In the search for potential immunomodulators methyl L‐(—)‐thiazolidine‐4‐carboxylate (2), 2‐amino‐2‐thiazoline (12), and 2‐aminothiazole (19) were transformed into derivatives of various bicyclic systems. Thus, from compound 2 derivatives of perhydrothiazolo[3,4‐a]pyrazine 4 and 5, perhydrothiazolo[4,3‐c]‐[1,4]oxazine 7, and perhydroimidazo[1,5‐c]thiazole 9a,b, from compound 12 derivatives of 2,3‐dihydro‐thiazolo[2,3‐b]pyrimidine 13a,b, and from compound 19 derivatives of imidazo[2,1‐b]thiazole 21, 22, 24, and 25 were prepared. 6‐(p‐Sulfamoylphenyl)‐7‐oxoperhydroimidazo[1,5‐c]thiazole‐5‐thione (9a) was found to exhibit immunorestoration activity.