Studies on Pd(II)-Catalyzed Synthesis of (<i>Z</i>)-α-Haloalkylidene-β-lactones from Cyclocarbonylation of 2-Alkynols and the Subsequent Coupling Reactions

A good regio- and stereoselectivity was observed for the PdCl2-catalyzed cyclocarbonylation of 2-alkynols with CuCl2 affording (Z)-α-chloroalkylidene-β-lactones. The highly optically active (Z)-α-chloroalkylidene-β-lactones could be easily prepared from the readily available optically active propargylic alcohols. The Pd(II)-catalyzed cyclocarbonylation of 2-alkynols with CuBr2 was also studied. Although the yields of (Z)-α-bromoalkylidene-β-lactones were low, due to the relatively higher activity of the C−Br bond, the coupling reactions of (Z)-α-bromoalkylidene-β-lactones were quite smooth to afford the corresponding products in high yields. A rationale for this reaction is discussed.