molecules Article New Natural Oxygenated Sesquiterpenes and Chemical Composition of Leaf Essential Oil from Ivoirian Isolona dewevrei (De Wild. & T. Durand) Engl. & Diels Didjour Albert Kambiré 1 , Jean Brice Boti 1 , Thierry Acafou Yapi 1 , Zana Adama Ouattara 2 , Ange Bighelli 3 , Joseph Casanova 3 and Félix Tomi 3, * 1 2 3 * Laboratoire de Constitution et Réaction de la Matière, UFR-SSMT, Université Félix Houphouët-Boigny, Abidjan 01 BP V34, Ivory Coast; dakambire@gmail.com (D.A.K.); jeanbriceboti@hotmail.fr (J.B.B.); acafouth@yahoo.fr (T.A.Y.) Laboratoire de Chimie Bio-Organique et de Substances Naturelles, UFR SFA, Université Nangui Abrogoua, Abidjan 02 BP 801, Ivory Coast; zana1504@yahoo.fr Laboratoire Sciences Pour l’Environnement, Equipe Chimie et Biomasse, Université de Corse—CNRS, UMR 6134 SPE, Route des Sanguinaires, 20000 Ajaccio, France; bighelli@univ-corse.fr (A.B.); joseph.casanova@wanadoo.fr (J.C.) Correspondence: felix.tomi@univ-corse.fr Received: 12 November 2020; Accepted: 27 November 2020; Published: 29 November 2020



 Abstract: This study aimed to investigate the chemical composition of the leaf essential oil from Ivoirian Isolona dewevrei. A combination of chromatographic and spectroscopic techniques (GC(RI), GC-MS and 13 C-NMR) was used to analyze two oil samples (S1 and S2). Detailed analysis by repetitive column chromatography (CC) of essential oil sample S2 was performed, leading to the isolation of four compounds. Their structures were elucidated by QTOF-MS, 1D and 2D-NMR as (10βH)-1β,8β-oxido-cadin-4-ene (38), 4-methylene-(7αH)-germacra-1(10),5-dien-8β-ol (cis-germacrene D-8-ol) (52), 4-methylene-(7αH)-germacra-1(10),5-dien-8α-ol (trans-germacrene D-8-ol) (53) and cadina-1(10),4-dien-8β-ol (56). Compounds 38, 52 and 53 are new, whereas NMR data of 56 are reported for the first time. Lastly, 57 constituents accounting for 95.5% (S1) and 97.1% (S2) of the whole compositions were identified. Samples S1 and S2 were dominated by germacrene D (23.6 and 20.5%, respectively), followed by germacrene D-8-one (8.9 and 8.7%), (10βH)-1β,8β-oxido-cadin-4-ene (7.3 and 8.7), 4-methylene-(7αH)-germacra-1(10),5-dien-8β-ol (7.8 and 7.4%) and cadina-1(10),4-dien-8β-ol (7.6 and 7.2%). Leaves from I. dewevrei produced sesquiterpene-rich essential oil with an original chemical composition, involving various compounds reported for the first time among the main components. Integrated analysis by GC(RI), GC-MS and 13 C-NMR appeared fruitful for the knowledge of such a complex essential oil. Keywords: Isolona dewevrei; leaf essential oil; (10βH)-1β,8β-oxido-cadin-4-ene; cis-germacrene d-8-ol; trans-germacrene d-8-ol; cadina-1(10),4-dien-8β-ol 1. Introduction Isolona dewevrei (De Wild. & T. Durand) Engl. & Diels (synonym: Monodora dewevrei De Wild. & T. Durand; genus Isolona Engl., Annonaceae) is an evergreen shrub or a tree that can reach 15 m in height. Leaves are narrowly obovate to obovate or elliptic to narrowly elliptic, 10–17 cm long and 4–7 cm wide, with acuminated apex. The inflorescences appear on leafy branches and sometimes on older ones, whereas the fruits are ovoid (6–7 cm long, 4–5 cm in diameter), smooth but Molecules 2020, 25, 5613; doi:10.3390/molecules25235613 www.mdpi.com/journal/molecules Molecules 2020, 25, 5613 2 of 14 very finely ribbed, glabrous, green and become yellow at maturity [1]. The genus Isolona consists of 20 species widely distributed in the tropical rain forests of West and Central Africa, and Madagascar. Five species of this genus grow wild in Côte d’Ivoire: