Chromatographic enantiomer separation and circular dichroism spectra of chiral rhodanines

Abstract

Rhodanines [2-thioxo-2,3-dihydro-1,3-thiazol-4(5H)-ones] with isopropyl, sec-butyl and neopentyl substituents in position 3 and methyl and phenyl substituents in position 5 have been resolved into enantiomers by chromatography on microcrystalline, swollen triacetylcellulose. While the 5-methyl compounds showed good optical stability, the 5-phenyl analogues underwent racemization in ethanol solution with half-lives of 1–2 h at ambient temperature and were therefore resolved at –3 °C.

The UV spectra resemble those of N-acetyldithiocarbamates but with hypsochromic shifts because of the inclusion of the chromophore in a five-membered ring. Transition energies and absorption intensities were reasonably well reproduced by CNDO/S calculations.

The CD spectra showed bands corresponding to the UV bands but also another band at ca. 260 nm, which may be ascribed to an n→π* transition located in the carbonyl group. The absolute configurations of the 5-methyl compounds were assigned on the basis of the signs of the n→π* bands, using Ripperger's quadrant rule for dithiocarbamates. The mechanisms for generation of rotational strengths in the 5-phenyl compounds are more complex and no safe assignments of absolute configurations could be made.

The CD spectrum of 3-neopentyl-5-methylrhodanine in ethanol was found to be insensitive to the temperature from ambient to –97 °C, indicating a 1:1 equilibrium between the anti and syn forms, whereas the 5-phenyl analogue showed a considerable temperature dependence, indicating a more biased equilibrium.