Preparation and properties of 2-methyleneoxetanes

The methylenation of beta-lactones 5 with dimethyltitanocene provides a versatile, reliable, and highly chemoselective entry to 2-methyleneoxetanes 7. The conversion proceeds selectively in the presence of alkenes, unprotected alcohols, and a variety of other carbonyl moieties, A study of conditions for the optimization of this reaction is delineated. In addition, the first X-ray structure of a 2-methyleneoxetane, which shows its similarity to related p-lactones, is reported. Reactivity studies of 2-methyleneoxetanes are presented in which it is demonstrated that these compounds are attacked at C-4 with a nucleophile; then, subsequently, the resultant enolate reacted with an electrophile. An interesting dichotomy of reactivity was observed when methyleneoxetane 7c was treated with electrophiles. Reaction of 7c with acetic acid gave acetoxyoxetane 19. When 7c was exposed to bromine, dibromoketone 20 resulted.