FACIAL SELECTIVITY IN CYCLOADDITIONS OF A CHIRAL KETENE ACETAL UNDER MICROWAVE IRRADIATION IN SOLVENT-FREE CONDITIONS - CONFIGURATIONAL ASSIGNMENT OF THE CYCLOADDUCTS BY NOESY EXPERIMENTS AND MOLECULAR MECHANICS CALCULATIONS

(R)-4-Phenyl-2-methylene-1,3-dioxolane (1) undergoes 1,3-dipolar and Diels-Alder cycloadditions under microwave irradiation within 3 min with excellent yields, a simple purification procedure, and an interesting facial selectivity. The stereochemistry of these cycloadducts has been inferred by NOESY experiments and molecular mechanics calculations. X-ray structure determination was required in one case. Thermal isomerization of the cycloadducts was performed affecting only the spiro-carbon. The Diels-Alder adducts in the presence of a slight amount of acid yield the corresponding open chain esters.