The structure and conformation of the 9,10-annelated-1,4,5,8-tetraoxadecalins (3,4 and 5) were investigated in solution using NMR spectroscopy, in the crystal using X-ray diffraction analysis and by computation, using the MM2 force field, suitably parameterized to include structural manifestations of the anomeric effect. The study focused on the geometry, viz., the structural parameters of the anomeric moieties and the ring conformation and degree of puckering, as affected by changes in the dihedral angle of the 9,10-annelating system. Very good agreement between the three methods was obtained, showing increased chair deformation, as the 9,10-substituents approach coplanarity.
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X-ray diffraction data for compounds 3, 4 and 5 were measureda t ca. 2O“C on a CAD4 tifractometer eqmppedu rltha graphitem onochromatoru. sing MoKa (h=O7 107 ;i> radlatlona nd the 61 scan techmque with scan range of 0 9 + tan 0” The data for compound 3 were collected on an upgraded ficker diffractometera t 128K, usmg MoKa tiation and the o-28 scans from 1.3’ below Ka, to 1 6” above Kaz25 Possible detenoraaon of the analyzed crystals was tested by detectmg frequently the mtenslties of three standard reflections of cbfferent zones of the reciprocal space, and was found neghglble durmg the measurements The data sets were not corrected for absorption or secondary extmcnon effects Several very strong reflections wluch appeared to suffer from extmctlon, VDZ, 2,2,2; -6,0,2, 4,0,2, -1,1,2; -2,0,4, -5.1.5 and -8,0,6 m 4, were excluded from the final calculations The cell constants and pertment details of the expenmental condltlons are summarized m Table 3 and ORTEP plots of 3, 4 and 5 are given m Figure 1 The three structures were solved by direct methods (SHELXS-86) 26 Their refinements were camed out by large-block least-squares( SHELX-76):’ mcludmg the positional and amsotroplc thermal parameterso f all the nonhydrogen atoms. The hydrogens were included m the structuref actor computations m calculatedp ositions, and were assigned a fixed lsotroplc temperaturef actor of U=O0 5 i2 (4 & 5) or U=O0 4 i2 (3) The final refinements were based only on those observations that sattsfied the condttlon F’, > 3&), usmg expenmental weights [w=d2(F,)] and mmmnzmg w(AF)’ They converged smoothly at relatively low discrepancy factors of R=O0 4 Fmal atomic coordmates of 3, 4 and 5 are hsted m Tables 4-6 and the resultmg bond lengths, bond angles and torsion angles m Tables 7-9 The crystallographic atom-labeling scheme used 1s ldentlcal with the molecular one shown m 1 Three crystallographlcally mdependent molecules conmbute to the asymmetnc unit of 5 two he m general postnons, while the third 1s located on a twofold axls of rotation at (0 5,y.O7 5) which comcldes with the C(15)-C(20) bond The correspondmga toms m the three rhfferent species are marked by unpnmed, pnmed and doubly pnmed labels, respectively Acknowledgments. The calculations were performed on the IBM 4381 and CYBER 180/990 computers at the Computational Center of Tel-Aviv University, the staff of which provided valuable assistance We thank Mrs Sarah Wemman for skillful technical assistance This work was started durmg the tenure of one of us (B F) of the Francqul Vlsltmg Chair at the State Umverslty of Gent The Francqul Foundation 1s gratefully acknowledged for thus nommatlon This research has been supported by the Basic Research Fund of the Israel National Academy of Sciences