DIOXIRANE OXIDATION OF 3-ARYLIDENEFLAVANONES - DIASTEREOSELECTIVE FORMATION OF TRANS,TRANS SPIROEPOXIDES FROM THE E-ISOMERS

Oxidation of the E isomers of the 3-arylideneflavanones 1 by dimethyldioxirane in acetone solution at ambient temperature led to spiroepoxides trans,trans-2 in high yields (greater than or equal to 170%) and complete diastereoselectivity. Steric interaction with the axial aryl group in the 3-arylideneflavanone E-1. directs the attack of the dioxirane to the opposite side during the oxygen transfer to afford exclusively the trans,trans-2 epoxides, which clearly demonstrates the advantage of dimethyldioxirane as oxidant. In comparison, these substrates give under Weitz-Scheffer conditions (alkaline hydrogen peroxide) both diastereomeric epoxides (dr ca. 3:1) in poor yields (less than or equal to 130%), while m-CPBA produces trans,trans-2 spiroepoxides preferentially, but also in low yields (less than or equal to 140%).(1,2) Attempted epoxidation of the Z isomers afforded instead the 3-benzoylflavones 3 and/or 3-benzoylflavanones 4 in low yields. With the much more reactive methyl(trifluoromethyl)dioxirane, the E-1 isomers also gave the trans,-trans-2 spiroepoxides diastereoselectively in high yields, but the Z isomers suffered complete decomposition. Presumably the sterically hindered Z isomers encumber the oxygen atom transfer by the dioxirane and radical-type oxidation dominates for these reluctant substrates.