Homoserine lactone preparation

A number of synthetic methods to prepare all the three main classes of AHLs have appeared in the literature. Initially the methods were developed to prepare the authentic AHLs with defined stereochemistry to confirm the identity of the natural signal molecule. Subsequently, when some of these molecules, e.g. AT-(3-oxododecanoyl)-L-homoserine lactone were found to impact on eukaryotic signalling systems [ 16,50-52], detailed studies not only of their preparation but also of their structural analogues were undertaken by many laboratories. 

Azetidine-2-carboxylic acid (2) is commercially available. It is readily prepared as the racemate by refluxing 2,4-dibromobutyric acid ester with benzhydrylamine in acetonitrile. If benzyl 2,4-dibromobutyrate is treated with benzhydrylamine, the resulting benzyl TV-benz-hydryl-D,L-azetidine-2-carboxylate is hydrogenolytically processed to D,L-azetidine-2-car-boxylic acid in a one-step reaction. 101,107 Resolution of the racemate can be performed by the method of Vogler 108 via fractional crystallization of the Z-D,L-Aze-OH-H-Tyr-N2H3 salt thereby the salt of the D-imino acid precipitates first from methanol. 96 A stereoselective synthesis of A-tosyl-L-azetidine-2-carboxylic acid can be achieved by a two-step reaction from N-tosyl-L-homoserine lactone. 94 ... 

Methionine carboxymethylsulfonium salts These derivatives of methionine (isomers) are prepared by treating proteins with iodoacetic acid the reaction is most specific for methionine at acid pH ( 3.5). These derivatives are not affected by performic acid oxidation (see under methionine sulfoxide), but are degraded by acid hydrolysis to give methionine, carboxymethyl-homocysteine, homoserine and homoserine lactone (Gundlach et al. 1959). 

In Ramalingam and Woodard s synthesis (117) the protected aspartate semialdehyde 113 was synthesized from (2S)-aspartic acid and then converted to the labeled alcohols 115 using (R)- and (S)-alpine boranes (Scheme 33). Hydrolysis then led directly to (2S,4R)- and (2S,4S)-[4- H,]homoserine lactones 112. Since the enzyme aspartase may be used to prepare samples or (2S, 3R)-[3- H J- and (2S, 3S)-[2,3- H2]aspartate (see Section IX), the synthesis, or a modification using more direct reduction methods, could be... 

S,3/ )-[2,3- H2]Homoserine lactone 112c, Ha = Hc = H, prepared as in Section V, Scheme 35, was converted to the diketopiperazine 354 (Scheme 89). This was converted to the corresponding bromide and the bis(lactim ether) was cyclized to the spiro compound 355, He = Although the cyclization was not entirely stereospecific, hydrolysis of 355, = H, gave... 

S-Sulfocysteine was prepared by the method of Inglis and Liu (1970). N -Mono- and dicarboxymethyllysine and 1-, 3-, and 1,3-carcarboxymethylhis-tidine were prepared by the method of Gundlach et aL (1959a). Homoserine lactone was prepared by the method of Ambler (1965). S-Carboxyethyl-cysteine was prepared by the method of Weil and Seibles (1961). Other standards were commercially available or donated. ... 

Solutions of homoserine and its lactone slowly reach equilibrium under most conditions at room temperature. As a result, samples of these derivatives prepared for analysis often gradually change in concentration even during the chromatographic steps, and color values usually reflect such losses. Acidic conditions favor lactone formation, and trifluoroacetic acid at 20°C for 1 hr can be used to convert essentially all of the homoserine (free or in peptides) to the lactone form (Ambler 1965). Alkaline conditions favor homoserine formation, but may often be too harsh for many peptide procedures (see Ambler 1965). 

A purified preparation of homoserine deaminase from rat liver was found by Binkley and Olson" not to attack serine or threonine. The latter finding would appear to rule out the possibility that threonine is an intermediate in reaction 19. Although some activation by AMP and glutathione was observed, neither substance was considered to be essential. The enzyme did not attack n-homoserine nor the lactones of L-homoserine or DL-homoserine. 

The formation of a-ketobutyric acid from homoserine (reaction 5) is based on the observation of Carroll et al. that homoserine or its lactone could be converted to this keto acid by the action of liver extracts under anaerobic conditions, whereas a-aminobutyric acid did not produce any keto acid upon incubation with the enzyme preparation under the conditions of cystathionine cleavage. 

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