Esters ring size effects

The Pd°-catalyzed transformation of enediynes represents a highly efficient and effective approach for the synthesis of polycyclic compounds, with different ring sizes being obtained by a variation of the tether [129]. In this respect, reaction of 6/1-270 led to the tricyclic product 6/1-271 as a single diastereomer. The initial step is a chemoselective hydropalladation of the propargylic ester moiety in 6/1-270 to give an alkenyl-Pd-species, according to the mechanism depicted in Scheme 6.71. A hexatriene is formed as a byproduct. 

Exocyclic reactions for aromatic carboxylic esters 174 Exocyclic reactions for aliphatic carboxylic esters 187 Endocyclic reactions for carboxylic esters 191 Carbon acid participation for carboxylic esters 195 Effective molarities 198 Ring size 199 Initiating nucleophile 200 Phosphate and sulphonate esters 200... 

This example illustrates the synthesis of cyclic compounds by intramolecular alkylation reactions. The relative rates of cyclization for ca-haloalkyl malonate esters are 650,000 1 6500 5 for formation of three-, four-, five-, and six-membered rings, respectively.28 (See Section 3.9 of Part A to review the effect of ring size on Sn2 reactions.)... 

A few scattered examples of hexahydro- and octahydro-l,3-diazocines have been reported, mainly in the context of studies on the effect of chain length on the cyclization of diamines. Reaction of the N- aryl-iV - aroyldiamines (194) with polyphosphoric ester gives the cyclic amidines (195) (77JCS(P2)2068). Yields of the diazocines (195 n =5) were about 40%, compared to 90% for the 1,3-diazepine (195 n =4). The basicities of the products decrease in the order of ring size 6 7>8 5, lower basicity reflecting non-planarity of the amidine system. 

This interesting reaction is especially useful for the synthesis of medium- and large-ring compounds from dicarboxylic esters, and is effective for ring sizes that cannot be made by the Dieckmann condensation or decarboxylation (Section 18-1 OB). Radical anions formed by addition of sodium to the ester... 

The importance of the ring size in the intramolecular reaction is the opposite from what was seen in the directly activated reaction, where the ring size has almost no effect on the rate of hydrolysis. In addition to the difference ring size plays in the intramolecular reaction, another major difference between the directly activated reaction and the intramolecular reaction is the importance of the buffer. For the directly activated ester reaction the buffer has no effect on the rate of hydrolysis, and in some cases causes a decrease in hydrolysis dne to nucleophilic attack of the carbonyl carbon by the buffer. The intramolecular reaction, however, can be greatly affected by the buffer present, with C02 and P04 having the greatest influence. ... 

Kozawa and Mori reported the synthesis of different ring size heterocycles from the same propargyl ester 670 by a ligand effect on Pd(0) (Scheme 207).284 When the reaction was carried out using P(o-tolyl)3 as a ligand, the carbapenam 671 was obtained in 57% yield, while the reaction of 670 using DPPF gave the carbacepham 672 in 56% yield. 

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