Stereochemistry of samarcandin-type sesquiterpenoid coumarins. Crystal structures of feshurin and nevskin

EXPERIMENTAL
Melting points of sesquiterpenoid coumarins feshurin (2, 216°C, CHCl
3
) and nevskin (4, 185°C, EtOAc) were
determined on a Boetius apparatus. Thermal studies of 4 were performed on an STA 409 PG Netzsch Simultaneous Analyzer
with a K-type thermocouple (Low RG Silver) and aluminum crucibles. Measurements were made in an inert N
2
atmosphere
at N
2
flow rate 50 mL/min in the range 25–400°C at heating rate 5 K/min. The sample weight was 5 mg. The measuring
system was calibrated with a standard set of KNO
3
, In, Bi, Sn, Zn, CsCl.
XSA. Single crystals of 2 and 4 for XSA were grown by slow evaporation of CHCl
3
and benzine–EtOAc, respectively,
at room temperature. Unit-cell constants were determined and refined on a CCD Xcalibur Ruby diffractometer (Oxford
Diffraction) using Cu K
D-radiation (300 K, graphite monochromator) [7]. Absorption corrections in both instances were
applied semi-empirically using the SADABS program [8]. Table 2 presents the principal parameters of the XSA and structure
refinements of 2 and 4.
The structures were solved by direct methods using the SHELXS-97 program suite and refined using the SHELXL-97
program. The results were plotted for visualization using the SHELXTL program [9]. All non-hydrogen atoms were refined
by anisotropic full-matrix LS (over F
2
). Positions of H atoms were located geometrically and refined with fixed isotropic
thermal parameters U
iso
= nU
eq
, where n = 1.5 for methyls and 1.2 for others and U
eq
 is the equivalent isotropic thermal
parameter of the corresponding C atoms. Hydroxyl H atoms were found in difference electron-density syntheses.
Peaks belonging according to our hypothesis to H
2
O or EtOAc solvate molecules appeared in difference electron-
density syntheses in the last refinement cycles of the structure of 4. The positions of these peaks were refined by isotropic LS
as isolated O and(or) C atoms with variable populations.
Data from the XSA were deposited as a CIF-file in the Cambridge Crystallographic Data Centre (CCDC).
Fig. 4. TG-DSC curves of nevskin (4).
50 100 150 200 250 300 350
-0.8
-0.6
-0.4
-0.2
0.0
0.2
30
40
50
60
70
80
90
100
TG, %
Temperature, Ñ
q
DSC
TG
DSC, W/mg
m
exp.
[1]

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ACKNOWLEDGMENT
We thank V. I. Vinogradova and N. D. Abdullaev for discussing the results. The work was sponsored by the AS RU
Basic Scientific Research Program, Grant FA-A7-T185.