New Techniques of Structure Elucidation for Sesquiterpenes
261
OH
O
OH
O
O
O
HO
OH
47
48
49
The relative configurations of the tricyclic sesquiterpenes isohirsut-1-ene (50)
and isohirsut-4-ene (51), obtained from an engineered Streptomyces strain, were
assigned by comparison of experimental and density functional theory
1 H and
13 C
chemical shifts [17]. To achieve this complex task, due to the hydrocarbon nature
of the structures, DFT calculations were carried out for eight possible stereoisomers
of each compound at the B3LYP/6-31+G(d,p) level of theory followed by scaling
the calculated isotropic values for each atom. Mean absolute deviations between the
calculated and experimental chemical shifts as well as DP4 [18] statistic values were
considered in the discernment of the correct structures [17].
50 (isohirsut-1-ene)
51 (isohirsut-4-ene)
The absolute configuration of a nor-sesquiterpenoid named artarborol (52),
isolated from the Mediterranean shrub Artemisia arborescens, was established using
a combination of chemical derivatization, NMR data, molecular modeling, and
quantum–mechanical calculations [19]. Comparison of the experimental
13 C NMR
chemical shifts with those calculated by the DFT MPW1PW91/6-31G(d,p) method
validated the stereochemical assignment. Given that the structure contains a ninemembered ring that confers flexibility, the study required the evaluation of all possible
conformers and the averaging of their calculated chemical shifts. This protocol was
recommended for sesquiterpenes containing flexible medium-sized rings such as
germacranes, humulanes, and caryophyllanes that exist in conformational equilibria
[19].
O
HO
O
52 (artarborol)
53 ((1R,2S,4S,5R,9R,11R)-3-ishwarone)
The structure of (1R,2S,4S,5R,9R,11R)-3-ishwarone (53) isolated from Peperomia scandens was elucidated by comparison between experimental and theoretical
NMR data that included chemical shift
13 C NMR calculations for four stereoisomers. The molecular modeling protocol also involved the vibrational frequencies
data calculated with the B3LYP functional and the cc-pVTZ basis set [20].
261
OH
O
OH
O
O
O
HO
OH
47
48
49
The relative configurations of the tricyclic sesquiterpenes isohirsut-1-ene (50)
and isohirsut-4-ene (51), obtained from an engineered Streptomyces strain, were
assigned by comparison of experimental and density functional theory
1 H and
13 C
chemical shifts [17]. To achieve this complex task, due to the hydrocarbon nature
of the structures, DFT calculations were carried out for eight possible stereoisomers
of each compound at the B3LYP/6-31+G(d,p) level of theory followed by scaling
the calculated isotropic values for each atom. Mean absolute deviations between the
calculated and experimental chemical shifts as well as DP4 [18] statistic values were
considered in the discernment of the correct structures [17].
50 (isohirsut-1-ene)
51 (isohirsut-4-ene)
The absolute configuration of a nor-sesquiterpenoid named artarborol (52),
isolated from the Mediterranean shrub Artemisia arborescens, was established using
a combination of chemical derivatization, NMR data, molecular modeling, and
quantum–mechanical calculations [19]. Comparison of the experimental
13 C NMR
chemical shifts with those calculated by the DFT MPW1PW91/6-31G(d,p) method
validated the stereochemical assignment. Given that the structure contains a ninemembered ring that confers flexibility, the study required the evaluation of all possible
conformers and the averaging of their calculated chemical shifts. This protocol was
recommended for sesquiterpenes containing flexible medium-sized rings such as
germacranes, humulanes, and caryophyllanes that exist in conformational equilibria
[19].
O
HO
O
52 (artarborol)
53 ((1R,2S,4S,5R,9R,11R)-3-ishwarone)
The structure of (1R,2S,4S,5R,9R,11R)-3-ishwarone (53) isolated from Peperomia scandens was elucidated by comparison between experimental and theoretical
NMR data that included chemical shift
13 C NMR calculations for four stereoisomers. The molecular modeling protocol also involved the vibrational frequencies
data calculated with the B3LYP functional and the cc-pVTZ basis set [20].