[3 + 2] Cycloadditions of carbonyl oxides to imines: An alternative approach to the synthesis of 1,2,4-dioxazolidines

Carbonyl oxides, derived from the ozonolysis of vinyl ethers, readily undergo [3 + 2] cycloadditions with imines, affording the corresponding 1,2,4-dioxazolidines 6a-z in the isolated yields of 14-97%. High degrees of stereoselectivity are observed in certain cases, suggesting that this [3 + 2] cycloaddition proceeds by a concerted process. In competition experiments, a relative order of reactivity towards carbonyl oxides was tentatively deduced: nitrone > imine > thioketone > carbonyl compound. The structure of the 1,2,4-dioxazolidine 6d was unambiguously determined by the X-ray crystallographic analysis. 1,2,4-Dioxazolidines were found to undergo ring cleavage reactions on either thermolysis or treatment with base.