Effect of the α-substituted chiral side chain on the helical conformation of N-substituted poly(p-benzamide)

Abstract

The effect of the side chain chiral center position on the secondary structure of oligomers and polymers of N-substituted 4-aminobenzoic acid was studied. Monomer 1 was prepared from l-alanine methyl ester hydrochloride in seven steps. The poly(p-benzamide) having an α-substituted side chain at the nitrogen atom (α-Me-PA) was obtained from the polymerization of 1 with triethylsilane, CsF, and 18-crown-6 in THF. The circular dichroism (CD) studies in solution demonstrated that α-Me-PA adopted a right-handed helical conformation in protic polar solvents, such as water and methanol, and a random structure in chloroform, indicating that solvophobic interactions played an important role in the helical folding of α-Me-PA. The tetramer of (S)-4-(sec-butylamino)benzoic acid was synthesized as a model compound of α-Me-PA. The single crystal X-ray analysis of the tetramer revealed that the right- and left-handed helical molecules existed in a one-to-one ratio in the crystal, and that the screw pitch of the right-handed helix was shorter than the left-handed helix.