2 research outputs found
A Macrocyclic Dimeric Diterpene with a <i>C<sub>2</sub></i> Symmetry Axis
An unprecedented macrocyclic dimeric diterpene containing a <i>C<sub>2</sub></i> symmetry axis was isolated from <i>Acacia schaffneri</i>. This compound, named schaffnerine, was characterized as (5<i>S</i>,7<i>S</i>,8<i>R</i>,9<i>R</i>,10<i>S</i>,17<i>S</i>,5′<i>S</i>,7′<i>S</i>,8′<i>R</i>,9′<i>R</i>,10′<i>S</i>,17′<i>S</i>)-7,8:7,17′:16,17:17,7′:7′,8′:16′,17′-hexaepoxy-7,8-<i>seco</i>-7′,8′-<i>seco</i>-dicassa-13,13′-diene (<b>1</b>) from its spectroscopic data. Comparison of its experimental vibrational circular dichroism spectrum with that calculated using density functional theory, at the B3LYP/DGDZVP level, assigned its preferred conformation and absolute configuration. The latter was confirmed by evaluation of the Flack and Hooft parameters obtained after single-crystal X-ray diffraction analysis
Methodology for the Absolute Configuration Determination of Epoxythymols Using the Constituents of <i>Ageratina glabrata</i>
A methodology to determine the enantiomeric
excess and the absolute
configuration (AC) of natural epoxythymols was developed and tested
using five constituents of <i>Ageratina glabrata</i>. The
methodology is based on enantiomeric purity determination employing
1,1′-bi-2-naphthol (BINOL) as a chiral solvating agent combined
with vibrational circular dichroism (VCD) measurements and calculations.
The conformational searching included an extensive Monte Carlo protocol
that considered the rotational barriers to cover the whole conformational
spaces. (+)-(8<i>S</i>)-10-Benzoyloxy-6-hydroxy-8,9-epoxythymol
isobutyrate (<b>1</b>), (+)-(8<i>S</i>)-10-acetoxy-6-methoxy-8,9-epoxythymol
isobutyrate (<b>4</b>), and (+)-(8<i>S</i>)-10-benzoyloxy-6-methoxy-8,9-epoxythymol
isobutyrate (<b>5</b>) were isolated as enantiomerically pure
constituents, while 10-isobutyryloxy-8,9-epoxythymol isobutyrate (<b>2</b>) was obtained as a 75:25 (8<i>S</i>)/(8<i>R</i>) scalemic mixture. In the case of 10-benzoyloxy-8,9-epoxythymol
isobutyrate (<b>3</b>), the BINOL methodology revealed a 56:44
scalemic mixture and the VCD measurement was beyond the limit of sensitivity
since the enantiomeric excess is only 12%. The racemization process
of epoxythymol derivatives was studied using compound <b>1</b> and allowed the clarification of some stereochemical aspects of
epoxythymol derivatives since their ACs have been scarcely analyzed
and a particular behavior in their specific rotations was detected.
In more than 30 oxygenated thymol derivatives, including some epoxythymols,
the reported specific rotation values fluctuate from −1.6 to
+1.4 passing through zero, suggesting the presence of scalemic and
close to racemic mixtures, since enantiomerically pure natural constituents
showed positive or negative specific rotations greater than 10 units