Bicyclic and Tricyclic Compounds with β‐Amino Alcohol Groups as Chiral Ligands in the Enantioselective Reaction of Diethylzinc with Aldehydes

Abstract

AbstractReduction of the enantiomerically pure allyloxy esters 9 or ent‐9 with DIBAH afforded the corresponding aldehydes which were treated in situ with chiral or achiral hydroxylamines 8 to give nitrones 10. These underwent a spontaneous intramolecular 1,3‐dipolar cycloaddition, affording the bicyclic compounds 11 and 12, respectively. In an analogous manner, a mixture of the tricyclic compounds 14 and 15 was prepared. Treatment of compound 16 with cyclohexene oxide afforded a mixture of diastereomers 17 and 18. Diastereomers 14 and 15 as well as 17 and 18 could be separated by chromatography. X‐ray analyses of compounds 11Ff, 17, and 11Af · HCl were performed. The bicyclic and tricyclic compounds were used as chiral ligands in the reaction of diethylzinc with aldehydes 19, in particular with benzaldehyde (19a). Using bicyclic compounds with a tertiary β‐hydroxyalkyl substitutent at the N atom as ligands, ee's in the range of 78 to 95% were found. Whereas for the best ligands 11Ae and Af the enantioselectivity in the reaction of 4‐tolualdehyde was only slightly decreased, with the aliphatic aldehydes 19c and d distinctly lower enantioselectivities were determined.