N-Substituted cyclododecanone thiosemicarbazones Ⅰ, prepared from cyclododecanone, undergo oxidative cyclisation on treatment with manganese dioxide giving 2-(1,11-undecylidene) -5-substituted imino-△ ^3-1,3,4-thiadiazolines Ⅱ. The structures of Ⅱ were confirmed by IR, 13^C NMR and elemental analysis. A by-product was separated from the reaction mixture and its structure was assigned as 2-(1,11- undecylidene)-5-imino- △ ^3-1,3,4-thiadiazoline Ⅲ by IR, 13^C NMR, EIMS and elemental analysis. A two-step reaction mechanism for the oxidative cyclisation of Ⅰ on manganese dioxide was proposed to explain the formation of Ⅱ and Ⅲ.

Key words: THIOSEMICARBAZONE P, DODECAMEMBER RING COMPOUNDS, MANGANESE DIOXIDE, OXIDATION REACTION, SYNTHESIS METHODS, CYCLIZATION, REACTION MECHANISM, THIADIAZOLE P