Interaction of Fused 1,2,4-Triazinone with Diborane/Oxidation: A New Route for the Synthesis of Partially Saturated and Aromatic Pyrazolo[5,1-c][1,2,4]triazines

Reactions of 8-carboxyethyl-4-oxo-3-tert-butyl-4,6-dihydropyrazolo[5,1-c][1,2,4]triazine and 8-butyryl-4-oxo-3-tert-butyl-4,6-dihydropyrazolo[5,1-c][1,2,4]triazine with B₂H₆ in Et₂O at 0°C led rapidly to the selective formation of 8-alkyl-substituted derivatives. Addition of boron trifluoride led to reduction of the triazine ring, with formation of 1,2,3,4-tetrahydropyrazolo[5,1-c][1,2,4]triazines. The latter were spontaneously oxidized by atmospheric oxygen, with formation of 1,4-dihydro derivatives, or could be converted to the corresponding aromatic pyrazolo[5,1-c][1,2,4]triazines using oxidative nitration/elimination sequence. These transformations represent a new route for the synthesis of previously inaccessible partially saturated and aromatic pyrazolo[5,1-c][1,2,4]triazines.