X-ray analysis has determined the structure of tetronolide, which has a unique Spiro y-lactone group.Tetrocarcins are novel antitumor antibiotics produced by Micromonospora chaZcea KY11091. Mild hydrolysis of tetrocarcin A (C67Hg6N20214), a major component of tetrocarcins, gave the component 1 and two moles each of L-amicetose and L-digitoxose. 1) Further hydrolysis of the component 1 gave the novel nitrosugar and the aglycone, tetronolide. The molecular formula was determined as C32 H 4G 0 8 Cfound:m/e 552.2697,required:m/e 552.27231 by a high resolution mass measurement. We report here an X-ray analysis of tetronolide.Recrystallization of tetronolide from ethyl acetate containing trace amount of methanol gave well formed crystals; orthorhombic with space group P212121, a=20.146(5), b=22.679(4), c=7.845(2) i, ~=3584 A O3 ,Dx=l.lgg.cm -3 , z=4. The intensity data were collected on a Rigaku four-circle automated diffractometer with graphite monuchromated MO Kcr radiation. 3577 independent reflexions (/Fo(>~.OU(/FO)),! with 28250' were obtained and corrected for Lorentz and polarization effects but not for absorption and extinction. The structure was solved by MULTAN 78.2) The resulting r: map revealed the locations of all non-hydrogen atoms. All hydrogen atoms and the disordered solvent molecules were located by the difference syntheses. Block-diagonal least-squares refinement of positional and thermal parameters reduced R to 0.083. Although the absolute configuration has not been determined, a perspective view of one of its enantiomers is shown in Fig.1 and the corresponding chemical structure is shown in Fig.2. Tetronolide exhibits several unique structural features: first, it has a thirteen-membered macrocyclic ring; second, the macrocyclic ring can be also recognized as a fourteen-membered macrolide which COntainS a sPiro Y-lactone structure; third, it has a B-diketone part and can form metal complexes. There 1s an intramolecular hydrogen bond between a carbonyl group and a hydroxyl group as shown by dotted lines in the figures. Two cyclohexene rings take half-chairconformation and the cyclohexane ring does chair conformation. The crystal structure and the nature of the metal complexes of tetronolide will be discus; in detail elsewhere.