PI-FACIAL DIASTEREOSELECTION IN [4-CYCLOADDITIONS OF 3,4-EPOXY-2-METHYLENEOXOLANES WITH OXADIENES - A SHORT SYNTHESIS OF SPIROKETALS(2])

Citation
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
Citations number
62
Categorie Soggetti
Chemistry Inorganic & Nuclear
Journal title
ISSN journal
00404020
Volume
50
Issue
27
Year of publication
1994
Pages
8035 - 8052
Database
ISI
SICI code
0040-4020(1994)50:27<8035:PDI[O3>2.0.ZU;2-N
Abstract
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.