Cyclization electrocydization

The reactions of alkenes and related compounds are grouped here into nine sections. The first five deal essentially with photoisomeriza-lion processes—geometrical isomerization about a carbon-carbon double bond, concerted (electrocydic) cyclization, concerted shifts (usually of hydrogen) along the ir-system, the di-jr-methane reaction. 

Ketone semicarbazones undergo acid catalyzed reversible cyclization to 3,3-disubstituted 1,2,4-triazolidinones (76JHC1257). The only example available in the literature where a fused triazoline was obtained involves the nitrosation of ethyl 2-isopropylidenehydrazonothiazole-4-acetate, resulting in the formation of a 3,3-dimethyl-2H-s-triazolo[3,4-f ]thiazole (Scheme 14) (78JHC40l).The reaction may be classified as a 1,5-electrocydization. 

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