Mechanism of cyclization of substituted 2′-hydroxychalcones to flavanones

Abstract

The conversion of 2′-hydroxy-(3), 2′,4′-dihydroxy-(4), 2′,3-dihydroxy-(5), and 2′,4-dihydroxy-chalcone (6) in aqueous alkali; and of (3) and 2′-hydroxy-4-nitrochalcone (7) in alkaline methanolic medium into the corresponding substituted flavanones has been studied kinetically and spectroscopically. It has been found that results from both methods are consistent with the existence of chalcones in the trans-s-cis conformation and indicate a mechanism that involves general acid attack to the ionized form of the 2′-hydroxychalcones, rotation through the CO–C_α bond and annelation to the flavanone.