NMR-based conformational analysis of perezone and analogues

Complete assignment of the (1)H NMR chemical shift and coupling constant values of perezone (1), O-methylperezone (2) and 6-hydroxyperezone (3) was carried out by total-line-shape-fitting calculations using the PERCH iterative spectra analysis software (PERCH Solutions Ltd., Kuopio, Finland). The resulting simulated spectra for the three compounds showed strong similarity to their corresponding experimental spectra. Particularly, all vicinal, allylic and homoallylic coupling constant values for the side chain of the three compounds were very similar, thus revealing that the conformation of these three molecules in solution is indeed almost identical. This fact is in agreement with extended side chain conformations over folded chain conformations because 1, 2 and 3 undergo completely different intramolecular cycloaddition reactions. In addition, results of double pulsed field gradient spin echo NOESY 1D experiments performed on perezone (1) were unable to provide evidence for folded conformers.

Conformational analysis of perezone and dihydroperezone using vibrational circular dichroism

The vibrational circular dichroism (VCD) spectra of perezone and dihydroperezone measured from CDCl(3) solutions were quite similar, suggesting analogous conformations for both molecules. Their absolute configurations were confirmed by comparison of the experimental VCD spectrum of each compound with curves generated from theoretical calculations using density functional theory (DFT) at the B3LYP/DGDZVP level of theory taking into account their conformational mobility. Conformational analysis of the 8-(R) enantiomer showed 19 low energy conformers in a 2.4 kcal/mol energy range, while for 8-(R), with the saturated side alkyl chain, 34 conformers were considered in the first 2 kcal/mol. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field, all MMFF94 conformers were geometrically optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by reoptimization and calculations of their vibrational frequencies at the B3LYP/DGDZVP level. Good agreement between the theoretical 8-(R) enantiomers and experimental VCD curves were observed for both.

Benzodihydrofurans from Cyperus teneriffae

Three new benzodihydrofurans (1-3) and seven known aromatic compounds (4-10) were isolated from the roots of Cyperus teneriffae. Vibrational circular dichroism spectroscopy was used to define the absolute configuration of 1.

Unimolecular rearrangements of ketene-O,O-acetals and fragmentations occurring in the gas phase

Gas phase skeletal rearrangements of regioisomeric 3-cyano-2-methoxy-3a-alkylfuro[2,3-b]- and [3,2-b]indoles were evidenced by product ions [M-32](+•), consistent with loss of methanol, on electron ionization in their mass spectra. The rearranged products occurring in gas phase were demonstrated to have elemental composition and fragmentation properties identical to those of authentic samples of 2-indolyl cyanomalonates. Isotopic labeling experiments support the formation mechanism of the [M-32](+•) ion. Additional thermal gas-phase reaction products were characterized by comparison with an authentic sample.

Myrtenal, a Controversial Molecule for the Proper Application of the CIP Sequence Rule for Multiple Bonds

This communication highlights the need of building hierarchical digraphs for the unequivocal assignment of stereochemical descriptors of (-)-myrtenal, a naturally-occurring oxygenated monoterpene whose absolute configuration (AC) is sometimes misrepresented in its structural formulae. Differentiation between duplicated atoms and phantom atoms for the proper application of the sequence rules is shown to be an essential step to get a proper construction of hierarchical digraphs.

Myrtenal, a controversial molecule for the proper application of the CIP Sequence Rule for multiple bonds

This communication highlights the need of building hierarchical digraphs for the unequivocal assignment of stereochemical descriptors of (-)-myrtenal, a naturally-occurring oxygenated monoterpene whose absolute configuration (AC) is sometimes misrepresented in its structural formulae. Differentiation between duplicated atoms and phantom atoms for the proper application of the sequence rules is shown to be an essential step to get a proper construction of hierarchical digraphs.

Absolute configuration of (-)-myrtenal by vibrational circular dichroism

The VCD spectrum of the monoterpene (-)-myrtenal (1) was compared with theoretical spectra using ab initio density functional theory (DFT) calculations at the B3LYP/6-31G(d,p), B3LYP/6-31G+(d,p), B3LYP/6-311G+(d,p), B3LYP/DGDZVP, and B3PW91/DGTZVP levels of theory. Conformational analysis of 1 indicated that the lowest energy conformer was s-trans-C2-C10, which contributes more than 98.5% to the total conformational population regardless of the employed level of theory. The use of a recently developed confidence level algorithm demonstrated that VCD spectra calculated for the main conformer, using the indicated hybrid functionals and basis set, gave no significant changes, from where it follows that B3LYP/DGDZVP calculations provide a superior balance between computer cost and VCD spectral accuracy. The DGDZVP basis set demanded around a quarter the time than the 6-311G+(d,p) basis set while providing similar results. The spectral comparison also provided evidence that the levorotatory enantiomer of myrtenal has the 1R absolute configuration.

