New Techniques of Structure Elucidation for Sesquiterpenes
259
Artemisinin (25) has been the subject of an investigation that included the
1 H
and
13 C NMR calculation of absolute shielding constants with the continuous set of
gauge transformations (CSGT) method at the DFT/B3LYP level of theory and the
6-311+G(2d,p) basis set. The calculated magnetic shielding tensors were converted
into chemical shifts by taking into account the
1 H and
13 C absolute shielding tensors
for tetramethylsilane calculated at the same level of theory [14].
OH O
O
O
O
O
O
O
O
O
OH
O
HO
OH
O
O
O
O
O
OH
HO
O
O
O
O
O
OH
O
22
23
24
25 (artemisinin)
7
2
6
2
O
O
HO
HO
O
O
O
OH
HN
O
H
OH
O
28
29
1
3
0
3
The structures of fifteen sesquiterpenoids were revised as depicted in 32–46 as
part of an extensive investigation that examined more than one-hundred halogenated
compounds [15]. The lack of efficient methods to calculate
13 C NMR data for carbons
bearing heavy atoms was resolved by the parametric corrections of DFT-computed