Reactivity of four-membered cyclic nitrones in 1,3-cycloaddition reactions. X-Ray crystal structure of 7-(2,6-dichlorophenyl)-N,N-diethyloctahydro-6-methyl-1,3-dioxo-2-phenylazeto[1,2-b]pyrrolo[3,4-d]isoxazole-6-carboxamide†‡