(SS)-2-(p-tolylsulfinyl)norborneno-p-benzoquinones: A new type of facially perturbed enantiopure quinones

The syntheses and asymmetric Diels-Alder reactions of (SS)-2-(p-tolylsulfinyl)norboreno-p-benzoquinones 10 and 11 with cyclopentadiene are reported. The cycloadditions allowed the highly stereoselective obtention of the four possible endo adducts 12-15, optically pure synthetic equivalents of norborneno-p-benzoquinone-cyclopentadiene bisadducts. The detailed study of the H-1-NMR spectra of the adducts pointed out the anisotropic effects exerted by the sulfinyl moiety on the spectroscopic behavior of these rigid systems. In all cases, the pi-facial selectivities were fully controlled by the sulfinyl group being possible to reverse the diastereoselection in thermal conditions and in the presence of ZnBr2. The stereoselective synthesis of the cage compound 5, precursor of garudane, was achieved from cycloadduct endo-syn-13.