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Section IBasic principlesThe side chain (ethylamine) consists of two carbons attached to an amine group.
Theα-carbon is bound to the amine group and theβ-carbon is covalently linked
to the catechol ring. The size and nature of the functional groups on the termi-
nal amine and theα-carbon determine whether an agent is active at eitherα-or
β-adrenoceptors or is an agonist or antagonist. In addition, only catechols (two
adjacent –OH groups on the benzene ring) are metabolized by catechol-O-methyl
transferase (COMT); derivatives without this feature may have a longer duration of
action. Similarly, monoamine oxidase (MAO) will metabolize only drugs with a single
amine group, preferably a primary amine, although adrenaline, a secondary amine
(see mini-dictionary below), is a substrate (see Chapter 12 ).Barbiturates
Barbiturates are derivatives of barbituric acid, which is formed by a condensation
reaction (i.e. water is also formed) between malonic acid and urea. Barbiturates
are weak acids, but also have imino groups present (see Figure8.3). Thio barbi-
turates have, as their name suggests, a thio (=S) substitution for the keto group
(=O). The types of hydrocarbon groups on the 5-carbon determine the duration of
pharmacological action. Oxybarbiturates undergo less hepatic metabolism than the
corresponding thiobarbiturate.