Hafnium Triflate as an Efficient Catalyst for Direct Friedel-Crafts Reactions of Chromene Hemiacetals

The direct Friedel-Crafts reaction of chromene hemiacetals with indoles, furans and sterically hindered anilines has been accomplished with high selectivity and excellent yields in the presence of a catalytic amount of hafnium triflate [Hf(OTf)(4), 0.1 mol%, 0-40 degrees C]. The mild conditions tolerate various sensitive functional and protecting groups, and the products were confirmed unambiguously from their spectra and by single-crystal X-ray analysis. This direct Friedel-Crafts reaction of chromene hemiacetals should inspire and encourage the consideration of hafnium triflate in the development of mild reaction conditions for the efficient derivatization of hemiacetal-containing compounds.