Conformation of esters

Likewise, carboxylate esters (47) have less double-bond character in their C—O bonds relative to the C—N in amides. There are two additional factors that should be taken into account when one considers the conformations of esters. 

Evidently, very special circumstances will be required to make the E conformation of esters so stable that atropisomers can be isolated at ambient temperatures. 

Solvation effects on the conformation of esters of three /i-snbstituted 1-phenyletha-nols with 2-flnoro-2-phenyl acetic acid (FCDA) were studied both experimentally (in five solvents ranging from CDCb to DMSO) and quantum mechanically. Semi-empiri-cal (AMI of MJS Dewar and PM3 of JJP Stewart) and ab initio (RHF/3-21 G) calculations were undertaken. Energy maps for the conformers of the esters as a function of the dihedral angles alpha (F-C-alpha acid-C=0) and beta (CO-O-C-alcohol-H) were obtained. Solvent effect calculations, through the self-consistent reaction field on the most stable conformers, were also carried out (Hamman et al., 1996). 

Let us mention that there are still other relations between conformation and dipole moments. Relative stability of conformational isomers was often brought in connection with their dipole moments. For instance, the preferred sp conformation of esters (3a), or of carboxylic acids, was often explained by the lower dipole moment6 compared to the conformation 3b, or in other words by the electrostatic repulsion7-9 of partial dipoles in 3b. Reasoning of this kind is usually based on anticipated rather than measured moments that of 3b cannot be directly measured. Interaction of more distant dipoles is commonly neglected, viz both its effect on the conformation and on the total /z (see, for example, Klinot and coworkers10). This is in fact the fundamental assumption underlying all analyses of dipole moments. 

The extremely stable and general sp conformation of esters and carboxylic acids (3a) is one of the remarkable features of stereochemistry. It was observed uniformly2 in all other molecules with the group O—C=0 and also with their heteroatom analogues with S, Se or Te. In contrast with the rigid conformation, the conjugation in the ester molecule, expressed by the formulas 69, is relatively weak and can be just detected in the dipole moment values164. 

Here are the two conformations drawn out for ethyl acetate. When the ethyl group (= R) and O are cis, not only can one oxygen lone pair interact with the C-O ft, but the other lone pair can also donate into the O of the C=0 bond. This is not possible when Et and O are trans they are no longer anti-pcripla-nar. The cis conformation of esters is generally the preferred one, even in formate esters, where the alkyl group ends up in what is clearly a more sterically hindered orientation. 

Fig. 2 Conformations of ester groups in 1,4-dihydropyridines with respect to orientation of carbonyl group relative to the neighboring C = C bond of the ring a cis-cis (sp,sp), b cis-trans (sp,ap), c trans-trans (ap,ap)...

Oxygen donor substrates continue to be extensively studied. Conformations of esters, (427) aldehydes and ketones, (428, 429) ketones, (430, 431) cyclohexanones, (432, 433) carboxylates, (434) benzene-1,2-dioxydiacetate, (435) borneols, (436) troponeiron tricarbonyl, (437) and nitrosopiperidines (438) have been deduced. It is... 

Principles of conformational analysis may be invoked to rationalize the face-selectivity of these compounds. Note, however, that there are two broad types of auxiliary those that contain a second carbonyl and those that do not. The former may function by chelating the metal of the Lewis acid catalyst, while the latter can only act as monodentate ligands to the metal of the Lewis acid. Figure 6.13a illustrates the probable transition state conformations of ester dienophiles when bound as monodentate ligands to the Lewis acid catalyst, M (auxiliaries 6.12a-f). The C(=0)-0 bond prefers the Z (or cis) conformation for a variety of reasons [163], but the preference is large probably >4 kcal/mole. Because of this constraint, the C-0 bond may be considered to be similar to a double bond (hence the E/Z or cis/trans designation). A subtle consequence of this constraint is the effect it has on... 

E,Z-conformation of esters and related compounds Esters and carboxyhc acids often prefer to orient substituents towards the carbonyl, despite the steric consequences (Figure 6.92). The Z-isomer is preferred by 5kcal/mol in the gas phase for methyl formate. The difference is even larger ( 8kcal/mol) for methyl acetate. ... 

Figure 6.92 Two conformations of esters which one is more stable ...

Culvenor C J 1966 The conformation of esters and the acylation shift . NMR evidence from pyr-rolizidine alkaloids. Tetrahedron Lett 1091-1099... 

CuLVENOR, C. J. The Conformation of Esters and the Acylation Shift NMR Evidence from Pyrrolizidine Alkaloids. Tetrahedron Letters 1966, 1091. 

The discussion is organized in the following order First the advantages of HRC scheme, relative to the industrial (i.e., heterogenous) process are briefly commented on second, the relevance of celMose activation and the physical state of its solution to optimization of esterification are discussed. Finally, the use of recently introduced solvent systems and synthetic schemes, designed in order to obtain new, potentially useful cellulose esters with controlled, reproducible properties is reviewed. A comment on the conformity of these methods with the concepts of green chemistry is also included. 

Thus, when acid 76 was crystallized as a salt with (S)-(-)-l-phenylethylamine ([S]-PEA), the X-ray structure showed that the conformational enantiomer 76a was trapped in the crystal, displaying O - H and O - Ht distances of 2.47 A and 3.41 A, respectively. The conformation of 76a placed the carbonyl oxygen and Hj, closer to the ideal values mentioned in Figure 7.26 as compared to H. A significant preference for Hj, was demonstrated after photolysis at 0 °C and diazomethane workup, when ester 77a (B) was obtained in 65% ee after 90% conversion. Figure 7.27 illustrates the minimal atomic displacements required for reaction by comparing the X-ray structure of the reactant with that of the product, and with a structure obtained at 50% conversion. Better chemical results were obtained by photolysis of 76a with (/ )-CEA, which gave 90% ee of ester of 77a (B) after diazomethane workup. 

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