Mechanisms Dieckmann cyclization

The mechanism of the Dieckmann cyclization, shown in Figure 23.6, is the same as that of the Claisen condensation. One of the two ester groups is converted into an enolate ion, which then carries out a nucleophilic acyl substitution on the second ester group at the other end of the molecule. A cyclic /3-keto ester product results. 

Mechanism of the Dieckmann cyclization of a 1,7-diester to yield a cyclic /3-keto ester product. 

Dideoxyribonuclcotide, 1113 Dieckmann, Walter, 892 Dieckmann cyclization, 892-893 mechanism of, 892-893 Diels, Otto Paul Hermann. 492... 

Malonate anions react at low temperatures (0—25 C) with cyclic a-haloketones via a Favorskii-type mechanism leading to jS-keto-esters (153) in reasonable yields (Scheme 17). " Reactions at higher temperatures result in simple 5n2 substitution. Specific Dieckmann cyclizations of alkyl pimelates can be effected by attaching one end of the substrate to a polymer support. ... 

Detailed mechanisms of Dieckmann cyclizations on PS have been elucidated by isotopic labeling experiments/ In many cases scrambling of " C-carbonyl carbon atoms by reversible condensations accompanies formation of the desired products, as shown in equation (32). Yields of six-membered j -keto esters from cyclization of PS-bound unsymmetrical diesters have been good, but attempts at PS-bound synthesis of the nine-membered jS-keto ester failed, as have attempts to perform that cyclization in solution. Small amounts of cyclodimers were formed as shown in equation (33). 

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. 

Key Mechanism 22-12 The Claisen Ester Condensation 1071 22-13 The Dieckmann Condensation A Claisen Cyclization 1074 22-14 Crossed Claisen Condensations 1074 22-15 Syntheses Using /3-Dicarbonyl Compounds 1077 22-16 The Malonic Ester Synthesis 1079 22-17 The Acetoacetic Ester Synthesis 1082 22-18 Conjugate Additions The Michael Reaction 1085 Mechanism 22-13 1,2-Addition and 1,4-Addition (Conjugate Addition) 1085... 

Keto esters are capable of undergoing cyclization reactions similar to the Dieckmann condensation. Write a mechanism for the following reaction. 

The mechanism of a Dieckmann condensation is identical to the mechanism we described for the Claisen condensation. An anion formed at the a-carbon of one ester in Step 1 adds to the carbonyl of the other ester group in Step 2 to form a tetrahedral carbonyl addition intermediate. This intermediate ejects ethoxide ion in Step 3 to regenerate the carbonyl group. Cyclization is followed by formation of the conjugate base of the j8-ketoester in Step 4, just as in the Claisen condensation. The j8-ketoester is isolated after acidification with aqueous acid. 

The Thorpe-Ziegler cyclization is characterized by a relatively straightforward mechanism. As demonstrated below, deprotonation of dinitrile 1 results in the formation of the anionic species 2. Intramolecular cyclization in a manner similar to the well-known Dieckmann condensation yields 3. Workup under aqueous conditions then produces imine 4, which immediately tautomerizes to the conjugated enamine 5. 

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