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

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• 作者：Janosch Recker ; Dennis J. Tomcik ; and Jon R. Parquette
• 刊名：Journal of the American Chemical Society
• 出版年：2000
• 出版时间：October 25, 2000
• 年：2000
• 卷：122
• 期：42
• 页码：10298 - 10307
• 全文大小：185K
• 年卷期：v.122,no.42(October 25, 2000)
• ISSN：1520-5126

文摘

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