Acyl groups, conformation

A very interesting steric effect is shown by the data in Table 7-12 on the rate of acid-catalyzed esterification of aliphatic carboxylic acids. The dissociation constants of these acids are all of the order 1(T, the small variations presumably being caused by minor differences in polar effects. The variations in esterification rates for these acids are quite large, however, so that polar effects are not responsible. Steric effects are, therefore, implicated indeed, this argument and these data were used to obtain the Es steric constants. Newman has drawn attention to the conformational role of the acyl group in limiting access to the carboxyl carbon. He represents maximum steric hindrance to attack as arising from a coiled conformation, shown for M-butyric acid in 5. 

Acyl groups are common in bacterial polysaccharides. The parent acids are fatty acids, hydroxy acids, and amino acids. The simplest acid, formic acid, has only been found as the amide. The occurrence of O-formyl groups had been reported, but proved to be incorrect. A-Formyl groups have been found in different polysaccharides for example, in the 0-specific side-chains of the LPS from Yersinia enlerocolitica 0 9, which are composed of 4,6-dideoxy-4-formamido-D-mannopyranosyl residues. The formyl group can assume two main conformations, s-cis (41) and s-trans (42), which are... 

Dipole moments of azolides have been reviewed, with emphasis on the conformation of the acyl group. 441 Unfortunately, structural and conformational studies on azolides by X-ray structure analysis are almost totally lacking, although they would be of great interest with regard to the conformations and to the bond lengths and bond orders in these systems. Only an X-ray analysis of V-acetyl-4-bromopyrazole 451 has been reported. 

The close packing of the acyl groups associated with the inclination of the lipid A backbone with respect to the fatty acid orientation seems to constitute a common and characteristic feature of the lipid A conformation. This specific (endotoxic) conformation is very likely to influence greatly the tendency of the amphiphilic lipid A to adopt peculiar supramolecular structures. 

The carbonyl group is bonded to an sp -hybridized X atom. As a general rule, a threefold barrier should characterize these compounds, and the preferred conformations of the acyl group are not easy to predict since they depend also on the heterocyclic ring conformation. Carbonyl derivatives of... 

As a general rule, it may thus be deduced from the results so far reported that acetyl- and benzoyloxiranes exist in solution as a mixture of two conformations, classified as distorted s-gauche, yet one of them may approach a distorted s-cis form in the presence of bulky acyl groups. 

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