Poly(β‐Amino acids). VI. Synthesis and conformational properties of poly[(r)‐3‐pyrrolidinecarboxylic acid]

Racemic and optically active 3‐pyrrolidinecarboxylic acids (β‐proline) were synthesized, and their polymers, poly[(RS)‐β‐proline] and poly[(R)‐β‐proline], were prepared by the polycondensation reaction of the p‐nitrophenyl esters. Model compounds, N‐cyclopentylcarboxylic acid pyrrolidide and N‐cyclopentylcarbonyl‐(R)‐3‐pyrrolidinecarboxylic acid pyrrolidide, were synthesized to elucidate the conformation of the polymer. The solution properties of poly[(R)‐β‐proline] and the model compounds were investigated by means of circular dichroism (CD) and NMR spectroscopy. The spectral patterns of the polymer and model compounds were similar in various solvents. Poly[(R)‐β‐proline] and poly[(RS)‐β‐proline] showed identical NMR spectra. These results suggest that poly[(R)‐β‐proline] may exist in a random conformation consisting of mixtures of cis and trans amide bonds. The conformational study of cyclopentanecarboxylic acid pyrrolidide by NMR spectroscopy with a shift reagent, Eu(fod)3, in CDCl3 implied that the plane containing the amide group bisects the cyclopentane ring. This suggests that each amide plane in the polymer in chloroform may also bisect the pyrrolidine ring.