Aminoselenazoles

Hofmann (1), of the Zurich School, was the first to have tried unsuccessfully to prepare the unsubstituted parent compound, selenazole much later, in 1955, Metzger and Bailly (2) were equally unsuccessful in trying to prepare selenazole from 2-aminoselenazole by reduction of the diazo compound,... 

The replacement of selenoamide by selenourea in the Hantzsch s synthesis. (1st method) leads to 2-aminoselenazoles 2, 14. 15). This series of compounds has been well developed, mainlv because selenourea is much more easily accessible than the selenoamides, but also because a wide variety of a-halogenated carbonyl compounds are available for the Hantzsch s evdization reaction (Scheme 5). 2-Aminoselenazole itself was prepared from commercially available chloroacetaldehyde semihydrate... 

The hydrolysis of 2-benzamidoselenazoles in aqueous phosphoric or sulfuric acid gives 2-aminoselenazOles identical to those prepared from selenourea (Scheme 12). 

Aminoselenazoles are crystalline compounds, and with the exception of the nonsubstituted derivative (1) they are stable and practically odorless. They are more basic than 2-aryl- or 2-alkylselenazoles. The hydrochlorides are not easily hydrolyzed in aqueous solution. Some of the K-substituted compounds are oily liquids. 

From the point of view of reactivity, there is little difference betvv een 2-amino-selenazoles and aryl- Or alkyl-2-aminoselenazoles, except that the A"-arvl derivatives are generallv less basic and that their salts are more easily hydrolyzed. 

Thus reactions of the selenazole ring are mainly limited to the 5-position. especially for 2-aminoselenazoles and 2-hydrazinoselenazoles that undergo electrophilic substitution. 

In the reaction of 4-substituted 2-aminoselenazoles with ethyl propiolate and dimethylacetylene dicarboxylate. the major products obtained from such a condensation are substituted 7H-selenazolo[3,2-a]pyrimidin-7-ones (5) and not the alternative isomeric substituted 5H-selenazoles[3,2-a]pyrimidin-5-ones (6). Distinction between the alternative structures was based on infrared, ultraviolet, and NMR data (Scheme... 

Sulfonamides prepared from 2-aminoselenazoles by normal methods sometimes possess activity comparable to that of sulfathiazole (45, 47). 

The ability of various selenium heterocycles to check the loss of orthophosphate caused by irradiation of ATP has been studied by Brucker and Bulka (92). They found that only 2-amino-4,5-dimethyiselenazole shows radioprotective properties, while other 2-aminoselenazoles, selenosemicarbazides, and acetone selenosemicar-bazones possess no such activity but are in addition very sensitive to radiation (93). 

Amino-5-bromoselenazole is mentioned in a patent without further details. It is stated to be a starting material for pharmaceutical products. In analogy to the corresponding thiazole compounds, it is stated to be prepared by heating 2-aminoselenazole in aqueous hydrobromic acid under reflux and slow addition of an equivalent amount of bromine. 

The most important 2-aminoselenazoles. along with their physical properties are shown in Table X-2. 

In the reaction of 2-aminoselenazole with ethyl propiolate, in addition to compound type 7 (50% yield), compounds 8 and 9 were also isolated. Compound 7 is probably formed through the cis isomer of 8 (Scheme 55). 

Aminoselenazoles, electrophilic substitution in 5-position, 240 alkyl substituted in 4 and 5-positions, preparation. 224... 

Benzamidoselenazole, bromination, 243 2-Benzamidoselenazoles, hydrolysis to substituted selenoureas, 228 hydrolysis to 2-aminoselenazoles, 229 Benzothiazole, 30... 

Ortho esters, in synthesis of symmetrical trimethine thiazolocyanines, 54 Oxazolone, for neutrocyanines, 27 Oxidation potentials, of dyes, 75 of mesosubstituted dyes, in relation with absorption, 77 of polymethine dyes, 72 Oxidoreduction, relation between sensitizers, and silver halides, 78 4-Oxo-disubstituted 2-aminoselenazoles, table of products, 262 Oxonols, nomenclature of, 26 in synthesis of dimethine neutrocyanines, 62... 

Aminoselenazoles (1) with DMAD gave 7f/-selenazolo[3,2-a]pyri-dinidin-7-ones (2, R = E), whereas EP yielded 2 (R = H) and the Michael-type adducts 3 and 4.590... 

Direct bromination of 2-aminoselenazoles affords the corresponding 5-bromo derivatives, stable only as hydrobromide salts. Diazonium salts also react with selenazoles to give the 5-azo derivatives. 

Selenourea and a-halocarboxyl compounds react similarly to give 2-aminoselenazoles (50) (53CR(237)906). Selenourea and ethyl bromopyruvate or methyl formylchloroacetate afford aminoselenazoles (51) and (52), respectively (75JHC675). Deamination of 4,5-diphenyl-2-aminoselenazole (53) to the 2-unsubstituted compound (57) is outlined in Scheme 21 (75CS(8A)39>. 

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