Chelate opening-closing process

Werner 50) and his students. He was the first to recognize that racemiza-tion of complexes such as M(AA)3, where AA is a bidentate, can take place by a chelate ring opening-closing process. Such a mechanism and two others are shown in Figure 9. These are called intramolecular processes, because racemization takes place without the complete departure of a ligand from the first coordination sphere. 

A so-called open transition state is another possibility. One transition state (33) leading to the product 10 is depicted in the margin. The aldehyde is attacked at its z-face and the heteroaryl substituent is avoiding the bulky oxazolidinone auxiliary. The reason for the fonna-tion of open transition states is not clear in this case, because zinc is a chelating metal ion and should form closed transition states of the Zimmerman-Traxler type. However, it is known that A-acetyl oxazolidinone derivatives such as 9 exhibit no chirality transfer in the aldol process. In the cases described in the literature, both possible diastereomers were obtained in nearly equal amounts. It is true that in the synthesis of 1 the selectivity is fairly low (4 1), but the major product is the unfavored one. 

Biochemical experiments and crystal structures of apo, mono, and diferric serum transferrin greatly enhance our understanding of the mechanism by which Tfs strongly chelate ferric ion and then release it within the endosome. These processes are inherently related the iron-binding steps occur in the opposite order to the steps leading to iron release from transferrin. Binding of iron to the tyrosine residues and to the synergistic anion while the Tf lobe is in the open conformation appears to be the first step. " " Once the ferric-loaded domain samples the closed... 

Figure 14 Iron release from transferrin. Iron is coordinated through four protein residues (D63, H249, Y95, and Y188) and the synergistic bidentate carbonate anion. The lobe continually samples the open and closed conformations. Partial loss of the carbonate, aspartate, and histidine are the first steps in the process. Coordination of the anion or chelator (A/C) forms a quaternary complex between the protein, iron, carbonate and the chelator/anion. Decay of the quaternary complex yields apo transferrin.

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