Absolute configuration of tropane alkaloids from Schizanthus species by vibrational circular dichroism

The absolute configuration (AC) of 6beta-hydroxy-3alpha-senecioyloxytropane (1), 3alpha-hydroxy-6beta-tigloyloxytropane (2), 3alpha-hydroxy-6beta-senecioyloxytropane (3), and 3alpha-hydroxy-6beta-angeloyloxytropane (4) was assigned as (1R,3R,5S,6R) using density functional theory (DFT) calculations at the B3LYP/DGDZVP level of theory in combination with experimental vibrational circular dichroism (VCD) measurements and comparison with the spectra of similar tropanes. The AC of 1 followed from a sample isolated from Schizanthus grahamii, while those of the mixture of 2 and 3, isolated from the same source, were determined by comparing the VCD measurement to a weighted calculation of the individual VCD spectra according to a 69:31 ratio of 2:3 determined by (1)H NMR signal integration. In turn, Schizanthus pinnatus provided a 7:3 mixture of 1:4 whose AC was determined using the experimental VCD absorptions in the 1150-950 cm(-1) spectral region which were compared with those observed for 1-3 and with those described for other 3alpha,6beta-tropanediol derivatives.

Antifeedant and cytotoxic activity of longipinane derivatives

The polyoxygenated longipinane derivatives 1- 8 were tested as antifeedant compounds against the herbivorous insects Spodoptera littoralis, Rhopalosiphum padi, and Myzus persicae. Compounds 1-3 and 8 exhibited significant antifeedant activity against S. littoralis and M. persicae. The antifeedant activity against S. littoralis increased moderately after the C-8 hydroxy group in 3 was removed to afford 1 and increased strongly after the remaining two hydroxy groups were acetylated to afford 2. Compound 1 was active on M. persicae. Compounds 1, 3, and 4, with an unsaturated six-membered ring, exhibited an increase in post-ingestive effects on S. littoralis ranging from antifeedant in the case of 1 to toxic for compounds 3 and 4. These compounds did not have any phytotoxic effect on Lactuca sativa. When tested on a panel of tumoral cells, compounds 2 and 6 exhibited moderate selective cytotoxic effects on the p53 null lung carcinoma cells H1299, which were not affected by the drug paclitaxel. In addition, vibrational circular dichroism (VCD) was applied to the representative longipinene derivative 2 to verify its absolute configuration, and the sensitivity of the VCD methodology was evaluated by comparing spectra of the three diastereoisomers (4 R,5 S,7 R,9 R,10 R,11 R)-7,9-diacetyloxylongipin-2-en-1-one (2), (4 R,5 S,7 S,9 R,10 R,11 R)-7,9-diacetyloxylongipin-2-en-1-one, and (4 R,5 S,7 R,9 S,10 R,11 R)-7,9-diacetyloxylongipin-2-en-1-one.

Preparation and absolute configuration of (1R,4R)-(+)-3-oxo-, (1S,4S)-(-)-3-oxo- and (1R,3S,4R)-(+)-3-acetyloxy-5-oxo-1 ,8-cineole

