Page - (000037) - in Biomedical Chemistry: Current Trends and Developments

deprotonation of the introduced group (as in the formation of imines; Scheme 1.1.20c). If the nucleophile is anionic, no positive charge is generated. In both cases, a negative charge is generated at the most electronegative atom, the carbonyl oxygen, which is usually neutralized through protonation (Scheme 1.1.20). Scheme 1.1.20 illustrates some of typical nucleophilic addition reactions of different nucleophiles to aldehydes and ketones. All the reactions depicted in Scheme 1.1.20 are called “direct” or [1,2]- additions. A special case of nucleophilic addition reactions is the Michael or [1,4]-conjugate addition. This reaction is quite frequent in biochemical pathways and consists of a nucleophile addition to the β– position of an α,β-unsaturated carbonyl system (Scheme 1.1.21). Scheme 1.1.20: Nucleophilic addition reactions: a) with a hydride ion as nucleophile; b) with an alcohol as nucleophile; c) with an amine as nucleophile; and d) with a hydrazine as nucleophile.