Biosynthesis of elsinochromes C and D. Pattern of acetate incorporation determined by 13C and 2H nmr

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Abstract
Elsinochromes C and D have been isotopically labelled by supplementing cultures of the producing fungus with sodium [1-13C]-, [2-13C]-, [1,2-13C]-, or [2-13C, 2-2H3]acetate and the distribution of isotope has been determined by 13C and 2H nuclear magnetic resonance spectroscopy. The pattern of 13C labelling is consistent with assembly of the elsinochrome carbon skeleton from two heptaketide chains with loss of the terminal carboxyl groups, and dimerization to generate the 1,2-dihydrobenzo (ghi)-perylene ring system. Elsinochrome D was converted to the more soluble triacetate to examine the fate of 2H presented to the culture as [2-13C, 2-2H3]acetate. Deuterium labelling was restricted to the C-14 and C-16 methyl groups, a result consistent with these belonging to the "starter" C2 units of polyketide intermediates. Carbon-13 enrichment data for elsinochromes labelled from [1,2-13C]acetate indicated that the polyketide chains were formed from a highly enriched precursor pool with subsequent dilution accounting for a much lower average incorporation. Presence of spin–spin coupling between linked carbons derived from separate polyketide precursors implies that dilution occurs after the dimerization step. From the combined evidence a plausible pathway for elsinochrome biosynthesis is deduced.