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.