Enantiomerically pure (1S,4S)-(-)-3-oxo-1,8-cineole (-)-2 and (1R,4R)-(+)-3-oxo-1,8-cineole (+)-2 were prepared for the first time and their absolute configurations assigned by vibrational circular dichroism (VCD) measurements. Thus, treatment of cineole 1 with chromyl acetate gave rac-2 which after sodium borohydride reduction and acetylation provided racemic 3-endo-acetyloxy-1,8-cineole, rac-4. Enantioselective hydrolysis using porcine liver esterase (PLE) gave a mixture of 3-endo-hydroxy-1,8-cineole (-)-3 and 3-endo-acetyloxy-1,8-cineole (+)-4. After chromatographic separation, (-)-3 was oxidized to (+)-2, while (+)-4 was hydrolysed to (+)-3 and then oxidized to (-)-2. The absolute configuration of either ketone 2 was established by VCD spectroscopy in combination with density functional theory (DFT) calculations at the B3LYP/DGDZVP level of theory, from where it followed that the (+)-2 enantiomer corresponds to (1R,4R)-1,3,3-trimethyl-5-oxo-2-oxabicyclo[2.2.2]octane and the (-)-2 enantiomer to the (1S,4S) molecule which is also in agreement with the absolute configuration deduced by the Mosher method for the starting chiral alcohols. Some literature inconsistencies are clarified. In addition, the enantiomerically pure monoester (1S,3S,4R,5R)-(-)-3-acetyloxy-5-hydroxy-1,8-cineole 6 and the ketoester (1R,3S,4R)-(+)-3-acetyloxy-5-oxo-1,8-cineole 7 were prepared from meso-diacetate 5 by enantioselective asymmetrization also using PLE.

Preparation and Absolute Configuration of (1R,4R)-(+)-3-Oxo-, (1S,4S)-(-)-3-Oxo- and (1R,3S,4R)-(+)-3-Acetyloxy-5-oxo-1,8-cineole

Enantiomerically pure (1S,4S)-(-)-3-oxo-1,8-cineole (-)-2 and (1R,4R)-(+)-3-oxo-1,8-cineole (+)-2 were prepared for the first time and their absolute configurations assigned by vibrational circular dichroism (VCD) measurements. Thus, treatment of cineole 1 with chromyl acetate gave rac-2 which after sodium borohydride reduction and acetylation provided racemic 3-endo-acetyloxy-1,8-cineole, rac-4. Enantioselective hydrolysis using porcine liver esterase (PLE) gave a mixture of 3-endo-hydroxy-1,8-cineole (-)-3 and 3-endo-acetyloxy-1,8-cineole (+)-4. After chromatographic separation, (-)-3 was oxidized to (+)-2, while (+)-4 was hydrolysed to (+)-3 and then oxidized to (-)-2. The absolute configuration of either ketone 2 was established by VCD spectroscopy in combination with density functional theory (DFT) calculations at the B3LYP/DGDZVP level of theory, from where it followed that the (+)-2 enantiomer corresponds to (1R,4R)-1,3,3-trimethyl-5-oxo-2-oxabicyclo[2.2.2]octane and the (-)-2 enantiomer to the (1S,4S) molecule which is also in agreement with the absolute configuration deduced by the Mosher method for the starting chiral alcohols. Some literature inconsistencies are clarified. In addition, the enantiomerically pure monoester (1S,3S,4R,5R)-(-)-3-acetyloxy-5-hydroxy-1,8-cineole 6 and the ketoester (1R,3S,4R)-(+)-3-acetyloxy-5-oxo-1,8-cineole 7 were prepared from meso-diacetate 5 by enantioselective asymmetrization also using PLE.

A new longipinene diester from Stevia monardifolia Kunth

Nonpolar and medium polarity fractions from whole plant methanolic extracts of Stevia monardifolia afforded the new 7beta-angeloyloxy-8alpha-isovaleroyloxylongipin-2-en-1-one 1 along with known 7beta,8alpha-diangeloyloxylongipin-2-en-1-one 2 and 7beta,8alpha-diangeloyloxylongipinan-1-one 3. Alkaline hydrolysis of a mixture of 1 and 2 gave 7beta,8alpha-dihydroxylongipin-2-en-lone 4 which was subjected to a single crystal X-ray diffraction study. The new compound 1, which is the third 7beta,8alpha-dihydroxylongipin-2-en-1-one diester natural product isolated from a Stevia especies, was fully characterized by one- and two-dimensional NMR spectroscopy and its absolute configuration was confirmed as the 4R,5S,7S,8S,10R,11R enantiomer by vibrational circular dichroism (VCD) measurements in comparison to calculation at the B3LYP/DGDZVP level of theory.

