Methionine, chiral methyl

The amino acid methionine is formed by a melhylation reaction of homo cysteine with iV-methyltetrahydrofolate. The stereochemistry of the reactior has been probed by carrying out the transformation using a donor with a "chiral methyl group" that contains protium (H), deuterium (D), and tritium (T isotopes of hydrogen. Does the methylation reaction occur with inversion oi retention of configuration ... 

Synthesis of methionine and AdoMet carrying a chiral methyl group of known configuration. 

Divalent sulfur compounds are achiral, but trivalent sulfur compounds called sulfonium stilts (R3S+) can be chiral. Like phosphines, sulfonium salts undergo relatively slow inversion, so chiral sulfonium salts are configurationally stable and can be isolated. The best known example is the coenzyme 5-adenosylmethionine, the so-called biological methyl donor, which is involved in many metabolic pathways as a source of CH3 groups. (The S" in the name S-adenosylmethionine stands for sulfur and means that the adeno-syl group is attached to the sulfur atom of methionine.) The molecule has S stereochemistry at sulfur ana is configurationally stable for several days at room temperature. Jts R enantiomer is also known but has no biological activity. 

Methyl isovalerate, azeotropic mixtures with butyl alcohols, 4 395t Methyl ketones, acetic anhydride used in synthesis, 1 148 Methyllithium, 14 249 15 147 Methylmagnesium chloride, 16 319 (R)-(—)-Methylmandelic acid chloride, chiral derivatizing reagent, 6 76t Methyl mercaptan production, 15 17 3-Methylmercaptopropionaldehyde (MMP), intermediate in methionine synthesis, 1 268, 269, 276... 

The laboratories of Floss and Arigoni laid the experimental foundation for the stereochemical analysis of methyl group transfer reactions, on the basis of the synthesis from chiral [ H, H, H]acetate of methionine and S-adenosylmeth-ionine carrying a chiral [ H, H, H]methyl group [335, 341), for example. 

It should be noted that the sulfur atom in methionine sulfoxide is a new chiral center, hence a pair of diastereosimers is present in the intermediates. This is a minor complication that is absent from an alternative method for the masking of the thioether its more recently proposed conversion to a tert.sulfonium salt by the reaction of an activated N-acyl derivative of the amino acid with methyl p-toluenesulfonate ... 

Supported liquid membranes (SLMs) consisting of 5% tri-n-octylphosphine oxide (TOPO) dissolved in di-w-hexylether/n-undecane (1 1) have been used in the simultaneous extraction of a mixture of three stUbene compounds (dienestrol, diethylstilbestrol, and hexestrol) in cow s milk, urine, bovine kidney, and liver tissue matrices [183]. The efficiencies obtained after the enrichment of 1 ng/1 stilbenes in a variety of biological matrices of milk, urine, liver, kidney, and water were 60-70, 71-86, 69-80, 63-74, and 72-93%, respectively. A new method to contribute to the discrimination of polyphenols including resveratrol with synthetic pores was proposed [184]. The work [185] evaluated two types of commonly available chiral detectors for their possible use in chiral method development and screening polarimeters and CD detectors. Linearity, precision, and the limit of detection (LOD) of six compounds (trans-stilbene oxide, ethyl chrysanthemate, propranolol, 1-methyl-2-tetralone, naproxen, and methyl methionine) on four common detectors (three polarimeters and one CD detector) were experimentally determined and the limit of quantitation calculated from the experimental LOD. trans-Stilbene oxide worked well across all the detectors, showing good linearity, precision, and low detection limits. However, the other five compounds proved to be more discriminating and showed that the CD detector performed better as a detector for chiral screens than the polarimeters. 

The preparation of l-[ C]methionine (Comar et al. 1976 Langstrom et al. 1987 Schmitz et al. 1995) and l-[4-ornithine (Ding et al. 1989) from [ C]methyl iodide and hydrogen [ C] cyanide, respectively, are examples of syntheses where the chiral center is established before introduction of the label (O Fig. 41.36). 

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