INVESTIGATION OF THE EFFECTS OF THE STRUCTURE AND CHELATE SIZE OF BIS-OXAZOLINE LIGANDS IN THE ASYMMETRIC COPPER-CATALYZED CYCLOPROPANATIONOF OLEFINS - DESIGN OF A NEW CLASS OF LIGANDS

A set of novel, C-2-symmetric bis-oxazoline ligands has been synthesiz ed by mounting two oxazoline rings onto an optically active 1,3-diaxol ane backbone. This design allows for the control of both orientation a s well as the proximity of the oxazolinyl R-groups around the reactive site, As a result of the twist imparted by the 1,3-dioxolane ring, th e stereogenic oxazolinyl substituents can be brought either toward or away from the complexed metal in a controllable fashion, Starting from L-amino alcohols and either L- or D-tartaric acid, two sets of ligand s (6b-e and 7a,b) were synthesized and evaluated in the copper-catalyz ed cyclopropanation of olefins, The comparison of benzyl and isopropyl derivatives of these ligands with previously reported five- and six-m embered bis-oxazolines clearly indicates the beneficiary effect of the larger chelate size and the chiral tether of the tartrate-derived lig and. The effect, of the different oxazolinyl groups along with the dif ferent substituents on the diaxolane tethers was also investigated. Th e influence of the alkyl group of the diazoacetate was studied, and th e diazoacetate derived from (-)-8-phenylmenthol was found to be superi or to (-)-menthyl diazoacetate. The cyclopropanation of vinyl acetate, a relatively unexplored substrate fur this reaction, furnished cyclop ropanol derivatives in good optical purify.