THERMAL AND CATALYZED DIELS-ALDER REACTIONS WITH CHIRAL 2-SUBSTITUTED-1,3-DIENES - CONFORMATIONAL MODELS FOR DIASTEREOFACIAL SELECTIVITY

The thermal or BF3-catalyzed Diels-Alder reactions of chiral 2-substituted-1,3-dienes bearing a hydroxy or a protected hydroxy group at the allylic position with various dienophiles have been studied in order to determine the regio- and stereoselective control elements. Good regio- and facial selectivities were ob;served in thermal reaction of dienes bearing a free hydroxy group. The stereoselectivity was reversed and strongly lowered when the hydroxy soup was protected as an ether. However the regio- as well as the stereoselectivity of the cycloaddition of these diene ethers with alpha,beta-unsaturated aldehydes or ketones are remarkably enhanced by the addition of a Lewis acid since in a few cases a unique isomer was formed. These experimental results have been rationalized and could be explained by conformational preferences of the dienes in the transition state. In particular, the reversal of facial selectivity for dienes having a free hydroxy group has been attributed to a hydrogen bonding interaction between the diene and the dienophile.