A New Longipinene Diester from Stevia Monardifolia Kunth

Nonpolar and medium polarity fractions from whole plant methanolic extracts of Stevia monardifolia afforded the new 7β-angeloyloxy-8α-isovaleroyloxylongipin-2-en-1-one 1 along with known 7β,8α-diangeloyloxylongipin-2-en-1-one 2 and 7β,8α-diangeloyloxylongipinan-1-one 3. Alkaline hydrolysis of a mixture of 1 and 2 gave 7β,8α-dihydroxylongipin-2-en-1-one 4 which was subjected to a single crystal X-ray diffraction study. The new compound 1, which is the third 7β,8α-dihydroxylongipin-2-en-1-one diester natural product isolated from a Stevia especies, was fully characterized by one-and two-dimensional NMR spectroscopy and its absolute configuration was confirmed as the 4R,5S,7S,8S,10R,11 R enantiomer by vibrational circular dichroism (VCD) measurements in comparison to calculation at the B3LYP/DGDZVP level of theory.

Dihydrofurochromones from Prionosciadium thapsoides

Chemical investigations of Prionosciadium thapsoides roots led to the isolation of the new dihydrofurochromones (S)-(+)-4'-O-angeloyl-5-O-methylvisamminol (1) and (S)-(+)-4'-O-senecioyl-5-O-methylvisamminol (2), together with the known coumarins jatamansin, buchtormin, isopteryxin, isosamidin, psoralen, and bergapten. The structures of 1 and 2 were determined by 1D and 2D NMR experiments, and their absolute configuration was established by chemical correlation with (+)-5-O-methylvisamminol (3), whose structure was secured by single-crystal X-ray diffraction analysis and its absolute configuration was established as S by vibrational circular dichroism spectroscopy in comparison to DFT B3LYP/DGDZVP calculations.

Absolute configuration of eremophilanoids by vibrational circular dichroism

The absolute configurations (AC) of natural occurring 6-hydroxyeuryopsin (1), of its acetyl derivative 2, and of eremophilanolide 8 were confirmed by comparison of the experimental vibrational circular dichroism (VCD) spectra with theoretical curves generated from density functional theory (DFT) calculations. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field. All MMFF94 conformers were further optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by calculations of their vibrational frequencies at the B3LYP/6-31G(d,p); the VCD spectra of 2 and 8 were also calculated at the B3PW91/DGDZVP level of theory. Good agreement between theoretical and experimental VCD curves unambiguously verified the 4S,5R,6S absolute configuration for 1 and 2, and the 1S,4S,5R,6S,8S,10S configuration for 8.

Antifeedant and phytotoxic activity of the sesquiterpene p-benzoquinone perezone and some of its derivatives

The sesquiterpene p-benzoquinone perezone (1), isolated from Perezia adnata var. alamani (Asteraceae), and its non-natural derivatives isoperezone (2), dihydroperezone (3), dihydroisoperezone (4), and anilidoperezone (5) were tested as antifeedants against the herbivorous insects Spodoptera littoralis, Leptinotarsa decemlineata, and Myzus persicae. Compounds 1-5 exhibited strong antifeedant activity against L. decemlineata and M. persicae, and elicited a low response by S. littoralis. Antifeedant activity on L. decemlineata and M. persicae increased when the hydroxyl group at C-3 in perezone (1) was changed to C-6 to give isoperezone (2). The same effect was found with hydrogenation of the double bond of the alkyl chain of (1) to yield dihydroperezone (3). In contrast, hydrogenation of this double bond in isoperezone (2) to give dihydroisoperezone (4) led to a reduction in antifeedant activity. Determination of the phytotoxic activity of 1-5 revealed that 3 had a significant inhibition effect on Lactuca sativa radicle length growth.

Antifeedant and phytotoxic activity of hydroxyperezone and related molecules

The insect antifeedant and toxic activity of hydroxyperezone (1), its derivatives 2-9, along with 3-hydroxy- (10) and 6-hydroxythymoquinone (11) were studied against Spodoptera littoralis, Leptinotarsa decemlineata, and Myzus persicae. The antifeedant tests showed that L. decemlineata was the most sensitive insect, followed by M. persicae, while S. littoralis was not deterred by compounds 1-11. Leucohydroxyperezone tetraacetate (3), oxoperezinone (6), dihydroleucoperezinone diacetate (7), 3-hydroxy- (10) and 6-hydroxythymoquinone (11) showed strong activity against L. decemlineata. 1 and 7 exhibited moderate deterrent activity against M. persicae, while 1 and dihydroleucohydroxyperezone tetraacetate (4) acted as post-ingestive antifeedants to S. littoralis. The phytotoxic activity of compounds 1-11 was also evaluated. Hydroxyperezone (1) strongly inhibited seed germination at 24 h, while the activity of 3-8 and 10 was moderate. The level of radicle growth inhibition obtained with compounds 1-5 and 8-11 was significant (< 50%).

