2H-CHROMENES - SYNTHESIS, SEPARATION OF ENANTIOMERS, CIRCULAR-DICHROISM AND THERMAL RACEMIZATION

The novel 2H-chromenes 1-3 have been synthesized by reduction of the a ppropriate lactones with diisobutylaluminium hydride and subsequent re action of the resulting lactols with the corresponding alcohols. The p reparation of 4-'7 was achieved from suitably substituted benzopyryliu m salts and corresponding alcohols, while 8-13 were obtained by heatin g of appropriate hemiacetals with corresponding alcohols. The preparat ive separation or enrichment of enantiomers of 1-13 was accomplished b y enantioselective Liquid chromatography on triacetyl- or tribenzoylce llulose. The relative configuration of 1-3 was deduced by comparison o f the circular dichroism spectra of their enantiomers. The barriers fo r the interconversion of enantiomers were determined by thermal racemi zation. Gibbs energies of activation Delta G(not equal) for reversible cleavage of the C(sp(3))-O bond in 1-13 have been found in the range of 101-134 kJ/mol and are rationalized qualitatively by steric and ele ctronic effects in the transition state.