The heterogeneous catalytic hydrogenation of two isomeric cinchona alkaloids, cinchonidine and cinchonine, was studied over Pt/Al(2)O(3) in 1N H(2)SO(4) solution under 100 bar H(2) at 25 degrees C. Cinchonidine was transformed into two diastereomeric hexahydroderivatives by hydrogenation of ring A