Conformational Analysis of Carboxylic Esters and Amides

The C(0)-0 bond length in esters is significantly shorter than the C(sp )-0 bond of aryl or vinyl ethers. In a study of the geometry of thioesters in crystal structures by Zacharias et al. [27] it was found that the bond length does not differ from that of the corresponding bond in aryl and vinyl monosulfides (O 1.336 [21] vs. 1.367 A S 1.767 vs. 1.763 A [27]). This suggests that there is more electron delocal- 

A study similar to that of the carboxylic esters was done for carboxylic amides by Chakrabarti and Dunitz [28]. A general conclusion for the amides from primary, secondary and tertiary C(a) amines is that the C(a)-CGff) bond of an alkyl substituent avoids the synperiplanar arrangement to the C(0)-N bond. For the (er(-alkyl substituents one of the C - C bonds is therefore always in an antiperiplanar position 

The conformation about the C-N bond has been studied in cyclic amides (O) [29 a] and enamines (D) [29 b]. Gilli et al. did a similar analysis and expanded the selection of fragments with thioamides (S), amidines (N), and anilines (A) [29 c]. The 90 molecular structures included in the study map a pathway whereby the rotation 

A study by Kaftory and Agmon describes a pathway for the simultaneous inversion of the two neighboring nitrogen atoms in the 1,2,4-triazolidinedione ring [30]. 

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