Catalyzes a syn cycloisomerization. (S)-muconolactone = cis,cis-muconate + H(+) Aromatic compound metabolism; beta-ketoadipate pathway; 5-oxo-4,5-dihydro-2-furylacetate from catechol: step 2/3. Homooctamer. Belongs to the mandelate racemase/muconate lactonizing enzyme family.