5.5- Disubstituted 2,5-dihydro-1,2,3-triazines

Sodium borohydride reduction of 4,6-disubstituted 1,2,3-triazines in methanol afforded 2,5-dihydro-1,2,3-triazines (329) in good yields. 1,2,4,5-Tetrazines with mild reducing agents give dihydro derivatives. 

When the dihydro-1,2,4-triazinone (194) is heated at 180 °C under reducing conditions there is rupture of the 1,2-bond followed by ring closure, producing 4,5-diphenyl-imidazolinone (195). Such reductive ring closures of benzotriazine 1-oxides (196) can give benzimidazoles, in particular 2-(4-thiazolyl)benzimidazoles (197) ( thiabendazole ). Similar reactions take place with 1,3,5-triazines. In fact, primary amines cleave the compounds completely with evolution of ammonia and formation of iV.iV -disubstituted formamidines. With suitable primary amines, though, this reaction can be designed to produce imidazolines or benzimidazoles (Scheme 113). 

Similarly prepared from l,2-diamino-3-chlorocyclopropenylium ions 9 and 4,6-disubstituted 5-dialkylamino-l,2,3-triazincs 10 were the derivatives of l,2-diamino-3-(l,2,3-triazinium-2-yl)cyclopropcnylium dications 11, which upon hydrolysis gave the 1,2-diamino-3-(5-oxo-2,5-dihydro-l,2,3-triazin-2-yl)cyclopropenylium ions 12. " °... 

Dihydro-1,2,3,-triazines (129b,c) were also prepared in reasonable yield by sodium borohydride4 (in MeOH) or catalytic (H2, Pd-C) reduction211,212 of 4,6-disubstituted 1,2,3-triazine 2-oxides (128b,c). 

Anions of methylene-active compounds (1,3-dione enolates and ester enolates), ketene acetals, and even electron-rich five-membered heterocycles comprise another group of nucleophiles that attack triazine rings, preferably in the form of l,2,3-triazin-2-ium salts. 4,6-Disubstituted-2-methyl-l,2,3-triazinium iodides add malonic ester anion at the C-5 position to form 4,6-disubstituted-2-methyl-5-bis(ethoxy-carbonyl)methyl-2,5-dihydro-l,2,3-triazines in 57-76% yield <1992CPB2283, 1996CHEC-II(6)483>. The following cases have been published more recently. 

In addition to these cases, the oxidation of 4,6-disubstituted-2-methyl-2,5-dihydro-1,2,3-triazines 21q-s by I2 gives 2-methyl-l,2,3-triazinium iodides 114a-c <1990TL7193>, whereas the treatment of 21t-v with bromine followed by LTA generated the N-2-demethylated 1,2,3-triazines 17z and 188a and 188b (Equation 63) <1992CPB2283>. 

In addition to the cases of displacement of 5-amino functions in 2-EWG-substituted l,2,3-ttiazin-2-ium salts yielding 2-EWG-substituted 2,5-dihydro-l,2,3-ttiazin-5-ones (EWG = COOMe, 2,3-(dialkylamino)cyclopropenylium, picryl) <1979CB1535, 1984CHEC(3)369>, examples of hydrolysis of 4,6-disubstituted-5-dialkylamino-2-methyl-l,2,3-triazinium salts 89a and 89d-g generating 2-methyl-2,5-dihydro-l,2,3-triazin-5-ones 22c-g have been reported (Equation 79) 1985BCJ1073> (A), <2002HC0325> (B), <2003H(59)477> (C)). 

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