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Aryl Amides and Aryl Ureas

Aryl amides are oligomers that consist of either diaminobenzene and dicarboxylic acid benzene derivatives, or aminobenzoic acid derivatives (Fig. 8.3) [118]. Various functional groups have been introduced to hydrogen bond to the backbone amide to limit the number of low energy conformations, making the aromatic and amide components coplanar. The interactions responsible for structural stabilization are almost exclusively non-cooperative, although cooperative interactions are possible between nonadjacent units (see Chapter 1). Linear [119-121], cyclic [122-124], crescent-shaped [125, 126], and helical [125-129] aryl amides have all been reported. However, bioactivity has only been introduced into the linear extended aryl amides. Aryl amides can be synthesized by Fmoc-based solid phase chemistry but are usually obtained through solution-phase techniques [130]. Carboxylic acids that have been converted to the corresponding acid chlorides are reacted with aromatic amines to produce aryl amide oligomers. Furthermore, a convergent segment doubling strategy can be employed whereby two oligomer segments are efficiently linked in a single step [130]. However, aromatic oligoamides that fold during synthesis can reduce yields because steric hindrance prevents access to the reagents.

Aryl ureas are structurally similar to aryl amides, possessing backbones rigidi-fied by H-bonding groups. Aryl ureas differ from aryl amides by the insertion of an amine moiety between the aromatic rings and carbonyls of the linking amides (Fig. 8.3). Bioactive aryl ureas have been demonstrated to adopt extended conformations. The aryl urea extended conformation is stabilized by H-bonding interac-

Aryl urea Aryl amide

Fig. 8.3 Chemical structures of the monomers for aryl amide and aryl ureas.

Aryl urea Aryl amide

Fig. 8.3 Chemical structures of the monomers for aryl amide and aryl ureas.

tions. The presence of an additional H-bond interaction makes the backbone more rigid compared to aryl amides. Several procedures have been described for the synthesis of urea oligomers, including on solid support via activated carbamate intermediates [131-133] and single-pot reactions using carbonyl diimada-zole couplings [134].

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