Abstract The C-formylation of 1, 3-dimethyl-2 (3H)-benzimidazolone and 4-methyl-2H-1, 4- benzoxazin-3 (4H)-one was performed using 1, 1-dichloromethyl methyl ether at the Friedel- Crafts reaction conditions. The formylation of 3-methyl-2 (3H)-benzoxa-and-thiazolone at the 6-position was carried out by modified Duff's method with hexamethylenetetramine in trifluoroacetic acid.