Methyl -cysteinate

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

Lenthionine has the characteristic shiitake flavor. It is formed from the precursor, lentinic acid 14 by complex reactions involving a C-S lyase enzyme.30 Cyclic polysulfides occur in other Basidiomycete mushrooms (Genus Micromp-hale and Colly bid), in some red alga, and in seeds of Parkia speciosa. The latter contain lenthionine and 1,2,4-trithiolane (1,2,4-trithiacyclopentane) 17 as well as compounds with 4, 5, or 6 sulfur atoms.31 These seeds are valued in Indonesia for a unique, onion-like odor. Djenkolic acid and dichrostachinic acid S -[(2-carboxy-2-hydroxyethylsulfonyl)-methyl]cysteine are converted by a C-S lyase enzyme to cyclic polysulfides djenkolic acid yields 1,2,4-trithiolane and 1,2,4,6-tetrathiepane the latter is also formed from dichrostachinic acid.32... 

The acid-catalysed oxidation of a protected S-methyl cysteine, which gives poor diastereoselectivity when oxidized in conventional solvents, shows density-dependent diastereoselectivity as shown in scheme 6.1 [8], Here, tert-butyl hydrogen peroxide (TBHP) is used as the oxidant and the reaction is catalysed by an Amberlyst resin (a solid acid). By tuning the pressure at which this reaction was carried out, almost 100% selectivity to one diastereomer could be achieved (Figure 6.4). 

Figure 6.4 Effect of pressure on the diastereoselective oxidation of a protected methyl cysteine. Reproduced by Permission of the Royal Society of Chemistry...

L cysteine (118) giving dihydroaemginoic acid (119), reduction of the carboxyl group to the aldehyde 121 and condensation of the latter with L-iV-methyl-cysteine. Details will be given for the procedure worked out by Zamri and Abdallah 397). The first condensation step was effected in a phosphate buffer (pH 6.4) to minimize epimerization at C-4. Then the carboxyl group was reacted with iV,(9-dimethyl-hydroxylamine (120) using diethylcyanophosphonate as condensation agent. Reduction with LiAlHa yielded the aldehyde 121, which then was treated with L-A-methyl-cysteine. A mixture of the four stereoisomers of (66), (4 R,2"5,4"R), (4 5,2"5,4"R), A R,2 R,A R), A S,2 R,A R) in a ratio of 2 1 2 5 was obtained. 

In all of the above reactions, a chiral center of the alkene was located in the allylic position. However, as shall be demonstrated next, more distant chiral centers may also lead to highly selective cycloadditions with 1,3-dipoles. In two recent papers, the use of exocyclic alkenes has been applied in reactions with C,N-diphenylnitrone (165,166). The optically active alkenes 109 obtained from (S)-methyl cysteine have been applied in reactions with nitrones, nitrile oxides, and azomethine ylides. The 1,3-dipolar cycloaddition of 109 (R=Ph) with C,N-diphenyl nitrone proceeded to give endOa-1 Q and exOa-110 in a ratio of 70 30 (Scheme 12.36). Both product isomers arose from attack of the nitrone 68 at the... 

S)-a-methyl-cystein derivatives 190,191) using (190) as chiral auxiliary reagent,... 

The enzyme contains five posttranslationally modified amino acids near the active site N-methyl-histidine, 5-methylarginine, 2-methylglutamine, 2-methyl-cysteine, and thioglycine in a thiopeptide bond. The latter may be the site of radical formation.4583 13... 

The metal ion-promoted hydrolysis of a number of bidentate esters such as methyl 2,3-diaminopropionate (8), methyl histidinate (9), methyl cysteinate (10) and the ethyl ester of ethylenediaminemonoacetate (11) have been studied. The first three esters give very thermodynamically stable metal complexes in solution with pendant ester functions.64"71 Typical kinetic data for these systems are given in Tables 9 and 10. 

Blood proteins, such as hemoglobin, may be used in tests of human exposure because blood is readily and safely accessible. For example, the exposure of mice to ethylene oxide or dimethylnitrosamine was estimated by measuring alkylated residues in hemoglobin. The method was subsequently extended to people exposed occupationally to ethylene oxide by measuring 7V-3-(2-hydroxycthyl) histidine residues in hemoglobin. Similarly methyl cysteine residues in hemoglobin can be used as a measure of methylation. 

The preparation of protected (/ )-2-methyl-cysteine by Fukuyama starts with the enantio-selective discrimination of the prochiral ester groups in 6 with pig liver esterase (Scheme 3) [5]. The ester function of the resulting product 7 is selectively reduced (7 16). Cyclization to the )9-lactone gives compound 17. Attack of the thioacetate at the )9-lactone methylene carbon atom provides the (/f)-compound 18. Selective reduction of the carboxylic acid function in 7 gives the (S)-compound 19 in an analogous fashion. 

Pattenden s synthesis of (/ )- and (S )-2-methyl-cysteine [12] is based on Seebach s self regeneration of chirality [13]. Scheme 5 shows the synthesis of the (/ )-isomer 5b. 

From the (/ )-cysteine methyl ester 22 the thi-azolidine 9 is obtained, which can be deproto-nated with LDA. The attack of the methyl iodide on the enolate takes place from the side opposite to the bulky terr-butyl group (23 10). The auxiliary chiral center is removed under acidic conditions and (/ )-2-methylcysteine methyl ester hydrochloride Sb X FICI is obtained. (S)-2-Methyl-cysteine methyl ester hydrochloride ent-5b X FlCl can be prepared in the same way from (Sl-cysteine. 

For the synthesis of Thiangazole, Pattenden and co-workers prepared the oxa-zole derivative 25.3 [Scheme 5.25] from the a-methyl cysteine derivative 25.1 in four steps.55 The final step in the sequence involved acidolytic cleavage of both the N-Boc and S-Boc protecting groups with anhydrous HC1 in ether. However, in certain circumstances, an N-Boc group can be cleaved in the presence of an S-Boc group and a /erf-butyl ester [Scheme 5.26).56... 

O -KG = a-ketoglntarate ACmC = 5-(L-a -aminoadipoyl)-L-cysteinyl-L-S-methyl-cysteine ACOY = S-(L-a-... 

A,A-dialkyldithiocarbamates produce stable, square-planar rhodium(II) complexes of the type [Rh(S2CNR2)2] (R = Me, Et) similar anionic hfr(maleonitriledithiolate) are also known. Neutral complexes are formed by the thiol containing amino acids cysteine, methyl cysteine, or penicillamine. It is noteworthy that these paramagnetic [Rh(aminoacid)2] complexes can easily be prepared from dimeric, diamagnetic [Rh(02CMe)2MeOH]2. ... 

CH3SH Methionine, methionine sulfoxide, methionine sulfone, S-methyl cysteine... 

Outside of some occasional catalyst deactivation, palladium catalysts are noted for their general resistance to catalyst poisons. They are also relatively unaffected by chloride and bromide ions and only moderately inhibited by iodide ions which makes palladium quite useful for the hydrogenolysis of carbon-halogen bonds. The most striking example of this inertness to catalyst poisons is illustrated by the hydrogenolysis of methyl cystine to methyl cysteine over palladium (Eqn. 11.13),a reaction that occurs on one of the most powerful of all... 

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