Synthesis, characterization, and solution properties of 5‐ethyl‐substituted poly( L ‐prolines). Synthesis of cis‐ and trans‐5‐ethyl‐ L ‐proline and optically active poly(cis‐5‐ethyl‐ D ‐proline)

This article reports a continuation of our work on the substituent effects on the preferred helical conformations and the mutarotation of substituted poly(L‐prolines). The size of the substituent has been increased from a methyl group to an ethyl group in the 5 position. The purpose is twofold: (i) according to our theoretical conformational energy calculations, an ethyl group in the 5 position can exert a greater steric effect than can a methyl group; and (ii) the rotation‐isomerization of the ethyl group introduces a new intriguing fact to the problem. The cis isomer of 5‐ethylproline was synthesized by catalytic hydrogenation of Δ′‐2‐ethylpyrroline‐5‐carboxylic acid, whereas for trans‐5‐ethylproline, a chemical separation method involving p‐toluenesulfonyl chloride was used. The resolution of cis‐5‐ethylproline and the assignment of absolute configurations have been carried out by fractional crystallization and circular dichroism spectroscopic techniques, respectively. Poly(cis‐5‐ethyl‐D‐proline) was obtained from its corresponding N‐carboxyanhydrides via a ring opening polymerization.