Hydroxyamino acid, hydrolysis

The imidazolidine-4-one 29, obtained by heating the methanol solution of an a-aminoamide and a 4-substituted benzaldehyde, is a mixture of two diastereomers which can be separated by chromatography. They are oxidized by MCPBA, separately or as a mixture, into the two diastereomeric l-hydroxyimidazolidine-4-ones 30. Hydrolysis of 30 by ethanolic HC1 and hydroxylamine hydrochloride gives the optically pure TV-hydroxyamino acid amide 31 (Scheme 9).[12]... 

Ruveda and co-workers have shown that the /5-hydroxyleucine, of which the aryl ether function in frangulanine is constructed, is present in the erythro-L-form (21). Dihydrofrangulanine was reduced with lithium in methylamine to an enol ether of /3-hydroxyleucine which on hydrolysis generated the free amino acid. It was shown that the hydroxyleucine in the hydrolysate is degraded by snake venom l-aminooxidase but not by pig kidney D-aminooxidase. Inasmuch as reo-hydroxyamino acids are attacked by neither enzyme it follows that the /3-hydroxyleucine, of hydrolytic origin, is the L-erythro form. 

Hymenocardine (58), (32, 40) has a p-hydroxy-w-aminoacetophenone unit (81), instead of the usual styrylamine, in its cyclic system which can be recognized in addition to N,N-dimethylisoleucylvaline and tryptophan in its acid hydrolysate. Mild alkaline hydrolysis results in ring opening via /3 elimination on the hydroxyamino acid and severance of the phenolate to a tetrapeptide 82 whose structure was determined by mass spectrometry and further hydrolysis. It is the only peptide alkaloid in which /3-hydroxy valine is involved in the aryl ether bridge. 

That the hydroxyamino group is in all cases attached to the 8 carbon is evident from the fact that periodate oxidation yields nitrosodimer (rather than N2O, which would arise from an N -hydroxyamino acid (42, 89)), performic acid oxidation affords glutamic acid (136) and, finally, deacylation followed by catalytic hydrogenation, reaction with fluoro-dinitrobenzene and hydrolysis gives N8-dinitrophenylornithine (63). 

Acid hydrolysis affords two residues of L-2-amino-6-hydroxyamino-hexanoic acid, (-)-3-hydroxy-2-methylpentanoic acid, CO2, m-cresol and... 

L-serine. The latter three fragments arise by slow decomposition of a primary unit of 2-(2-hydroxy-6-methylphenyl)oxazoline. Acid hydrolysis also liberates /ra s-octadec-2-enoic acid, but the cis isomer is obtained by periodate cleavage and hence represents the natural configuration. The length of the n-A2-unsaturated fatty acid side chain ranges from C14 to C20 with Cig representing the main component (85%). No method has been found to separate mycobactins P with different side chains. Saponification yields mycobactic acid P and a neutral substance, cobactin P. Reductive hydrolysis in HI gives the hydroxyamino acid as L-lysine. 

Serine is one of the two hydroxyamino acids, the other being threonine. Serine has two major pathways of catabolism. The first, and apparently predominant, direction in many mammals is catalyzed by serine dehydratase, where water is removed between the alpha and beta carbons of serine. A rearrangement of the double bond forms an amino acid with spontaneous hydrolysis to form pyruvate and ammonia. Pyruvate then can be metabolized as discussed in previous chapters. This enzyme is primarily active in the liver, where the ammo-... 

For complete hydrolysis of bonds between hindered amino acid residues the above mentioned 16 hour period is not sufficient. In addition to isoleucine and valine synthetic intermediates can contain residues with bulky and acid resistant blocking groups, such as S-benzylcystein. Should the latter be preceded or followed by a hindered amino acid, then the time of hydrolysis must be considerably prolonged. Since the hydroxyamino acids and also tryptophan gradually decompose under these conditions, it might be necessary to carry out more than one analysis, with shorter and longer hydrolysis times. 