Isolona campanulata, I. cooperi, I. deightonii, I. soubreana and I. dewevrei. I. cooperi and I. campanulata are used in Ivorian herbal medicine to treat bronchial ailments, skin diseases, hematuria, infertility and to facilitate childbirth [1,2]. Reported studies carried out on solvent extracts of I. campanulata and I. cooperi have led to the isolation and identification of various alkaloids, sterols and sesquiterpenes [3–5]. Concerning the volatile constituents of Isolona species, the chemical compositions of essential oils from I. cooperi and I. campanulata were determined. The main constituents of leaf and stem bark oils from I. cooperi were (Z)-β-ocimene and γ-terpinene, while the composition of root bark oil was dominated by 5-isopentenylindole and (E)-β-caryophyllene [6]. The leaf oil from I. campanulata was rich in sesquiterpenes and its composition was dominated either by eudesm-5-en-11-ol or by (E)-β-caryophyllene and α-humulene [7]. In previous works, we investigated and reported for the first time the chemical compositions of leaf, root and stem bark essential oils from I. dewevrei, dominated by germacrene B/germacrene D and by cyperene, respectively. From the leaf oil, four new compounds were isolated and characterized as 6,12-oxido-germacra-1(10),4,6,11(12)-tetraene, (5αH,10βMe)-6,12-oxido-elema-1,3,6,11(12)-tetraene, germacra-1(10),4,7(11)-trien-6,12-γ-lactone and (1βH,5βH)-6,12-oxido-guaia-6,10(14),11(12)-trien-4α-ol [8]. The structure of germacrene D-8-one, another new natural compound, was also elucidated after isolation from the stem bark essential oil of the plant [9]. Continuing the chemical characterization of essential oils of aromatic and medicinal plants from Côte d’Ivoire [10–14], we now report on the chemical composition of the leaf essential oil from I. dewevrei, along with isolation and structure elucidation of three new natural sesquiterpenes as well as description of NMR data of a fourth sesquiterpene. 2. Results and Discussion Two leaf essential oil samples (S1 and S2) from I. dewevrei growing wild in Côte d’Ivoire were obtained by hydrodistillation of fresh leaves and the yields calculated on a weight basis (w/w) were 0.105 and 0.121%, respectively. The oil samples were first analyzed by a combination of GC(RI), GC-MS and 13 C-NMR, following a computerized method developed at the University of Corsica. This method allowed identification of components present at a content as low as 0.4–0.5% and compiled in our laboratory-made 13 C-NMR spectral data library [15,16]. Although various constituents were identified by the mean of the three techniques, several others, some of which were present at appreciable amounts, remained unidentified. Special attention was paid to four of them that belong to the oxygenated sesquiterpene family, according to their apolar and polar retention indices: compounds 38 (retention indices measured on apolar and polar capillary column, respectively (RIa/RIp) = 1534/1853; 7.3 and 8.7%), 52 and 53 (RIa/RIp = 1657/2355; 10.4 and 9.9%) and 56 (RIa/RIp = 1676/2276; 7.6 and 7.2%). Therefore, essential oil sample S2, which had a higher amount (2.9 g) and contained the four unidentified compounds, was subjected to repetitive column chromatography (CC) in order to perform structural elucidation. In parallel, analysis of CC fractions by GC(RI), GC-MS and 13 C-NMR led to the identification of several minor components. 