Preparation and properties of 2-methyleneoxetanes

The methylenation of beta-lactones 5 with dimethyltitanocene provides a ver satile, reliable, and highly chemoselective entry to 2-methyleneoxetanes 7. The conversion proceeds selectively in the presence of alkenes, unprotecte d alcohols, and a variety of other carbonyl moieties, A study of conditions for the optimization of this reaction is delineated. In addition, the firs t X-ray structure of a 2-methyleneoxetane, which shows its similarity to re lated p-lactones, is reported. Reactivity studies of 2-methyleneoxetanes ar e presented in which it is demonstrated that these compounds are attacked a t 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 wit h acetic acid gave acetoxyoxetane 19. When 7c was exposed to bromine, dibro moketone 20 resulted.