N-protected aspartic acids

Finally, as an example of regioselective aminolysis, a recent report by Conde and Lopez-Serrano deserves to be cited. They used CalB for the regioselective mono-amidation of N-protected aspartic acid diethyl esters with butylamine [96]. 

There is stereoselectivity here, as in the last example, and both are determined by the substrate, that is the molecule being hydroxylated. The reagent MoOPH is large and it attacks the less hindered side of the substrate under kinetic control. Some control can be achieved in the interesting hydroxylation of aspartic acid enolates.41 The lithium enolate of the dimethyl ester of N-protected aspartic acid 259 gives high svn selectivity in hydroxylation next to only one of the two ester groups 260. 

Notes Yields were determined by NMR. The only other constituent of the crude reaction mixture was the N-protected aspartic acid. 

If (he carboxylic acid concerned is a protected aspartic acid, an N-linked asparagine results, an observation that suggested that the procedure could be used for the preparation of glycopeptides. Accordingly, coupling of NPG 52 and aspartic acid 53 in acetonitrile solvent yields the N,N-diacyl derivative 54, which can be chemoselectively AT-deacetylated with piperidine in DMF to give asparagine 55 [19] (Scheme 13a). 

Alternatively, oxazolones have been used as reagents to activate and to couple N-protected dicarboxylic amino acids wherein the carboxylate moiety acts as the nucleophile. For example, 2,4-dimethyl-5(4//)-oxazolone 255 reacts with N-benzyloxycarbonyl-L-aspartic acid to give a mixture of the anhydrides 256 and 257. Subsequent reaction of 256 and 257 with phenylalanine methyl ester hydrochloride and A-methylmorpholine produces a mixture of the a-isomer 258 and p-isomer 259 of Al-benzyloxycarbonyl-aspartylphenylalanine methyl ester (Scheme 7.83). °... 

The N-hydroxypyridine-2-thione esters of Boc-protected amino acids undergo decarboxylation on photolysis in the presence of a hydrogen-atom-transfer reagent such as t-butyl mercaptan in 80-95% yield. Decarboxylation of the side-chain carboxy groups of suitably protected aspartic and glutamic acids can also be effected, but yields are lower (50-80%). ... 

Apoptotic/necrotic transformation of excitable cells. Effects of dipeptides directed to support stability of cellular structures increase the reliability of cellular functions under normal conditions and especially during oxidative stress, which accompanies effect of several extreme factors. It was found in experiments on individual neurons that carnosine prevents cell death induced by excitotoxic compounds, N-methyl-D-aspartic acid (NMDA) or kainate [93-95] or experimental hypoxia/reoxigenation [96]. Apoptosis induced by exposure of cerebellum neurons to kainic acid (see Table 6), was arrested if the cells were pre-incubated with carnosine or anserine and simultaneously heavy necrotic processes were substitute by light (reversible) necrosis. At the same time, N-acetylcamosine or homocaraosine did not reveal protecting action [94,95]. 

Alternatively, tetramethylthietane amine is condensed with an N-protected form of D-alanine to give alanyl amide. This is then coupled to a protected analogue of L-aspartic acid to give a crude form of alitame. The crude product is then purified. 

N-Protection of amino acids. For the synthesis of /-butyloxycarbonyl-)3-benzyl-L-aspartic acid by the reaction of the amino acid derivative with /-butyl azidoformate, Polzhofer2 found that use of triethylamine as base gives a higher yield (75%) than use of magnesium oxide (40%). The yield using /-butyl 4-nitro-... 

Cw-4-(Phosphonomethyl)-2-piperidinecarboxylic acid (19) is a powerful and selective N-methyl-D-aspartic acid (NMDA) antagonist. The phosphate analogue, (21) has now been described, having been obtained via the Af-protected diphenyl ester (20) by... 

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