Further Evidence for the Mechanism of Formation of Coumarin by Perkin Reaction from salicylaldehyde and a Novel Synthesis of 1,1-diphenyl-2(2'-hydroxyphenyl) ethene from O-α,α-diphenylacetylsalicylaldehyde with Et3N

The mechanism of formation of coumarin (1) by Perkin reaction from salicylaldehyde was shown to involve exclusively a basecatalyzed intramolecular aldol-type condensation of O-acetyl salicylaldehyde (2) as an obligatory intermediate followed by dehydration. Direct evidence in support of this contention was provided by the formation of coumarin and 3-phenyl coumarin(5) as the sole products in the reactions of O-acetyl salicylaldehyde(2)with Et3N or NaOAc and O-α-phenyl acetyl salicylaldehyde (4) with Et3N, respectively. A novel synthesis of 1,1-diphenyl-2(2'-hydroxyphenyl)ethene(8a) was achieved by the reaction of O-α,αdiphenylacetylsalicylaldehyde (6) with Et3N. A plausible mechanism of formation of 8a in the above reaction is proposed. The formation of 8a is assumed to involve an intermediate β-lactone (9) formed through two consecutive intramolecular condensation. Base-catalyzed decarboxylation of 9 finally gives 8a.