5 • Additional Problems
5 • Additional Problems
Which of the following structures are identical? (Green = Cl.)
Assign R or S configuration to each chirality center in pseudoephedrine, an over-the-counter decongestant found in cold remedies (blue = N).
Chirality and Optical Activity
Which of the following objects are chiral?
(a) A basketball (b) A fork (c) A wine glass (d) A golf club (e) A spiral staircase
(f) A snowflake
Erythronolide B is the biological precursor of erythromycin, a broad-spectrum antibiotic. How many chirality centers does erythronolide B have? Identify them.
Assigning Configuration to Chirality Centers
Assign R or S configurations to the chirality centers in ascorbic acid (vitamin C).
Xylose is a common sugar found in many types of wood, including maple and cherry. Because it is much less prone to cause tooth decay than sucrose, xylose has been used in candy and chewing gum. Assign R or S configurations to the chirality centers in xylose.
On reaction with hydrogen gas in the presence of a platinum catalyst, ribose (Problem 5-54) is converted into ribitol. Is ribitol optically active or inactive? Explain.
One of the steps in fat metabolism is the hydration of crotonate to yield 3-hydroxybutyrate. The reaction occurs by addition of – OH to the Si face at C3, followed by protonation at C2, also from the Si face. Draw the product of the reaction, showing the stereochemistry of each step.
The dehydration of citrate to yield cis-aconitate, a step in the citric acid cycle, involves the pro-R “arm” of citrate rather than the pro-S arm. Which of the following two products is formed?
The first step in the metabolism of glycerol, formed by digestion of fats, is phosphorylation of the pro-R – CH2OH group by reaction with adenosine triphosphate (ATP) to give the corresponding glycerol phosphate plus adenosine diphosphate (ADP). Show the stereochemistry of the product.
One of the steps in fatty-acid biosynthesis is the dehydration of (R)-3-hydroxybutyryl ACP to give trans-crotonyl ACP. Does the reaction remove the pro-R or the pro-S hydrogen from C2?
The naturally occurring form of the amino acid cysteine (Problem 5-63) has the R configuration at its chirality center. On treatment with a mild oxidizing agent, two cysteines join to give cystine, a disulfide. Assuming that the chirality center is not affected by the reaction, is cystine optically active? Explain.
Assign R or S configurations to the chiral centers in cephalexin, trade-named Keflex, the most widely prescribed antibiotic in the United States.
Chloramphenicol, a powerful antibiotic isolated in 1947 from the Streptomyces venezuelae bacterium, is active against a broad spectrum of bacterial infections and is particularly valuable against typhoid fever. Assign R or S configurations to the chirality centers in chloramphenicol.
Allenes are compounds with adjacent carbon–carbon double bonds. Many allenes are chiral, even though they don’t contain chirality centers. Mycomycin, for example, a naturally occurring antibiotic isolated from the bacterium Nocardia acidophilus, is chiral and has [α]D = −130. Explain why mycomycin is chiral.
Long before chiral allenes were known (Problem 5-68), the resolution of 4-methylcyclohexylideneacetic acid into two enantiomers had been carried out. Why is it chiral? What geometric similarity does it have to allenes?
We’ll see in Chapter 11 that alkyl halides react with hydrosulfide ion (HS−) to give a product whose stereochemistry is inverted from that of the reactant.
Draw the reaction of (S)-2-bromobutane with HS− ion to yield 2-butanethiol, CH3CH2CH(SH)CH3. Is the stereochemistry of the product R or S?
Ketones react with sodium acetylide (the sodium salt of acetylene, Na+– : CCH) to give alcohols. For example, the reaction of sodium acetylide with 2-butanone yields 3-methyl-1-pentyn-3-ol:
Imagine that a reaction similar to that in Problem 5-76 is carried out between sodium acetylide and (R)-2-phenylpropanal to yield 4-phenyl-1-pentyn-3-ol: