Efficient Enzymatic Process for the Production of (2S)-4,4-Difluoro-3,3-dimethyl-N-Boc-proline, a Key Intermediate in the Synthesis of HIV Protease Inhibitors

(2S)-4,4-Difluoro-3,3-dimethyl-N-Boc-proline (3) is a key intermediate for the synthesis of HIV protease inhibitors. Here, several approaches for the preparation of enantiopure 3 and its analogues are disclosed. Among these methods, one strategy relies on resolving the racemic methyl ester of 3 through a protease-catalyzed enantioselective hydrolysis. Despite the fact that this resolution was applied to prepare kilogram quantities of optically pure acid 3 for clinical trials, this process suffered from low efficiency, high cost and difficulties in improvement by medium engineering. An alternative much more efficient and cost-effective enzymatic process was therefore developed by switching the protective group of the proline esters from a Boc to a benzyl moiety. This new process has a much higher throughput (6.3 mmol/h/L vs 0.11 mmol/h/L), and the cost of the process was also dramatically reduced to only 5% of the protease resolution process.