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The influence of protective groups on diastereofacial selectivity of Diels-Alder cycloaddition reactions
B W Gung1, D A Mareska, A Karipidis
1Department of Chemistry, Miami University, Oxford, OH 45056, USA.
Abstract:
The diastereofacial selectivity of both inter- and intramolecular Diels-Alder reactions with dienes having an allylic stereogenic center has been studied by varying the allylic oxygen protective group. Four different hydroxy protective groups were investigated including benzyl, t-butyldiphenylsilyl, triethylsilyl, and t-butyldimethylsilyl ethers. For inter-molecular reactions, the benzyl ether derivative gave the highest pi-facial selectivity through a transition state in which the allylic stereocenter favors the CH eclipsed conformation. For intramolecular cycloadditions, the t-butyldimethylsilyl group gave a slightly higher selectivity than the benzyl ether derivative through a transition state in which the allylic stereocenter favors the CO eclipsed conformation. The opposite diastereofacial selectivity observed for inter- and intramolecular reactions is explained by considering both steric and electronic effects.