Investigations of New Biomimetic Transamination Systems and Inclusion Complexation with Cyclodextrins

Abstract: Two new transamination systems are described. The first example is a dibenzo[a,d]- cycloheptene containing system, designed to accomplish a [1,3]-hydride transfer. Several catalysts have been examined for this purpose. tris-Pentafluorophenyl borane was found to be the most efficient catalyst. The stereochemistry of several closely related dibenzo[a,d]- cycloheptene have also been examined, displaying a solvent dependent in the preferred conformation. The second transamination system consists of a thioxanthene dioxide moiety, to afford facile [1,3]-proton transfer. Several imines of a-keto esters were synthesised, to result in imines of a-amino esters upon rearrangement in 75%-quantitative yields. Simple amines, amidines and guanidines were examined with respect to their catalytic activity in the transamination reaction. DBU, benzamidine, and a pseudo-C2-symmetric guanidine proved to be successful catalysts. Asymmetric induction was also evaluated with the pseudo-C2- symmetric guanidine, in a maximum asymmetric induction of 45% e.e. The unusually high dynamic stability of the 1:2 complex between 1,4-dimethylbicyclo- octane and a-cyclodextrin has been studied by various NMR methods combined with computa- tional methods. Hydrophobicity of the guest molecule together with a highly preorganised structure of the guest and the host molecules are suggested as main origines for the observed high dynamic stability.