Diels−Alder Reactions of Chiral 1,3-Dienes

Structurally related 1,3-dienes 3a − f attached to a chiral carbon were treated with maleic anhydride under a variety of conditions. The initial Diels−Alder adducts were not isolated as they spontaneously rearranged to the isomeric isoindolones 6 and 7 . The rate of the rearrangement was dependent on the relative stereochemistry of the initial cycloadducts. The π-facial selectivity of the Diels−Alder reaction was determined by analyzing reaction mixtures of the isomeric isoindolones by 1 H and 13 C NMR spectroscopy and HPLC. A gradual increase in the π-facial selectivity was observed when the homoallylic hydroxyl group of the diene was endowed with a larger protecting group or slightly increased by performing the reactions in ether saturated with LiClO 4 . These effects were rationalized by proposing a model based on 1,3-allylic strain in the transition state for the dienes 3a − f .