Involved in the biosynthesis of the spirotetramate antibiotics pyrroindomycins. Catalyzes the intramolecular cyclization forming the spiro-conjugate moiety in pyrroindomycins, via an exo-selective [4+2] cycloaddition reaction. 4-[(1R,2R,4aS,5S,8aR)-2-[(2R,3R,5E,7E)-3-ethyl-2-hydroxy-5,7-dimethylnona-5,7-dien-1-yl]-5-hydroxy-1-methyl-1,2,4a,5,6,7,8,8a-octahydronaphthalene-1-carbonyl]-2-methylidene-5-oxo-2,5-dihydro-1H-pyrrol-3-olate = (1S,3R,6R,8R,9R,11R,14S,15S,19R,20R)-8-ethyl-9,15-dihydroxy-3,4,6,20-tetramethyl-21,23-dioxo-24-azapentacyclo[20.2.1.0(1,6).0(11,20).0(14,19)]pentacosa-4,12,22(25)-trien-25-olate kcat is 342.6 min(-1). Antibiotic biosynthesis. Homodimer. The N-terminal 10-AA residues are absolutely required for enzymatic activity. Cells lacking this gene lose the ability to produce pyrroindomycins. Reaction occurs via a unique trapping mechanism whereby the lid-like action of the N-terminal tail imposes conformational constraints on the beta-barrel catalytic core, which enhances the proximity and polarization effects of reactive groups (1,3-diene and alkene) to drive cyclization in a regio- and stereo-specific manner. Truncated N-terminus.