Serine esters

The earliest method developed for the preparation of nonracemic aziridine-2-car-boxylates was the cyclization of naturally occurring (3-hydroxy-a-amino acid derivatives (serine or threonine) [4]. The (3-hydroxy group is normally activated as a tosyl or mesyl group, which is ideal for an intramolecular SN2 displacement. The cyclization has been developed in both one-pot and stepwise fashion [4—9]. As an example, serine ester 3 (Scheme 3.2) was treated with tosyl chloride in the presence of triethylamine to afford aziridine-2-carboxylate 4 in 71% yield [9]. Cyclization of a-hydroxy- 3-amino esters to aziridine-2-carboxylates under similar conditions has also been described [10]. 

FIGURE 2 Polymers that can be derived firom L-serine. (a) Poly-(serine ester) was obtained by ring opening polymerization of N-protected serine-p-lactones (19). (b) Poly(serine imine) has appar-... 

Zhou, Q. X., and Kohn, J., Preparation of poly(L-serine ester) A structural analog of conventional poly-L-serine, Macromole-cules. In Press. 

Serine esters can be O-alkylated without concomitant A -alkylation, when the amino group is protected as its trityl derivative. The reaction is generally high yielding under relatively mild conditions [33], particularly with the more reactive allyl and benzyl halides. 

Serine.—Its ester is contained in the fractions which distil between ioo° and 130° at O 5 mm. The mixed esters contained in this fraction are treated with a small quantity of water and then with five to six volumes of petroleum ether, which precipitates serine ester as an oil the oil is then shaken up with petroleum ether to remove admixtures as far as possible and is hydrolysed with baryta water. On removal of the baryta it crystallises when the solution is concentrated, and it is purified by treatment with alcohol, which dissolves other substances which are also present, and recrystallisation from water. Its )8-naph-thalene sulpho-derivative. 

The free ester, obtained from the sodium salt as a thick oil, on reduction with aluminium amalgam gave N-benzoyl serine ester —... 

Cherney, R. J. Wang, L. Efficient Mitsunobu reactions with N-phenylfluorenyl or N-trityl serine esters./. Org. Chem. 1996, 63, 2544— 2546. 

Although transfer of the acetate group from its inital site, bound as a serine ester, to its reactive position on Cys-1305(a) can occur via an intermediate in which the acetate is attached to the phosphopantetheine thiol, evidence suggests that this reaction is not kinetically competent and that the biologically significant mechanism utilizes a direct transfer from Ser-819( ) to Cys-1305(a) [73]. Transfer of the malonyl group from its initial position at Ser-5421 (/3) to its reactive position as a phosphopantetheine thioester occurs directly [69]. 

The anionic polymerization of A(-benzyloxycarbonyl-L-serine 3-lactone leads to poly(A -acyl-L-serine ester) My, = ca. 40 000), from which poly(L-serine ester hydrochloride) can be obtained by hydrogenation. ... 

Despite the relatively high reactivity of the serine hydroxy group, serine esters and serine-containing peptides with free hydroxy groups have been used as amine components in solution synthesis. Syntheses with free serine hydroxy groups can be performed with azides, without the risk of 0-acylation.b l Mixed anhydrides, active esters, and carbodiimides have also been successfully used for the coupling reactions. Therefore, for this couphng reaction, any over activation has to be avoided. 

The enzyme cholinesterase, effecting the degradation of ACh, consists of two active sites, the anionic and the esteratic site (Figure 9.1). In the first step, ACh is attached to the anionic site and the acetyl group of the ACh molecule is transferred to the amino acid serine. The resulting serine ester is then cleaved hydrolytically and the enzyme is regenerated. ... 

Oxazolidines are subject to ring-chain tautomerism. A variety of substituted oxazolidines in the solid state exist in the chain form, based on C NMR experiments <85X5919,92X4979). In solution, the two forms are in equilibrium, the position of which depends on the solvent and the substituents. Oxazolidines prepared from meta- and para-substituted benzaldehydes and 2-amino-2-methyl-propanol, norephedrine, norpseudoephedrine, and serine esters all give good linear plots for the equation log -l-log Ax=h where a are the Hammett-Brown values <93X6701,93JOC1967). 

Alternatively, amino acids can be polymerized by linkages other than the conventional peptide bond, yielding pseudopoly(amino acids) [26]. For example, the amino acid serine can be used to produce poly(serine ester), poly(serine imine), or conventional polyserine ... 

The advantage of this form of covalent catalysis presumably arises from the relative ease of positioning a nucleophile that is part of the protein itself and can be more easily handled in the interaction than a water molecule. Additionally, the serine ester intermediate is more reactive than the amide substrate, so that an attack by water is easier in the second step. 

Zhou, Q.X. and Kohn,J. (1990) Preparation of poly(L-serine ester) Astmctural analogue of conventional poly(Ij-serine). Macromolecules, 23, 3399-3406. 

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