The determination of a chlorinated benzofuran pharmaceutical intermediate by HPLC-MS with on-line derivatization

An HPLC-MS method for the analysis of 2-chloromethylbenzofuran, a pharmaceutical intermediate alkylating reagent employed in the preparation of a second generation HIV protease inhibitor, N-(2(R)-hydroxy-l(S)-indanyl)-2(R)-phenylmethyl-4(S)-hydroxy-5-(1-(4-(2- benzo[b]furanylmethyl)-2(S)-N'-9-t-butyl-carboxamido)-pipera zinyl))-pentaneamide, is described. Preliminary analysis of the protease inhibitor by HPLC-MS indicated that the quality of the drug substance was influenced by the composition of the chloromethylbenzofuran. Direct analysis of the chloromethylbenzofuran by LC-MS using atmospheric pressure ionization was unsuccessful, necessitating an alternative approach. The method described incorporated post-column derivatization of the chloromethyl benzofuran using a modification of the drug substance process chemistry yielding a derivative amenable to MS analysis using atmospheric pressure chemical ionization (APCI). This allowed measurement of an impurity in the chloromethylbenzofuran which was incorporated into the protease inhibitor.