Page - (000041) - in Biomedical Chemistry: Current Trends and Developments

involves the acetylation of a serine residue in the enzyme through a transesterification reaction (Scheme 1.1.26). Esterification is an acyl substitution reaction where an acyl group is transferred to the oxygen atom of the alcohol. In the synthesis of aspirin, acetic anhydride is used for efficiency reasons as it is a much better acetylating agent than acetic acid (Scheme 1.1.25). Scheme 1.1.25: Simplified mechanism for the synthesis of aspirin from salicylic acid and acetic anhydride. This reaction is normally performed under acid catalysis with sulfuric or phosphoric acids. Transesterifications are acyl substitution reactions where the nucleophile is an alcohol and the electrophile is an ester. Essentially, the alkoxyl moiety in the ester is substituted by another. In the case of aspirin, the alcohol/nucleophile is the side chain of the serine residue in the COX enzyme which substitutes the alcohol moiety of the salicylic acid (Scheme 1.1.26). Scheme 1.1.26: Simplified mechanism of the transesterification reaction of aspirin with the serine residue. 1.1.1.4.7 Carbonyl Condensation Reactions As we have seen before, a condensation reaction joins two molecules