Selenazolium salts

Alkaline deacylation of 2-acyl-4-aryl-l,3-selenazoles (46) results in removal of the acyl group, and the parent compounds (47) are produced in good to fair yields (Scheme 11) [76], The selenazolium salts are activated toward nucleophiles at the 2-position. The salts (48) react with DMSO-d6 to give the 2-deuteroxy derivatives (49) (Scheme 12) [77],... 

Alkylthio-selenazoles are alkylated on nitrogen using Meerwein salts to produce the corresponding 2-alkylthio-3-methyl-l,3-selenazolium salts. The alkylthio group in these salts can be replaced with selenium using NaHSe to afford 3-methyl-l,3-selenazole-2-selenone 70 (Scheme 5) <2003NJC1622>. 

The chemical shifts of the protons of the methyl groups of A-methyl-benzo-selenazolium salts have been compared with those for the protons of the methyl groups in similar compounds. Benzoselenazole, tritiated in the 2-position, has been prepared, and the triton chemical shift has been shown to be the same as that of the corresponding proton chemical shift. Benzoselenazol-2-yI 2-formyl-fur-5-yl sulphide has been synthesized from potassium benzoselenazole-2-... 

Carbocyanines with three methine groups can be prepared using Pvo equivalents of selenazolium quaternary salt and one equivalent of ethyl orthoformate in pyridine solution (53). 

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],... 

Selenaaromatic rings in selenazolium and selenopyrylium salts are transformed to other rings in reactions other then cycloaddition. Examples are conversion of 1,2,4-selenadiazolium salt (67) to 1,3,5-selenadiazine (68) (Scheme 17), oxidative ring contraction of selenopyrylium salt (81) to selenophene (82), or replacement of selenium in the 2-selenachromylium salts (83) by two nitrogen atoms resulting in formation of benzodiazepines (84) (Scheme 21) [116],... 

Another approach for the synthesis of the isoselenazolium salt 88 is based on the formation of one Se-N bond by cyclization of 3-(methylselanyl)prop-2-enamide 87 using phosphorus pentachloride (Scheme 30) <1993SUL55>. Earlier, in a similar manner, starting from iV-alkyl- or iV-phenyl-2-(methylselanyl)benzamides, the 3-chlorobenziso-selenazolium chlorides 20 (Scheme 3) were obtained in 80-90% yield <1976BSF1124>. 

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