Divergent Annulations of 5,6-Dihydro-4H-1,2-Oxazine N-Oxides and Enol Diazoacetates for the Switchable Chemoselective Synthesis of Fused 1,2-Oxazine Derivatives

Reactions of 5,6-dihydro-4H-1,2-oxazine N-oxides with enol diazoacetates were studied. A particular reaction path depends on the amount of catalyst and the order of the addition of the substrates. Use of Rh2(Oct)4 (2 mol.%) leads to chemoselective [3+3]-annulation producing 1,2-oxazine-fused 1,2-oxazine derivatives. With lower catalyst loadings (0.03 mol.%) enol diazoacetates are converted to cyclopropene derivatives, which in situ react with 1,2-oxazine N-oxides via tandem [3+2]-cycloaddition-rearrangement producing oxazine-fused aziridines. Both transformations showed a wide substrate scope and produced target products with good yields and diastereoselectivity, thus allowing selective preparation of these rare heterocyclic systems. A mechanistic rationale for observed chemo- and stereo- selectivities was proposed.