Diastereofacial selectivity of Diels-Alder reactions of carbohydrate-derived dienes and their carbocyclic analogs

The Diels-Alder reactions of two dienes derived from carbohydrates and their carbocyclic analogs are described. The dienes (2S)-3,4-dihydro-2-methoxy-6-vinyl-2H-pyran (1) and (4R)-3,4-dihydro-4-methoxy-6-vinyl-2H-pyran (2) were synthesied from tri-O-acetyl-D-glucal. Cycloaddition reactions of these dienes, each of which contains a single methoxy group at either the allylic or anomeric position, were carried out with a series of dienophiles including N-phenylmaleimide, dimethyl acetylenedicarboxylate, diethyl azodicarborylate, and N-phenyl-1,2,4-triazoline-3,5-dione. The incorporation of an amine base in the Diels-Alder reactions was found to be essential for the formation of good yields of unrearranged cycloadducts. The diastereofacial selectivities observed for dienes 1 and 2 are compared with those of the analogous carbocyclic dienes methoxy-1-vinylcyclohexene derivatives 3 and 4. Cycloadditions to both the carbohydrate-derived dienes and their carbocyclic analogs were found to aoccur with a preference for addition of the dienophile to the face of the diene opposite the methoxyl group in every case