Structure of an anti-Aldol Addition Product of Benzaldehyde and a Pseudoephedrine-Derived O-Silyl Ketene N,O-Acetal

. [

1 (sin0/A)max = 0.54 A--; h from -7 to 7, k from -18 to 18, l from -19 to 19.Three standard reflections (053, 1i4 and 018) showed no variations greater than those predicted by counting statistics.6162 reflections were measured, of which 5887 were independent.Goodness of fit for merging was 0.87 (Rmerge = 0.019 for 177 reflections with exactly two observations).All reflections were used in solution and refinement of the structure.The Si atoms were found from a Patterson map and the remaining atoms located by successive structure factor-Fourier calculation; the y coordinate of atom Sil was fixed at 0.7 to define the origin.Fo 2 magnitudes were used in   Discussion.There are two nearly identical molecules in the asymmetric unit of this compound (Fig. 1); this discussion will use average values of bond distances and angles for the two, indicated by sample e.s.d.'s within square brackets.Moreover, the molecules have a partial (non-crystallographic) symmetry to them, extending from the Si atom in both directions around the ring to the methyl C atoms, so that many bond distances quoted are averages of Fig. 1.ORTEPII (Johnson, 1976) drawings of (a) molecule 1 and (b) molecule 2, with 50% probability ellipsoids showing the numbering system.H atoms are shown as arbitrary small circles.
Fig. 2.An ORTEPII (Johnson, 1976) 1.469 [12] A expected.The angles show a similar agreement, with the average value of the difference between corresponding angles in the two molecules being 1.2 ° in the central ring system.There are no close intramolecular contacts across the ring, but all atoms are at approximately van der Waals distances from each other.
Intermolecular contacts are also all at van der Waals or greater distances (Fig. 2).There are two H...H interactions less than 2.4 A, one 2.34 and one 2.39 A, the shorter being between an H atom of molecule 1 and a different H atom in the same molecule, one unit cell away along a.There are four H...O interactions at van der Waals distances (2.6 A) or closer, suggestive of C--H...O hydrogen bonding.All of these contacts are to the carbonyl O atoms, clearly the best receptor in the molecule, with the shortest being from H11B to 033, at 2.49 A.
Generous financial support from the National Science Foundation is gratefully acknowledged.
[ C I F r e f e r e n c e : H H 0 5 8 9 ]

full-matrix least-squares refinement, which mini- mized Yw(Fo 2 -F~R) 2. H atoms were placed at calcu- lated positions (C--H =0.095 A) with staggered geometry. Positional and anisotropic displacement parameters of all non-H atoms (y of Sil fixed) were refined; H-atom parameters were not refined, but the atoms were repositioned once near the end of the refinement. For 5616 reflections with Fo2> 0 R = 0.054, for 4305 reflections with Fo2> 3~r(Fo 2) R =
0.039, wR (on F 2) = 0.006; S = 1.

14. In the final difference Fourier map 0.45 _> Ap _> -0 . 3 4 e A,-3. Atomic scattering factors and values for f' and f" were taken from Cromer & Waber (1974) and Cromer (1974). Programs used were those of the CR YM crystallographic computing system (Duchamp, 1964) and ORTEP (Johnson, 1976). Final refined parameters of the atoms are listed in Table 1 with selected distances and angles in Table 2.* The structure solution and preliminary refinement were with the data averaged in point group 2/m (Rin t = 0.028 for 2530 duplicates, good- ness of fit = 0.94 for 2826 multiples). The data were re-averaged in point group 2 for the final refinement. Structure factors with f " = 0.071 for Si were better by about 2% for both R and goodness of fit, than for -0.071, so final refinements were based on the positive value. * Lists of assigned H-atom parameters, anisotropic dis- placement parameters, complete distances and angles, and observed and calculated structure factors have been deposited with the British Library Document Supply Centre as Supple- mentary Publication
N o .S U P 55198 (32 pp.).

Copies may be obtained through The Technical Editor, International Union of Crystallography, 5 Abbey Square, Chester CHI 2HU, England.
packing drawing projected down a showing the contents of a unit cell, with the unit cell outlined.The c axis is horizontal.Atoms are shown as 50% probability ellipsoids, with Si atoms in molecule 1 shaded.H atoms are not shown.