Biosynthesis of Aurachins A-L in Stigmatella aurantiaca: A Feeding Study

The isolation of aurachins A-L (1-11) from Stigmatella aurantiaca strain Sg a15 is described. Their structures and relative configurations were deduced from spectroscopic data, in particular NMR. Three structural types were identified: A-type aurachins (1, 2, 6) are C-3 oxygen-substituted quinolines carrying a farnesyl residue on C-4, C-type aurachins (3, 4, 7-11) are C-4 oxygen-substituted quinolines carrying a farnesyl residue on C-3, and C-type aurachin E (5) has a [1,1a,8,d]imidazoloquinoline structure. Feeding of C-13-labeled precursors showed that the quinoline ring is constructed from anthranilic acid and acetate, and the farnesyl residue from acetate by both the mevalonate and nonmevalonate pathways. Further, feeding of labeled aurachin C (3) indicated the A-type aurachins are derived by a novel intramolecular 3,4-migration of the farnesyl residue that is induced by a 2,3-epoxidation and terminated by a reduction step. O-18-Labeling experiments indicated the new oxygen substituents originate from atomospheric oxygen. On the basis of these results a biosynthetic scheme covering all aurachins is proposed. It is further proposed that quinolones with an unorthodox substitution pattern, such as the 2-geranylquinolones from Pseudonocardia sp. and the 3-heptylquinolones from Pseudomonas sp., are formed by related rearrangement mechanisms.