MACROLIDE BIOSYNTHESIS .7. INCORPORATION OF POLYKETIDE CHAIN ELONGATION INTERMEDIATES INTO METHYMYCIN

Administration of [1-C-13] propionate to cultures of Streptomyces venezuelae SC 2366 gave methymycin (1), which was shown by C-13 NMR analysis to be labeled at the predicted sites, C-1, C-3, C-5, C-9, and C-11. Similarly, incorporation of [1,2-C-13(2)]acetate gave methymycin labeled at C-7 and C-8. A series of presumptive intermediates of polyketide chain elongation was also successfully incorporated. Thus, feeding of (2S,3R)-[2,3-C-13(2)]-2-methyl-3-hydroxypentanoyl N-acetylcysteamine (NAC) thioester 7a gave both methymycin (1) and neomethymycin (2) labeled as expected at C-10 and C-11. In a complementary experiment, (2S,3R)-[3-H-2,3-C-13]-2-methyl-3-hydroxypentanoyl NAC thioester 7b was incorporated into 1 and 2 without loss of deuterium. Finally, incorporation of (4R,5R)-[2,3-C-13(2)]-4-methyl-5-hydroxy-2-heptenoyl NAC thioester 10a gave 1 and 2 labeled at C-8 and C-9. These results support a processive model of polyketide chain assembly in which the functionality and oxidation level are adjusted subsequent to each condensation step.