2.1. Structure Elucidation of Unidentified Compounds 2.1.1. Structure Elucidation of Compound 38 Compound 38 was obtained with a great degree of purity (GC: 98.7%) in the sub-fraction F4.3.1 (15 mg). The exact mass measured by GC-QTOF-MS was 220.1821 g/mol, corresponding to the empirical formula C15 H24 O (calculated mass = 220.1822 g/mol). The 1 H-NMR, 13 C-NMR and DEPT spectra were in agreement with the C15 H24 O formula, which involved four unsaturation degrees (Table 1) (Supplementary Materials, Figures S1–S8). Molecules 2020, 25, 5613 3 of 14 Table 1. NMR data of compound 38. Compound 38 13 C C δ (ppm) DEPT 1 86.63 C 2 30.71 CH2 3 30.13 CH2 4 5 6 7 8 133.78 122.89 51.98 54.01 81.75 C CH CH CH CH 9 43.42 CH2 10 11 12 13 14 15 41.18 33.27 21.81 19.82 19.71 22.65 CH CH CH3 CH3 CH3 CH3 1 H, 13 C-NMR 1H δ (ppm) Multiplicity (J (Hz)) 1 H–1 H COSY a 2.15 b 2.29 a 2.00 b 2.17 5.57 2.25 1.16 4.27 a 1.02 b 2.21 2.02 1.45 0.94 0.87 1.07 1.59 m m m m quint (1.5) m t (9.3) d (5.2) dd (11.0, 3.8) dd (11.0, 5.2) m dsept (9.3, 6.7) d (6.7) d (6.7) d (7.4) br s 2b, 3a, 3b 2a, 3a, 3b 2a, 2b, 3b 2b, 3a, 3b 6 5, 7 6, 8, 11 7, 9a, 9b 8, 9b, 10 8, 9a, 10 14, 9a, 9b 7, 12, 13 11 11 10 - HMBC H → C NOESY 1 H–1 H 1, 3, 4, 6, 10 1, 3, 4, 6, 10 1, 2, 4, 5, 15 1, 2, 4, 5, 15 1, 4, 6, 7, 15 1, 2, 4, 5, 7, 10 5, 6, 8, 11, 12, 13 6, 7, 9, 10, 11 7, 8, 10, 14 1, 7, 8, 10, 14 1, 2, 6, 8, 9, 14 6, 7, 8, 12, 13 7, 11, 13 7, 11, 12 1, 9, 10 3, 4, 5 2b, 3a 2a, 3b 2a, 3b 2b, 3a, 15 6, 13, 15 5, 11, 13 2b, 8, 9a, 14 7, 9a, 14 9b, 8, 7, 14 9a, 10 9b, 14 6, 12, 13 11, 13 5, 6, 11, 12 7, 8, 9a, 10 3b, 5 and DEPT spectra indicated the occurrence of a tri-substituted double bond (C4, 133.78 ppm and C5, 122.89 ppm) and two carbons bearing the oxygen atom (C, 86.26 ppm and CH, 81.09 ppm), belonging to an oxide sub-structure. Taking into account the four unsaturation degrees and this double bond, compound 38 bears a tricyclic structure. NMR spectra of 38 evidenced an isopropyl group (H11, 1.45 ppm, dsept: 9.3, 6.7 Hz; H12, 0.94 ppm, d: 6.7 Hz) and H13, 0.87 ppm, d: 6.7 Hz), a methyl group (H15, 1.59 ppm, broad s) linked to a sp2 quaternary carbon and another one (H14, 1.07 ppm, d: 7.4 Hz) linked to a sp3 methine. Starting from the methine linked to the oxygen atom (CH, 81.75 ppm, 4.27 ppm, d, 5.2 Hz) the HMBC correlations evidenced the oxa-bicyclo[2.2.1]heptane (oxa norbornane) substructure bearing the isopropyl group on C7 and the methyl group on C10. Correlation plots observed on the COSY spectrum between H7 and H11 on the one hand and between H10 and H14 on the other hand confirmed the position of both substituents on the oxa-norbornane framework. The last four carbons, including two sp2 carbons and two sp3 carbons, constituted the third cycle, obviously cyclohexenic. HMBC correlation plots allowed the positioning of the cyclohexene sub-structure vs. the oxa-norbornane moiety as well as the position of the methyl group on the double bond. Therefore, the molecule under investigation possesses a bicyclo[4.4.0]decane skeleton with an oxide function between C1 and C8, drawing an oxa-norbornane sub-structure that bears a methyl on C10 and an isopropyl group on C7. The second cycle is a cyclohexene with a vinylic methyl on C4. Therefore, this molecule may be considered as a 1,8-oxido-cadin-4-ene. Eight stereoisomers may be drawn, four of these display a cis stereochemistry of the bicyclo[4.4.0]decane ring junction, the last four display a trans stereochemistry of the ring junction. The relative configurations of the ring junction and those of carbons bearing the methyl and isopropyl groups were determined through NOESY correlations. Indeed, the observed correlation between H7 