Effect of temperature on the acidolysis of N-acyl-N,alpha,alpha-trialkyl glycine amides as related to the nature of substituents

The C-terminal amide bond of N-acyl-N,alpha,alpha-trialkyl glycine amides is labile to acid and this has been currently assigned to steric crowding within the amino acid residue. However, our previous work has shown that in the acidolysis of some of these compounds steric hindrance seems to play a less important role than what one would expect. Thus, the cleavage of two sets of such compounds bearing different degrees of crowding was investigated at five different temperatures in order to clarify the effect of structure on reactivity in terms of enthalpy and entropy of activation. The compounds exhibited an Arrhenius-type behaviour, and both enthalpies and entropies of activation were calculated by taking advantage of the transition state theory. In addition, the kinetic data were analysed in terms of isokinetic relationships in order to find evidence to support that the compounds react under the same mechanism. The changes in the reaction rate are governed by the changes in both the enthalpy and the entropy of activation, which are related to bond energy and steric hindrance, respectively. In general, the entropies of activation are very negative for all compounds investigated, which reflects large steric constrictions associated with the formation of the transition state. In addition, they are very sensitive to the structure of the substrates.

Kinetic investigation of the effect of the amino acid side chains in the selective acidolysis of N-acyl-N,alpha,alpha-trialkyl glycine amides

Accurate kinetic measurements of the rate constants for the acidolysis of five N-acetyl-N-(4-methoxybenzyl)-alpha,alpha-trialkyl glycine cyclohexyl amides in TFA were performed at 25 degrees C and the reactions monitored by HPLC. In all cases the results were consistent with a first order behaviour with respect to the substrate. No direct correlation was obtained with these data between the rate constant values and structure, but a good correlation coefficient was obtained when a multiple regression analysis was applied by taking advantage of a Taft equation using appropriate polar and steric substituent parameters. In a plot of the values observed for log k against those calculated by this equation all five points fell very close to the line of perfect correlation. The calculated sensitivity coefficients to polar and steric contributions were used to discuss the experimental results and showed that the acidolysis were comparatively less affected by steric effects than expected.

Synthesis and modification of dibenzylglycine derivatives via the Suzuki-Miyaura cross-coupling reaction

The objective of this paper is to describe in details of various available methods to prepare C(alpha,alpha)-dibenzylglycine (Dbzg) and then include our work involving the synthesis of side chain Dbzg derivatives. alpha,alpha-Disubstituted amino acids (alpha,alphaAAs) are important members in the family of modified amino acids. Replacement of the alpha-hydrogens of glycine 1 by alkyl groups leads to alpha,alphaAAs. The steric hindrance of the quaternary centre of Aib 2 combined with the helix-forming capacity has attracted the attention of structural biologists and protein crystallographers. Dbzg 3 is a special structural variant of Aib. The presence of two benzyl groups at C(alpha)-position not only impart rigidity to the peptide backbone in which it is incorporated, but also acts as a useful vehicle for studying pi-pi interactions. Although several C(alpha,alpha)-disubstituted glycines such as C(alpha,alpha)-diethyl glycine (Deg), C(alpha,alpha)-dipropyl glycine (Dpg) etc. have been studied in detail, not much has been known about Dbzg because of limited availability of synthetic procedures. Various Dbzg derivatives 19a-f have been prepared using ethyl isocyanoacetate 14 as a glycine equivalent (eq.). A useful and simple methodology has been developed using the Suzuki-Miyaura cross-coupling reaction for the modification of Dbzg derivatives 17d, 19d, 22. Using this 'Building Block Approach' (Accounts of Chemical Research 36, 2003, 342) one can generate a variety of Dbzg derivatives 20a-f and 23a-e, which may find useful applications in combinatorial synthesis and QSAR studies.