CERCOSPORA-BETICOLA PHYTOTOXINS - CEBETINS THAT ARE PHOTOACTIVE, MG2+-BINDING, CHLORINATED ANTHRAQUINONE XANTHONE CONJUGATES

Two Cercospora beticola toxins, isolated from cultures of the sugar beet pathogen, were identified as tautomeric compounds and named cebetin A (1) and cebetin B (2). The structure of cebetin B (2) and its absolute configuration were determined by single-crystal X-ray diffraction. The structure of cebetin A (1) was elucidated on the basis of UV-visible, NMR, and mass spectral data. The molecular backbone of cebetins is totally different from that of the known phytotoxin cercosporin and comprises partially hydrogenated xanthone and anthraquinone moieties connected through two seven-membered rings. The xanthone and anthraquinone systems contain respectively a chlorine atom and an epoxide function. Cebetin B crystallizes in the ortborhombic space group P22(1)2, with a = 14.449 (3) angstrom, b = 22.269 (2) angstrom, c = 14.061 (2) angstrom, V = 4524.3 angstrom3, Z = 4, and D(x) = 1.369 g/cm3. The structure was determined from 3670 diffractometer data and refined to a final R = 0.066. In the crystal, cebetin B molecules exist as a dimeric complex, (C31H21O13Cl.2CH3OH)2Mg2, Where the two molecules of cebetin B share two Mg2+ ions and are related by a 2-fold axis. Spectroscopic data showed that cebetin A is a metal-free keto-enol tautomer of cebetin B. Both cebetins A and B at a concentration of 1 ppm were lethal to sugar beet cells in suspension culture in the presence of light. This effect was not observed in the absence of light.