100157-26-4

Through a trans-lactonization reaction, galbonolide B (1) was converted to 3 with the chiral secondary alcohol at C13 exposed for derivatization. Two independent methods were employed to determine the absolute chirality at C13. Both of these methods established S chirality at C13. Since the relative stereochemistry of galbonolide B had been determined from the X-ray structure, the absolute stereochemistry of galbonolide B was therefore formally established to be structure 1, which contradicted earlier speculations in the literature. A total synthesis of galbonolide B has been completed. A highly selective method was developed for the assembly of the peculiar diene unit using Martin's sulfurane reagent for the dehydration of the preceding tertiary alcohol 20. The chiral center at C4 was installed by 'contra-steric' enolate chemistry. A novel macro-Dieckmann cyclization was employed to generate the macrocycle. The desired configuration at C2 was obtained from the kinetic protonation of the corresponding enolate. Finally, a seldom used protecting group, 2,4,6-trimethylbenzylidene acetal, was employed for the glycol unit. It exhibited extremely facile hydrolysis under mildly acidic conditions without causing any decomposition of synthetic intermediates.

(Chemical Equation Presented) An ester enolate rearrangement/silicon mediated fragmentation cascade was used to convert 7 and 8 into the alcohol 9. The alcohol 9 was converted into the allylic alcohol 12 and then to the ester 14. A further ester enolate r