The total synthesis of the potent PAF antagonist ginkgolide B has been accomplished. The complex architecture of ginkgolide B which includes six rings, eleven stereogenic centers, ten oxygenated carbons, and four contiguous fully substituted carbons is a daunting challenge for chemical synthesis. The synthesis of ginkgolide B was accomplished through a stereoselective intramolecular photocycloaddition of enone 5 to construct the congested core of the molecule. The photocycloaddition substrate was prepared through technology for the construction of carboalkoxycyclopentenones previously reported from these laboratories. Regioselective cyclobutane fragmentation and further functionalization of the photoadduct 4 provided the key pentacyclic intermediate. Acid-catalyzed rearrangement and epoxide opening were key transformations in the production of ginkgolide B from the pentacyclic intermediate.