The hydrolysis of nitriles (58) formed in this way is usually difficult (Scheme 19). In the presence of cone, sulfuric acid mainly amides of a-keto acid oximes (59) were obtained (1), Concentrated hydrochloric acid yields amides of N-hydroxyamino acids (60) (130) while in dilute acid the desired products (1) are formed (1, 79). Taking into account the instability of N-hydroxyamino nitrile (58) and the possibility of its polymerization, the first stage of hydrolysis is carried out at lower temperature (79). The Strecker method has also been applied to the synthesis of several cyclic N-hydroxyamino acids (70, 71) (134). 

Lau and Schollkopf 184) prepared a-mono- or disubstituted nitrone esters (195) (Scheme 42) by metalation of (191) with potassium / r/-butylate in THF and subsequent alkylation using alkyl halides. Hydrolysis of (195) then furnishes a,a-disubstituted N-hydroxyamino acids (67,196) and their methyl esters (76). 

The splitting of nitrones may be carried out by hydrolysis with hydrochloric acid (155,178,182), hydrazinolysis (182,186) or most suitably hydroxylaminolysis (183) (Scheme 44). Esters (191-193, 212) are easily converted into N-hydroxyamino acid esters (76, 77,130, 213) by action of hydroxylamine or its crystalline salts such as hydroxylamine hydrochloride (184), oxalate or toluenosulfonate (183) without disturbing the ester function. The salts of the last type are easily crystallized. 

Among other reactions, the bis-metallated species (151) derived from nitroalkanes condense with dialkyl carbonates to give comp>ounds (152), in 60—80% yield, which can serve as precursors of both a-amino-acids and a-hydroxyamino-esters as well as a-keto-esters. Oxazolin-5-ones (153) can be alkylated at the 4-position by alkyl halides in hot DMF containing HMPA and ethyldi-isopropylamine. Yields are good (60—90%) for allylic, benzylic, and propargylic halides but otherwise poor (e.g. 32% with EtI) under these conditions acid hydrolysis of the products affords substituted a-amino-acids. Mesoionic l,3-oxazol-5-ones (154), obtained from imidoyl chlorides and acyl-tetracarbonylferrates, react with alcohols to give N-acyl-a-amino-acid esters. ... 

Oppolzer s camphor-based sultams 92 proved itself as an efficient, robust auxiliary for enolate amination with 1-chloro-l-nitroso cyclohexane 486 as an electrophile. Thus, sultams 92 were first deprotonated with NaHMDS, and to the sodium enolates thus formed was added a solution of the blue nitrosochloride 486. Decolorization occurred immediately, and the mixture was quenched with hydrochloric acid to give hydroxylamines 487, in all cases as essentially pure diastereomers. The reductive cleavage of the nitrogen-oxygen bond was achieved with zinc dust to yield a-aminoacyl sultams 488. By mild hydrolysis with lithium hydroxide, the chiral auxiliary 91 was removed and recovered under concomitant formation of a-amino acids 490. Any racemization was avoided by applying this procedure, even in the case of the labile substrates with R equals a phenyl or / r -methoxyphenyl substituent. On the other hand, the auxiliary could be cleaved at the stage of hydroxylamines 487, so that not only a-amino acids 490 but also a-hydroxyamino acids 489 became available with excellent enantiomeric purity (Scheme 4.103) [232]. 

Gruen (757) found that the presence of most of the amino acids tested during hydrolysis does not affect the tryptophan recovery. Of the hydroxyamino acids, threonine had no effect at all, while tyrosine has a small effect. On the other hand serine was found to reduce the recovery of tryptophan by about 10% due to a deamination reaction of serine. This produces pyruvic acid which, as a ketoacid, interacts with the indole nucleus of tryptophan (287). Whereas cysteine had no effect, cystine caused a substantial loss of tryptophan (about 40%), and was itself recovered on the analyzer partly in the reduced cysteine form. These results would indicate that a major factor in the loss of tryptophan during acid hydrolysis of a protein is degradative oxidation by cystine. 

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