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 (2 R ,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 (9 S ,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.