Poly(p-benzamide)s 1 bearing a chiral side chain on the nitrogen atom were synthesized by chain-growth polycondensation methodology. The polyamides exhibited well-defined molecular weights with narrow polydispersities. Solutions of the polyamides in several organic solvents (CH(3)CN, CHCl(3), and CH(3)OH) showed dispersion type CD signals characteristic of coupled-oscillator and much larger as compared with the corresponding monomer. The CD signals were dependent on the temperature and molecular weight of the polyamides but independent of the solvent, as far as examined. An exciton model analysis of the absorption and CD spectra provided a clear-cut picture for the secondary structure of these polyamides in solution that the N-alkylated poly(p-benzamide)s possess a right-handed helical conformation ((P)-helix). In the solid states, the results of X-ray crystallographic analysis of 4-(methylamino)benzoic acid oligomers substantiated that they have a helical conformation with three monomer units per turn.

Hexapole helicenes 1, which contain six [5]helicene substructures, were synthesized by Pd-catalyzed [2+2+2]cycloadditions of aryne precursor 6. Among the possible 20 stereoisomers, which include ten pairs of enantiomers, HH-1 was obtained selectively. Density functional theory (DFT) calculations identified HH-1 as the second most stable isomer that quantitatively isomerizes under thermal conditions into the most stable isomer (HH-2). Both enantiomers of HH-2 can be separated by chiral HPLC. Single-crystal X-ray diffraction analyses revealed a saddle-like structure for (P,M,P,P,M,P) HH-1 and a propeller-like structure for (P,M,P,M,P,M) HH-2. Because of the helical assembly and the resulting steric repulsion, the structure of HH-1 is significantly distorted and exhibits the largest twisting angle reported so far (up to 35.7° per benzene unit). Electrochemical studies and DFT calculations indicated a narrow HOMO-LUMO gap on account of the extended π-system. Kinetic studies of the isomerization from HH-1 to HH-2 and the racemization of enantiomerically pure HH-2 were conducted based on H NMR spectroscopy, HPLC analysis, and DFT calculations.

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[reaction: see text] Introduction of an electron-withdrawing group on the aromatic ring of N-methylacetanilide decreased the ratio of the cis conformer, and the ratio correlates well with the Hammett sigma values of the substituents. These steric properties can be applied to achieve amide conformational switching by protonation at the aromatic substituent of 4-[bis(dimethylamino)]-N-methylacetanilide or N-[p-(dimethylamino)phenyl]-N-phenylacetamide.

The stereochemical alteration caused by N-methylation of an aromatic amide structure is an efficient means of changing the biological activity of a molecule, as we have found during the drug design of synthetic retinoids' and synthetic cytokinins.1 2 N-Methylbenzanilide (1) exists predominantly in a cis amide conformation in solution and in the crystal, while benzanilide (2) exists in a trans conformation.3 Though the cis conformation is superficially less favorable, this seems a rather common phenomenon intrinsic to aromatic /V-methylamides. The cause of this real stabilization of the cis structure is unclear and is under study. The cis preference can be used for the fixation of a molecule in a shape that seems less favorable from simple stereochemical considerations. In this paper, we present some examples of molecular construction by using an /V-methylamide structure as a splint or a scantling in a molecule.

The crystal structures of N,N‘-diphenylguanidine (1) and its N-methylated derivatives were investigated, and the conformational properties of these molecules were utilized to construct water-soluble oligomers with multilayered aromatic structures. N,N‘-Diphenylguanidine (1) afforded two types of crystals, chiral (P212121) and racemic (P21/c), upon recrystallization from EtOH. In both crystals, 1 exists in the (E,Z) conformation, in which one C−N bond (length:  1.28−1.30 Å) attached to a phenyl ring shows double-bond character. In contrast, N,N‘-dimethyl-N,N‘-diphenylguanidine (4a) exists in the (Z,Z) conformation with the two aromatic rings facing each other. As judged from the crystal structures of several N-methylated compounds, the conformational preferences of diphenylguanidines appear to be related to those of aromatic anilides. N,N,N‘,N‘‘-Tetramethyl-N‘,N‘‘-diphenylguanidinium iodide (6) afforded chiral crystals, like 1 and N-methyl-N,N‘-diphenylguanidine (2). The absolute structure of each enantiomeric propeller conformation of 6 was determined by X-ray analysis using the Bijvoet difference method. The Z-conformational preference of 4 allowed us to synthesize oligomeric di- or tetraguanidines (9−12) which have multilayered aromatic structures both as a crystal and in organic and aqueous solvents.

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