Structure of quinomycin antibiotics.

Sir: In the course of screening for antitumor antimetabolites,l) L.J. HANKA of these laboratories submitted lyophilized beer from a control culture lacking antimetabolite activity, for in vivo assay against PS-3 leukemia" in mice. The lyophilized beer solids significantly prolonged the life span of the leukemic mice and our efforts were enlisted to isolate the active principle. The lipophilic active principle was readily extracted with solvents; mycelial cake was the richest source of active extract. Solvent extracts had reasonable in vitro activity and bioautography distinguished the activity from all the antibiotics in the Upjohn collection of known antibiotics. Mycelial extracts were defatted with petroleum ether and purified by silica gel chromatography, preparative thin-layer chromatography, and counter current distribution yielding a homogenous crystalline antibiotic, U-48, 160. Infrared mull spectra suggested a very close relationship with quinomycin A (echinomycin)3) from which U-48, 160 had already been differentiated by bioautography leaving the probability that the antibiotic was a member of the quinoxaline4) family. Although the ultraviolet spectrum and optical rotation of U-48, 160 were consistent with the quinoxaline antibiotics,5,6) NMR spectra, both proton and 13C, apparently differentiated the antibiotic from the reported structures shown (Fig. 1). An apparent S-CH3 (s, 6 2.1) was inconsistent with published* quinomycin and triostin structures." Since structures of the minor components of the quinoxaline family were based on comparison with quinomycin A, the structure determination" of which was accomplished without the benefit of NMR, we investigated the proton and 13C spectra of that antibiotic. To our surprise, the spectra betrayed the presence of the S-CH3 in a sample which had been found identical to authentic echinomycin** by bioautography on 3 systems as well as comparison of infrared spectra. The 100 MHZ proton spectrum*** (Fig. 2) of quinomycin A shows the presence of 64 protons (4 exchangeable) and agrees well with the expected" resonances for quinoxaline, alanine, serine, and N-methylvaline fragments of the molecule. The 2 N-CH3's adjacent to the bridge section are present but the 2 expected') isolated