Synthesis of Optically Pure Compounds by Enantiotopically Differentiating Monoacetalization of Prochiral Diketones. Part II. Fragmentation of β‐Keto‐Acetals

Treatment of β‐keto‐acetals, derived from non‐enolisable β‐diketones, with sulfonic acids in boiling benzene resuots in a smooth retro‐Claisen ‐type fragmentation. The acetal‐C‐atom is thereby transformed into a carboxylic ester via a dialkoxycarbenium ion, which is dealkylated by the sulfonate counter‐ion. Application of this reaction to the diastereomeric monoacetals 3 and 4 , derived from cis ‐9‐methyl‐decalin‐1,8‐dione ( 1 ), followed by transesterification with CH 3 OH, yields optically pure 4‐(2′‐methyl‐3′‐oxocyclohexyl)butyrate 9 ((+)− 9 from 3 , (−)‐ 9 from 4 ) and the monosulfonate of Meso ‐2,3‐butanediol (−)‐ 13 ( Scheme 2) . Unexpectedly, this cleavage proceeds as well with monoacetal 26 , obtained by acetalization of trans ‐9‐methyl‐decalin‐1,8‐dione ( 27 ) with 2,2‐dimethyl‐1,3‐propanediol ( Scheme 7) . Some attempts, aiming at an isomerization of the cis ‐ and trans ‐decalin derivatives 3 and 24 , or 25 and 26 , via the postulated carboxonium intermediate, were not successful.