Enzyme-catalyzed enantiomeric resolution of N-Boc-proline as the key-step in an expeditious route towards RAMP

Abstract For the preparation of both enantiomers of N -carbamoyl-2-methoxymethylpyrrolidine, the precursors of Enders’ chiral auxiliaries, SAMP and RAMP, enzyme-catalyzed kinetic resolution of racemic N -carbamoyl, N -Boc, N -Cbz proline esters and prolinols were examined. B. licheniformis protease (subtilisin) preferentially hydrolyzed the ( R )-carbamoylproline ester with an enantiomeric ratio (E) of 10. To a hydrophobic N -Cbz proline ester, subtilisin showed lower selectivity ( E =2.8), and in contrast, a purified protease (isozyme A) from the earthworm showed the preference of ( S )-enantiomer ( E =13.6). In a practical sense, C. antarctica lipase B (Chirazyme L-2) was effective for the hydrolysis of both N -Boc and N -Cbz derivatives with E >100. The e.e. of ( R )- N -carbamoyl-2-methoxymethylpyrrolidine was raised to be >99.9% by recrystallization at the N -Boc-prolinol stage, which was derived from the ( R )- N -Boc-proline methyl ester (98.7% e.e.) through a preparative-scale enzyme-catalyzed resolution (49% yield) of the racemic substrate.