New Techniques of Structure Elucidation for Sesquiterpenes
291
at the PBEPBE/6-31G* level of theory in gas phase and in methanol [100]. The
calculated ECD spectra of sarcanolides A (221) and B (222) closely matched their
experimental ECD curves and the molecular models of both structures allowed the
interpretation of the exciton couplings, which arose from the interaction of the α,βunsaturated γ -lactone and the α,β-unsaturated ketone chromophores [100].
O
O
O
HO
OH
OH
OH
O
O
O O
O
O
O
HO
OH
OH
O
O
O O
221 (sarcanolide A)
222 (sarcanolide B)
4 Techniques Based on X-Ray Diffraction
Single-crystal X-ray crystallography is one of the most powerful techniques for the
elucidation of the three-dimensional molecular structures that has been extremely
useful for the field of natural products. While most advances in X-ray diffraction
analysis have concentrated on the improvement of speed and accuracy of data collection, some developments have focused on the generation of enhanced tools for the
determination of the absolute configuration of organic compounds. These approaches
have been valuable for natural products that crystallize in non-centrosymmetric space
groups. The calculation of the Flack parameter is based on the comparison obtained
from a complete derivation of a Bijvoet intensity ratio by way of mean-square Friedel
differences [101], while the Hooft protocol includes probabilistic methodology for
the determination of the absolute structure of an enantiopure compound and is also
based on Bijvoet-pair intensity differences [102]. Therefore, in both cases the collection of a larger data set is required to include pairs of redundant reflections related to
each other by inversion through the origin. Furthermore, a reliable absolute configuration assignment can be made by using Cu Kα radiation. In both methods, if the
analyzed structure is solved in the correct absolute configuration, either the Flack
and/or the Hooft parameters will be near zero. On the contrary, if the structure is being
solved in the opposite absolute configuration, both parameters will be increased up
to the unit. Inversion of the model structure in the crystallographic software can be
applied to test this attribute. This section describes the structure elucidation of new
sesquiterpenoids using X-ray crystallography for which their absolute configuration
determination was achieved by calculating the Flack and/or Hooft parameters.
291
at the PBEPBE/6-31G* level of theory in gas phase and in methanol [100]. The
calculated ECD spectra of sarcanolides A (221) and B (222) closely matched their
experimental ECD curves and the molecular models of both structures allowed the
interpretation of the exciton couplings, which arose from the interaction of the α,βunsaturated γ -lactone and the α,β-unsaturated ketone chromophores [100].
O
O
O
HO
OH
OH
OH
O
O
O O
O
O
O
HO
OH
OH
O
O
O O
221 (sarcanolide A)
222 (sarcanolide B)
4 Techniques Based on X-Ray Diffraction
Single-crystal X-ray crystallography is one of the most powerful techniques for the
elucidation of the three-dimensional molecular structures that has been extremely
useful for the field of natural products. While most advances in X-ray diffraction
analysis have concentrated on the improvement of speed and accuracy of data collection, some developments have focused on the generation of enhanced tools for the
determination of the absolute configuration of organic compounds. These approaches
have been valuable for natural products that crystallize in non-centrosymmetric space
groups. The calculation of the Flack parameter is based on the comparison obtained
from a complete derivation of a Bijvoet intensity ratio by way of mean-square Friedel
differences [101], while the Hooft protocol includes probabilistic methodology for
the determination of the absolute structure of an enantiopure compound and is also
based on Bijvoet-pair intensity differences [102]. Therefore, in both cases the collection of a larger data set is required to include pairs of redundant reflections related to
each other by inversion through the origin. Furthermore, a reliable absolute configuration assignment can be made by using Cu Kα radiation. In both methods, if the
analyzed structure is solved in the correct absolute configuration, either the Flack
and/or the Hooft parameters will be near zero. On the contrary, if the structure is being
solved in the opposite absolute configuration, both parameters will be increased up
to the unit. Inversion of the model structure in the crystallographic software can be
applied to test this attribute. This section describes the structure elucidation of new
sesquiterpenoids using X-ray crystallography for which their absolute configuration
determination was achieved by calculating the Flack and/or Hooft parameters.