Page - (000042) - in Biomedical Chemistry: Current Trends and Developments

together to form a bigger one and liberates a small one (e.g. water, methanol, acetic acid). Carbonyl condensations occur with two carbonyl compounds. This type of reaction is a very important one, adding to the chemical versatility that carbonyl groups grant to a system. Carbonyl condensations are essentially nucleophilic substitutions that allow easy carbon-carbon bond formation in relatively mild conditions (certainly within the reach of an enzyme), adding chain formation and polymerization to the list of enzyme-catalysed reactions. 1.1.4.7.1 Aldol Reaction An aldol reaction involves carbonyl compounds such as aldehydes or ketones in which at least one species has an α-proton (Scheme 1.1.27). Although it is not formally a condensation reaction unless dehydration happens, it is often named as aldol condensation in biochemical fields. The “proper” aldol condensation product though is the result of a dehydration of the formed aldol. Scheme 1.1.27: General mechanism for an aldol condensation reaction. The aldol adduct suffers a dehydration for which the mechanism is not specified since the precise step sequence may vary according to catalyst used. First of all, the enolate is generated from a ketone or an aldehyde by an α-deprotonation with a base. The enolate is a strong nucleophile and a nucleophilic addition occurs unto an aldehyde (or ketone) acting as electrophile. The resultant anion, the aldolate, is protonated to produce the aldol, a β-hydroxy-aldehyde or ketone. The aldol may then eliminate a water molecule yielding an α,β-unsaturated aldehyde or ketone (Scheme 1.1.27).