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

[reaction: see text]. Retrocycloadditions of 4H-4-alkyl-5-(trialkylsilyloxy)-1,3-dioxins proceed smoothly in refluxing toluene to afford (Z)-2-(trialkylsilyloxy)-2-alkenals with complete stereoselectivity. 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.