Folding dendrons: The development of solvent-, temperature-, and generation-dependent chiral conformational order in intramolecularly hydrogen-bondeddendrons

The synthesis of intramolecularly hydrogen-bonded dendrons with stereogenic terminal groups derived from (1S,2S)-(+)-thiomicamine up to the third gene ration is described. Circular dichroism (CD) studies reveal that the equili bria interconverting two diastereomeric helical conformations (M and P heli ces) relating a pair of anthranilamide termini depends on solvent, temperat ure, and dendrimer generation. A conformational preference for M-type helic ity 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 al l solvents and at all temperatures investigated; however, at the second gen eration the equilibrium begins to bias a single diastereomeric helical conf ormation along the periphery that becomes maximal at low temperatures and i n poor solvents. At the third generation, the helical bias is intrinsically higher so that the conformational preference of the termini becomes much l ess sensitive to solvent and temperature, and the unfolding process becomes more difficult. We propose that nonbonded repulsive interactions that incr ease with generation and in poor solvents couple the motions and conformati onal preferences of each pair of terminal groups through their correlated r otations and contribute to the stability of the M helical conformation of t he terminal groups. This represents the first example of well-defined asymm etric secondary structure occurring in a dendrimer system.