Synthesis, Regiochemistry, and Reactions of Dichlorocyclobutenones¹

Dichlorocyclobutenones previously obtained only with difficulty were prepared by the cycloaddition of di-chloroketene with alkynes in the presence of phosphorus oxychloride in fair to good yields. The regiochemistry of addition can be rationalized on the basis of electronic factors except in the case of (trimethylsily))acetylenes which exhibit unusual regioselectivity. Thermolysis of the cyclobutenones in the presence of alcohol led to electrocyclic ring cleavage, giving vinylketene intermediates which were trapped by the alcohol; on the other hand, reaction with alkoxide took place at room temperature and led either to ring opening or allylic substitution.