Carbonyl

"The in situ generation of an iminium ion from a carbonyl compound lowers the LUMO energy of the system. Iminium catalysis is comparable to Brønsted- or Lewis acid activation of carbonyl compounds. The LUMO energy is lowered, theα-CH acidity increases, and nucleophilic additions including conjugate additions as well as pericyclic reactions are facilitated."

"Having seen a variety of types of Claisen condensations, we can now ask how this process may be logically analyzed for synthetic use. Three facts are available to help us: (1) Claisen condensations always form 1,3-dicarbonyl compounds; (2) one of the reaction partners in a Claisen condensation must be an ester, whose alkoxide group is lost in the course of the condensation; and (3) the other reaction partner (the source of the nucleophilic enolate) must contain at least two acidic hydrogens on an a-carbon. In addition, if a mixed condensation is being considered, one reaction partner should be incapable of self-condensation (e.g., it should lack α-hydrogens). If we are given the structure of a target molecule and wish to determine whether (and, if so, how) it can be made by a Claisen condensation, we must analyze it retrosynthetically with the preceding points in mind. For example, let us consider whether 2-benzoylcyclohexanone can be made by a Claisen condensation."

"β-Dicarbonyl compounds such as ethyl 3-oxobutanoate (acetoacetate) and diethyl propanedioate (malonate) are versatile synthetic building blocks for elaborating more complex molecules. Their unusual acidity makes it easy to form the corresponding anions, which can be used in nucleophilic displacement reactions with a wide variety of substrates. Their hydrolysis produces 3-ketoacids that are unstable and undergo decarboxylation on heating."

"Reaction of the stabilized anions derived from β-dicarbonyl compounds and related analogs with α,β-unsaturated carbonyl compounds leads to 1,4-additions. This trans formation, an example of Michael addition, is base-catalyzed and works with α,β-unsaturated ketones, aldehydes, nitriles, and carboxylic acid derivatives, all of which are termed Michael acceptors. (...) β-Dicarbonyl anions, like ordinary enolate anions, undergo Michael additions to α,β-unsaturated carbonyl compounds. Addition of a β-ketoester to an enone gives a diketone, which can generate six-membered rings by intramolecular aldol condensation (Robinson annulation)."

"In organic synthesis, the carbonyl group is intimately involved in many reactions that create new carbon-carbon bonds. The carbonyl group is electrophilic at the carbon atom and hence is susceptible to attack by nucleophilic reagents. Thus, the carbonyl group reacts as a formyl cation or as an acyl cation. A reversal of the positive polarity of the carbonyl group so it acts as a forrnyl or acyl anion would be synthetically very attractive. To achieve this, the carbonyl group is converted to a derivative whose carbon atom has the negative polarity. After its reaction with an electrophilic reagent, the carbonyl is regenerated. Reversal of polarity of a carbonyl group has been explored and systematized by Seebach."