and H14 located the isopropyl group in the exo position vs. the oxa-norbornane sub-structure as well as the methyl 14 in the endo position. In parallel, the NOESY correlation between H6, H11 and H13 corroborated a cis junction of the bicyclo[4.4.0]decane framework. The structure of compound 38 was elucidated as (10βH)-1β,8β-oxido-cadin-4-ene, a diastereoisomer of cis and trans-cadinene ethers, which displays a trans junction of the bicyclo[4.4.0]decane skeleton and a different stereochemistry to the isopropyl/methyl groups (Figure 1). β Molecules 2020, 25, 5613 β β 4 of 14 Figure 1. Structure of compound 38. 2.1.2. Structure Elucidation of Compounds 52 and 53 Sub-fraction F5.3.3 (26 mg) exhibited a single chromatographic peak—(99.4% on GC apolar and polar columns). In contrast, the 13 C-NMR spectrum of this sub-fraction displayed two series of 15 carbon signals easily distinguishable by their relative intensities (compounds 52 + 53). The ratio calculated by the mean of protonated carbons’ ’relative intensities was 7/3 (compounds 52/53). The scanning of the chromatographic peak afforded super imposable mass spectra and the exact mass measured was 220.1823 g/mol, corresponding to C15 H24 O formula (calculated mass = 220.1822 g/mol). In addition, the two series of 13 C chemical shifts corresponding to these compounds were very similar. Indeed, each compound displayed six sp2 carbon signals, which consisted of two quaternary carbons, three methines and an ethylenic methylene (109.34 and 112.25 ppm, respectively; 52 and 53). The nine other signals belonged to sp3 carbons and each structure of compound was constituted of three methines, of which one carbon linked to an oxygen atom (69.57 and 73.13 ppm, respectively; 52 and 53), three methylenes and three methyl groups (Table 2) (Supplementary Materials, Figures S9–S17). Therefore, the 13 C-NMR and DEPT spectra corroborated the C15 H24 O formula, which involved four unsaturation degrees. Each compound exhibited six sp2 carbons that belonged to three double bonds, which were obviously monocyclic; therefore, they may be considered more precisely as methylene cyclodecadienols and the various observations suggested the presence of two epimers. Compounds 52 and 53, which co-eluted on apolar and polar columns, look uneasily separable by chromatographic techniques at our disposal. Therefore, the NMR extraction technique was used on the sub-fraction that contained only the two compounds (99.4%, ratio 7/3) for their structural elucidation [17,18]. This technique consisted of first assigning the 1 H and 13 C chemical shifts of each compound, taking into account the relative intensities of their signals and using the HSQC spectrum. Then, the specific correlations of each isomer were plotted on the other 2D-NMR spectra, i.e., COSY, NOESY, HMBC. Lastly, the determination of their respective structure was achieved by using the specific correlations belonging to each compound. Concerning the major compound 52, NMR spectra evidenced an isopropyl group: H11 (1.69 ppm, m), H12 (0.97 ppm, d: 6.7 Hz) and H13 (0.87 ppm, d: 6.7 Hz); as well as a methyl group linked to a sp2 quaternary carbon (H14, 1.71 ppm, broad s) and an exocyclic methylene (H15, 4.78 and 4.82, br d: 2.3Hz). Therefore, the molecule contained the cyclodecadiene structure, substituted by a hydroxyl group. In addition, two deshielded methine signals (C8, 69.57 and C7, 57.56 ppm) suggested a first carbon linked to the hydroxyl group and a carbon in α of the previous carbon, probably deshielded by the isopropyl