6H-Dibenzo[b,d]pyrans. I. Synthesis

The preparation of 2′,4′-dihydroxy- and 2′,6′-dihydroxybiphenyl-2-carboxylic acid lactones (3 and 4) is described. Grignard addition to or direct boron trifluoride etherate – sodium borohydride reduction of these lactones yields respectively these 6,6-substituted (e.g. 6) or the 6,6-unsubstituted (13a) 6H-dibenzo[b,d]pyrans. Evidence that the reductive route proceeds without ring cleavage is presented.