Competitive α and β cyclization of
2‘-hydroxychalcone epoxides affords
2-(α-hydroxybenzyl)-3-coumaranone and/or 3-hydroxyflavanones, which depends on the conditions
employed. Epoxidation
of 2‘-hydroxychalcones by dimethyldioxirane followed by either base- or
acid-catalyzed ring closure
provides a novel, general, and efficient method for the synthesis of
trans-3-hydroxyflavanones, which
includes also the naturally occurring derivatives. Extension of
this two-step procedure to 1-(2-hydroxyphenyl)-2-alken-1-ones was also accomplished. A strong
preference for α cyclization was
observed in the case of β-unsubstituted or -monoalkylated
α,β-enones, while both 2,2-dimethyl-3-hydroxychromanones and 2-(1-hydroxy-1-methylethyl)-3-coumaranones were
obtained from the β,β-dimethylated substrates.