Characterization of a trace by-product of the synthesis of the protease inhibitor tipranavir (PNU-140690)

Tipranavir(TM) (PNU-140690) is a protease inhibitor under clinical investig ation for the treatment of human acquired immunodeficiency syndrome (AIDS). During scale-up synthesis of clinical quantities of the bulk drug, a color ed, transient by-product of the final coupling reaction was observed. Quant ities of this colored, transient chemical species were too low (<<0.1%) for characterization by conventional spectroscopic methods. It was, however, p ossible to isolate sufficient material for characterization based on mass s pectrometry and submicro inverse-detection gradient (SMIDG) nmr methods by methanol stripping of silica gel that had been used in purification of bulk drug. This process afforded an enriched feedstock from which small quantit ies of this highly colored and unstable (halflife < 18 hours in methanol an d < 10 minutes in acetone) trace contaminant could be isolated by semiprepa rative reversed phase hplc. The impurity was identified as an unstable Zinc ke salt formed by the condensation of two molecules of the anilino precurso r and the pyridine used as a base in the final step of the synthetic proces s. Following identification of this impurity, efforts were undertaken to en gineer it out of the synthetic process.