Galbonolide B

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 rearrangement furnished the stereochemical identity of galbonolide B 1. Base mediated macrocyclization of the acetate ester 16 followed by base mediated alkylation and acetal deprotection gave galbonolide B 1.

Galbonolides A and B are antifungal compounds, which are produced by Streptomyces galbus. A multimodular polyketide synthase (PKS) was predicted to catalyze their biosynthesis, and a methoxymalonyl-acyl carrier protein (methoxymalonyl-ACP) was expected to be involved in the biosynthesis of galbonolide A. Cloning of a methoxymalonyl-ACP biosynthesis locus (galGHIJK) and the flanking regions has revealed that the locus is colocalized with beta-ketoacyl synthase (KAS)-related genes (orf3, 4, and 5), but separated from any multimodular PKS gene cluster in S. galbus. A galI-disruption mutant (SK-galI-5) is unable to produce galbonolide A, but can synthesize galbonolide B, indicating that galGHIJK is involved in the biosynthesis of galbonolide A. A disruption mutant of orf4 is severely impaired in the production of both galbonolides A and B. These results indicate that galGHIJK and the KAS genes are involved in the biosynthesis of galbonolides, although they are not colocalized with a multimodular PKS gene cluster. We further propose that a single galbonolide PKS generates two discrete structures, galbonolides A and B, by alternatively incorporating methoxymalonate and methylmalonate, respectively.

The galbonolide antibiotics are non-glycosylated heptaketide 14-membered macrolides. These antibiotics exhibit broad-spectrum fungicidal activities, including against the human pathogen Cryptococcus neoformans. Previously, galbonolides B and E were isolated from the marine actinomycete Streptomyces sp. LZ35. By bioinformatics analysis, the putative galbonolide biosynthetic gene cluster, gbn, was identified in the genome of strain LZ35. In order to verify that the core genes (gbnA-E) are sufficient for synthesizing the basic structure of galbonolide as previously proposed, we performed the heterologous expression of gbnA-E in a "clean background" host Streptomyces coelicolor ZM12, in which all the native polyketide synthase genes have been deleted. As expected, the production of galbonolide B (1) was detected in the transformant. To the best of our knowledge, this is the first report that demonstrates the essential role of gbnA-E in the biosynthesis of galbonolides by heterologous expression. This heterologous expression system would be helpful to generate novel galbonolide derivatives by co-overexpression of unusual biosynthesis extender units.