Reaction of Resorcinol with α,β-Unsaturated Ketones

Products of the acid-catalyzed reaction of resorcinol with α,β-unsaturated ketones have been found to fall into two classes. The first type of product is illustrated by 4-(3,4-dihydro-7-hydroxy-2,4,4-trimethyl-2H-1-benzopyran-2-yl)-1,3-benzenediol, 1, formed in 91% yield by the reaction of resorcinol with 4-methyl-3-penten-2-one (mesityl oxide). The second type of product is illustrated by (C₂-symmetric) 2,2‘-spirobi(7-hydroxy-4,4-dimethylchroman), 4, formed in 85% yield by the reaction of resorcinol with 2,6-dimethyl-2,5-heptadien-4-one (phorone). A number of examples of reactions leading, in quite good yields, to products analogous to 1 are presented, as well as some reactions that fail. Flavan 1 is identical to the compound formed by the acid-catalyzed reaction of acetone with excess resorcinol. Compound 1 and a steroidal analogue of 1 have been found to be fluorescent.