One-Pot Synthesis of α-Haloketones Employing a Membrane-Based Semibatch Diazomethane Generator

The crucial structural motive in viral protease inhibitors such as atazanavir and darunavir is a chiral aminoalcohol structure. The structure is generally introduced during the synthesis of the protease inhibitor via an alpha-chloroketone intermediate. The alpha-chloroketone can be synthesized in a multistep sequence from naturally occurring L-phenylalanine. Herein, we report a one-pot synthesis of an alpha-chloroketone starting from N-Boc-L-phenylalanine in a novel type of tube-in-flask semi-batch diazomethane generator. Activation of the amino acid to the mixed anhydride was carried out in the flask, while diazomethane was generated from in situ formed N-nitroso-N-methylurea within a gas-permeable tubing contained inside the flask. The diazomethane diffused through the gas-selective membrane into the flask, and reacted with the anhydride to the diazoketone (Arndt-Eistert reaction). The addition of aqueous hydrogen chloride provided the alpha-chloroketone and destroyed any excess of diazomethane. The desired product was isolated by extraction in excellent purity and yield (90%-96%).