1 13 (72s/o) 1 4 (54%)rhodium catalysis. The existing postulate that diazo compounds initially cyclopropanate furans and the production of both types of products from 1 suggest a unified mechanism for these reactions involving the acylcyclo-propane. An explanation for the favored conversion of this adduct to the dihydrofuran product rather than a dienal is then required (eq 3). Ring opening to the zwitterion with cyclic dicarbonyls could be aided by "spiroactivation".'4 NMR experiments aimed at identifying intermediates in the cycloadditions to give 3 or 6 have thus far given no evidence for such cyclopropanes. An alternative mechanism that has been proposed previously for dipolar addition involves direct generation of the zwitterion.' This is difficult to reconcile with the known preference of furan for electrophilic attack a t the 2-position. (14) Danishefsky, S. Acc. Chem. Res. 1979,12,66.In summary, the dipolar cycloaddition of cyclic diazo 1,3-diketones provides a rapid entry into polyheterocyclic systems. The influence of ring size and breadth of application of this reaction will be established in the future.It is already apparent, however, that the method provides an expeditious synthetic route toward such natural heterocycles as aflatoxin.'6 Aflatoxin E , Acknowledgment. The support of NIH grant GM-38226 and the Johnson Matthey Metals Loan Program is gratefully acknowledged. Supplementary Material Available: Experimental procedures and spectral data, ORTEP diagrams and tables of crystallographic data, atomic positional and thermal parameters, and bond lengths and angles for 14 (12 pages); listing of observed and calculated structure amplitudes (5 pages). Ordering information is given on any current masthead page. (15) Woff, S.; Hohann, H. M. R. Synthesis 1988,10,760. Townwnd, C. A.; Whittamore, P. R. 0.; Brobst, S. W. J. Chem. SOC., Chem. Commun. 1988,726. Biichi, G.; Weinreb, S. M. J. Am. Chem. SOC. 1971,93, 746.Received M a y 8 , 1 9 9 1 Smenochromenes A-D (2-5) are four unusual macrocyclic chromenes that can be derived by cyclization of farneayl hydroquinone. The structure of smenochromene A (2) was determined by X-ray analpis and the structures of the remaining compounds were elucidated by interpretation of spectral data. The unusual geometry of smenochromene A gives rise to some unexpected spectral data. The sponge Smenospongia sp. also contains smenodiol (6), which is related to compounds previously found in this genus.
Ossamycin is a cytotoxic agent of undetermined structure that was originally isolated in 1965 from culture broths of Streptomyces hygroscopicus var. ossamyceticus. Its overall structure and relative stereochemistry have now been determined by single crystal X-ray diffraction studies. Absolute stereochemistry was established according to the previously determined configuration of its aminosaccharide constituent, ossamine. The aglycone of ossamycin possesses a 24-membered macrolide ring system onto which is incorporated both a 6,6-spiroketal and a 5-membered hemiketal ring system. The overall three-dimensional structure possesses features in commonwith the related macrocyclic antibiotics dunaimycin, cytovaricin, and A82548A.Ossamycin is a fermentation-derived natural product that was originally reported in 1965 as a novel cytotoxic agent isolated from culture broths of Streptomyces hygroscopicus var. ossamyceticus.^However, its structure has remained unknown until now. Preliminary studies in 1969 had shown that ossamycin contained an unusual aminodeoxysaccharide that was given the name, ossamine.2) This amino sugar and its enantiomer have been prepared by several total syntheses and are well characterized.2~5) Aminodeoxy sugars such as ossamine are typically found in macrolides and other polyketide-derived fermentation products.6) Ossamine itself has been recently proposed as a constituent of the novel tetracyclic insecticidal macrolide, spinosyn G (factor G of the A83543 complex).7) As a result of our continuing interest in this large and structurally diverse class of polyketide-derived natural products, we have now determined the structure of ossamycin by single crystal diffraction X-ray crystallography (Fig. 1).Isolation and Characterization The sample ofossamycin was obtained by fermentation of its producing organism, Streptomyces hygroscopicus var. ossamyceticus, that was obtained from the American Type Culture Collection (Rockville, Md.) where it had been originally deposited by the Bristol-Myers Company under accession number 15420. Ossamycin was isolated FEB. 1996 from the mycelial mass by extraction with acetonitrile, separation using an HP-20ss non-functionalized resin, and then chromatography, first on silica gel and then by reversed-phase HPLC. The isolated sample was fully characterized by physico-chemical and spectroscopic methodsand shownto be identical to a sample that had been previously received from the Bristol Laboratories by HPLC(including comparison by co-injection), mass spectrometry, and NMR(in acetone-6) spectroscopy.After the structure of ossamycin had been established by X-ray crystallography, XHand 13C NMRassignments were madeon the basis of ID comparisons with the corresponding spectra of cytovaricin and A82548Acom-bined with DEPT, COSY, TOCSY, HMQC, HMBC,
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