Dipolar [3+2] Cycloaddition of Nitrones to 3-Aroylpyrrolo[2,1-c][1,4]benzothiazine-1,2,4-triones

The dipolar [3+2]-cycloaddition reactions of 3-aroylpyrrolo[2,1-c][1,4]benzothiazine-1,2,4-triones with diarylnitrones lead to the formation of benzo[b]isoxazolo[5',4':2,3]pyrrolo[1,2-d][1,4]thiazines. The reaction proceeds stereoselectively to form exclusively the (2aS*,3R*,5aS*) diastereomers. Density functional theory computations for the model reaction showed that the (2aS*,3R*,5aS*) diastereomers are both kinetically and thermodynamically preferred over the alternative (2aS*,3S*,5aS*) diastereomers. At room temperature in solution, the resulting cycloadducts dissociate into the starting reagents, with stability highly dependent on the electronic properties of the substituents in the nitrone moiety.