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[摘要]：The absolute configuration (AC) of sargaol acetate (2), isolated from the brown alga Stypopodium fiabelliforme, was determined using vibrational circular dichroism (VCD) measurements and density functional theory (DFT) calculations. Conformational searches using the Monte Carlo stochastic algorithm for 2 and three model compounds with a smaller side chain (diisoprenyl, isoprenyl and ethyl) provided 2172, 596, 82 and 12 conformations, respectively. In the last two cases the moderate number of conformations allowed conformational distributions assessment using single point energy calculations and considering a Boltzmann behavior. In the larger model molecules a prior selection from the original conformational set was done based on structure diversity as implemented in the Spartan'04 modeling software, keeping only 100 conformations in each case. All final conformers where further subjected to geometry optimizations and vibrational calculations providing weighted theoretical IR and VCD spectra which were compared to the experimental spectrum. In all cases only very weak bands are observed above 1300 cm(-1), the strong bands around 1200 cm(-1) clearly show that the AC of 2 is R, and only models including isoprenyl units allowed good prediction of bands in the 1000-1100 cm(-1) spectral region. |
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