Di- and Tetrahydro-1,2,3-triazines

The C-C-C-N-N-N mode b of cyclization is represented by the cyclization of l-aryl-3-alkyl- (or aryl-) (3-hydro-xypropyO-triazenes giving 3,4,5,6-tetrahydro-l,2,3-triazinium salts 31 (Equation (103), Section 9.01.9) and by the preparation of l-alkyl-l,4,5,6-tetrahydro-l,2,3-triazines 30 (Equation 104). There is flexibility in the choice of substituents (alkyl, aryl). Highly functionalized 2,3,4,5-tetrahydro-l,2,3-triazin-4,5-diones (CA 2,3-dihydro-l,2,3-triazine-4,5-diones) 41 are available from the cyclization of 3-diazo-2-oxopropionic acid derivatives, but variation seems to be limited to the choice of the N-3 substituent (Equation 106). 

2-Aryl-2,5-dihydro-l,2,3-triazines, also with electron-withdrawing substituents, are available via 1,3-dipolar cycloaddition of electron-poor dipolarophiles (DMAD, maleonitrile, etc.) to 1,2,3-triazole 1-oxides 1987CC706, 1990J(P1)3321 or 1,2,3-triazol-l-ium imides (Equations (126) and (127) Section 9.01.10). The electronic properties of the aryl groups introduced with the triazolium precursor may vary widely. If alkenes are used as dipolarophiles, thermolysis of the adducts has to be combined with oxidation by manganese dioxide 1993JCM78 . 

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