Polarized ketene dithioacetals 63 : stereoselective synthesis of α-ylidene-γ-butyrolactones

A facile method for highly stereoselective synthesis of α-arylidene (4a-4d,4f-4h and 4j) and α-ethylidene (4e,4i)-γ-butyrolactones has been developed by acid catalyzed lactonization of the corresponding α-ylidene-γ, δ-unsaturated esters 3a-aj . The required esters 3a-3j were obtained by regioselective reduction of the corresponding α-oxo- α-allyl (or substituted allyl) ketene dithioacetals 1a-1j with sodium borohydride and subsequent boron-trifluoride etherate catalyzed methanolysis of the resultant carbinol acetals 2a-j . Treatment of α-acetylketene dithioacetals 1e and 1i with methylmagnesium iodide afforded the carbinol acetals 5b and 5c respectively which under above sequence of reactions yielded the corresponding α-isopropylidene-γ-butyrolactones 7a and 7b in good yields