Chiral reagents for asymmetric construction of carbon frameworks

Abstract

Chiral titanium reagent prepared in situ by mixing dichlorodiisopropoxy titanium and a tartrate-derived chiral 1, 4-diol catalyzed various carbon-carbon bond forming reactions as a chiral Lewis acid; for example, asymmetric cycloaddition reactions such as Diels-Alder reaction and [2+2] cycloaddition reaction. In the presence of the titanium catalyst and MS 4A, 3-alkenoyl-l,3-oxazolidin-2-ones reacted with various dienes to give the Diels-Alder adducts with high enantioselectivity. Hydronaphthalene moieties of mevinic acids were synthesized by applying the catalyst to the intramolecular Diels-Alder reaction. As a β-hydroxy acrylic acid equivalent, 3-(3-borylpropenoyl)-l,3-oxazolidin-2-ones were prepared and utilized in the above asymmetric Diels-Alder reaction. The boryl group of the adducts thus produced was readily converted to hydroxyl group oxidatively. Asymmetric [2+2] cycloaddition reaction occurred by the treatment of a mixture of 3-alkenoyl-l,3-oxazolidin-2-ones and vinylic and acetylenic sulfides with the titanium catalyst. Various cyclobutanes, cyclo-butenes and methylenecyclobutanes were prepared in high optical purity. © 1992, Walter de Gruyter. All rights reserved.