Intramolecular ene reactions catalyzed by NbCl5, TaCl5 and InCl3

Abstract

Niobium pentachloride, tantalum pentachloride and indium trichloride efficiently catalysed the cyclization of (R)-citronellal to a mixture of isopulegol and neoisopulegol in good yields. A comparative study was carried out which demonstrated that NbCl5 is the most active Lewis acid whereas InCl3 is the most selective one. The selectivity of the reactions varied according to the Lewis acid used and to the solvent. The ene reaction of a 1,7-diene was also investigated. In this case, all Lewis acids tested showed excellent selectivity.