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