The reaction begins with molecule 2 and the Grubbs catalyst dissolved in dichloromethane. A Grubbs catalyst essentially joins two ends, both of which are double bonds, and connects them to form a single double bond, creating one diene from two. In this case, it closes the two ends of the ring in an alkene metathesis reaction. This produces molecule 13. However, this is not the desired final product since the double bond is not in the correct place. Therefore, a rhodium chloride hydrate catalyst is used. The rhodium metal essentially shuffles hydrogens around because it is able to pull them off and delocalization allows the double bond to shift accordingly. Then when the rhodium deposits the hydrogen, the double bond forms in the correct place, forming the desired final product, pavidolide B.