C-15-functionalized eudesmanolides from Mikania campanulata

The aerial parts of Mikania campanulata Gardner afforded the nine new eudesmanolides (1S,4S,5S,6S,7S,10R)-1-hydroxy-15-acetoxyeudesm-11(13)-en-6,12-olide (1), (1S,4S,5S,6S,7S,10R)-1,4-dihydroxy-15-acetoxyeudesm-11(13)-en-6,12-olide (2), (1R,4S,5S,6S,7S,10R)-1,4-dihydroxy-15-acetoxyeudesm-11(13)-en-6,12-olide (3), (1R,4S,5S,6S,7S,10R)-1-hydroxy-4,15-diacetoxyeudesm-11(13)-en-6,12-olide (4), (1S,4R,5S,6S,7S,10R)-1,15-diacetoxy-4-hydroxyeudesm-11(13)-en-6,12-olide (5), (1S,5S,6S,7S,10R)-1-hydroxy-15-acetoxyeudesma-4(15),11(13)-dien-6,12-olide (6), (1S,5S,6S,7S,10R)-1,15-dihydroxyeudesma-3,11(13)-dien-6,12-olide (7a), (1S,2R,5S,6S,7S,10R)-1,2,15-trihydroxyeudesma-3,11(13)-dien-6,12-olide (8a), and (1R,6S,7S,10R)-1,15-dihydroxyeudesma-4,11(13)-dien-6,12-olide (9a), among which the last three were characterized as their corresponding peracetates (7b-9b). The structures of 1-6 and 7b-9b were elucidated using 1D and 2D NMR measurements, while that of major constituent 1 was confirmed by single-crystal X-ray diffraction analysis, and its absolute configuration evidenced from vibrational circular dichroism measurements, which were compared to results obtained from density functional theory calculations. The chemistry of the large genus Mikania is briefly analyzed.

A new eremophilanolide from Senecio sinuatus Gilib

From the hexane extracts of Senecio sinuatus roots, the new 3beta-angeloyloxy-6beta-hydroxyeremophil-1(10)-en-8beta,12-olide (3), along with the known compounds 3beta-angeloyloxy-6beta-hydroxyeremophil-1(10)-ene (1), 3beta-senecioyloxy-6beta-hydroxyeremophil-1(10)-ene (2), and 3beta-angeloyloxy-6beta,8alpha-dihydroxyeremophil-1(10)-en-8beta,12-olide (4), were isolated. Complete 1H and 13C NMR chemical shift assignments of 1-4 were achieved using one- and two-dimensional NMR techniques, including gHMQC and gHMBC experiments. A Monte Carlo search, followed by B3LYP/6-31G*DFT calculation, provided the theoretical conformations of the eremophilane rings, which were in agreement with results derived from 1H-1H NMR coupling constant analysis, and confirmed by NOESY experiments.

1H, 13C and 15N NMR assignments of phenazopyridine derivatives

Phenazopyridine hydrochloride (1), a drug in clinical use for many decades, and some derivatives were studied by one- and two-dimensional (1)H, (13)C and (15)N NMR methodology. The assignments, combined with DFT calculations, reveal that the preferred protonation site of the drug is the pyridine ring nitrogen atom. The chemoselective acetylation of phenazopyridine (2) and its influence on the polarization of the azo nitrogen atoms were evidenced by the (15)N NMR spectra. Molecular calculations of the phenazopyridines 2-4 show that the pyridine and phenyl groups are oriented in an antiperiplanar conformation with intramolecular hydrogen bonding between the N-b atom and the C-2 amino group preserving the E-azo stereochemistry.

Conformational evaluation and detailed 1H and 13C NMR assignments of eremophilanolides

Extensive application of 1D and 2D NMR methodology, combined with molecular modeling, allowed the complete 1H and 13C NMR assignments of eremophilanolides from Senecio toluccanus. Comparison of the experimental 1H, 1H coupling constant values with those generated employing a generalized Karplus-type relationship, using dihedral angles extracted from MMX and DFT calculations, revealed that the epoxidized eremophilanolides 1 and 2 show conformational rigidity at room temperature, whereas molecules 3-6, containing an isolated double bond, are conformationally mobile.