Oxadiazole derivatives R 0290Isothiazolo[3,2-b]-1,3,4-oxadiazole-5,5-dioxide: Synthesis of a New Heteropentalene System. -Cyclocondensation of 2-methyl-thiocyanato-2-butenal with aroyl hydrazides gives the novel azomethine imines (III). Their oxidation with hydrogen peroxide is studied under different reaction conditions. Electrocyclization of hydrosultams (V) derived from oxidation products (IV) provides hitherto unknown heteropentalenes (VI). -(SCHMIDT, S.; KOLBERG, A.; HENNIG, L.; HUNGER, J.; SCHULZE*, B.; Heterocycles 65 (2005) 11, 2705-2720; Inst. Org. Chem., Univ. Leipzig, D-04103 Leipzig, Germany; Eng.) -Mais 13-141