Nature and Chiral Environments

Although the different enantiomers of a chiral molecule have the same physical properties, they usually have different biological properties. For example, the (+) enantiomer of limonene has the odor of oranges, but the (-) enantiomer has the odor of pine trees. 

More dramatic examples of how a change in chirality can affect the biological properties of a molecule are found in many drugs, such as fluoxetine, a heavily prescribed medication sold under the trade name Prozac. Racemic fluoxetine is an extraordinarily effective antidepressant but has no activity against 

The hand-in-glove fit of a chiral substrate into a chiral receptor is relatively straightforward, but it s less obvious how a prochiral substrate can undergo a selective reaction. Take the reaction of ethanol with NAD catalyzed by yeast alcohol dehydrogenase. As we saw at the end of Section 9.13, the reaction occurs with exclusive removal of the pro-R hydrogen from ethanol and with addition only to the Re face of the NAD carbon. 

The hundreds of different pharmaceutical agents approved for use by the U.S. Food and Drug Administration come from many sources (see the Chapter 5 Focus On). Many drugs are isolated directly from plants or bacteria, and others are made by chemical modification of naturally occurring compounds, but an 

The S enantiomer of ibupro-fen soothes the aches and pains of athletic injuries much more effectively than the R enantiomer. 

Copyright 2010 Cengage Learning. All Rights Reserved May not be coped, scanned, or diq licated, in whole or in part Due to electronic rights, sc ie third party content maybe sup re sed tom the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall leamii e qierience. Cei age Leamii reserves the right to remove additional cortent at stny time if subsequent rights restrictions require it. 

---

In these systems, chirality and sub-molecular motion stem directly from each other, thus showing an even more intimate relationship. Many different kinds of stimuli have been used to provoke such changes in co-conformation including pH-change [41-47], redox processes [41,47-49], the nature of the environment [50], photochemistry [51,52], temperature (entropy-driven shuttling) [53] and reversible covalent chemistry [54]. 

---