The Dendritic Effect on Enantioselectivity of Organocatalytic Reactions and the Effect of Local Compaction on a Titanium Mediated Allylation of Aldehydes.

摘要

The synthesis, study and catalytic properties of chiral terminated dendrons based on an intramolecularly folded pyridine-2,6-dicarboxamide branch unit is reported. Two differently terminated chiral dendrons were prepared and studied for their chiroptical properties: 1.) N(pi)-methyl histidine dendrons were prepared and tested as organocatalysts in the kinetic resolution of secondary alcohols and an intramolecular acyl rearrangement of oxindoles to generate all carbon, quaternary stereocenters, and 2.) biphenyl and tetra-phenyl-core-based, chiral terminated dendrons were synthesized and tested for their catalytic properties in the titanium mediated, asymmetric addition of allyltributyltin to aldehydes. In the former project (organocatalysis), the dendrons were, after much synthetic difficulty, synthesized up to their second generation and exhibited a slightly positive dendrimer effect from G0 to their second generation hierarchy. Using circular dichroism (CD) spectroscopy, the dendrons exhibited a positive Cotton-effect (CE) (P-helical bias) that was enhanced not only from the first to second generation, but also from organic to aqueous solvents. In the latter project (asymmetric allylation of aldehydes), various amide termini bi/tetra-phenyl core-based dendrons were prepared and showed a cooperative effect (in terms of enantioselectivity) when the tetraphenyl core was used. CD studies showed that the dendrons, when attached to the aforementioned cores, developed a bias toward an M-helical sense, which was much more pronounced in the tetraphenyl dendron.