4- -1,2,3-selenadiazoles

The synthesis of 1,2,3-selenadiazoles by the action of selenium dioxide on semi-carbazones, and their pyrolysis to the corresponding acetylenes (see Vol. 2, p. 720 Vol. 3, p. 673) has been applied to several examples incorporating steroid, phenanthrenyl, fluorenyl, and naphthyl residues. The method may prove useful as a route to pharmacologically active 17-ethynyl-steroids. A large number of aryl-1,2,3-selenadiazoles as well as the three isomeric 4-[(2-, 3-, and 4-)pyridyl]- 

3- selenadiazoles have been similarly obtained. The n.m.r. spectra of aryl- 

Synthesis.—As in the past, the majority of the synthetic routes to 1,2,4-thia-diazoles are ultimately based on ring-closures involving suitably placed imino- and mercapto-groups (Type C). Additional types of syntheses, classified according to the fragments from which the hetero-ring is constructed (see Vol. 3, p. 679) are listed as methods F and G. 

Kutschy, P. Kristian, M. Dandarova, and J. Kovac, Collect. Czech. Chem. Commun., 1981, 46, 1160. 

3-selenadiazoles have been recorded. Coupling constants 

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The above examples illustrate reactions at an electron-deficient carbon atom. Other 1,1-bielectrophiles allow the direct introduction of a heteroatom into the resultant heterocycle. The most widely applicable and versatile methods for the synthesis of 1,2,5-thiadiazoles and 1,2,5-selenadiazole rely on this approach. 

JMC1164), and when (148) was treated with sulfur monochloride, the aromatic system (150) was obtained (81JOC4065). The corresponding 1,2,5-selenadiazole was isolated when (148) was heated with selenium dioxide (52% yield). 

The 3-chloroacrylaldehyde (218) (Type 1, Scheme 6) reacted readily with 2-amino-l,3,4-selenadiazole (219). The final product was the [l,3,4]selenadiazolo[3,2-a]pyridinium salt (220) (76ZC337X... 

Benzoselenolo[2,3-ii]benzoselenophenes synthesis, 4, 116, 1078 Benzoselenolopyrylium cations oxidative ring contraction, 4, 969 [l]Benzoselenolo[3,4-ii][l,2,3]selenadiazoles synthesis, 6, 354 Benzo[h]selenophene, 2-acetyl-synthesis, 4, 966 Benzo[h]selenophene, 2-bromo-acylation, 4, 948... 

Bis(pyridiniumthio)quaternary compounds gastric acid secretion and, 2, 519 Bis(selenadiazoles) decomposition, 6, 349... 

Naphthoquinone 1-methide thermal dissociation, 3, 785 N aphtho[2,3 -d ][2,1,3]selenadiazole... 

Phenanthro[l,2-d][l,2,3]selenadiazole, 10,11 dihydro- H NMR, 6, 348 synthesis, 6, 353 Phenanthro[b]thiophenes synthesis, 4, 914 Phenanthro[4,5-bcd]thiophenes synthesis, 4, 883, 907, 914 Phenanthro[9,10-ej[l, 2,4]triazines synthesis, 3, 434 Phenarsazin synthesis, 1, 561 Phenazine dyes, 3, 196-197 nitration, 3, 177 UV Spectra, 2, 127 Phenazine, 3-amino-2-hydroxy-in colour photography, 1, 374 Phenazine, 1-chloro-nucleophilic substitution, 3, 164-165 5-oxide... 

Selenadiazole-4-carboxylic acid ethyl ester photolysis, 6, 351... 

Selenadiazole-5-carboxylic acid, 4-methyl-ethyl ester... 

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