The study of chiral methyl groups

A later synthesis from Arigoni s laboratory uses a very ingenious, completely chemical process [124]. There are two crucial steps in which the geometries of two transition states in a pyrolytic reaction determine the overall outcome. In the first of these (which is an ene reaction ) a triple bond, -C = C-3H acquires H from one direction only, forming 

In another chemical synthesis, (2R)-[2H]glycine, 147, has been converted to (R )-acetate in excellent ( optical ) yield [126] (earlier methods were less efficient as noted by Floss and Tsai [127]). The conversion of bromoacetic acid, 148, to ethanol occurred with inversion of configuration. Other, enzymatic, syntheses of chiral acetates have been reported and are summarized by Floss and Tsai [127], 

While the syntheses described here should produce chiral acetate samples, methods are needed to prove the existence of chirality and to determine its extent. The first method was devised independently by the Comforth and Arigoni groups and is still widely used [122,128,129]. It depends on the use of two enzymes, malate synthase and fumarase. Thus, overall, the sample of acetic acid is first converted to its CoA derivative (either with acetate kinase and phosphotransacetylase or chemically with ClCOOC2H5 and Co ASH) and then to malate and fumarate  

The method relies on an isotope effect so that H is removed in preference to 2H and 3H in the malate synthase reaction. In other words, the hydrogen isotope which is eliminated (in formation of the methylene group of malate) is determined by an intramolecular isotope effect such effects normally follow the sequence kH k2H k3H. Whether 3H is located in the pro-R or pro-S position of malate is then determined from the known selectivity of fumarase. 

The species, 152, derived by elimination of 3 H is of no practical interest. Tritium is only present at tracer levels so there are large numbers of CH22HCOOH molecules also present these will give rise to 152 by elimination of H. As for 150 and 151, if the malate synthase shows a normal isotope effect ( H k2H), 150 will predominate over 151. 

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