Asymmetric Induction in the Cycloaddition of a Masked p-Benzoquinone to Cyclic Nitrones

1,3-Dipolar cycloadditions of cyclic nitrones to achiral and chiral p-benzoquinone monoketals have shown poor chemo- and stereoselectivity. To overcome these problems, a highly efficient strategy has been set up on the basis of the temporary conjugate addition of thiophenol to the dipolarophile. The phenylthio derivative 10b, available in both enantiopure forms, has demonstrated to be effective as a masked chiral synthetic equivalent of p-benzoquinone in the model reaction.