vicinal alkylation–carboxymethylation of electron-poor alkenes by radical-chain reactions with O-alkyl O-silyl ketene acetals and their [3+2] annulation by reaction with O-cyclopropylcarbinyl O-silyl ketene acetals

O-Silyl ketene acetals of the type H2CC(OR)OSiMe2But, in which R is a tertiary or secondary alkyl group, react with electron-poor alkenes to bring about vicinal alkylation–carboxymethylation of the latter. When R is a cyclopropyldimethylcarbinyl group such reactions take a more complex course involving ring opening of the cyclopropylcarbinyl radical and lead ultimately to [3+2] annulation of the alkene.