SYNTHESIS, CHARACTERIZATION, AND SOLUTION PROPERTIES OF 5-ETHYL-SUBSTITUTED POLY(L-PROLINES) - SYNTHESIS OF CIS-5-ETHYL-L-PROLINE 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 DELTA prime -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.