‘Three‐Component Reaction’ with aromatic thioketones, phenyl azide, and dimethyl fumarate

The reaction of thiobenzophenone (= diphenylmethanethione; 8a ) or 9 H ‐fluorene‐9‐thione ( 8b ) and methyl fumarate ( 9 ) in excess PhN 3 at 80° yields a mixture of diastereoisomeric thiiranes 10 and 11 ( Scheme 1 ). A mechanism involving the initial formation of 1‐phenyl‐4, 5‐dihydro‐1 H ‐1, 2, 3‐triazole‐4, 5‐dicarboxylate 12 by 1, 3‐dipolar cycloaddition of PhN 3 and 9 is proposed in Scheme 2 . The diazo compound 13 , which is in equilibrium with 12 , undergoes a further 1, 3‐dipolar cycloaddition with thioketones 8 to give 2, 5‐dihydro‐1, 3, 4‐thiadiazoles 14 . Elimination of N 2 yields the thiocarbonyl ylide 15 which cyclizes to the corresponding thiirane. Desulfurization of the thiiranes 10 and 11 with hexamethylphosphorous triamide leads to the olefinic compounds 16 ( Scheme 3 ). The crystal structures of 10a , 11a , and 16b were determined.