Stable isotope feeding studies reveal a steroid 5(6→7)abeo ring contraction biogenesis for the antibiotic solanioic acid produced by cultures of the fungus Rhizoctonia solani

Culture feeding experiments with [1-13C]-acetate, [2-13C]- acetate, and [1,2-13C]-acetate have shown that the steroid ring B contraction involved in the biogenesis of the unprecedented carbon skeleton of the antibiotic solanioic acid (1) by the fungus Rhizoctonia solani involves cleavage of the C-5/C-6 bond. The study revealed that 9-epi-solanioic acid (4), which spontaneously converts to solanioic acid (1), is also produced by the cultures and it may be the actual natural product.