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A tandem CuAAC/ring cleavage/4 + 2 annulation reaction of terminal ynones, sulfonyl azides, and oximes has been developed to synthesize functionalized dihydrooxazines under mild conditions. In particular, intermediate N-sulfonyl acylketenimines are the first example of a 4π-system participating in 4 + 2 cycloadditions, and dihydrooxazines can convert to 2-aminopyridines through ring cleavage under basic conditions.
Yang et al. (2021) studied this question.