Y-Eliminations

Pyridoxamine phosphate serves as a coenzyme of transaminases, e.g., lysyl oxidase (collagen biosynthesis), serine hydroxymethyl transferase (Cl-metabolism), S-aminolevulinate synthase (porphyrin biosynthesis), glycogen phosphoiylase (mobilization of glycogen), aspartate aminotransferase (transamination), alanine aminotransferase (transamination), kynureninase (biosynthesis of niacin), glutamate decarboxylase (biosynthesis of GABA), tyrosine decarboxylase (biosynthesis of tyramine), serine dehydratase ((3-elimination), cystathionine 3-synthase (metabolism of methionine), and cystathionine y-lyase (y-elimination). 

Carba-sugars of the a-L-altro and P-D-gluco modifications were prepared from 149 by way of 155. 0-Mesylation of 155 with an excess of mesyl chloride and pyridine resulted in formation of the cyclohexenealdehyde 159, accompanied by y -elimination. Reduction of 159 with sodium borohydride gave the cyclohexenemethanol 160, which is the antipode oP 141. 

Similarly, phenols with unsaturated side chains can form five- and six-membered rings. In these systems the quaternary carbon imposes the (3, y-elimination. As with the above... 

Scheme 161 Cyclopropane formation by cathodic y-elimination of a 1,3-dibromide.

Eq.(18) has two complex roots with real part equal to zero, and consequently it is possible to deduce a relation between x and y. By substituting Eq.(18) into Eq.(12) one obtains a parametric equation xo = fi y )- Eliminating xo between xo = fi y ) and Eq.(13), the parametric equations of self-oscillating behavior are deduced ... 

This pyridoxal-phosphate-dependent enzyme [EC 4.2.99.9], also known as cystathionine y-synthase, catalyzes the reaction of O-succinyl-L-homoserine with L-cysteine to produce cystathionine and succinate. The enzyme can also use hydrogen sulfide and methanethiol as substrates, producing homocysteine and methionine, respectively. In the absence of a thiol, the enzyme can also catalyze a /3,y-elimination reaction to form 2-oxobu-tanoate, succinate, and ammonia. 

When the reaction shown in Scheme 22 was applied to cyclic 1,2- or 1,3-snlfates, instead of the corresponding 1,2- or 1,3-dilithio componnds, olefins or cyclopropanes were obtained through the corresponding p or y-elimination processes, respectively. AcmaUy, the second reaction has been nsed to prepare snbstitnted cyclopropanes 64 in a... 

A typical example showing the feasibility of cyclobutanes to undergo y-elimination is the sodium hydride induced elimination in 64, which provides a useful route to some electronega-tively substituted bicyclo[t.l.O]butanes 65.24... 

In other cases, sulfenic acid elimination can involve y-hydrogen atoms with the formation of cyclopropane derivatives, y-Elimination is favored when DMSO is the reaction solvent. An example involving l-methylsulfinyl-2-ethyl-3-phenyl propane [14198-15-3] is shown in equation 13 (45) ... 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

Pyridoxal-5 -phosphate participates in many reactions with a-amino acids, including transaminations, a decarboxylations, racemizations, a,/3 eliminations, j8,y eliminations, aldolizations, and the (3 decarboxylation of aspartic acid. 

The observation of this y-elimination reaction has raised the question, whether in DNA the related C(l ) radical can also lead to strand breakage. However due to the negative charge at the phosphate group in 3 -position, the rate of the y-elimi-nation reaction (54) should be considerably slower than that of reaction (53). 

Methionine 7-lyase, in contrast, has been purified from Aeromonas sp., Pseudomonas putida and Clostridium sporogenes (361. This enzyme effects a a,y-elimination of methanethiol and ammonia from L-methionine with the direct formation of 2-ketobutyrate ... 

The corresponding halocrotonate esters react with EtO- mostly via the j8,y-elimination-addition route. However, the isolation of a 2-butyne... 

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