group. This was confirmed by correlations observed on the HMBC spectrum, which evidenced that the isopropyl group was linked to C7. Multiplicity of vinylic proton signals (H5: 5.79 ppm, d, 16.1 Hz; H6: 5.56 ppm, dd, 16.1, 9.8 Hz) demonstrated a CH=CH double bond. According to the correlations observed on the HMBC spectrum, this double bond was located between C7 and the quaternary carbon (C4, 148.76 ppm) of the exocyclic C=CH2 . This was confirmed by correlation plots observed on the COSY, which also showed two proton groups formed by the sequences H1-H2-H3 and H5-H6-H7-H8-H9. The HMBC correlations of the hydrogens H1, H3 and H9 completed the structure of compound 52 as 4-methylene-germacra-1(10),5-dien-8-ol. Molecules 2020, 25, 5613 5 of 14 Table 2. NMR data of compounds 52 and 53. Compound 52 C δ 13 C (ppm) 1 132.27 CH 2 29.36 CH2 3 34.61 CH2 4 5 6 7 8 148.76 137.36 130.18 57.56 69.57 C CH CH CH CH DEPT 9 47.28 CH2 10 11 12 13 14 132.55 28.47 20.52 21.58 19.34 C CH CH3 CH3 CH3 15 109.34 CH2 δ 1H (ppm) 5.14 a 1.99 b 2.46 a 2.13 b 2.48 5.79 5.56 1.93 4.12 a 2.39 b 2.56 1.69 0.97 0.87 1.71 a 4.78 b 4.82 Multiplicity (J (Hz)) br dd (10.7, 4.8) m m m m d (16.1) dd (16.1, 9.8) dt (9.8, 2.5) m dd (14.0, 2.3) dd (14.0, 5.3) m d (6.7) d (6.7) br s br d (2.3) br d (2.3) Compound 53 COSY 1 H–1 H 2a, 2b 1, 2b, 3a, 3b 1, 2a, 3a, 3b 1, 2a, 2b 1, 2a, 2b 6 5, 7 6, 8, 11 7, 9a, 9b 8 8 7, 12, 13 11 11 15b 15a 1 H–1 H HMBC H → C NOESY 2, 9, 10, 14 1, 3, 4, 5, 10, 15 1, 3, 4, 5, 10, 15 1, 2, 4, 5, 15 1, 2, 4, 5, 15 3, 4, 6, 7, 15 4, 5, 7, 8, 11 5, 6, 8,11, 12, 13 6, 7, 9, 10, 11 1, 7, 8, 10, 14 1, 7, 8, 10, 14 6, 7, 8, 12, 13 7, 11, 13 7, 11, 12 1, 2, 8, 9, 10 2, 3, 4, 5 2, 3, 4, 5 2b, 6, 9b 3b, 14 1, 3a, 15b 2b, 6, 15b 2a, 5, 8, 14 3b, 7, 8, 14 1, 2b, 9b, 11, 13 5, 8, 12, 13, 14 3b, 5, 7, 14 9b, 14 1, 6, 9a, 11 6, 9b, 12, 13 7, 11, 13 6, 7, 11, 12 2a, 3b, 5, 7, 8, 9a 3b, 5, 15b 2b, 3a, 15a δ 13 C (ppm) 130.27 30.58 32.26 148.79 127.64 134.81 53.96 73.13 45.68 134.16 27.44 20.64 21.86 19.53 112.25 δ 1 H (ppm) Multiplicity (J (Hz)) NOESY 1 H–1 H 5.63 a 2.16 b 2.19 a 2.22 b 2.46 6.06 5.93 2.12 4.09 a 2.02 b 2.70 1.97 0.99 0.94 1.44 4.71 4.88 br dd (10.6, 5.0) m m m m d (16.3) dd (16.3, 9.8) m m dd (14.2, 4.0) dd (14.2, 6.8) m d (6.8) d (6.8) br s br d (2.2) br d (2.2) 2b, 6, 9b 3b, 14 1, 3a, 15b 2b, 6, 15b 2a, 5, 14 3b, 7, 14 1, 2b, 9b, 11, 13 5, 12, 13, 14 9a, 13 8, 9b, 14 1, 6, 9a, 11 6, 9b, 12, 13 7, 11, 13 6, 8, 11, 12 2a, 3b, 5, 7, 9a 3b, 5, 15b 2b, 3a, 15a Molecules 2020, 25, 5613 6 of 14 The (E) stereochemistry of the intracyclic double bonds was evidenced by the value of the coupling α β β constant (16.1 Hz) for C5=C6 and by the occurrence of a correlation plot between H1 and H9 in the NOESY spectrum for C1=C10. The relative stereochemistry of the isopropyl and hydroxyl groups was determined through (i) the values of coupling constants of signals of geminated hydrogens; (ii) NOE spatial correlations observed between various protons. Indeed, H6 appears as a dd (JH5-H6 = 16.1 Hz, and JH6-H7 = 9.8 Hz). In turn, the signal of H7 is a dt (JH6-H7 = 9.8 Hz, JH7-H11 = 2.5 Hz and JH7-H8 = 2.5 Hz). Assuming that the isopropyl group adopts an equatorial position, H7 is axial and the coupling constant value JH7-H8 = 2.5Hz locates H8 in equatorial position. Therefore, H7 and H8 display a cis stereochemistry as well as the isopropyl and hydroxyl groups and 52 is cis-germacrene D-8-ol. This point is corroborated by the observation in the NOESY spectrum of a correlation plot between H7 and H8, confirming that both protons are