双语化学Synthesis and reactions of β-dicarbonyl compounds More chemistry of enolate anions.ppt

mechanism 19.9 Michael additions Robinson annulation (conjugate addition followed by aldol cyclization) Conjugate addition followed by Claisen ester cyclization gives cyclic diketones Acetoacetic ester synthesis? α,β-unsaturated nitriles are ideal for conjugate addition tropinone synthesis I (Michael addition) tropinone synthesis II (Mannich reaction) Mechanism: * Chapter 19 Synthesis and reactions of β-dicarbonyl compounds: More chemistry of enolate anions 19.1 introduction β –dicarbonyl compounds: Compounds having two carbonyl groups separated by an intervening carbon atom. Specific enol equivalent from 1,3-dicarbonyl compounds Why donot enols formed from 1,3-dicarbonyl compounds react immediately with themselves by the aldol reaction? These enols are very stable. ii) The carbonyl groups in the unenolized fraction are poorly electrophilic ester and ketone groups. 19.2 Crossed aldol reaction of active hydrogen compounds with aldehydes and ketones - the Knoevenagel condensation. Mechanism: E1cB More examples: More about the base: secondary amine R2NH The electrophile could be the iminium ion 19.3 Claisen condensation: the synthesis of β-keto esters mechanism A wrong mechanism Double-headed curly arrow Notice: When planning a reaction with an ester and an alkoxide anion, it is important to use an alkoxide that has the same alkyl group as the alkoxyl group of the ester. Esters having only one αhydrogen do not undergo the usual Claisen condensation. However, they can be converted to β-keto esters by reactions that use very strong bases. Dieckmann condensation – intramolecular Claisen condensation In general, the Dieckmann condensation is useful only for the preparation of 5- and 6-membered rings. 19.3A Crossed Claisen condensations Crossed Claisen condensations are possible when one ester component has no α hydrogens and, therefore, is unable to form an enolate ion and undergo self-condensation. Reactive esters that cannot enolize 19.3B Acylation of other carbanions 1