Biomimetic synthesis of the tetracyclic core of berkelic acid

Acid-catalyzed condensation of 2,6-dihydroxybenzoic acid 3 with ketal aldehyde 14 in methanol at 25 degrees C, followed by CH2N2 esterification, gave a 4: 1: 4: 1 mixture of diastereomers 15b-18b in 60% yield. Equilibration of this mixture with TFA in CDCI3 provided tetracycle 15b ( 83% yield) with the complete skeleton of berkelic acid. A similar condensation at 0 degrees C afforded 15b - 18b and a reduction product 19b, which was probably formed by a 1,5-hydride shift.