Chiral Benzothiopyrans by Organocatalysis

Significance

Reported is the first catalytic asymmetric synthesis of benzothiochromenes starting from readily available precursors under mild, metal-free conditions. The reaction tolerates aryl, alkyl and ester cinnamic aldehydes and proceeds in good to excellent yields and excellent enantio­selectivities. Benzoic acid was found to be required for high enantioselectivity. X-ray crystal structure analysis of one product established the absolute configuration at C-2 as R. A standard organocatalytic mechanism involving addition to a chiral iminium ion was suggested and supported by NMR analysis.