Esters conformation

The (5)-selectivity for 6-MeCL in Novozym-435-catalyzedhydrolysis and transesterification reactions was unexpected. Recent molecular modeling studies by Veld et al. suggest that the cisoid ester conformation in lactones of ring size below eight could be responsible for the switch in enantioselectivity [88]. In contrast, the ester... 

Parameters required for the description of polyesters I are taken from a recent paper (Abe, A, J. Am. Chem. Soc. 1984, 106, 14) which dealt with the dipole moments of dialkyl esters of dicarboxylic acids. Since the ester groups are all assumed to be in the trans configuration, short-range interactions between consecutive rigid cores are unimportant. As for the rotation around the O-C — C-C bond, the six-state scheme (termed model I in the above paper) is employed. The statistical weight parameter a representing the relative importance of the reversed ester conformations with respect to the normal ones is set equal to unity. The three-state scheme (termed model II) proposed alternatively in the above reference is examined for chains with n = 5 and 6 for comparison. In this model the C O /CC eclipsed form is assumed to be intrinsically more stable than the C 0 /CH form a stabilzation energy E( 1) of 5.0 kj mol-1 is adopted. 

In this procedure the acylating molecule is the active ester formed between the amino acyl moiety and the hydroxy group of HOAt. 40 Compared to the earlier methods utilizing HOSu 41 and HOBt, 42 HOAt maintains the known ability to suppress racemization, but is much more effective in catalyzing peptide-bond formation. This efficiency of HOAt may be attributed to the possible assistance of both N2 and N7 to the amine nucleophilic attack (Scheme 2). If the preferred active ester conformation found in the crystalline state (the heterocyclic At plane lies approximately perpendicular to that of the carboxylic ester moiety)143 is preserved in solution, then assistance to nucleophilic attack on both faces of the ester is likely. 

Further ab initio molecular orbital calculations on the reaction profile for the base-catalyzed hydrolysis of dimethyl phosphate in various ester conformations have provided support for this theory (5 6) ... 

The title of this paper asks if theories fit the facts. So far it has been argued that for cyclic esters, conformational effects make it exceptionally difficult to control the variables to test any theory but by showing that ligand reorganisation processes in reaction intermediates (as opposed to stable phos-phoranes) are limited then at least some simplification of mechanisms is possible. It is pertinent now to examine the one assumption that was so valuable in explaining the differences in hydrolysis patterns of 5-membered phosphates and phosphonates, viz, the C-P bond provides a barrier to pseudorotation. 

Fig. 10. Absorption spectra of four l-alkyl-4-carbomethoxy-pyridinyl radicals, (R), in thin films at 77 K. The three main bands are at 6S0 nm, 400 nm and 300 nm. The 650 nm band (charge-transfer absorption of pimer) shifts from 623 nm for I-CH3 to 675 nm for l-lCHsfsC, along with a two-fold intensity decrease. The 400 nm band is composed of the pyridinyl radical absorption of the pimer, and absorptions of the two ester conformational isomers of the pyridinyl radical monomer. The 300 nm band varies with the different relative contributions of the pimer and the monomer...

Ergosteryl tosylate, 239 Esters, conformation, 25 —, hydrolysis, 18,24-26 Ethers, cleavage, 50 —, oxidation, 52 Ethoxycarbonylhydrazine, 337 Ethyl formate, 181 Ethyleneimines, 117 Ethylidenetriphenyiphosphorane, 153... 

Model B describes the case in which a chelating group is present in the dienophile, as shown in Figure 40. In contrast to model A, the Lewis acid complex is now anti-s-cis, although the (Z)-ester conformation is still intact. The s-cis conformation is also observed in two crystal structures of chelated complexes, although one of these (c/. Figure 25) is somewhat unusual. "... 

Benzoic acid esters, conformation, 52 phenyl rotation, 67... 

These results have suggested that discrimination between the operators may be based upon the degree to which the repressor protein restricts phosphate ester conformational freedom in the complex. We suggest that specific, tightly bound complexes retain the inherent phosphate ester conformational flexibility of the operator itself whereas more weakly bound operator-protein complexes restrict the phosphate ester conformational freedom in the complex relative to the free DNA. 

T BUrgi, A Baiker. Model for Enantioselective Hydrogenation of a-keto ester over Chirally Modified Platinum Revisited Influence of the a-keto ester Conformation. J Catal 194 445-451,2000. 

Phytochrome Isolated bile pigments Biliverdin dimethyl ester Conformation ... 

Chem. Descrip. Polyglyceryl mixed veg. fatty acid esters conforms to hydrogenated veg. oil NF CAS 68334-28-1 EINECS/ELINCS 269-820-6 Uses Food emulsifier, crystal inhibitor in veg. oils lubricant in pharmaceuticals (clinical nutrition, coating, delivety/absorption enhancement, dermatologicals, infant formulas, nutritional/sports supplements, suppositories) lubricant in cosmetics (creams/lotions, sun care preps.) Regulatory FDA 21 CFR 172.854(c)... 

Substituent polarity effects are apparent in the conformational equilibria of cyclohexyl acetate and monochloro- dichloro-, and trichloro-acetates as evidenced by their H n.m.r. spectra. The inversion barriers decrease, and the proportions of axial ester conformer increase, with successive chlorine substitution of the acetate. The conformations of various trans-1,2-disubstituted cyclohexanes have been examined by n.m.r. and a discussion of the observed inversion barriers in terms of steric and dipole-dipole interactions has been given. A new description of these conformational effects is proposed. 

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