Morphine and its O-methylated derivative, codeine, are among the most effective painkill...

Morphine and its O-methylated derivative, codeine, are among the most effective painkillers known. However, they possess two serious drawbacks: they are addictive, and repeated use induces a tolerance to the drug.

Many morphine analogs have been prepared in an effort to find drugs that are equally effective as painkillers but that have less risk of physical dependence and potential abuse. Following are three of these.

(a) List the structural features common to each of these molecules. (Chapters 1–2, 4–6)

(b) The Beckett-Casey rules are a set of empirical rules to predict the structure of molecules that will bind to morphine receptors and act as analgesics. According to these rules, to provide an effective morphine-like analgesic, a molecule must have (1) an aromatic ring bonded to (2) a quaternary carbon and (3) a nitrogen at a distance equal to two carbon–carbon single bond lengths from the quaternary center. Show that these structural requirements are present in the four molecules shown in this problem. (Chapters 1, 4)