SYNTHESIS AND CYCLIZATION OF 1-(2-HYDROXYPHENYL)-2-PROPEN-1-ONE EPOXIDES - 3-HYDROXYCHROMANONES AND -FLAVANONES VERSUS 2-(1-HYDROXYALKYL)-3-COUMARANONES

Competitive alpha and beta cyclization of 2'-hydroxychalcone epoxides affords 2-(alpha-hydroxybenzyl)-3-coumaranone and/or 3-hydroxyflavanon es, which depends on the conditions employed. Epoxidation of 2'-hydrox ychalcones by dimethyldioxirane followed by either base- or acid-catal yzed ring closure provides a novel, general, and efficient method for the synthesis of trans-3-hydroxyflavanones, which includes also the na turally occurring derivatives. Extension of this two-step procedure to 1-(2-hydroxyphenyl)-2-alken-1-ones was also accomplished. A strong pr eference for alpha cyclization was observed in the case of beta-unsubs tituted or -monoalkylated alpha,beta-enones, while both 2,2-dimethyl-3 -hydroxychromanones and 2-(1-hydroxy-1-methylethyl)-3-coumaranones wer e obtained from the beta,beta-dimethylated substrates.