Stereoselective preparation of (Z)-2-(trialkylsilyloxy)-2-alkenals by retrocycloaddition reactions of 4H-4-alkyl-5-(trialkylsilyloxy)-1,3-dioxins.: Useful reactants for Lewis acid catalyzed [4+3] cyclizations

[GRAPHICS] Retrocycloadditions of 4H-4-alkyl-5-(trialylsilyloxy)-1,3-dioxins proceed smoothly in refluxing toluene to afford (Z)-2-(trialkylsilyloxy)-2-alkenals with complete stereos electivity. These enals undergo Sasaki-type [4 + 3] cyclizations with dienes in the presence of Lewis acids, in many instances with excellent regio- and/or stereoselectivity.