Studies on Pd(II)-catalyzed synthesis of (Z)-α-haloalkylidene-β- lactones from cyclocarbonylation of 2-alkynols and the subsequent coupling reactions

(Chemical Equation Presented) 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 CuBr ₂ 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.