Helical aromatic urea and guanidine

N,N'-Dimethyl-N,N'-dipheylurea (1) and N,N'-Dimethyl N,N'-diphenylguanidinium bromide (2) each have folded structures with the two phenyl groups located face-to-face. Oligomeric compounds 3 and 4, having five benzene rings connected at the mera positions by cis-urea or guanidino groups, form aromatic layered helices in the crystal state. The crystals are racemic with a I : 1 enantiomer ratio. They exhibit stacked (3) or zigzag chain (4) packing, reflecting displaced parallel structure for the terminal phenyl groups of neighboring molecules of 3 and tilted T-shaped structure for those of 4, respectively. (C) 1997 Elsevier Science.