Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Cover Picture (Angew. Chem. Int. Ed. Engl. 4/1989)
The cover picture shows the hypothetical transition state of a new, highly stereoselective addition of allyl or ester enolate groups to the re side of aldehydes. These reactions are made possible by titanium(IV) cyclopentadienyl complexes with two identical carbohydrate ligands and a transferable group. The inexpensive chiral auxiliary 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose proved to be astonishingly efficient. The sugar moieties are shown in the conformation determined by X-ray structure analysis and NMR measurements for the corresponding CpTiCl complex. In the transition state, however, the “piano stool” arrangement of the titanium ligands changes to a more trigonalbipyramidal geometry. Although this model does not allow an explanation or visualization of the enantiofacial differentiation achieved with this reagent, the different conformations of the enantiotopic glucose ligands, as well as the stability of the complex due to the cyclopentadienyl ring, seem to be responsible for the high induction. Further details on the synthesis of homoallyl alcohols, β-hydroxy acids, and threo-β-hydroxy-α-amino acids according to this principle are reported by R. Duthaler et al. in four communications on page 494 ff.