The genus Aspergillus includes several hundred species of mold, which collectively produce a large number of natural products. Many of these natural products have been found to possess interesting biological properties. Two of these natural products were both synthesized from a common tricyclic precursor (compound 4 below) with an intricate shape. Tricyclic compound 4 was made from bicyclic compound 1 using AIBN and n-Bu3SnH to start a radical cyclization reaction. Draw a mechanism for the cyclization step in which bicyclic intermediate 2 is converted into tricyclic intermediate 3.