oxide (dimer of acetone) and phorone and isophorone (trimers). In the Meyer–Schuster rearrangement, the starting compound is a propargyl alcohol. Another synthetically valuable method to access α,β-unsaturated carbonyls is via Selenoxide elimination. Cyclic enones can be prepared via the Pauson–Khand reaction. Synthesis using regiospecific enolate formation and masked functionality Regiospecific formation is the controlled enolate formation by the specific deprotonation at one of the α-carbons of the ketone starting molecule. This provides one of the best understood synthetic strategies to introduce chemical complexity in natural product and total syntheses. A prominent example of its use is in the total synthesis of progesterone illustrated in Figure "Regiospecific enolate formation in the total synthesis of progesterone". When ketones are treated with base, enolates can be formed by deprotonation at either α-carbon. The selectivity is determined by both the steric and electronic effects on the α-carbons as well as the precise base used (see figure ""Masked functionality" for regiospecific enolate formation" for an example of this). Enolate formation will be thermodynamically favoured at the most acidic proton which depends on the electronic stabilization of the resulting anion. However, the selectivity can be reversed by sterically hindering the thermodynamic product and therefore kinetically favouring deprotonation at the other α-carbon centre. Traditional methods for regioselective enolate formation use either electronic activating groups (e.g. aldehydes) or steric blocking groups (e.g. 1,2-ethanedithiol protected ketone). An enone can also serve as a precursor for regiospecific formation of an enolate, here the enone is a “masked functionality” for the enolate. This process is first described by Gilbert Stork who is best known for his contributions to the study of selective enolate formation methods in organic synthesis. Reacting an enone with lithium metal generates the enolate at the α-carbon of the enone. The enolate product can either be trapped or alkylated. By using “masked functionality”, it is possible to produce enolates that are not accessible by traditional methods. The “masked functionality” approach to regiospecific enolate formation has been widely used in the total synthesis of natural products. For example, in the total synthesis of the steroid hormone progesterone, Stork and co-workers used the “masked functionality” to stereospecifically construct one of the quaternary carbons in the molecule. --> Enals An enal (or alkenal) is an organic compound containing both alkene and aldehyde functional groups. In an α,β-unsaturated enal, the alkene is conjugated to the carbonyl group of the aldehyde (formyl group).