27 • Additional Problems
27 • Additional Problems
The following model is that of cholic acid, a constituent of human bile. Locate the three hydroxyl groups, and identify each as axial or equatorial. Is cholic acid an A–B trans steroid or an A–B cis steroid?
Propose a biosynthetic pathway for the sesquiterpenoid helminthogermacrene from farnesyl diphosphate.
Identify the following fatty acid, and tell whether it is more likely to be found in peanut oil or in red meat:
Propose a mechanistic pathway for the biosynthesis of caryophyllene, a substance found in clove oil.
Suggest a mechanism by which ψ-ionone is transformed into β-ionone on treatment with acid.
Isoborneol is converted into camphene on treatment with dilute sulfuric acid. Propose a mechanism for the reaction, which involves a carbocation rearrangement.
Fats, Oils, and Related Lipids
Plasmalogens are a group of lipids found in nerve and muscle cells. How do plasmalogens differ from fats?
Cardiolipins are a group of lipids found in heart muscles. What products would be formed if all ester bonds, including phosphates, were saponified by treatment with aqueous NaOH?
Terpenoids and Steroids
Assume that acetyl CoA containing a 14C isotopic label in the carboxyl carbon atom is used as starting material and that the mevalonate pathway is followed. Identify the positions in α-cadinol where the label would appear.
Assume that acetyl CoA containing a 14C isotopic label in the carboxyl carbon atom is used as starting material and that the mevalonate pathway is followed. Identify the positions in squalene where the label would appear.
Assume that acetyl CoA containing a 14C isotopic label in the carboxyl carbon atom is used as starting material and that the mevalonate pathway is followed. Identify the positions in lanosterol where the label would appear.
Flexibilene, a compound isolated from marine coral, is the first known terpenoid to contain a 15-membered ring. What is the structure of the acyclic biosynthetic precursor of flexibilene? Show the mechanistic pathway for the biosynthesis.
Draw the most stable chair conformation of dihydrocarvone.
Draw the most stable chair conformation of menthol, and label each substituent as axial or equatorial.
Propose a mechanistic pathway for the biosynthesis of isoborneol. A carbocation rearrangement is needed at one point in the scheme.
Digitoxigenin is a heart stimulant obtained from the purple foxglove Digitalis purpurea and used in the treatment of heart disease. Draw the three-dimensional conformation of digitoxigenin, and identify the two –OH groups as axial or equatorial.
Diethylstilbestrol (DES) has estrogenic activity even though it is structurally unrelated to steroids. Once used as an additive in animal feed, DES has been implicated as a causative agent in several types of cancer. Show how DES can be drawn so that it is sterically similar to estradiol.
Cembrene, C20H32, a diterpenoid hydrocarbon isolated from pine resin, has a UV absorption at 245 nm, but dihydrocembrene (C20H34), the product of hydrogenation with 1 equivalent of H2, has no UV absorption. On exhaustive hydrogenation, 4 equivalents of H2 react, and octahydrocembrene, C20H40, is produced. On ozonolysis of cembrene, followed by treatment of the ozonide with zinc, four carbonyl-containing products are obtained:
Propose a structure for cembrene that is consistent with its formation from geranylgeranyl diphosphate.
α-Fenchone is a pleasant-smelling terpenoid isolated from oil of lavender. Propose a pathway for the formation of α-fenchone from geranyl diphosphate. A carbocation rearrangement is required.
Fatty acids are synthesized by a multistep route that starts with acetate. The first step is a reaction between protein-bound acetyl and malonyl units to give a protein-bound 3-ketobutyryl unit. Show the mechanism, and tell what kind of reaction is occurring.
Propose a mechanism for the biosynthesis of the sesquiterpenoid trichodiene from farnesyl diphosphate. The process involves cyclization to give an intermediate secondary carbocation, followed by several carbocation rearrangements.