ACID-CATALYZED ADDITION OF N-ARYL IMINES TO DIHYDROFURAN - POSTULATED DEPENDENCE OF THE REACTION-MECHANISM ON THE RELATIVE FACE OF APPROACH OF REACTANTS

Imines 1a and 1b react with dihydrofuran (DHF) under Lewis acid catalysis via Diels-Alder-type addition to form the tetrahydroquinoline derivatives 3a, b and 4a, b. Besides these compounds, the methanol adduct 2 of 1a, or 1a in the presence of methanol, gives the methanol-containing tetrahydrofuran derivative 7a and the hexahydrofuro[3,2-b]furan derivatives 5 and 6. In the presence of methanol, 1b gives 3b and 4b, and also 7b and 8. The products 3a, b, 5, 6 and 7a, b originate from approach of the Si (or Re) face of 1a, b onto the Si (or Re) face of DHF; 4a, b and 8 derive from interaction of the Si (or Re) face of 1a, b with the Re (or Si) face of DHF. The dependence of the product distribution on the polarity of the solvent suggests that a concerted mechanism pre-dominates in the former mode and a zwitterionic one in the latter. In the addition of 1a or 2 the mechanistic preference may be exclusive.