Polyfunction amino acids

Test results for a series of reactions of polyfunctional amino acids with CDI in aqueous solution are reported in reference [51]. Serine and threonine did not poly-condense. Instead, via the iV-imidazolyl-carbonyl amino acids, L-2-oxooxazolidine-4-carboxylic acid or L-(+)-fraw,s-5-methyl-2-oxooxazolidine-4-carboxylic acid were obtained. 

Cross-linked polyamides may be prepared from polyfunctional amino acids, triamines, or tricarboxylic acids. 

The selective protection and liberation of the a-amino function, the carboxy group, and the various side-chain functional groups of polyfunctional amino acids constitute some of the most fundamental problems in peptide chemistry. 

Trimethylsilylation has been used to derivatize ammo acids for GC analysis for more than two decades, but no silylation procedure of general application exists All amino acids, including Gin and Asn, can be derivatized (Fig. 6), but problems of silylation occur with polyfunctional amino acids because of the different reactivities of — NH2, —OH, —SH, and —COOH functionalities and the ready hydrolysis of the tnmethylsilyl (TMS) group in the derivatives and the silylahng reagents With some of the amino aads, di- and trisubstituted TMS derivatives are formed (Husek and Macek, 1975, Drozd, 1975) In spite of these difficulties. 

Since in pyoverdines and in pseudobactins both ester bonds and amidic linkages other than with the. a-amino groups of the amino acids have been encountered it is essential to account for the functional groups of polyfunctional amino acids. Several examples relating to pyoverdine I-III (3) (ref. 18) will be given here ... 

Keywords Diels-Alder reactions of chiral polyfunctional carbocyclic nucleosides and cyclopropane amino acids... 

The condensation of amino acids likewise may produce cyclic and/or linear products the same is true of virtually all polyfunctional condensation reactions. The conversion of cyclic monomers and dimers (or other cyclic low polymers) to chain polymers was discussed in the preceding chapter the reverse reaction may often occur as well. Thus the alternative ring and chain products which may be produced by condensation of a bifunctional monomer usually are interconvertible, but with varying degrees of facility. 

It was only around 1850 that the first amines were discovered by Wurtz [2], who considered them as alkylated (or arylated) derivatives of NH3. Nowadays, it is well known that the amine function is widespread among biologically important compounds, but mostly it is present in polyfunctional molecules such as amino acids, alkaloids, etc. Simple amines are very rare in nature, with the exception of tri-ethylamine and the trimethylammonium ion which come from the putrefaction of proteins. 

The elimination/addition reaction already proceeded at room temperature when the dichloropropionic acid had been linked as an ester to the support, but required heating when an amide linkage had been chosen. When amines with low nucleophilicity were used, such as aniline or a-amino acid esters, higher reaction temperatures were also beneficial. Occasional by-products for this reaction sequence were acrylic acid derivatives or the corresponding hydrogenated products (2-thiopropionic acid derivatives). These by-products were usually formed when a very small excess of amine was used in the elimination/addition step. Both the thiols and the amines used in this reaction sequence could be polyfunctional, as illustrated by the examples sketched in Fig. 3. 

Recently, Sames and co-workers showed an interesting application, in which it was demonstrated that the Shilov chemistry permits heteroatom-directed functionalization of polyfunctional molecules [16]. The amino acid valine (10) was allowed to react in an aqueous solution of the oxidation catalyst PtCU and Cu(ii) chloride as stoichiometric oxidant (Scheme 3). At temperatures >130 °C a catalytic reaction was observed, and a regioselective C-H functionalization delivered the hydroxyvaline lactone 11 as a 3 1 mixture of anti/syn isomers. It was noted that the hydroxylation of amino acid substrates occurred with a regioselectivity different from those for simple aliphatic amines and carboxylic acids. The authors therefore proposed that the amino acid functionalization proceeded through a chelate-directed C-H activation. 

The exchange of halogen for the amino group is important in the formation of other polyfunctional compounds, particularly the amino acids. In several of these transformations with aqueous or liquid ammonia, it has been shown that the presence of ammonium salts minimizes the formation of secondary and tertiary aminesExcellent directions for the synthesis of a-amino acids (C,-C ) from a-halo acids and ammonia are given. The methods have been reviewed. Long-chain amino acids are prepared by this and other procedures. ... 

Coombes and Katchalski [29] have considered a slightly more complex version of this mechanism in which a second propagation coefficient operates above a critical degree of polymerization. Katchalski et al. [30] calculated the molecular weight distribution obtained in a system following scheme (12) but also including a bimolecular termination step. Various authors have analysed more complex systems in which the initiator is a polymeric species. Thus Gold [31] has shown that initiation by a poly a-amino acid with a Poisson distribution leads to a polymeric product with an over-all Poisson distribution, and Katchalski et al. [32] demonstrated that in multichain polymers synthesized from polyfunctional initiators Poisson distributions also arise. 

Some of dicarboxylic acids can also be distilled, without decomposition, under reduced pressures. This is at least a theoretical ground for the possibility of their direct GC analysis. Few successful attempts have been described, but these analytes require on-column injection of samples and extremely high inertness of chromatographic systems. Many types of polyfunctional carboxylic acids (hydroxy-, mercapto-, amino-, etc.) cannot be analyzed in free, underivatized form, owing to either non volatility and/or absence of thermal stability. These features are the principal reasons for the conversion of carboxylic adds, before their GC analysis, into less polar derivatives without active hydrogen atoms. 

Enzymes are composed of amino acid chains linked via peptide bonds and can be considered as polyfunctional and (multi-)charged macromolecules with a defined, more or less rigid three-dimensional structure. A typical enzyme molecule with a molecular weight of 30,000 Da resembles a compact spherical particle of approx. 4 nm in diameter. Many enzymes, especially those which are regulated by various effectors, consist of more than one amino acid chain (sub-... 

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