Structure of an 11-Membered Cyclic Silyl Enol Ether from Condensation of Methacrolein and a Pseudoephedrine-Derived O-Silyl Ketene N,O - Acetal

The title compound, (4S,5S,8R, IOZ)-2,2,5,6,8,10- hexamethyl-4-phenyl-1,3-dioxa-6-aza-2-silacycloun-dec-10-en-7-one, crystallizes with two virtually identical molecules in the asymmetric unit, each with the same absolute configuration. Bond distances and angles are normal; the closest approaches across the ring between non-bonded atoms are all greater than t Contribution No. 8557.

3.35 .A,.The amide and enol ether groups are planar to within ___ 0.06 A.

C o m m e n t
The pseudoephedrine-derived O-silyl ketene N,Oacetal (1) undergoes facile aldol addition reactions with aldehydes to form 9-membered cyclic siloxane condensation products (Myers, Widdowson & Kukkola, 1992).a,/3-Unsaturated aldehydes react differently, forming, in the case of methacrolein, the 11-membered cyclic silyl enol ether (2) (Myers & Subramanian, 1992).The structure of this crystalline material has been established unambiguously by X-ray crystallography.The title compound, (4S,5S,8R, IOZ)-2,2,5,6,8,10hexamethyl-4-phenyl-1,3-dioxa-6-aza-2-silacycloundec-10-en-7-one, crystallizes with two virtually identical molecules in the asymmetric unit, each with the same absolute configuration.Bond distances and angles are normal; the closest approaches across the ring between non-bonded atoms are all greater than t Contribution No. 8557. 0108-2701/92/112090-03506.00 Fig. 1 shows the two independent molecules; both drawings have the same orientation relative to the least-squares plane of all their atoms, emphasizing their great similarity.Fig. 2 is a packing drawing showing the molecules in the unit cell.Bond distances and angles in the two independent molecules agree well; average values are quoted here, together with the higher or highest e.s.d.'s of the individual bonds.The Si--C and Si---O lengths average 1.842 (6) and 1.636 (3) A. The angles at silicon differ from 109.5 ° by an average of 3.1 ° .There is some evidence of opening of the ring at the internal O atoms, where angles average 125.2 (9)°; the C---O distances, however, are normal [1.443 (5)A to C1, 1.376 (6)A to C17].Other internal angles in the 11-membered ring are close to expected values.The grouping C8--N(C10)---Cl1(O3)--C12 is nearly planar in both molecules, with similar deviations from the least-squares plane.In molecule A, the two 'end' atoms, C8 and C12, are 0.057 A out of the plane and the inner atoms (N, C10, C11 and 03) are -0.011,-0.045, -0.006 and -0.052A out of the plane respectively.In molecule B, the comparable distances are + 0.047, and -0.017, -0.030, -0.009 and -0.037/~.The non-bonded distances across the ring are all at or greater than van der Waals distances: O1...C10 3.42 and 3.48 A, O2-..C11 3.30 and 3.33 A, O1...O3 3.66 and 3.67/~, 02.-.03 3.51 and 3.56 A. Intermolecular contacts are also all at van der Waals distances or greater, the shortest (O2b...H4b) being 2.65 A. The shortest H-..H contact is 2.38 A compared with an expected value of 2.4 A.   The y coordinate of Sia was fixed to define the origin.The absolute configuration of the molecule about the C 1--C2 bond was known so only the relative configurations at other chiral sites had to be determined, f " was ignored for all atoms, butf' was used for Si.
We gratefully acknowledge generous financial support from the National Science Foundation.0108-2701/92/112092-03506.00 Lists of structure factors, anisotropic thermal parameters, H-atom coordinates and complete geometry have been deposited with the British Library Document Supply Centre as Supplementary Publication No. SUP 55244 (23 pp.).Copies may be obtained through The Technical Editor, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England.[CIF reference: HH1008] SCHAEFER, VIJAYA SUBRAMANIAN AND ANDREW G. MYERS Division of Chemistry and Chemical Engineeringt and The Beckman Institute, Mail Code 139-74, California Institute of Technology, Pasadena, California 91125, USA (Received 20 February 1992; accepted 25 June 1992)

Fig. 1 .
Fig. 1.An ORTEP drawing of molecules A (top) and B (bottom) with 50% probability ellipsoids showing the numbering system for the non-H atoms.H atoms are shown as spheres of arbitrary small diameter.

Table 2 .
Selected bond lengths (A) and angles (o)