INTRAMOLECULAR DIELS-ALDER REACTIONS OF SILYL ACETAL TRIENES - DIENOPHILE-CONTROLLED CYCLOADDITIONS OF MATCHED AND UNMATCHED DIMETHYLATED SUBSTRATES

Thermal intramolecular Diels-Alder reaction of a mixture of silyl acetal trienes S and 6 followed by HF-mediated removal of the silicon tether gives in high overall yield the cyclic diol-esters 9 and 10. Compound 10 is converted selectively under acid catalysis into hydroxylactone 11. Th structures of 9 and 11 are established by X-ray crystallography. The highly stereoselective nature of the IMDA reactions of S and 6 implied by the formation of 9 and 11 is interpreted in terms of transition-states whose geometry is constrained predominantly by the dienophile stereocentre.