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Abstract:
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The absolute configuration of semisynthetic (2)-3a,6b-acetoxytropane
1, prepared from (2)-6b-hydroxyhyoscyamine 2, has been determined using vibrational
circular dichroism (VCD) spectroscopy. The vibrational spectra (IR and VCD) were calculated
using DFT at the B3LYP/DGDZVP level of theory for the eight more stable conformers
which account for 99.97% of the total relative abundance in the first 10 kcal/mol
range. The calculated VCD spectra of all considered conformations showed two distinctive
spectral ranges, one between 1300 and 1200 cm21, and the other one in the 1150–
950 cm21 region. When compared with the experimental VCD spectrum, the first spectral
region confirmed the calculated conformational preferences, whereas the second
region showed little change with conformation, thus allowing the determination of the
absolute configuration of 1 as (3S,6S)-3a,6b-diacetoxytropane. Also, the bands in the
second region showed similarities between 1 and 2 in both the experimental and calculated
VCD spectra, suggesting that these bands are mainly related to the absolute configuration
of the rigid tropane ring system, since they show conformational independency,
no variations with the nature of the substituent, and are composed by closely
related vibrational modes. |