The stereochemistry of the four stereoforms of 5‐chloro‐3‐methylmuconolactones could be deduced from NMR and stability data, and from the comparison with authentic (4R, 5S)‐5‐chloromuconolactone. Muconolactone isomerase of Alcaligenes eutrophus JMP 134 was shown to catalyze syn ‐elimination of hydrogen chloride from (4R, 5S)‐5‐chloro‐3‐methylmuconolactone, (4R, 5S)‐5‐chloro‐3‐methyl‐muconolactone and (4R, 5S)‐5‐chloromuconolactone to form 3‐methyl‐cis ‐dienelactone, 3‐methyl‐trans ‐dienelactone and a 3:1 mixture of cis‐ and trans‐dienelactone, respectively. 3‐methyl‐trans‐dienelactone was a substrate of pJP4‐encoded dienelactone hydrolase of A. eutrophus JMP 134, whereas 3‐methyl‐cis‐dienelactone transformation was negligible indicating a restricted substrate specificity of this enzyme. Both substrates were transformed into 3‐methylmaleylacetate which in turn was a substrate for maleylacetate reductase. This compound was shown to possess a cyclic structure (4‐hydroxy‐3‐methylmuconolactone) under acidic conditions.
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