In the course of our screening program for biologically active substances, we found that Streptomyces tsukubaensis No. 9993 produced a potent immunosuppressant, tentatively designated FR 900506. The structure of FR 900506, C_<44>H_<69>NO_<12> mp127-9°, has been elucidated as shown 1 to be 23-membered-ring lactone having allylic and, cyclohexyl propenyl groups on the ring by means of chemical degradation and spectral evidences. The absolute stereostructure of 1 has been determined by X-ray diffraction study and by isolating (s)-pipecolinic acid(2) from 6N HCl hydrolysis products of 1.
