Intramolecular reactions diester condensation

Another important reductive coupling is the conversion of esters to a-hydroxyketones (acyloin condensation).267 This reaction is usually carried out with sodium metal in an inert solvent. Good results have also been obtained for sodium metal dispersed on solid supports.268 Diesters undergo intramolecular reactions and this is also an important method for the preparation of medium and large carbocyclic rings. 

The intramolecular Claisen condensation of diesters, or Dieckman reaction, occurs readily to give five- or six-membered rings, and it has been extensively used for cyclopentanone and cyclohexanone derivatives. 

The intramolecular condensation reaction of diesters, the Dieckmann condensation, works best for the formation of 5- to 7-membered rings larger rings are formed with low yields, and the acyloin condensation may then be a faster competitive reaction. With non-symmetric diesters two different products can be formed. The desired product may be obtained regioselectively by a modified procedure using a solid support—e.g with a polystyrene 10 ... 

Intramolecular Claisen condensations can be carried out with diesters, just as intramolecular aldol condensations can be carried out with diketones (Section 23.6). Called the Dieckmann cyclization, the reaction works best on 1.6-diesters and 1,7-diesters. Intramolecular Claisen cyclization of a 1,6-diester gives a five-membered cyclic /3-keto ester, and cyclization of a 1,7-diester gives a six-membered cyclic /3-keto ester. 

One typical radical reaction is a coupling reaction. Oxidative decarboxylation coupling reaction of carboxylic acids by electrolysis (Kolbe electrolysis), intramolecular coupling reaction of diesters with Na (acyloin condensation), formation of pinacols from ketones or aldehydes with Na or Mg are well known classical methods [1,2]. Recently, oxidative... 

Dieckmann reaction (Section 23.9) the intramolecular Claisen condensation reaction of a 1,6-or 1,7-diester, yielding a cyclic p-keto ester. 

The Claisen reaction is a carbonyl condensation that occurs between two ester components and gives a /3-keto ester product. Mixed Claisen condensations between two different esters are successful only when one of the two partners has no acidic a hydrogens (ethyl benzoate and ethyl formate, for instance) and thus can function only as the acceptor partner. Intramolecular Claisen condensations, called Dieckmann cyclization reactions, provide excellent syntheses of five- and six-membered cyclic /3-keto esters starting from 1,6- and 1,7-diesters. 

Intramolecular Claisen reactions are known as Dieckmann reactions, and these work well to give 5- or 6-membered rings. A 1,6-diester forms a 5-membered ring, while a 6-membered ring is formed by the condensation of a 1,7-diester. 

The addition of the phthalimido anion to triesters of 2-phosphonopropenoic acid at room temperature gives an initial adduct, which is unexpectedly inert to benzaldehyde reaction but which adds to a second molecule of the phosphonic diester to form a second carbanion the latter undergoes an intramolecular WEH condensation to give the heterocyclic system 371 indolizine and quinolizine phosphonic diesters have been similarly obtained from maleimide, succinimide and glutarimide anions ". The ability of piperidine to add to the compounds 372 decreases in the order R R = (BuO)H (exothermic reaction) > (BuO)2 > (BuO)Me > Bu2 (the last requires heat) ". Ethenylphosphonic diamides (372 R = r2 = NR2) are unreactive to nucleophiles (by virtue of reduced electrophilic character at phosphorus and thus reduced electron attraction from the carbon-carbon double bond) but are reactive to electrophilic reagents. ... 

Intramolecular base-promoted cyclization of suitable diesters (Scheme 52) is one of the earliest routes to benzo[a]quinolizidines, which was exploited in the synthesis of racemic emetine developed by Oppenshaw <53JCS2463>. Reaction of tetrahydroisoquinoline (255) with ethyl a-formylbutyrate followed by hydrogenation afforded (256), which was cyclized by base to yield (257), the benzo[a]-quinolizidine portion of the alkaloid. On this occasion only one of the two possible Dieckmann condensation products was observed, although this has not always been the case with related cyclizations <73BSF3476>. 

B.ii. The Dieckmann Condensation. Just as there is an intramolecular version of the aldol condensation, there is an intramolecular version of the Claisen condensation but it has been given a different name, the Dieckmann condensation. 109,110 reaction involves intramolecular cyclization of an a, co-diester such as... 

Just as there is an intramolecular version of the aldol condensation (see Section 22.2), there is an intramolecular version of the Claisen condensation. A simple illustration is treatment of an a,(0-diester such as diethyl 1,6-hexanedioate (77, the diethyl ester of adipic acid called diethyl adipate see Chapter 20, Section 20.9) with sodium ethoxide in refluxing ethanol. When this experiment is done, triphenylmethide (19 introduced in Section 22.1) is used as the base, and reaction in the solvent benzene gives the isolated flnal product (86) in 70% yield. Note that benzene is rarely used now-a-days because it is a suspected carcinogen and its use is being phased out. This reaction sequence uses the same mechanism as the Claisen condensation. 

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