ALPHA-METHYLENE BETA-LACTONES AS MASKED ALLENES AND ALLENE EQUIVALENTS - SOME SELECTED CHEMICAL-TRANSFORMATIONS

Selective chemical transformations have been performed on a number of alpha-methylene beta-lactones to illustrate that these highly functionalized heterocycles serve as useful building blocks in organic synthesis. Thus, thermal decarboxylation of these ''masked allenes'' provides a useful regioselective preparation of allenes with a predetermined substitution pattern. Nucleophilic addition takes place at the carbonyl group with ring opening, but free-radical addition, e.g., thiophenol, and electrophilic addition, e.g., bromine, proceed at the exomethylene group with preservation of the beta-lactone ring. [4 + 2] Cycloaddition with cyclopentadiene affords the expected beta-lactones, which on thermal decarboxylation lead to 2-alkylidenenorbornenes. This regio- and stereoselective transformation establishes the alpha-methylene beta-lactones as valuable allene equivalents. The alpha-methylene beta-lactones serve also as dipolarophiles by undergoing readily 1,3-dipolar cycloaddition with diazoalkanes. Reaction with Lawesson's reagent affords a hitherto unknown alpha-methylene beta-(S)-lactone.