In a purification study of novel vasodilator FK409 from the fermentation broth of Streptomyces griseosporeus, we found the existence of FR-900411, a precursor of FK409 in the broth, suggesting that FK409 was derived from FR-900411 via a chemical reaction. We therefore isolated FR-900411 from the broth. The structure of FK409 and FR-900411 were determined to be 1 and 2 respectively, on the basis of spectroscopic and chemical evidence.