Pd-Catalyzed Cyclization to Form Benzo-pyranones from Enediynes

Significance

This paper describes the synthesis of substituted dibenzo[b,d]pyran-6-ones by a Pd-catalyzed, one-step, tandem cyclization reaction from enediynes. For the 14 cases studied, yields vary from poor to modest; furthermore, interesting chlorinated and minor byproducts are formed in a few cases. A mechanism involving the formation of a vinyl palladium intermediate which undergoes 6-endo cyclization to yield the product is proposed. A 5-exo cyclization to the observed chlorinated product is observed when the terminal alkene substituent is a sterically hindered alkyl group or an electron-rich aryl ring. The starting methyl 2-[6-substituted-3(Z)-hexen-1,5-diynyl]benzo­ates were prepared in moderate to excellent yield by the well-established Sonogashira methodology.