NARGENICIN BIOSYNTHESIS - INCORPORATION OF POLYKETIDE CHAIN ELONGATION INTERMEDIATES AND SUPPORT FOR A PROPOSED INTRAMOLECULAR DIELS-ALDER CYCLIZATION

A series of proposed polyketide chain elongation intermediates has been synthesized as the N-acetylcysteamine (NAC) thioesters and incorporated into nargenicin (1) in C-13-labeled form by administration to both actively fermenting and resting cultures of Nocardia argentinensis. Thus, feeding of (2S,3R)-[2,3-C-13(2)]-2-methyl-3-hydroxypentanoyl NAC thioester 5a gave nargenicin bearing contiguous labels at C-16 and C-17, as established by C-13 NMR. In a complementary experiment, (2S,3R)-[3-H-2,3-C-13]-2-methyl-3-hydroxypentanoyl NAC thioester 5b was incorporated into nargenicin without loss of deuterium. Incorporation of the triketide substrate (2E,4R,5R)-[2,3-C-13(2)]-2,4-dimethyl-5-hydroxy-2-heptenoyl NAC thioester 8c gave nargenicin labeled at C-14 and C-15. Finally, feeding of the tetraketide dienoic ester, (2E,4E,6R,7R)-[2,3-C-13(2)]-4,6-di-methyl-7-hydroxy-2,4-nonadienoic acid NAC thioester 9d, gave nargenicin labeled, as expected, at C-12 and C-13. These results provide further confirmation for a processive model of polyketide chain elongation in which the stereochemistry and oxidation level are adjusted prior to each successive round of condensation-chain extension. The incorporation of the tetraketide precursor also lends support to a proposed intramolecular Diels-Alder mechanism for formation of the characteristic octalin ring system of 1.