CHAPTER TWENTY-SIX Lipids oH O OPO3 *CHCO2H—*CH2CCH2 CSCOA—>* CH3CCH2 CSCOA—* CHaCCH2CH2OPP一 O CHCO.H CH—C H OPP OPP FIGURE 26.8 Diagram showing the distribution of the C label (C)in citronellal biosynthesized from acetate in which the methyl carbon was isotopically enriched with 1C H3CO2H一 Citronellal Figure 26.8 traces the C label from its origin in acetic acid to its experimentally deter mined distribution in citronellal PROBLEM 26.11 How many carbon atoms of citronellal would be radioactively labeled if the acetic acid used in the experiment were enriched with C at C-1 instead of at C-2? Identify these carbon atoms A more recent experimental technique employs C as the isotopic label. Instead of locating the position of a C label by a laborious degradation procedure, the C NMR spectrum of the natural product is recorded. The signals for the carbons that are enriched in C are far more intense than those corresponding to carbons in which c is present only at the natural abundance level Isotope incorporation experiments have demonstrated the essential correctness of the scheme presented in this and preceding sections for terpene biosynthesis. Consider- able effort has been expended toward its detailed elaboration because of the common biosynthetic origin of terpenes and another class of acetate-derived natural products, the steroids 26.11 STEROIDS: CHOLESTEROL Cholesterol is the central compound in any discussion of steroids. Its name is a combi- nation of the greek words for"bile"(chole)and"solid"(stereos)preceding the charac teristic alcohol suffix -ol. It is the most abundant steroid present in humans and the most important one as well, since all other steroids arise from it. An average adult has over 00 g of cholesterol; it is found in almost all body tissues, with relatively large amounts present in the brain and spinal cord and in gallstones. Cholesterol is the chief constituent of the plaque that builds up on the walls of arteries in atherosclerosis. Cholesterol was isolated in the eighteenth century, but its structure is so complex that its correct constitution was not determined until 1932 and its stereochemistry not Back Forward Main MenuToc Study Guide ToC Student o MHHE Website1034 CHAPTER TWENTY-SIX Lipids Figure 26.8 traces the 14C label from its origin in acetic acid to its experimentally deter￾mined distribution in citronellal. PROBLEM 26.11 How many carbon atoms of citronellal would be radioactively labeled if the acetic acid used in the experiment were enriched with 14C at C-1 instead of at C-2? Identify these carbon atoms. A more recent experimental technique employs 13C as the isotopic label. Instead of locating the position of a 14C label by a laborious degradation procedure, the 13C NMR spectrum of the natural product is recorded. The signals for the carbons that are enriched in 13C are far more intense than those corresponding to carbons in which 13C is present only at the natural abundance level. Isotope incorporation experiments have demonstrated the essential correctness of the scheme presented in this and preceding sections for terpene biosynthesis. Consider￾able effort has been expended toward its detailed elaboration because of the common biosynthetic origin of terpenes and another class of acetate-derived natural products, the steroids. 26.11 STEROIDS: CHOLESTEROL Cholesterol is the central compound in any discussion of steroids. Its name is a combi￾nation of the Greek words for “bile” (chole) and “solid” (stereos) preceding the charac￾teristic alcohol suffix -ol. It is the most abundant steroid present in humans and the most important one as well, since all other steroids arise from it. An average adult has over 200 g of cholesterol; it is found in almost all body tissues, with relatively large amounts present in the brain and spinal cord and in gallstones. Cholesterol is the chief constituent of the plaque that builds up on the walls of arteries in atherosclerosis. Cholesterol was isolated in the eighteenth century, but its structure is so complex that its correct constitution was not determined until 1932 and its stereochemistry not * CH3CO2H *  14C * * CH O * * * * 2 7 531 8 6 4 Citronellal *CH3CO2H O 3CCH * 2CSCoA 2CO2H *CH * O OH *CH3CCH2CSCoA O *CH * OPO *CH3CCH2 OPP 3 *CH CH2 2 C O O OPP * * * OPP * * * * * * H * * * O 2 FIGURE 26.8 Diagram showing the distribution of the 14C label (*C) in citronellal biosynthesized from acetate in which the methyl carbon was isotopically enriched with 14C. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website