Synthesis of Optically Pure Compounds by Enantiotopically Differentiating Monoacetalization of Prochiral Diketones

Article abstract of DOI:10.1002/hlca.19820650303

Reaction of 9-methyl-cis-decalin-1,8-dione (2) and of its trans-isomer (3) with (2R,3R)-2,3-butanediol (8) gives in both cases the monoacetals in high yield.While no enantiotopic differentiation is found for 3, a strong preference for the (9S,10R)-monoacetal 9 is found for the cis-decalin-dione 2.The differentiation, reaching a maximal ratio of 9:1, is found to increase by conditions, which slow down the rate of reaction, e.g. lowering the temperature, the concentration of the acid catalyst, or by using a sterically encumbered catalyst.The use of this procedure for the preparation of optically active compounds is exemplified by the transformation of monoacetal 9 (obtained in 76percent yield from 2) into (9R,10R)-9-methyl-1-decalone (12).The differing behaviour of diketones 2 and 3 is discussed in terms of steric and electronic effects.