Transition-metal-free [3 + 2] cycloaddition of C,N-cycloazomethylimines with in situ formed isocyanates from dioxazolones: a facile synthesis of triazolinones

A facile and novel method was established for the synthesis of triazolinone scaffolds through [3 + 2] cycloaddition of C,N-cycloazomethylimines with dioxazolones in the presence of CH3ONa. In these transformations, isocyanates were formed in situ by the decarboxylation and Curtius rearrangement of dioxazolones. Importantly, the isocyanate formed from dioxazolone is employed for the first time for the cyclization instead of the widely used N-acyl nitrenes. The newly developed [3 + 2] cycloaddition is applicable to a broad range of substrates with high functional group tolerance, providing various triazolinones in good to excellent yields (up to 94%). Additionally, this method is easy to operate, green, environmentally friendly, and avoids highly toxic reagents.