60 CD励起子カイラリティー法による鎖状グリコール系の絶対構造決定(ポスター発表の部)

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To determine the absolute configuration and conformation of chiral acyclic 1,3- and 1,2-glycols, the CD exciton chirality method has been applied to various acyclic 1,3- and 1,2-bis(p-bromobenzoates). Acyclic anti-1,3-dibenzoates exhibit typical exciton split CD Cotton effects, the sign of which agrees with the sign of the screw sense between two benzoate chromophores in the conformation expanded in a zigzag form. For example, the CD spectrum of anti-(2S,4S)-8 exhibited bisignate Cotton effects of positive chirality: λ_<ext>253.5nm, Δε+22.1 and λ_<ext>237.0nm, Δε-12.3, A=+34.4. On the other hand, syn-dibenzoates showed a weak single Cotton effects irrespective of the asymmetric structure: e.g., syn-(2R,4S)-15, λ_<ext>240.5nm, Δε+4.1. Other 1,3-dibenzoates composed of primary and secondary alcoholic benzoates exhibit exciton split CD Cotton effects of half intensity in comparison with those of anti-1,3-dibenzoates: e.g., (2S)-1, λ_<ext>252.6nm, Δε+13.9 and λ_<ext>235.9nm, Δε-3.9, A=+17.8. These results provided the CD method for determination of the absolute stereochemistry of acyclic 1,3-glycols. The same method could be applied to acyclic 1,2-glycol dibenzoates. In the case of syn-(2S,3S)-21, CD spectrum showed positive first and negative second Cotton effects indicating positive exciton chirality in the stable conformation of 21: λ_<ext>252.8nm, Δε+11.8 and λ_<ext>238.2nm, Δε-8.1, A=+19.9. These results are consistent with the conclusion of the conformational analysis that the conformer C with a positive exciton chirality in Figure 5 is the most stable. The CD method obtained here was next applied to acyclic 1,2,3-triols. Dibenzoate syn-(2S,3S)-28 derived from triol (-)-26 exhibited typical exciton Cotton effects of positive chirality, λ_<ext>254.1nm, Δε+9.9 and λ_<ext>238.5nm, Δε-9.6, A=+19.5, while anti-(2S,3R)-31 showed single Cotton effect reflecting meso-structure of dibenzoate part. To determine the absolute configuration of meso compound 31, it was converted to dibenzoate 33 composed of terminal primary and secondary benzoates. The CD spectrum of 33 exhibited exciton Cotton effects which enabled us to determine the absolute configuration as indicated: λ_<ext>253.0nm, Δε-18.4 and λ_<ext>237.0nm, Δε+8.0, A=-26.4. It was thus clarified that the CD exciton chirality method is also applicable to acyclic polyol systems as in the case of cyclic compounds.
天然有機化合物討論会の論文
1991-09-07

60 CD励起子カイラリティー法による鎖状グリコール系の絶対構造決定(ポスター発表の部)

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