Electrocyclic Ring Closure of the Enols of Vinyl Quinones. A 2H-Chromene Synthesis

Thermolysis of enolizable vinyl quinones in polar, aprotic media provides 2H-chromenes. Experimental evidence supports a two-step mechanism in which enolization is followed by a thermal 6pi-electrocyclic reaction of an intermediate quinone methide. Application of this method led to the total synthesis of the reputed structure of an Ageratum juvenile hormone. When enolizable vinyl quinones are the products of Stille coupling, the chromene annulation product is obtained directly. [reaction: see text]