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310 Organic Chemistry Solutions Manual T. Machiguchi et al., J. Chem. But we can't use diagrams like that for mechanisms! The cycloaddition uses maleic anhydride as Soc., Chem. Commun., 1973, 196. two-electron component with a low LUMO. We could use one of the dienes in the ring to provide four electrons but their ends are far apart and the electron-deficient ring is a poor HOMO. If we include the sulfur atom we can provide eight electrons and an atom (S) with a large HOMO coefficient. The tricyclic product is clearly folded back on itself so that the triene in the seven- membered ring and the carbonyl groups in the anhydride are close. There must be an endo effect in [8 + 2] cycloadditions. S H 0 [8 2] 0 cycloaddition 0 H 0 H 0 Problem 15 This unsaturated alcohol is perfectly stable until it is oxidized with Cr(VI): it then immediately cyclizes to the product shown. Explain. H Cr(VI) H 0 Purpose of the problem Discovery of a common effect in intramolecular cycloadditions. Suggested solution J.-L. Gras and M. Bertrand, If the starting material were heated it would no doubt undergo a Diels-Alder reaction but the diene Tetrahedron Lett., 1979, 4549. and dienophile are poorly matched. Both have high-energy HOMOs but there isn't a low-energ LUMO in sight. Once the enone is formed, the energies match well and cycloaddition is fast. The 0 stereochemistry comes from an endo arrangement (diagram in frame in margin). H H H OH Cr(VI) H 0 0 Problem 16 Suggest mechanisms for these reactions and comment on the stereochemistry of the first product. 0 MeO 0 + 0 0 MeOH OH 0