Folding Dendrons:  The Development of Solvent-, Temperature-, and Generation-Dependent Chiral Conformational Order in Intramolecularly Hydrogen-Bonded Dendrons

Abstract

The synthesis of intramolecularly hydrogen-bonded dendrons with stereogenic terminal groups derived from (1 S, 2 S )-(+)-thiomicamine up to the third generation is described. Circular dichroism (CD) studies reveal that the equilibria interconverting two diastereomeric helical conformations (M and P helices) relating a pair of anthranilamide termini depends on solvent, temperature, and dendrimer generation. A conformational preference for M-type helicity along the periphery of the dendrons increased with increasing dendrimer generation and in poor solvents as observed by CD. Equilibration of these diastereomeric helical conformations is rapid at the first generation in all solvents and at all temperatures investigated; however, at the second generation the equilibrium begins to bias a single diastereomeric helical conformation along the periphery that becomes maximal at low temperatures and in poor solvents. At the third generation, the helical bias is intrinsically higher so that the conformational preference of the termini becomes much less sensitive to solvent and temperature, and the unfolding process becomes more difficult. We propose that nonbonded repulsive interactions that increase with generation and in poor solvents couple the motions and conformational preferences of each pair of terminal groups through their correlated rotations and contribute to the stability of the M helical conformation of the terminal groups. This represents the first example of well-defined asymmetric secondary structure occurring in a dendrimer system.