A and 4 molecules in the unit cell. Final atomic parameters have been obtained from a blockdiagonal

Transcription

1 Crystal and molecular structure of the m-bromobenzoate derivative of bisnorquassin H. LYNTON Department of Chemistry, Uniuersity of New Brunswick, Fredericton, New Britns~vick Received July 4, 1969 The molecular structure of the in-bromobenzoate derivative of bisnorquassin, C,,H2,0,Br, been determined by the heavy atom method. The compound crystallizes in the orthorhombic system, space group P2,2,2,, with unit cell dimensions n = A, b = f 0.02 A, c = A and 4 molecules in the unit cell. Final atomic parameters have been obtained from a blockdiagonal least-squares refinement using anisotropic temperature parameters. The final agreement residual for I665 observed reflections is R = The structure of bisnorquassin previously proposed by Findlay and Cropp, on the basis of spectroscopic and chemical evidence, is shown to be essentially correct. Canadian Journal of Chemistry, 48, 307 (1970) Introduction Bisnorquassin, C2,H2,0,, was first prepared by Robertson et al. (1,2) by the treatment of both quassin and norquassin with a warm mixture of concentrated hvdrochloric and acetic acids. The complete structure and relative stereochemistry of quassin, a bitter principle from Quassia atnara, has been established by Valenta et al. (3, 4). Early studies showed that bisnorquassin was not a simple bis-demethylated quassin, but in view of its ultraviolet (u.v.) absorption spectrum, possessed an extended system of conjugation. A structure for bisnorquassin has been given by Findlay and Cropp (5) which is adequately supported by spectroscopic and chemical evidence. This structure is not comwlete in that the stereochemical co~lfiguration about certain carbon centers is not established. This X-ray analysis was undertaken to provide the complete stereochen~ical structure of this interesting compound. Crystallograplzic and Inteizsity Data The specimen of the m-bromobenzoate derivative of bisnorquassin used in the structure analysis was supplied by J. A. Findlay and J. S. Tandon. The crystal data are given in Table 1. The compound is orthorhombic and the systematic absences establish the space group as P2,2,2,. Unit cell dimensions were measured from oscillation and Weissenberg photographs. The density measurements indicate 4 molecules of C2,H2,O,Br in the unit cell. All intensity data were collected from a needle shaped crystal of average length 0.25 mm and average cross section 0.15 mm. As the linear has TABLE 1 Crystal data System Orthorhombic Space group p Svstematic absences hoo: h = 2i2 + 1 Molecular weight Unit cell dimensions a b C A Volume 2370 AT Measured density (by flotation) 1.50gcmr3 Calculated density (assuming 4 molecules in the unit cell) 1.52 g cmw3 absorption coefficient for CuKa radiation is 29.7 cm-l, no absorption corrections were made. Using the multiple film pack technique, equiinclination Weissenberg photographs were obtained of layers Ilk0 - - ldc6. The total number of independent reflections with intensities above the minimum intensity limit for observation was The intensities of all reflections were estimated by visual comparison with a calibrated strip prepared using a suitable refleciion from the intensity crystal. Lorentz-polarization factors were applied to all data. Structure Analysis A 3-dimensional Patterson synthesis established the positions of the bromine atoms. Subsequent 3-dimensional Fourier syntheses then gave the coordinates of all the carbon and oxygen atoms. The refinement was by successive cycles of

2 TABLE 2 Fractional atomic coordinates, and e.s.d. in parenthesesy Atom CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 xla ulb zlc ; have been multiplied by lo4. TABLE 3 Anisotropic temperature parametersy block-diagonal least-squares,' employing only the observed reflections. The quantity minimized was Co(Fo - FC)'. The matrices used were 3 x 3 for the positional parameters and 6 x 6 for the anisotropic thermal parameters of each atom. The weighting scheme o = 1/[a + IF,[ + cjfoj2] was employed with the following values: a = and c = Scattering factor curves used were those given in International Tables (7). During the structure refinement the scale factor for each layer was adjusted by comparison of observed and calculated Fa's. The positions of the hydrogen atoms could not be deduced from a difference synthesis. No corrections were made for extinction, and in the last cycle, no parameter shift exceeded 0.2 times the estimated standard deviation. The final agreement residual, R = CI Fo - Fcj/jCIF, I, for observed reflections was IS = Results and Discussion The final atomic coordinates are given together with their estimated standard deviations (e.s.d.) in Table 2. The temperature parameters are given in Table 3. Figures 1 and 2 give the intramolecular bond lengths and angles, and Fig. 3 shows the general stereochemistry of the molecule. Some bond angles are omitted from Fig. 2 and are given lall computations were done on an I.B.M computer using programs by Ahn~ed et a/. (6). Atom PII PZZ P33 P23 P13 812

