Selenazole 2,4-dimethyl

Selenazole, 2-aryl-4-chloromethyl-hydrolysis, 6, 344 Selenazole, 2-diethylamino-nitration, 6, 341 Selenazole, 2,4-dimethyl-oxidation, 6, 341 Selenazole, 2-hydrazino-oxidation, 6, 342 Selenazoles, 6, 339-347 electrophilic substitution, 6, 340 hetero fused synthesis, 6, 344 mass spectra, 6, 340 mesoionic... 

The 2-[2-thienyl]selenazole is formylated in the 5-position by action of n-butyllithium. dimethyl formamide. and hydrolysis (106). 

TTie reaction of 2.4-dimethylselenazole with an excess of benzaldehyde gives a monostyryl derivative exclusively (12). Oxidation of 2,4-dimethyl-selenazole with selenium dioxide yields 4-methylselenazole (Scheme 30) (4). 

Nitration in the 5-position of 4-methyl- and 2,4-dimethylselenazoles with HNO3-H2SO4 is more rapid than for thiazoles [4-methyl-5-nitroselenazole. m.p. 45°C (19) 2.4-dimethyl-5-nitroselenazole. m.p. 115-120 C (decomp.) (19)]. Direct nitration of 2-amino 4-methyl-selenazole leads to nng rupture (19). 

Reactivity of 2-Amino Selenazoles with Ethyl Propiolate (54) and Dimethyl Acetylene Dicarboxylate... 

Under comparatively mild conditions, already by the action of warm nitric-sulfuric acid, 4-methyl- and 2,4-dimethyl-selenazole were nitrated in the 5-position. 4-Methyl-5-nitroselenazole has mp 45°C, and the 2,4-dimethyl analog has mp 115-120°C (decomp. 

The halogenation of selenazoles goes less smoothly than the nitration and sulfonation. For example the bromination of 2,4-dimethyl-selenazoie with cold bromine first gives an unstable monobromo derivative (mp 168°C). This is transformed easily into a compound of mp 205°C (decomp.) which Haginiwa assumes is the hydrogen bromide salt of 2,4-dimethyl-5-bromoselenazole. 

Because of the basic nitrogen atom, alkyl-selenazoles form quaternary salts. 2,4-Dimethyl-3-ethylselenazolium iodide (mp 157-158 0) was prepared by Brooker et al in 87% yield as colorless crystals by heating of 2,4-dimethylselenazole in excess ethyl iodide for 2 days. By reaction with the corresponding quaternary salts, the following cyanine dyes (8) were prepared" l 3-diethyl-4-methylselenazolo-2 -... 

Alkyl selenazoles have been sulfonated. Thus 2,4-dimethylselenazole with fuming sulfuric acid (5% S03) at 100°C gives 2.4-dimethyl-5-sulfonylselenazole [m.p. 238°C (decomp.) (26)], which is relatively unstable. decomposing to give metallic selenium. 

The quaternary salts of selenium-nitrogen heterocycles are labile to nucleophiles and can be converted to other heterocyclic systems by ring expansion [82, 103], An example is conversion of 1,2,4-selenadiazolium trifluoromethane sulfonate (67) into 1,3,5-selenadiazine (68) (Scheme 17) [104], 2,3-Dimethyl-1-benzo-l, 3-selenazolium tetrafluoroborate is readily condensed with aromatic aldehydes to 2-styrylselenazole [105] or treated with sodium hydride to give 3-methyl-2-methylene-2,3 -dihydro-1 -benzo-1,3 -selenazole [106],... 

Synthesis of 1,3-selcnazole derivatives has been achieved by reactions of dimethyl A -(ethoxycarbonylmethyl)iminodithio-carbonate with selenothioethanoic acid 3 -ester < 1997JA8592>. Nucleophilic attack of the carbanion derived from dimethyl A -(ethoxycarbonylmethyl)iminodithiocarbonate at the C=Se group, followed by the elimination of -butyl thiolate, affords an intermediate which forms the selenazole by intermolecular nucleophilic attack (Scheme 11) <2003NJC1622>. 

Reactions of primary selenoamides with dimethyl acetylenedicarboxylate afford 5-methoxycarbonylmethylene-4,5-dihydro-l,3-selenazol-4-ones 75 (Equation 17). Reactions with acetylenedicarboxylic acid in ethanol give 4-ethoxy-4-hydroxy-5-carboxymethylene, 5 dihydro-l,3-selenazoles 76 (Equation 18) <2006JHC79>. 

The meso-ionic selenazole (120) is an extremely reluctant dipolarophile, but when heated under reflux for one week with dimethyl acetylenedicarboxylate in benzene it gives a pyridone, presumably by loss of selenium from the intermediate (127). ... 

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