Environment-friendly organic synthesis using bismuth compounds. Bismuth triflate catalyzed synthesis of substituted 3,4-dihydro-2H-1-benzopyrans

A highly catalytic method for the synthesis of dihydrobenzopyrans from salicylaldehydes has been developed. An extension of this method to the synthesis of a pyrano2,3,b]benzopyran has also been achieved. Bi(OTf)₃·xH₂O (1 < x < 4) (0.1 mol%) smoothly catalyzes the condensation of substituted salicylaldehydes with 2,2-dimethoxypropane to give the corresponding substituted 3,4-dihydro-2H-1-benzopyrans as a mixture of diastereomers (9:1) in moderate yields. The relative configuration of the methoxy groups in the two diastereomers was established by NOE experiments. The advantages of this method include the use of an easy to handle, inexpensive and relatively non-toxic catalyst.