736 LIPIDS Reduction of camphor with sodium borohydride gives a mixture of stereoisomeric alcohols, of which one is borneol and the other isoborneol oH Borneol Isoborneol 26.11 Figure 26.8 in the text describes the distribution ofC(denoted by *)in citronellal biosynthesized from acetate enriched withC in its methyl group CH.H CH If, instead, acetate enriched with C at its carbonyl carbon were used, exactly the opposite distribu- tion of the C label would be observed CH- COH WhenCH, CO, H is used, C-2, C-4, C-6, C-8, and both methyl groups of citronellal are labeled. When CH3CO, H is used, C-1, C-3, C-5, and C-7 26.12 (b) The hydrogens that migrate in step 3 are those at C-13 and c-17(steroid numbering) As shown in the coiled form of squalene 2, 3-epoxide, these correspond to hydrogens at C-14 and C-18(systematic IUPAC numbering) (c) The carbon atoms that form the C, D ring junction in cholesterol are C-14 and c-15 of squalene 2,3-epoxide. It is the methyl group at C-15 of squalene 2, 3-epoxide that becomes the methyl group at this junction in cholesterol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
736 LIPIDS Reduction of camphor with sodium borohydride gives a mixture of stereoisomeric alcohols, of which one is borneol and the other isoborneol. 26.11 Figure 26.8 in the text describes the distribution of 14C (denoted by *) in citronellal biosynthesized from acetate enriched with 14C in its methyl group. If, instead, acetate enriched with 14C at its carbonyl carbon were used, exactly the opposite distribution of the 14C label would be observed. When 14CH3CO2H is used, C-2, C-4, C-6, C-8, and both methyl groups of citronellal are labeled. When CH3 14CO2H is used, C-1, C-3, C-5, and C-7 are labeled. 26.12 (b) The hydrogens that migrate in step 3 are those at C-13 and C-17 (steroid numbering). As shown in the coiled form of squalene 2,3-epoxide, these correspond to hydrogens at C-14 and C-18 (systematic IUPAC numbering). (c) The carbon atoms that form the C, D ring junction in cholesterol are C-14 and C-15 of squalene 2,3-epoxide. It is the methyl group at C-15 of squalene 2,3-epoxide that becomes the methyl group at this junction in cholesterol. HO H CH3 O H H H 14 15 18 HO H H 13 17 * * * * CH O * CH3CO2H CH3CO2H * CH * * O * * * * NaBH4 O OH H Borneol H OH Camphor Isoborneol Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
LIPIDS 737 (d) The methyl groups that are lost are the methyl substituents at C-2 and C-10 plus the methyl group that is C-1 of squalene 2, 3-epoxide O 26.13 Tracking theC label ofCH,CO, H through the complete biosynthesis of cholesterol requires a systematic approach. First, by analogy with Problem 26.11, we can determine the distribution of C (denoted by *)in squalene 2.3-epoxid Next, follow the path of the C-enriched carbons in the cyclization of squalene 2, 3-epoxide to Lanosterol Cholesterol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) The methyl groups that are lost are the methyl substituents at C-2 and C-10 plus the methyl group that is C-1 of squalene 2,3-epoxide. 26.13 Tracking the 14C label of 14CH3CO2H through the complete biosynthesis of cholesterol requires a systematic approach. First, by analogy with Problem 26.11, we can determine the distribution of 14C (denoted by *) in squalene 2,3-epoxide. Next, follow the path of the 14C-enriched carbons in the cyclization of squalene 2,3-epoxide to lanosterol. *** ***** * * * * * * * * * * O then on to cholesterol * * * * * * * * * * * * * * * * HO * * * * Lanosterol * * * * * * * * * * * * * * HO * * Cholesterol * * * * * * * * HO * * * * * * * *** * * * * * * * * * ****** O O H 2 10 CH3 CH3 CH3 LIPIDS 737 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website