Aromatic architecture based on cis conformational preference of N-methylated amides

Abstract

Aromatic secondary amides such as benzanilide (1) exist in trans-amide form both in the crystal and in solution, whereas N-methylbenzanilide (2) exists in cis form in the crystal, and predominantly in cis form in various solutions. Similar cis conformational preferences were observed in the aromatic N,N′-dimethylated urea (4) and guanidine (7), in which two aromatic rings are located face-to-face. The cis conformations of N-methylated amides, ureas and guanidines could be utilized to construct interesting aromatic architecture. First, N,N′,N″-trimethyl-N,N′,N″-triphenyl-1,3,5- benzenetricarboxamide (12) and a cyclic triamide 14 have crystal structures in which three N-phenyl groups exist with the same orientation(syn conformation). Second, several aromatic amides which have no fixed asymmetric center afforded chiral crystals by simple recrystallization. In the case of 1,2-bis(N-benzoyl-N-methylamino)benzene (18), two enantiomeric crystals could be distinguished morphologically or on the basis of symmetrical CD spectra in KBr, and by 1H-NMR in the presence of chiral 1, 1′-bi-2-naphthol. Third, aromatic multi-layered structures could be constructed by using N,N′-dimethylated urea or guanidino bonds. These unique layered structures could be applied to obtain aromatic molecules with potent DNA-binding ability.