The 1,3-dipolar cycloaddition of cyclic nitronates (5,6-dihydro-4H-1,2-oxazine N-oxides) with cyclooctyne affords oxazine-annulated spiroaziridine derivatives in mild conditions with high yields and diastereoselectivity via consecutive strain-promoted 3 + 2-cycloaddition/ring contraction. Isoxazoline N-oxides and silyl nitronates also react smoothly, albeit with a lower diastereoselectivity. DFT calculations reveal key features, such as half-boat conformation of the oxazine ring during cycloaddition, and control of the stereoselectivity during ring contraction by an anomeric effect in the O-N-O moiety.
Maksakova et al. (Mon,) studied this question.