[1,2,4]Triazines. 4 . Regiospecific ring closure reactions involving 6‐amino‐5‐hydrazino[1,2,4]triazin‐3(2 H )‐ones and orthoesters

6‐Amino‐5‐hydrazino[1,2,4]triazin‐3(2 H )‐one, 3 , was treated with a variety of orthoesters and concentrated acid forming mixtures of ring‐closed and open‐chain derivatives. These mixtures were converted in warm aqueous acid to the Mriazolo[5,1‐ d ][1,2,4]triazin‐3(2 H )‐ones, 11b, 14b , and 15b by a regiospecific ring closure at N‐4 of the [1,2,4]triazine ring and subsequent Dimroth‐like rearrangement of the initially formed s ‐triazolo[4,3‐ d ][1,2,4]triazin‐3(2 H )‐ones ( 11a, 14a, 15a ).