Diastereoselective Reduction of Derivatives of 3,4-Dihydro-1-methylidene-2-tartaroylisoquinoline

Various enamides were prepared from 3,4-dihydro-6.7dimethoxy-l -methylisoquinoline by reaction with (R, R)-tartaric acid derivatives.The enamides were reduced with hydrogen over a platinum catalyst to afford diastereomeric mixtures enriched in the (1R)-isomer.The diastereoselectivity of the reduction step was assessed by conversion of the mixtures into Nacetylsalsolidine and measurement of its specific rotation.The enamide (5c), in which the hydroxyl groups of the 2-tartaroyl group are unprotected, cyclizes to an aminal(9) in the presence of acids.The reduction of 5c with sodium borohydride in acidic media afforded a diastereomeric mixture enriched in the (IS)-isomer.The catalytic reduction of linear enamides using homogeneous catalysts of high enantiomeric purity was reported some years ago by Kagan and coworker^.'^In their first two reports, they demonstrated that a-Nacylaminoacrylic acids were converted into the corresponding saturated a-Kacylamino acids in high optical yield.Also, it was shown3 that simple enamides would undergo asymmetric hydrogenation and that imines would undergo asymmetric hydrosilylation.Somewhat later ~c h i w a ~ showed that the enamide.2-acetyl-1,2,3,4-