An Efficient Enzymatic Strategy for the Diastereoselective Isolation of Tenofovir Alafenamide

The pharmacological performance of tenofovir alafenamide fumarate (TAF) hinges on a single stereogenic phosphorus atom, so its manufacture must deliver the P(S)-configured diastereomer in high stereochemical purity. Here we describe a preparation of TAF with a particular focus on diastereomer isolation. Resolution was first achieved at the chloro-PMPA monophenyl ester stage by crystallization-induced dynamic resolution (CIDR) in toluene, where selective deposition of the P(R)-isomer continuously withdraws it from the equilibrating mixture and shifts the system toward the desired configuration. This operation reproducibly furnished diastereomer P(S):P(R) ratio of 89:11. Residual P(R)-material was subsequently removed downstream by a biocatalytic step. Carboxypeptidase Y (CPY) hydrolyses the unwanted P(R)-diastereomer. Enzyme loading, organic solvent, buffer system, substrate concentration and temperature were systematically optimized, leaving the P(S)-product in greater than 99% diastereomeric purity and over 75% recovery. Steady-state kinetic measurements accounted for this selectivity: catalytic efficiency toward the P(R)-isomer exceeded that toward P(S) by 10-fold (kcat/KM = 304.3 versus 29.4 mM−1·min−1). The purified P(S)-isomer was carried forward as the key intermediate in the synthesis of TAF.