A sesquiterpene lactone of the germacrane series, mucrochin, has been isolated from the epigeal part of Tanacetopsis mucronata, and its structure has been studied by IR and mass spectral methods and PMR spectroscopy. It structure has been determined unambiguously by x-ray structural analysis as 5,6-dihydroxy5c~, 6f3, 7~, 813 (H)-germacra-l (lO), 4(15), 1 (13)-trien-8,12-olide. A boat-chair conformation with a configuration of the 1D 14 15D5 type has been established.Continuing an investigation [1][2][3][4][5] of the plant Tanacetopsis mucronata, we have isolated a sesquiterpene lactone of the germacrane type with mp 181-183~ composition C15H2004, which has been called mucrochin. Its IR spectrum contained absorption bands at (cm -1) 3455 (-OH group), 1735 (C=O of a -y-lactone), 1670 (double bond conjugated with a ~,-lactone carbonyl), and 1658 (isolated C=C bond). In the mass spectrum we observed the peak of the molecular ion with m/z 264 (M +) and peaks of ions with m/z 246 (M + --H20) and 228 (M + --2H20) due to ejection of one and two molecules of water, and also fragments characteristic for breakdown of germacranolides under electron impact (see Experimental section).In .the PMR spectrum, two doublets, each with a 3j value of 3 Hz, appeared at 6.63 and 5.56 ppm. On the addition of very small amounts of trifluoroacetic acid they shifted in the downfield direction, and this showed that they belonged to the protons of two secondary hydroxy groups. The PMR spectrum also contained the signals of the protons of a methyl group at a double bond, of exocyclic methylenes, of protons geminal to a hydroxy group, of a proton on a double bond of the main ring, and of a lactone proton (Table 1). These facts make it possible to assume that mucrochin is a germacranolide. The presence of the resonance lines of two protons at 3.55 and 4.4-4.6 ppm showed the secondary nature of the hydroxy groups. Double-resonance experiments confirmed that they were located vicinally.Signals of the protons of an exomethylene group of a ~,-lactone ring appeared in the form of doublets at 6.46 and 5.77 ppm with 4J7,13 = 3.5, 4J7,13, = 3.0 Hz, which showed a trans-linkage of the "r-lactone ring of type S [6][7][8][9].