Scheme 3.18

Self-association can occur in any solvophobic foldamers at high enough concentrations and is often a problem encountered during characterization of conformations. In general, ill-defined aggregates are formed during such a process and it is quite difficult to limit self-association to a specific stage such as dimerization. An elegant approach toward duplex foldamers was recently reported by Furusho, Yashima, and colleagues in meta-linked oligoresorcinols 34 [132]. When the 5merH was crystallized from a hydrophobic solvent mixture, chloroform/acetonitrile, it formed a single helix in the solid state (Fig. 3.8). Unlike the mPE oligomers, the aromatic units in meta-linked oligoresorcinols cannot stack upon one another due to steric congestion. It was clear from the crystal structure that a large surface of hydrocarbon was exposed in the single helix and was in contact with solvent molecules. When the compound was crystallized from

Fig. 3.8 (A) Single helical structure of 5merH. (B) Double helical structure of 5merH. (Reprinted with permission from Ref 132. Copyright 2006, American Chemical Society, Washington, DC.)

water, nonpolar solvents were no longer available, and unfavorable exposure of hydrophobic surface was avoided by forming a double helix. Geometrical manipulation through rigid and amphiphilic backbone once again demonstrates its power in limiting self-association to a selective, specific process. The double helical structure was maintained in aqueous solution as shown by UV, NMR, and CD spectroscopy.

Scheme 3.19

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