β′-chloro-β-hydroxy ketones in high yields. It was speculated that ringcleavage of the cyclobutanone ring with titanium(IV) chloride gave trichlorotitanium enolate having a tertiary alkyl chloride moiety and then aldol reaction of the titanium enolate proceeded. A trialkylsilylmethyl group at the 2-position of cyclobutanone facilitated the ringcleavage. Synthesis of substitutedcyclopentenone from an obtained