[reaction: see text] This communication describes the first examples of tandem endo-regioselective and stereospecific oxacyclizations of 1, 5-diepoxides to oxepane products and a similar tandem oxacyclization of 1,5,9-triepoxides to fused bisoxepane cyclic carbonates. A mechanism for these biomimetic oxacyclizations is proposed in which the epoxides act as both electrophilic and nucleophilic reaction partners.