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Highly functionalized quinolines and pyridines could be synthesized by BF3⋅OEt2-mediated reactions of vinyl azides with N-aryl and N-alkenyl aldimines, respectively. The reaction mechanism could be characterized as formal [4+2]-annulation, including unprecedented enamine-type nucleophilic attack of vinyl azides to aldimines and subsequent nucleophilic cyclization onto the resulting iminodiazonium ion moieties.

Triflimide (Tf2NH) specifically catalyzed reactions of alcohols and vinyl azides, enabling efficient construction of amides with C–C bond formation through nucleophilic attack of vinyl azides onto the putative carbocation intermediates derived from alcohols are described.

The enamine-type vinyl azides attack carbon electrophiles such as imines (II), aldehydes (IV), and carbocations, which are generated in situ from alcohols (XV) in the presence of BF3·OEt2.

Concise synthesis of 1,2,4-oxadiazoles was achieved by heating N-benzyl amidoximes with K3PO4 in DMF at 60 °C under an O2 atmosphere via benzylic C–H oxygenation. On the other hand, aerobic treatment of N-benzyl amidoximes with Cs2CO3 in DMSO at 100 °C could result in oxidative skeletal rearrangement to deliver quinazolinones as a major product. This orthogonal product selectivity could be realized by difference of the reaction temperature as well as selection of the solvents and inorganic bases.

This method opens access towards highly substituted isoxazole and pyrazole skeletons under transition metal-free conditions.

Oxidative skeletal rearrangement of 5-aryl-4,5-dihydro-1,2,4-oxadiazoles into quinazolinones is induced by molecular oxygen (under a dry air atmosphere) that likely proceeds via transient iminyl radical species. Concise syntheses of biologically active quinazolinone derivatives were demonstrated using the present strategy.

The conditions which are used for the aerobic oxidation process can also be applied to the direct C-3 formylation.