~ Pergamon Tetra&edron: Asymmetry, 1/ol. 8, No. 18, pp. 3073-3078, 2997 © 1997 Elsevier Science Ltd. All rights reserved. Printed in Great Britain PII: SO9b'7-4166(97)N3~%4 0957-4!66/97 S17.O9 + 0.00 Ennttmelective Diels-Aider reactions: room temperature bis(oxazoline)-zinc, -magnesium, and -copper triflate catalysts James M. Takacs,* Edward C. Lawson, Michael J. Reno, Mark A. Youngman and David A. Quincy Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588-0304, USA Abstract: A series of chiral bis(oxazoline) ligands, differing in the length of the chain connecting the chirai oxazoline subunits and in the nature of the substituent at the chiral center, were examined in the room temperature Zn(OTf)2, Mg(OTf)2 and Cu(OTf)2 catalyzed reaction of N-crotonyloxazolidinonewith cyclopentadiene. A 1,4-his(oxazoline) ligand proved best for Zn(OTf)2 and afforded product consistent with reaction via an octahedral model, while 1,3-bis(oxazoline) ligands were best for Mg(OTf)2 and Cu(OTf) 2. © 1997 Elsevier Science Ltd We have been interested in the use of chiral bis(oxazoline) ligands (box ligands) for certain iron, 2 rhodium,2 and palladium-catalyzed 3 carbocyclizations, and having access to these ligands, became interested in the use of their metal complexes in Lewis acid catalyzed reactions? The Lewis acid catalyzed Diels--Alder reaction provides a useful venue for testing approaches to the design of asymmetric catalysts, and in particular, the reactions of ot, l~-unsaturated N-acyloxazolidinones (e.g., 1) provide a useful standard for testing the efficacy of two point binding chiral Lewis acids. Several notable successes have been achieved in the Diels--Alder reactions of I and closely related substrates, particularly using C2-symmetric chiral box ligands in conjunction with copper(II), 5-!2 magnesium(II),l 2,23, ! 4 and iron(iiI) salts. ] !.!2,15 o O o o 1,,(a- CHs) ~,. (~,F~, (~,~-,, ...) b (FI = H) '~ r,too, uJ~ (ma/oO Zinc(if) salts constitute useful reagents for Lewis acid catalyzed transformations in organic synthesis, and at the onset of our studies, their absence from among the very successful chiral Lewis acid catalysts was surprising. 5 Furthermore, while a number of very efficient catalyst systems had been defined that afford very high enantiomeric excess at low to very low temperature, efficient catalysts that operate at higher temperatures could have practical implications for large scale reactions and perhaps in regard to substrate scope. We therefore set as a goal to look for efficient catalysts that operate at or near ambient temperature, with particular emphasis on finding an efficient chiral bis(oxazoline)Zn(II)- catalyst system. We reasoned that different metal salts should demand different optimal distances separating the two oxazoline moieties to accommodate intrinsic differences in ionic radii and/or coordination geometry. To test this hypothesis, chiral bis(oxazoline) ligands 3,16 4,17 and 5is (1,2-, 1,3- and 1,4-box ligands, respectively; RI--CH2Ph, CHPh2, Ph, t-Bu) were synthesized via the standard methods and screened in combination with Zn, Mg, and Cu salts as catalysts for the cycloaddition of cyclopentadiene with the * Corresponding author. Email: jtakacs@unlinfo.unl.edu