A one-pot synthesis of 2-imino-1,3,4-thiadiazolines was successfully achieved under mild conditions. The developed synthesis involves Fe(Pc)-catalyzed aerobic oxidation of acylhydrazides followed by P(NMe 2 ) 3 -mediated annulation of the in situ generated N-acyldiazene with isothiocyanates. The present annulation showed broad substrate scope with good functional group tolerance, and was effective on gram scale.aerobic oxidation, heterocycle, one-pot synthesis, organic synthesis S C H E M E 1 Previously reported preparations of 2-imino-1,3,4-thiadiazoline scaffold and the developed one-pot process for 2-imino-1,3,4-thiadiazoline. (a) Traditional methods. (b) Chang's 7 work. (c) This work. Su Been Kim and Sang Eun Baek contributed equally to this study. Dedicated to Professor Sukbok Chang on the occasion of his 60th birthday

The framework of 1,3,4-oxadiazine is crucial for numerous bioactive molecules, but only a limited number of synthetic methods have been reported for its production. In 2015, Wang’s group developed a 4-(dimethylamino)pyridine (DMAP)-catalyzed [2 + 4] cycloaddition of allenoates with N-acyldiazenes, which provided an atom-efficient route for 1,3,4-oxadiazines. However, the practicality of this method was limited by the instability of N-acyldiazenes as starting materials. Building upon our ongoing research about the aerobic oxidation of hydrazides and their synthetic applications, we hypothesized that aerobic oxidative cycloadditions using acylhydrazides instead of N-acyldiazenes may provide a more practical synthetic route for 1,3,4-oxadiazines. In this manuscript, we describe a one-pot synthetic protocol for 1,3,4-oxadiazines from acylhydrazides and allenoates. The developed one-pot protocol consists of aerobic oxidations of acylhydrazides into N-acyldiazenes using NaNO2 and HNO3, followed by the DMAP-catalyzed cycloaddition of allenoate with the generated N-acyldiazenes. A variety of 1,3,4-oxadiazines were produced in good to high yields. In addition, the practicality of the developed method was demonstrated by a gram-scale synthesis of 1,3,4-oxadiazine.