P. Pale et al., PI-FACIAL DIASTEREOSELECTION IN [4-CYCLOADDITIONS OF 3,4-EPOXY-2-METHYLENEOXOLANES WITH OXADIENES - A SHORT SYNTHESIS OF SPIROKETALS(2]), Tetrahedron, 50(27), 1994, pp. 8035-8052
Hetero Diels-Alder reaction of oxadienes with 3,4-epoxy-2-methyleneoxo
lanes gave the corresponding 3,4-epoxy-1,6-dioxaspiro[4,5]-dec-7-enes
with high stereoselectivity. Good yields of adducts were obtained in t
he presence of mild Lewis acis, such as zinc or stannous chloride. The
spiroketal adducts have been transformed chemio- and stereospecifical
ly by either hydrogenation, hydride reduction or acid catalyzed isomer
ization. The stereochemical outcome of the cycloadditions has been inv
estigated. The spiroketal adducts always result from an oxadiene addit
ion 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 fu
nction. Ab initio calculations suggested that 3,4-epoxy-2-methyleneoxo
lane and 3,4-epoxy-3-methyl-2-methyleneoxolane adopt envelopw conforma
tions with the oxygen atom of the oxolane moiety pointing toward the e
poxide ring. No significant distortion from planarity was calculated f
or the exocyclic double bond of these dienophiles.