3 LYNTON: CRYSTAL AND MOLECULAR STRUCTURE TABLE 3 (Concluded) Atom PI 1 I322 I333 I323 I313 I312 *All values have been multiplied by lo5; T = exp - [h2p11 + kzp P33 + klp~3 + f111)13 + hkp1~1. TABLE 4 Bond angles omitted from Fig. 2 Atoms Angle ('1 in Table 4. A final set of structure factors has been placed in the Depository of Unpublished Data.' The estimated standard deviations for all bonds are A except for c,-c,, and C24-C25 where the e.s.d. are 0.05 A. The e.s.d. for all bond angles are 2-3". Nothing can be established about 'Photocopies may be obtained, upon request, from: Depository of Unpublished Data, National Science Library, National Research Council of Canada, Ottawa, Canada. FIG. 1. Intramolecular bond distances.

4 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 TABLE 5 Best mean planes Atom -- Distance from plane (A) Benzene ring Equation 0.895~ ~ ~ = 0 C Clzvowzophore Equation ,~ ~~ = 0 Benzene ring and chvornophore Eq~~ation 0.875~ ~ ~ = C C C C C C C ( C C ( C possible hydrogen bonding as no intermolecular distances were found to be shorter than the usual van der Waals separation values. The gross structure proposed by Findlay and Cropp (5) is confirmed. The molecule has a cage conformation in which all substituent groups, except the hydrogen attached to C-6, point away from the center of the molecule. The hydrogens attached to C-10 and C-11 must be cis and not trans as suggested by Findlay and Cropp. They lie on the outward side of the 5-membered ring as does the methyl group attached to C-5 and the hydroxyl group attached to C-4. The hydrogen atom attached to C-1 is cis to the OH group at C-4. The chromophore extending from 0-3 through C-16, C-15, C-14, C-13 to C-12 is essentially planar and is coupled with the planar m-bromobenzoate substituent at C-12, the whole forming a large conjugated planar array in which no atom is more than 0.5 A out of the best mean plane (see Table 5). I wish to acknowledge the support of the National Research Council of Canada for this work, and I thank Mrs. Brenda LefTen for the estimation of the intensity data used in this analysis. 1. E. LONDON, A. ROBERTSON, and H. WORTHINGTON. J. Chem. Soc (1950). 2. K. R. HAMEN, D. B. JAQUISS, J. A. LAMBERTON, A. ROBERTSON, and W. E. SAVIGE. J. Chem. Soc (1954). FIG. 2. Intramolecular bond angles.

5 LYNTON: CRYSTAL AND MOLECULAR STRUCTURE FIG. 3. General shape of the bisnorquassin molecule. 3. Z. VALENTA, S. PAPADOPOULOU~, and C. PODESVA. PIPPY. N.R.C.C. Crystallographic Programs for the Tetrahedron, 15, 100 (1961). I.B.M./360 System, Divisions of Pure Physics and 4. Z. VALENTA, A. H. GRAY, D. E. ORR, S. PAPADO- Pure Chemistry, National Research Council of Can- ~ourous, and C. PODESVA. Tetrahedron, 18, 1433 ada, Ottawa, Canada. (1962). 7. International Tables for X-ray Crystallography, Vol. 5. J. A. FINDLAY and D. T. CROPP. Can. J. Chem. 46, 111. The Kynoch Press, Birmingham, England, (1968). Table A. 6. F. R. AHMED, S. R. HALL, C. P. HUBER, and M. E.