Novel conversions and coupling reactions of chalcones and their epoxides

Abstract

Chalcones undergo photoisomerization from trans- to the hitherto unknown cis-forms only when available phenolic hydroxy-functions are fully substituted. The cis-chalcones ere metastable and revert immediately to trans-isomers during acidic hydrolysis of protective acetal groups.

Both trans- and cis-isomers of fully O-substituted chalcones form stable trans-epoxides with alkaline hydrogen peroxide. These can serve as-synthetic intermediates, as in the synthesis of a pair of 2-hydroxy-3-methoxy-3-phenylpropiophenone racemates by acidic hydrolysis in methanol (S_N1 mechanism) and of α-hydroxychalcones (alkaline hydrolysis).

Stereoselective β-addition of phenolic nuclei via unstable epoxide intermediates (S_N2 mechanism) occurs with alkaline peroxide when a 4-hydroxy-function is present in the chalcone, giving structural isomers of 3,3-diaryl-2-hydroxypropiophenones.