Chiral Lewis Base Assisted Brønsted Acids

Significance

Chiral phosphonium salts such as fluorosulfonate A were designed as a novel class of cationic Brønsted acid catalysts. These catalysts are formed in situ by mixing a chiral phosphorus(III) Lewis base (e.g. 1) with an achiral strong Brønsted acid (e.g. FSO₃H) and are termed ‘chiral Lewis base assisted Brønsted acids’ (chiral LBBAs). Brønsted acid A was found to catalyze the enantioselective cyclization of 2-geranylphenols 2 to give the desired trans-fused cyclized products 3 with generally moderate to good dia­stereo- and enantioselectivities. However, high enantioselectivity could also be achieved (er up to 96.5:3.5).