Dieckmann condensation Subject

The intramolecular version of ester condensation is called the Dieckmann condensation.217 It is an important method for the formation of five- and six-membered rings and has occasionally been used for formation of larger rings. As ester condensation is reversible, product structure is governed by thermodynamic control, and in situations where more than one product can be formed, the product is derived from the most stable enolate. An example of this effect is the cyclization of the diester 25.218 Only 27 is formed, because 26 cannot be converted to a stable enolate. If 26, synthesized by another method, is subjected to the conditions of the cyclization, it is isomerized to 27 by the reversible condensation mechanism. 

This synthetic route is based on ring closure by Dieckmann condensation of 1,2-bis-carbalkoxyalkylpyrrolidines. It has gained special importance during the last few years, after application to several total syntheses of naturally occurring pyrrol izidine bases. The usual starting compounds employed in this route are esters of a-pyrrolidineacetic acid, proline, and their homologues, which are converted into N-substituted dialkyl dicarboxylates. The esters of the dicarboxylic acids are subjected to Dieckmann condensation and subsequent ketonic hydrolysis the resultant ketones are used in further reactions. 

Regioselective techniques were developed to direct the cyclization of unsymmetrical esters. An interesting example of this aspect of the Dieckmann condensation is the synthesis of ip-methylcarbapenems by Tanabe and co-workers, ip-methylcarbapenems is a structure that exists in potent and broad antibacterial compounds, such as meropenem and biapenem, which renders it synthetically interesting for the synthesis of some close analogues 30a-d. Thioesters 29 was subjected to cyclization under dehydration type Ti-Dieckmann condensation conditions 3.0 equivalents of the TiCU and 3.3 equivalents of the base BusN were used to give the vinyl... 

The atroposelective construction of the 6,6 -biaryl linkage was recognized as the key step in the synthesis of viriditoxin. Although many atroposelective couplings of complex molecules have been completed and have been the subject of many reviews,most of these approaches relied on either proximal stereogenic centers or covalently linked chiral auxiliaries. Our objective was to explore the influence of distal chiral centers on the catalytic oxidative dimerization of naphthopyran-2-ones using vanadium catalysis (Scheme 7). Assembly of naphthopyran-2-ones has been well studied and recently reviewed." The Staunton-Weinreb annulation or the Michael-Dieckmann condensation is ideally suited for the rapid... 

Much interesting work has been done in the last ten years on the bridging of pyrrole and piperidine rings. Early in their work on this subject Clemo and Metcalfe (1937) prepared quinuclidine (V) by the reduction of 3-ketoquinuclidine (IV), the latter resulting from the hydrolysis and decarboxylation of the product (III) of a Dieckmann internal alkylation, applied to ethyl piperidine-l-acetate-4-carboxylate (II), itself made by condensing ethyl piperidine-4-carboxylate (I) with ethyl chloroacetate. 

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