BIOSYNTHESIS OF THE C15 MACROLIDE ANTIBIOTICS .1. BIOCHEMICAL ORIGIN OF THE 4 OXYGEN-ATOMS IN BREFELDIN A

The biochemical origin of the four oxygen atoms present in brefeldin A (1), a 16-membered macrolide antibiotic, is determined by feeding experiments with sodium [18O2,2-3H]acetate and 36O2 in which the labeling regiochemistry of 1 is shown by EI mass spectral analysis. Brefeldin A is labeled only at C-1 and C-15 by [18O2]acetate. These two positions are proven to be equally 18O enriched by analysis of the EI mass spectral fragmentation pattern of a derivative of 1. The resulting equal C-1 and C-15 labeling is interpreted as evidence for a one-step, intramolecular mechanism of macrolide ring formation in vivo. The incorporation of [18O2]acetate into 1 is accompanied by loss of 46% of its 18O enrichment as determined from the dilution of the [2-3H]acetate reference label. This is interpreted to imply that the carbonyls of the intermediate polyketide may exist as ketone hydrates in vivo. Since the antibiotic was labeled at C-4 and C-7 by 36O2, but by two different molecules of oxygen as proven by the results of a feeding experiment in a 32O2-36O2 gas mixture, it is concluded that the bioorganic mechanism for formation of the cyclopentanol ring of 1 does not closely parallel prostaglandin biosynthesis. A new hypothesis for this ring formation is made, based on the oxidosqualene to lanosterol cyclization as precedent. The lack of C-15 labeling of 1 by 36O2 is further evidence against a C-l6 fatty acid biosynthetic origin for 1. © 1979, American Chemical Society. All rights reserved.