SYNTHETIC STUDIES ON O-HETEROCYCLES VIA CYCLOADDITIONS .3. REGIOCHEMICAL AND MECHANISTIC QUESTIONS IN REACTIONS OF POLARIZED DIARYL CARBONYL YLIDES

The unsymmetrical diaryloxiranes (9), (24), (25), and (26) undergo smooth cycloaddition with electron-deficient olefins under thermal or photochemical initiation, apparently via polarised carbonyl ylide intermediates. With unsymmetrical dipolarophiles the additions are surprisingly unregioselective; thus the oxirane (9) gives both adducts (19) and (20), (24) gives (36) and (37), (25) forms (34) and (35), and (26) affords the pairs (38) and (39), and (40) and (41). The structural variations in the intermediate series (42), (43), and (44) suggest a marked variation in frontier orbital levels, which is however not paralleled by changes in regioselectivity; formulation of (42)-(44) as 1,3-diradicals cannot be excluded.