Condensed Naphtho[1,8-de][1,3]thiazines 2. An Orientating Structure Modification of Naphtho[1,8-de][1 ,2,4]triazolo[3,4-b][1,3]thiazine Ring System

Treating 2-hydrazino[1,8-de][1,3]thiazine (3) with sodium nitrite in acetic acid solution under cooling afforded naphtho[1,8-de]tetrazolo[5,1-b][1,3]thiazine (4a) in 90% yield. On the other hand, refluxing 3 and methyl pyruvate (5a) neatly gave only the hydrazone intermediate 6a. Because of the possible occurrence of a side reaction, more vigorous process could not be used to ensure the ring-closure of 6a. More preferably, lower hydrazone homologs of 6a were easily prepared by the reaction of 3 and the corresponding ketones under mild conditions in 91-99% yields.