active salts formed i e the compounds of formula iia and lib above while causing the other to precipitate out of solution suitable solvents include acetone acetonitrile dioxane ethyl acetate tetrahydrofuran dimethoxyethane methanol ethanol 2 propanol and methylethylketone the preferred solvent is acetone the temperature of the foregoing resolution is first elevated to between 20 u00b0 c and 110 u00b0 c to ensure complete solution of the starting materials the solution is then allowed to cool to about 20 25 u00b0 c to afford the diastereoisomeric salt when s 1 cyclohexylethylamine is used as the resolving agent as described above the s 1 cyclohexylethylamine salt of r 2 benzylsuccinic acid 4 4 n t butoxycarbonyl methylamino piperidine amide precipitates out of solution and can be physically separated by methods well known to those skilled in the art the s 1 cyclohexylethylamine salt of the opposite enantiomer s 2 benzylsuccinic acid 4 4 n t butoxycarbonylmethylamino piperidine amide remains in solution when r 1 cyclohexylethylamine is used as the resolving agent the r 1 cyclohexylethylamine salt of s 2 benzylsuccinic acid 4 4 n t butoxycarbonyl methylamino piperidine amide precipitates out of solution while the r 1 cyclohexylethylamine salt of r 2 benzylsuccinic acid 4 4 n t butoxycarbonyl methylamino piperidine amide remains in solution neutralization of the cyclohexylethylamine salts of and r 2 benzylsuccinic acid 4 4 n t butoxycarbonyl methylamino piperidine amide or s 2 benzylsuccinic acid 4 4 n t butoxycarbonyl methylamino piperidine amide to form the corresponding optically active free acids may.