Abstract 1H, 3H-Imidazo [1, 5-c] thiazole-5, 7-[6H, 7aH]-dione and the corresponding 7- thione derivatives as well as 7, 8-dihydro-5H-imidazo [1, 5-c][1, 3] thiazine-1, 3-[2H, 8aH]- dione and the corresponding 3-thione derivatives were synthesized starting from L-cysteine and DL-homocysteine thiolactone, respectively. The second group of bicyclic compounds represents a new heterocyclic ring system. The structures of the compounds were ...