AbstractMethylenecyclobutane and higher alkylidenecyclobutanes containingno substituents in the ringundergoCriegeeozonolysis with formation of cyclobutanone as a major product. When thering contains α‐substituents, a primary ozonide is formed in which the (substituted) cyclobutyl ring is ruptured simultaneously with O‐O fission. The overall result is a rearrangement to a seven‐membered cyclic 3,4‐dioxa ketone (cf.Scheme 3). This rearrangement is mechanistically related to previously studied cyclopropyl systems in which the small ring is ruptured without the assistance of a ring substituent.At room temperature, the 3,4‐dioxacycloheptanone derivative slowly loses formaldehyde, leaving a γ;‐butyrolactone. Structurally related 3,4‐dioxacyclohexanones are relatively stable,i.e.they do not decompose into correspon