Synthesis of optically pure compounds by enantiotopically differentiating monoacetalization of prochiral diketones

Reaction of 9‐methyl‐cis‐decalin‐1,8‐dione (2) and of its trans‐isomer (3) with (2R,3 R)‐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,10 R)‐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 76% yield from 2) into (9 R, 10 R)‐9‐methyl‐1‐decalone (12). The differing behaviour of diketones 2 and 3 is discussed in terms of steric and electronic effects.