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Chapter 35 Suggested solutions for Chapter 35 309 Purpose of the problem and application of photochemical [2 + 2] cycloadditions. Suggested solution of the two starting materials could absorb the light to provide the SOMO for the CO₂Me This does not affect the stereochemistry of the reaction. There is no endo effect in CO₂Me 2] photocycloadditions so the molecules simply come together with the rings arranged in an fashion to give the least steric hindrance (diagram in frame in margin). Me H CO₂Me NaBH₄ CO₂Me MeO₂C H H 0 0 0 0 The stereochemistry of the reduction is easy to explain as the molecule is folded in such a way that only the bottom face of the carbonyl group is open to nucleophilic attack. The oxyanion can immediately cyclize to form the lactone. Clearly, this is possible only if the group up, but it is also possible only if the groups are on this side of the molecule. The formation of the lactone does prove the stereochemistry of the cycloaddition. MeO₂C 0 CO₂Me .CO₂Me .CO₂Me CO₂Me OMe H 0 0 H BH₃ 0 Problem 14 Thioketones, with a C=S bond, are not usually stable as we shall see in Chapter 46. However, this thioketone is quite stable and undergoes reaction with maleic anhydride to give the product shown. Comment on the stability of the starting material, the mechanism of the reaction, and the stereochemistry of the product. S H 0 S H 0 H Purpose of the problem Exploration of a new structure, revision of aromaticity, and encounter with [8 + 2] cycloadditions. Suggested solution This particular thioketone is stable because the C=S bond is very polarized by delocalization making the seven-membered ring an aromatic cation with six electrons in it. Though all the diagrams are correct, the one in the frame (margin) gives a better representation of the structure. S etc. S S S