Stereochemistry of the enzymic lactonisation of cis,cis-muconic and 3-methyl-cis,cis-muconic acid

Experiments with 4-methyl[3,5,6-H-2(3)]pyrocatechol 7 have shown that the product of its enzymic oxygenation, 3-methyl-cis,cis-muconic acid 8, undergoes enzymic cyclisation by syn addition of carboxy groups to distal double bonds to form the (S)-4-methylmuconolactone 9 in the bacterium Pseudomonas putida and the (S)-3-methylmuconolactone 11 in the fungus Aspergillus niger. Similarly, syn cyclisation of the cis,cis-muconic acid 15 has been shown to occur in A. niger to form the (S)-muconolactone 16. The relative and absolute stereochemistries of enzymic cycloisomerisation of muconic, 3-carboxymuconic and 3-methylmuconic acid in bacteria and fungi are here compared and discussed.