title: Unique Solvent-Dependent Atropisomerism of a Novel Cytotoxic Naphthoxanthene Antibiotic FD-594.
abstract: The absolute stereochemistry of FD-594 1, a new cytotoxic antibiotic, was determined by X-ray diffraction, and its conformation was studied by CD and NMR spectroscopy. The aglycon part of 1 was found to have (3R,6S,7S) configuration. Particularly interesting was the solvent-dependent atropisomerism of 1 and related compounds. The CD spectra of 1 exhibited in two solvent systems almost opposite mirror-image curves depending on the solvent. While a large negative Cotton effect (Deltaepsilon = -33.9, 279 nm) was observed in CHCl(3), a similar positive Cotton effect (Deltaepsilon = 38.9, 279 nm) appeared in methanol most probably due to dramatic conformational changes. Similar chiroptical reversal was observed in aglycon 2 and aglycon methyl ether 4. These results can be best described in terms of solvent-dependent atropisomerism. This constitutes the first observation of solvent-dependent atropisomerism of a natural product. The crucial factor that perturbs the stable conformation in different solvents is discussed on the basis of molecular mechanics calculations.
authors.affiliation: Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan, and Research Laboratories, Taisho Pharmaceutical Co. Ltd., 1-403, Yoshino-cho, Omiya-shi, Saitama 330-8530, Japan.
authors.forename: Tadashi
authors.initials: T
authors.lastname: Eguchi
authors.forename: Katsuya
authors.initials: K
authors.lastname: Kondo
authors.forename: Katsumi
authors.initials: K
authors.lastname: Kakinuma
authors.forename: Hidehiro
authors.initials: H
authors.lastname: Uekusa
authors.forename: Yuji
authors.initials: Y
authors.lastname: Ohashi
authors.forename: Kazutoshi
authors.initials: K
authors.lastname: Mizoue
authors.forename: Ya-Fang
authors.initials: YF
authors.lastname: Qiao