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Suggested solutions for Chapter 40 40 Problem 1 to Suggest mechanisms for these reactions. H Br CHBr₃ OMe BuLi t-BuOK Br Purpose of the problem Two simple carbene reactions initiated by base. Suggested solution Going to the right we must remove the rather acidic proton from CHBr₃ to give the carbanion. This bromide ion to give dibromocarbene and insertion into cyclohexene gives the product. Br Br Br Br H CBr₃ Br : Br Br Br Br The second reaction is very similar. α-Elimination of HCl gives a carbene that inserts into the couble bond. These are the simplest types of carbene reactions and are very common. H H H H H H OMe OMe OMe OMe Problem 2 Suggest a mechanism and explain the stereochemistry of this reaction. CuSO₄ CO₂Me 0 CO₂Me Purpose of the problem Another important carbene method used in the synthesis of a natural antibiotic. stract Suggested solution The diazo-compound decomposes to gaseous nitrogen and a carbene under catalysis by Cu(II). This reaction established the skeleton of cycloeudesmol (margin) Insertion into the exposed alkene gives the three-membered ring. The stereochemistry comes partly and was carried out by E. Y. Chen, from the 'tether' - the linkage between the carbene and the rest of the molecule that delivers the Tetrahedron Lett., 1982, 23, 4769, carbene to the bottom face of the alkene. The rest comes from the inevitable cis fusion between the at Sandoz. six- and three-membered rings. 0 CO₂Me 0 CO₂Me CO₂Me cycloeudesmol