Aminoacids Chelation

The aqueous chemistry of AT,0-chelated complexes appeared to be intermediate between that of the neutral AT V-chelates and anionic 0,0-chelates, with significant effects of the chosen N- and O-group (111). The aminoacidate complexes hydrolyzed... 

A conjugate of a poly(aminoacid) and a D03A chelate [177] has been reported to have a pH dependent relaxivity. Here, this originates from conformational changes of the poly( amino acid) carrier as the pH varies from 4 to 8, which affect overall and local rotational motions in the macromolecule. 

Formation of a metal-cyclic structure such as 414 is typical for aminoacids [1,714-722], which is explained by chelate effect. At the same time, complexes have been reported in which aminoacids behave as monodentate ligands with localization of the coordination bond either in N- (415) or O- (416) donor centres [714] ... 

We note that, with the lone participation of a carboxyl group in the coordination, the coordination modes examined earlier for the complexes of carbonic acids (Sec. 2.2.5.4, formulae 276 283) are possible. Additionally, a mixed binding amongst ligands is observed in many complexes of aminoacids, for example chelates 277, 414, and 0,0 -bridge 279 [714],... 

Only little is known about the structure of six-membered chelate rings in complexes with )3-aminoacids. The most probable is a distorted boat conformation with the more voluminous substituent neighboring the amino group in an equatorial arrangement. 

Arsenite oxidase was solved at higher resolution (1.64 A) offering a more reliable view of the active site (40). Two dithiolene chelates are symmetrically bound at normal distances (2.4 A) and a single oxo ligand is observed at 1.6 A. The absence of any other covalent link from the protein leaves the Mo as five coordinate (alanine replaces the aminoacid position normally occupied by the coordination of serine, cysteine or selenocysteine residues). This Mo environment was interpreted as indicating a reduced Mo site, possibly from photoreduction in the X-ray beam. 

Figure 5-1 Examples of Metal Chelates. Only the relevant portions of the molecules are shown. The chelate formers are (A) thiocarbamate, (B) phosphate, (C) thioacid, (D) diamine, (E) o-phenantrolin, (F) a-aminoacid, (G) o-diphenol, (H) oxalic acid. Source From K. Pfeilsticker, Food Components as Metal Chelates, Food Sci. Technol., Vol. 3, pp. 45-51, 1970.

Evans s oxazolidinones 1.116 and 1.117 are a class of chiral auxiliaries that has been widely applied [160, 167, 261, 411]. Deprotonation of 7/-acyl-l,3-oxa-zolidin-2-ones 5 30 and 5.31 smoothly gives chelated Z-enolates, which then suffer alkylation between -78 and -30°C on their least hindered face [167, 1036]. After hydrolysis, the corresponding enantiomeric acids are obtained according to the auxiliary that was used (Figure 5.21). Due to the low reactivity of lithium enolates, sodium analogs are preferred in some cases [411, 862, 1036], This methodology has been applied to the synthesis of chiral a-arylpropionic acid anti-inflammatory drugs [1037, 1038], natural products [1039, 1040], and a-substituted optically active 3-lactams en route to nonracemic a,a-disubstituted aminoacids [136,1041]. 

Oppolzer and Tamura [460, 861, 1076] have recommended 1-chloro-l-ni-trosocyclohexane as an electrophile for preparation of nonracemic a-aminoacids. a-Aminoacids are formed with an excellent enantiomeric excess from sodium eno-lates ofN-acylsultams 1.134 (R = R CH by a sequence of nitrosylation, hydrolysis and reduction of the intermediate hydroxylamine, and cleavage of the auxiliary with LiOH (Figure 5.39). The stereoselectivity of tins process is interpreted by attack of the electrophile on the face opposite to the nitrogen lone pair of the Z-chelated enolate 5.58 (Figure 5.39). 

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