engleski

2H-Chromenes: Synthesis, Separation of Enantiomers, Circular Dichroism and Thermal Racemization

The novel 2H-chromenes 1-3 have been synthesized by reduction of the appropriate lactones with diisobutylaluminium hydride and subsequent reaction of the resulting lactols with the corresponding alcohols. The preparation of 4-7 was achieved from suitably substituted benzopyrylium salts and corresponding alcohols, while 8-13 were obtained by heating of appropriate hemiacetals with corresponding alcohols. The preparative separation or enrichment of enantiomers of 1-13 was accomplished by enantioselective liquid chromatography on triacethyl- or tribenzoylcellulose. The relative configuration of 1-3 was deduced by comparison of the circular dichroism spectra of their enantiomers. The barriers for the interconversion of enatiomers were determined by thermal racemization. Gibbs energies of activation G for reversible cleavage of the C-O bond in 1-13 have been found in the range of 101-134 kJ/mol and are rationalized qualitatively by steric and electronic effects in the transition state.

Chiral 2H-chromenes; Enantiomer separation; Enantioselective liquid chromatography; Circular dichroism; Relative configuration; Barriers to racemization

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