Synthesis of Novel Tröger’s Base-Derived Helical Scaffolds

Tröger’s base (TB) is a chiral V-shaped molecule in which the aromatic rings are nearly perpendicular. The overarching goal of this project is to utilize the unique chirality and inherent shape of the Tröger’s base monomer to design, synthesize and study dimeric, tetrameric and octameric TB oligomers, which will form helical structures. We describe here the methodology for the synthesis of novel Tröger’s base diester monomer 13, which is highly soluble in most organic solvents compared to TB systems with methylene bridges. Chiral HPLC resolution of TB monomer 18, using a semiprep chiral AD-H column, gave access to pure enantiomers of the TB monomer. The (-)-enantiomer of 18 was used to synthesize the novel syn diester TB dimer 20, via double Buchwald-Hartwig coupling based phenazine formation. Energy minimization modeling of the syn dimer 20 using Web MO shows a potential binding cleft, which can ultimately be applied for the synthesis of desired tetrameric and octameric scaffolds. The chiral HPLC resolution of TB monomer 18 is expensive, time-consuming and has low scalability. This problem was solved by the synthesis of a menthone-based chiral auxiliary 27, which allows easy access to the enantiopure monomers of TB. The chirality of 27 was utilized to form the diastereomers of menthone TB 33, which were readily separable by column chromatography. These diastereomers were then hydrolyzed to give pure enantiomers of diol TB monomer 34.