Bromohelenalin CI 5H, , 0, Br crystallises in the orthorhombic space group P2,2,2, with Z = 4 in a unit-cell of dimensions, a = 8.873 f 0.004, b = 12.272 f 0.003, and c = 13.187 f 0.003 8. Intensities were measured on a diffractometer and the structure refined to a final R of 5.8% for 11 25 observed reflections. The conformation of the seven-membered ring is a deformed chair, with 1-and 5-junctions trans and the lactone junction is cis.The carbon skeleton is biogenetically abnormal, a methyl group having migrated from position 4 to 5. The bond distances and angles are normal. The standard deviations are 0.01-0.02 8 for the interatomic distances and 0.6-0.9" for the bond angles.THE structures of many Helenium species have been determined by chemical as well as X-ray diffraction studies, including the biogenetically abnormal, sesquiterpene lactone tenulin (I), the bitter principle of Helenium species,,+ which was deduced from chemical and spectroscopic studies: and the norsesquiterpene mexicanin-E (11), which was determined by chemical and n.m.r. s t u d i e ~. ~-~ These structures have been confirmed by X-ray analysis of bromoisotenulin (111) and bromomexicanin-E (IV) .lo Recently we have also determined the structure of 3-bromoanhydrodehydrodihydropulchellin l1 (V) by X-ray diffraction studies.Tenulin has been related to several other sesquiterpene lactones, the most important of which is helenalin,12 the main constituent of Helenizlm autumnale L. and several other Heleniwn species. A complex series of arguments based on the variety of transformation of helenalin, tenulin, and their congeners led to the con-E. P. Clark,
The crystal structure of O-(bromoacety1)tetrahydrodouglanine. C17H2,0,Br, a derivative of the sesquiterpene lactone, douglanine, has been determined by an X-ray diffraction study. The crystals are orthohombic, space group P2,212!, Z = 4 in a unit cell of dimensions, a = 7.657 f 0,005. b = 13.722 f 0.004, and c = 16.392 f 0.004 8.Intensities of 1497 independent reflections were measured on a diffractometer. The structure was solved by the heavy-atom technique, using Patterson and Fourier syntheses and refined by least-squares techniques to R 7.6% for 1075 observed reflections. Both six-membered rings are in the chair-form. The bond distances and angles are normal and mean standard deviations are of the order of h0.02 a for the bond distances and 0.9' for the bond angles.THE structures and stereochemistry of douglanine and relative stereochemistry of O-(bromoacety1)tetra-(C,,H,O,) (I) which is found with arglanine (11) have hydrodouglanine based upon that of the tetrahydrobeen found by ix., u.v., and n.m.r. studies.lY2 Catalytic douglanine from which it was prepared, is shown in (V).An X-ray analysis was undertaken to determine the structure independently, and also to establish whether douglanine has 1 cc-OH configuration or 1 P-OH configuration. The X-ray results corroborate the chemical study, and establish the 1 %-OH configuration originally assigned to doug1anine.l EXPERIMENTALThe bromoacetate was prepared by acylation of tetrahydrodouglanine (IV) (30 mg.) in a mixture of pyridine (0.5 ml.) and bromoacetyl bromide (0-5 ml.). The solution was set aside overnight, then diluted with water, extracted with chloroform, and purified by chromatography over silica gel (2 x 25 cm. column). It was recrystallized first from methanol containing a few drops of water then from ether (30 mg.) as colourless crystals, m.p. 132"; v 1735 (ester CO), 1780 (lactone CO) cm.-1 (Calc .
Bromoisotenulin crystallises in the monoclinic space group P2,, a = 8.732(3), b = 23.094(6), c = 10.237(5) 8, @ = 121~27(10)0,Z = 4, with two independent molecules in the asymmetric unit. Two sets of data were collected : photographic, 2300 reflections, and diffractometer. 1700 reflections. The structure w a s solved by Patterson and heavy-atom methods, and the diffractometer data refined by full-matrix least-squares t o R 0.075. The results confirm the structure (IV) suggested by Herz et a/. and supply the unknown stereochemistry and absolute configuration. The carbon skeleton is biogenetically abnormal, a methyl group having migrated from C(4) to C ( 5 ) .Both five-membered rings are trans-fused t o the seven-membered ring. Attention is drawn to an error in the published stereochemistry of the related compound, bromomexicanin E.THE sesquiterpene bitter principle tenulin, C,,H,,05, was isolated by Clark from several Heleniztm species. Its properties were and although much chemical work was done on this and similar compounds, little progress was made towards elucidating their structures. Barton and de May0 5 suggested formulae (I) and ( 11) for tenulin and isotenulin. Later, Braun et aZ.6 repeated much of this work, but indicated a different position for the carbonyl group in the unsaturated five-membered ring. In order to resolve this difference and to determine the detailed stereochemistry we undertook an X-ray diffraction study of crystals of the derivative,
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