Synthesis and characterization of two new chiral Kemp's acid derivatives: structures fixed by a peculiar system of N-H center dot center dot center dot O, C-H center dot center dot center dot O and C-H center dot center dot center dot N hydrogen bonds

摘要

New chiral imide-oxazoline derivatives of Kemp's acid were synthesized with the aim to produce new ligands suitable for catalytic asymmetric reactions. Compound 1 is C18H28N2O3, systematic name (1R, 5S, 7R)-7-[(S)-4-tert-butyl-4,5- dihydrooxazole-2-yl]-1,5,7-trimethyl-3-azabicyclo-[3.3.1]-nonane-2,4-dione. Compound 2 is C21H26N2O3, systematic name (1R, 5S, 7R)-7-[(S)-4-benzyl-4,5-dihydrooxazole-2-yl]1,5,7-trimethyl-3-azabicyclo-[3.3.1]-nonane- 2,4-dione. Both compounds contain two molecules with very similar conformations in the asymmetric unit. The two structures were characterized by single crystal X-ray diffraction. DFT calculations of two possible distinct conformations were performed to elucidate the differences between the preferred conformation in vacuo and the one observed in the single crystal. Also, charges on the key atoms of the compounds were compared with a common ligand used for asymmetric transfer hydrogenation.