Pi-facial diastereoselection in [4+2]-cycloadditions of 3,4-epoxy-2-methyleneoxolanes with oxadienes : a short synthesis of spiroketals

Hetero Diels-Alder reaction of oxadienes with 3,4-epoxy-2-methyleneoxolanes gave the corresponding 3,4-epoxy-1,6-dioxaspiro[4,5]-dec-7-enes with high stereoselectivity. Good yields of adducts were obtained in the presence of mild Lewis acis, such as zinc or stannous chloride. The spiroketal adducts have been transformed chemio- and stereospecifically by either hydrogenation, hydride reduction or acid catalyzed isomerization. The stereochemical outcome of the cycloadditions has been investigated. The spiroketal adducts always result from an oxadiene addition anti relative to the allylic epoxy substituent. When the oxadiene is substituted suitably as in crotonaldehyde, we demonstrated that the cycloaddition is totally endo selective relative to the enol ether function. Ab initio calculations suggested that 3,4-epoxy-2-methyleneoxolane and 3,4-epoxy-3-methyl-2-methyleneoxolane adopt envelopw conformations with the oxygen atom of the oxolane moiety pointing toward the epoxide ring. No significant distortion from planarity was calculated for the exocyclic double bond of these dienophile