New Techniques of Structure Elucidation for Sesquiterpenes
301
Fig. 5 Inflorescence of Stevia eupatoria Willd. Photograph courtesy of Dr. J. Martín TorresValencia
configuration of this ar-himachalene derivative was analyzed by X-ray diffraction
analysis of its acetyl derivative 276, for which the structure was solved by direct
methods and refined to a discrepancy index of 4.7%. Calculation of the Flack parameter gave an x = 0.10(2) value, while estimation of the Hooft parameter provided y
= 0.11(11). Inversion of the absolute configuration of the X-ray structure increased
these parameters to x = 0.9(2) and y = 0.89(11), respectively, confirming that
podocephalol acetate (276), and therefore podocephalol (275) had the (7R) configuration. The crystal data also allowed the identification of unusual supramolecular layers
in the cell package of 276. In addition, vibrational circular dichroism measurements
of 276 reinforced the assignment of the absolute configuration of these molecules
[115].
OR
275 R = H (podocephalol)
276 R = Ac (acetyl-podocephalol)
The tricyclic sesquiterpenoid rasteviol (277), obtained by reduction with sodium
borohydride in methanol of the natural product rastevione isolated from the roots
of Stevia serrata collected near Morelia, Mexico, was transformed into the novel
sesquiterpenoids 278–283 by means of multiple Wagner-Meerwein rearrangements
and hydride shifts that occurred when 277 was treated with Et 2 O:BF 3 (Scheme 1).
Four of these new compounds (279, 281–283) possess new hydrocarbon skeletons
301
Fig. 5 Inflorescence of Stevia eupatoria Willd. Photograph courtesy of Dr. J. Martín TorresValencia
configuration of this ar-himachalene derivative was analyzed by X-ray diffraction
analysis of its acetyl derivative 276, for which the structure was solved by direct
methods and refined to a discrepancy index of 4.7%. Calculation of the Flack parameter gave an x = 0.10(2) value, while estimation of the Hooft parameter provided y
= 0.11(11). Inversion of the absolute configuration of the X-ray structure increased
these parameters to x = 0.9(2) and y = 0.89(11), respectively, confirming that
podocephalol acetate (276), and therefore podocephalol (275) had the (7R) configuration. The crystal data also allowed the identification of unusual supramolecular layers
in the cell package of 276. In addition, vibrational circular dichroism measurements
of 276 reinforced the assignment of the absolute configuration of these molecules
[115].
OR
275 R = H (podocephalol)
276 R = Ac (acetyl-podocephalol)
The tricyclic sesquiterpenoid rasteviol (277), obtained by reduction with sodium
borohydride in methanol of the natural product rastevione isolated from the roots
of Stevia serrata collected near Morelia, Mexico, was transformed into the novel
sesquiterpenoids 278–283 by means of multiple Wagner-Meerwein rearrangements
and hydride shifts that occurred when 277 was treated with Et 2 O:BF 3 (Scheme 1).
Four of these new compounds (279, 281–283) possess new hydrocarbon skeletons