Lewis acid mediated highly regioselective intramolecular cyclization for the synthesis of β-lapachone

A highly regioselective intramolecular cyclization of lapachol mediated by Lewis acids including NbCl5, AlCl3, and FeCl3 was developed for synthesizing β-lapachone in excellent yields without any formation of the isomer α-lapachone. This procedure was efficient, mild, and easily scalable that avoided using highly hazardous concd H2SO4. In the case of ZrCl4 the cyclization was found to give α-lapachone as the main product. A possible mechanism for the Lewis acid mediated cyclization was also discussed.