Methionine oxidative lactonization

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). 

Figure 5.7 The possible biosynthesis of dithiophenes isolated from the roots of Arctium lappa (edible burdock). [O] = oxidation, [H] = reduction, — [H] = dehydrogenation (oxidation followed by the loss of water), [H2S] = addition of H2S or the biochemical equivalent to H2S, [CH3] = methylation due to the cofactor S-adenosyl methionine, SQL = sesquiterpene lactone (a guaianolide).

Griesbeck and co-workers further investigated PET reactions of sulfur-containing proteinogenic and non-proteinogenic amino acids and discovered an unusual solvent dependence of the product composition. Even more remarkable was the photochemical behavior of phthaloyl L-methionine 18 irradiation in acetone gave the tetracyclic lactone 20 in 72% yield (Scheme 6). In this particular case, PET oxidation at sulfur proceeds faster than a-decarboxylation vide infra). 

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