2-Ethoxycarbonyl-1-silacyclobutanes were synthesized by intramolecular C-H
insertion of carbenes generated photochemically from alpha-(di-tert-butylsi
lyl)-alpha-diazoacetates; they undergo smooth thermal ring-expansion by a 1
,3(C --> O) silyl shift to form 6-ethoxy-1-oxa-2-silacyclohex-5-enes.