Cyclocondensation reactions of racemic diastereomers of dimethyl-2-oxocyclohexanepropionic acids with (R)-phenylglycinol: access to both enantiomers of dimethyl cis-decahydroquinolines

Starting from racemic diastereomeric mixtures of dimethyl-2-oxocyclohexanepropionic acids (4-6) the synthesis of enantiopure 7,8-, 6,8-, and 5,8-dimethyl-substituted cis-decahydroquinolines (11, 13, and 15) and their enantiomers (ent-11, ent-13, and ent-15) is reported. The procedure involves a dynamic kinetic asymmetric transformation in the cyclocondensation of keto-acids 4-6 with (R)-phenylglycinol to give in each case two major oxazoloquinolone lactams (7a/7b, 8a/8b, 9a/9b), which differ in the absolute configuration of all the stereogenic centers except that of the chiral inductor. A subsequent two-step stereoselective removal of the phenylglycinol moiety with simultaneous reduction of the lactam carbonyl affords the enantiopure cis-decahydroquinolines in both enantiomeric series.