Enolate anions, esters, Dieckmann cyclization

Dieckmann cyclization of diethyl adipate can be described by the two successive equilibria shown below. Use SpartanView to obtain the energies of diethyl adipate, the keto ester, the keto ester enolate ion, ethanol, and ethoxide anion, and calculate AH° for both steps. Which step is more favorable ... 

Modem synthetic practice frequently requires the use of methods mote specific than those outlined above. Much attention has been focused on the mixed Claisen or Dieckmann reaction, i.e. the acylation of one ester by another, or its intramolecular equivalent, the regioselective cyclization of an unsymmetri-cal diester. A similar problem arises with the acylation of unsymmetrical ketones. This chapter thus describes the inter- and intra-molecular carbon-carbon bond-forming reactions in which a delocalized enolate anion (or close equivalent) reacts at an sp carbon atom in an addition-elimination sequence, as well as the acid-catalyzed equivalent employing an enol. In Table 1 we list the potential nucleophiles and the electrophiles that have been employed in these reactions, although not every possible combination has been reduced to synthetic practice. Table 2 gives details of acid-catalyzed acylations (see Section 3.6.4.3). 

The synthesis of the 10-methoxytetracyclic ketone 353 (Scheme 26) started from the readily available 3-methyl-5-methoxyindole 355, which after Boc protection, was brominated, and then condensed with the anion of the Schollkopf auxiliary 356 (from l-valine). Removal of the Boc-protecting group, followed in succession by A -methylation and hydrolysis, gave the required A -methyl-5-methoxy-D-tryptophan ethyl ester 357, which was then transformed into the key 10-methoxytetracyclic ketone 353, via N-benzylation, Pictet-Spengler condensation, and Dieckmann cyclization. Subsequent N-alkylation by the vinyl iodide 358 followed by Pd-catalyzed (enolate-driven)... 

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