Involved in the final steps of cholesterol and steroid degradation. Probably catalyzes the introduction of a double bound into the C ring of 5OH-HIC-CoA, leading to the formation of (5R,7aS)-5-hydroxy-7a-methyl-1-oxo-3,5,6,7-tetrahydro-2H-indene-4-carboxyl-CoA (Probable)