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

The four‐membered cyclic nitrones 1 and 2 react with dimethyl acetylenedicarboxylate to give the corresponding cycloadducts 3,4. Upon reaction of 2a with phenylacetylene, the 5′‐substituted isoxazoline derivative 5 was obtained. Reaction of 2a with methyl propynoate yielded a 1:4 mixture of the 4′‐ and 5′‐cycloadducts^δ, 7 and 6, respectively. Reaction of 2a‐c with N‐phenylmaleimide and of 2a with diethyl maleate yielded the corresponding azeto[1,2‐b]pyrrolo[3,4‐d]isoxazole derivatives, 8 and 9, respectively. The exclusive formation of the exo‐conformation of the cycloadducts was unequivocally proven by X‐ray analysis of 8a. Reaction of 2a with methyl 2‐propenoate (methyl acrylate) and 3‐buten‐2‐one (methyl vinyl ketone) gave regio‐ and stereoselectively the corresponding exo‐5′‐isoxazolidine cycloadducts, 10 and 11, respectively. The stereochemistry of the cycloadducts was deduced from the ¹H NMR data. Both alkyne and alkene dipolarophiles approach the four‐membered cyclic nitrones from the sterically less hindered side, resulting in a (regio‐ and) stereoselective formation of the cycloadducts obtained.