Ester preferred conformation

Mechanistically, it has been recently postulated that the C5-carboxylate ester prefers to occupy an axial position in the oxycarbenium intermediate, thereby favoring the formation of the 3H4 half-chair over the 4H3 conformer. Nucleophilic attack on the 3H4 half-chair intermediate occurs in a (1-lashion, providing the 1,2-m-mannuronates with excellent stereoselectivity (Fig. 6).64 The stereocontrol of mannuronate esters is independent of the type of donor employed and agrees well with this postulate.65... 

In the absence of distribution data (and knowledge of preferred conformation — at least in the axial phenyl examples), the significance of potency differences amongst the isomeric esters (LXXXIll-LXXXV) cannot, however, be judged. Of the more rigid reduced acridine congeners (LXXXVI), only the e-phenyl isomer has been obtained and this lacks hot-plate activity in mice [286]. 

The conformational characteristics of dimethyl esters of dicarboxylic acids are studies by the NMR and dipole moment method. Conformational energies of the internal CH2-CH2 bonds are determined from the observed 1H-1H vicinal coupling constants. Preferred conformations around the C-C bond are elucidated from the RIS analysis of dipble moments. With the RIS parameters thus established, the orientational correlation between the terminal ester groups is examined. The analysis provides the reason why the odd-even effect in the dipole moment is moderate, and attenuates rapidly with n in the ct.co-diester series. 

It is assumed that the preferred conformation of the substructure C=C—O—C of the configured ester enolate in the preferred transition state of the alkylation is that depicted in the center of the left-hand column of Figure 13.42. In the projection shown, the alkylating reagent reacts with the enolate from the front side for the reasons just stated. The reaction occurs with a diastereoselectivity of 97 3. Chromatography allows for the complete separation of the main diastereoisomer from the minor diastereoisomer. Reduction of the main diastereoisomer (for the mechanism, see Section 17.4.3) affords the alcohol B, a derivative of. S -a-benzyl propionic acid, with 100% ee. Hydrolysis of the benzylated esters without iso-... 

It has been noted that the photolysis of 67a and 67b gave cis-3-hydroxyproline esters 68 exclusively in moderate to good yields, whereas 67c-67d gave the isomeric compound 68. The selectivity for the formation of 68 was explained by considering the preferred conformation B and C due to intramolecular H-bondingas compared to A [24], as illustrated in Scheme 8.22. 

After the finding of a sweet taste in L-Asp-L-Phe-OMe (aspartame) by Mazur et at. (6), a number of aspartyl dipeptide esters were synthesized by several groups in order to deduce structure-taste relationships, and to obtain potent sweet peptides. In the case of the peptides, the configuration and the conformation of the molecule are important in connection with the space-filling properties. The preferred conformations of amino acids can be shown by application of the extended Hiickel theory calculation. However, projection of reasonable conformations for di- and tripeptide molecules is not easily accomplished. 

Fig. 9D.4 Preferred conformation of dimer B2-3 - O-gallate (B2G) and suggested conformational arrangements of the oenin intercalated between the galloyl ester group and catechol ring B of B2G (adapted from Berke and de Freitas 2005)...

Linear dichroism studies of benzoyloxy-steroids (in a stretched polythene film) suggest that the benzoate of a secondary alcohol exists in the preferred conformation (12) already accepted for the corresponding acetate esters. ... 

Fig. 14.10. Preferred conformation of a keto esters of chiral alcohols. > ...

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