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 B 2 H 6 in Et 2 O 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.