Utilities site selection influenced

Azlactone is commonly utilized as a precursor of a-quatemary a-amino acids and various heterocyclic compounds [28-30]. Because the enol form of azlactone has aromatic character, facile deprotonation from the C4-position affords the corresponding enolate under the influence of various bases. Interestingly, the enolate ion shows ambident reactivity and attacks the electrophile at either the C4-position (a-addition) or the C2-position (y-addition), thus acting as an a-amino enolate or an acyl anion equivalent, respectively (Fig. 1). The site-selectivity associated with this enolate seems to be heavily dependent on its stereoelectronic characteristics, and introduction of a bulky substituent into the Cl- or C4-position suppresses the nucleophilicity at the particular position. 

One possibility to solve this ambiguity is to use test reactions that allow the nature, strength, and number of active sites to be distinguished [7,72], Two points are important when choosing an appropriate test reaction (1) the reaction should proceed along one pathway, i.e., extensive side reactions should not occur, and (2) a low conversion should be maintained to directly measure intrinsic (differential) reaction rates and exclude the influence of product inhibition. However, even when all criteria are fulfilled, one should not forget that the information obtained is complex and can only be fuUy utilized if the adsorption/desorption and diffusion of the reactants and products and the reaction steps can be differentiated. Thus, it is insufficient to report only activity or the activity/selectivity pattern to deduce the acid-base properties. The reaction orders should be given in addition, with at least the rate of reaction normalized to the specific surface area of the catalytic material imder study. Ideally, a microkinetic model describes the reaction studied. 

---