The structure of the bicyclic monoterpene borneol is shown. Isoborneol, a stereoisomer of borneol, can be prepared in the laboratory by a two-step sequence. In the first step, borneol is oxidized to camphor by treatment with chromic acid. In the second step, camphor is reduced with sodium borohydride to a mixture of 85% isoborneol and 15% borneol. On the basis of these transformations, deduce structural formulas for isoborneol and camphor.
FIGURE 26.10
The biosynthetic conversion of squalene to cholesterol proceeds through lanosterol. Lanosterol is formed by a cyclization reaction of squalene-2,3-epoxide.
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Step 1: Squalene undergoes enzymic oxidation to the 2,3-epoxide. This reaction has been described earlier, in Section 16.14. Squalene On NADEL, enaye Squalene 2,3-epoxide Step 2: Cyclization of squalene 2,3-epoxide, shown in its coiled form, is triggered by ring opening of the epoxide. Cleavage of the carbon-oxygen bond is assisted by protonation of oxygen and by nucleophilic participation of the r electrons of the neighboring double bond. A series of ring closures leads to the tetracyclic carbocation shown. Squalene 2.3-epoxide Tetracyclic carbocation Step 3: Rearrangement of the tertiary carbocation formed by cyclization produces lanosterol. Two hydride shifts, from C-17 to C-20 and from C-13 to C-17, are accompanied by methyl shifts from C-14 to C-13 and from C-8 to C-14. A double bond is formed at C-8 by loss of the proton at C9 Tetracyclic carbocation formed in step 2 -Cont Step 4: A series of enzyme-catalyzed reactions converts lanosterol to cholesterol. The three highlighted methyl groups in the structural formula of lanosterol are lost via separate multistep operations, the C-8 and C-24 double bonds are reduced, and a new double bond is introduced at C-5 24 Lanosterol Cholesterol