1.1 Principles

The base catalyzed carbon-carbon bond formation is closely related to the carbon-carbon bond formation from organometallic reagents. In both methods, the negatively polarized carbon reacts with electrophilic carbon of carbonyl groups and related compounds.

The scope of the base-catalyzed reactions depends on three facts: (i) a wide range of organic compounds is able to form carbanions, (ii) these carbanions undergo reaction with electrophilic carbon in a variety of environments, and (iii) the basicity of the reagent used to abstract the proton may be widely varied.

1.2 Reactions of Enolates with Carbonyl Compounds

1.2.1 Reactions with Aldehydes and Ketones

1.2.1.1 Aldol Condensation

The reaction has become one of the most important methods for carbon-carbon bond formation. It consists of the reaction between two molecules of aldehydes or ketones that may be same or different. One of the reactants is converted into a nucleophile by forming its enolate in the presence of base and the second acts as an electrophile (Scheme 1).

Scheme 1

The geometry (( Z )- or ( E )) of the enolate depends on the reaction conditions and the nature of the substituents. Strong base (e.g. LDA), low temperature and short reaction time lead to kinetic enolate , while weak base (e.g. hydroxide ion), high temperature and longer reaction time favour the formation of thermodynamic enolate (Scheme 2)

Scheme 2

If the reactants are not the same, they can lead to the formation of diastereoisomers and their distribution depends on the reaction conditions and the nature of the substituents (Scheme 3).

Scheme 3