in the same side of the molecule. The structure of 52 is elucidated as 4-methylene-(7αH)-germacra-1(10)E,5E-dien-8β-ol or germacrene-d-8β-ol or α α cis-germacrene-d-8-ol (Figure 2). α α Figure 2. Structure of compounds 52 and 53. Similarly, all correlations observed on HMBC and COSY spectra for the minor component 53 led to the same structure of germacrene D-8-ol. The E stereochemistry of the intracyclic double bonds is evidenced similarly to 52. Thus, compounds 52 and 53 are epimers. Unfortunately, signals of H7 and H8 appeared as multiplets and therefore they were not useful for stereochemical investigation. Moreover, the NOESY spectrum was not very informative. However, two points may be highlighted; (i) the occurrence of a correlation plot between H8 and H11 (absent in the spectrum of 52) and (ii) the lack of correlation plot between H7 and H8, which are located in a trans antiperiplanar conformation, this plot being observed in the spectrum of 52. Lastly, considering that the cyclodecadiene moiety adopts a chair-boat-chair conformation, the deshielding (3.5 ppm) of C8 in 53 vs. 52 is in agreement with the axial/equatorial stereochemistry of the hydroxyl group (compared with menthol/neo-menthol, for instance). Compound 53 is named 4-methylene-(7αH)-germacra-1(10),5-dien-8α-ol or (7αH)germacrene d-8α-ol or trans-germacrene d-8-ol. 2.1.3. Structure Elucidation of Compound 56 Sub-fraction F5.3.1 (19 mg) contained compound 56 (98.3%), with RIs apol/pol = 1676/2276, suggesting an oxygenated sesquiterpene. The electron ionization (EI)-mass spectrum of compound 56 exhibited an m/z = 220 molecular ion peak (M•+ ) and an M•+ -18 peak (m/z = 202), characteristic of a sesquiterpene alcohol. However, no structure proposal emerged from GC-MS analysis with an acceptable fit (commercial MS libraries and home-made MS library). Therefore, structural elucidation was undertaken. The measured exact mass was 220.1823 g/mol, corresponding to C15 H24 O formula (calculated mass = 220.1822 g/mol). The 1 H-NMR, 13 C-NMR and DEPT spectra were in agreement with this formula, which involved four unsaturation degrees. These spectra also confirmed the presence of an alcohol function (C8, 65.54 ppm) (Table 3) (Supplementary Materials, Figures S18–S26). Four sp2 carbon signals including three quaternary carbons, involved in two C=C double bonds, Molecules 2020, 25, 5613 7 of 14 were observed. Taking into account the four unsaturation degrees, compound 57 obviously bears a bicyclic structure. The 1 H-NMR spectrum evidenced an isopropyl group: H11 (1.45 ppm, dsept: 9.3, 6.7 Hz), H12 (0.94 ppm, d: 6.7 Hz) and H13 (0.87 ppm, d: 6.7 Hz); confirmed by the COSY spectrum and two methyl groups linked to sp2 quaternary carbons (H14, 1.67 ppm, broad s; H15, 1.69 ppm, broad s). COSY correlations also evidenced two other hydrogen groups formed by the sequences H2-H3 and H5-H6-H7-H8-H9 and they indicated that the isopropyl group was linked to C7. Table 3. NMR data of compound 56. C δ 13 C (ppm) 1 130.31 C 26.70 CH2 2 DEPT δ 1H (ppm) Multiplicity (J (Hz)) a 1.99 m ddd (12.2, 3.6, 3.1) m m (1.5) br d (11.0) br dd (11.0, 4.3) m m dd (17.3, 4.1) dsept (7.0, 4.1) d (7.0) d (7.0) br s br s b 2.74 3 4 5 6 32.05 134.83 123.99 34.93 CH2 C CH CH a 2.04 5.45 2.86 7 48.03 CH 1.15 8 65.54 CH 9 41.81 CH2 10 11 12 13 14 15 119.79 27.14 18.50 21.79 18.74 23.62 C CH CH3 CH3 CH3 CH3 4.17 a 2.04 b 2.30 2.10 1.04 1.05 1.67 1.69 a COSY HMBC H → C 1 H–1 H NOESY 1 H–1 H a 2b, 3 1, 3, 4, 6, 10 2b, 3 2a, 3 1, 3, 4, 6, 10 2a, 3, 14 2a, 2b 6 5, 7 1, 2, 4, 5, 15 1, 4, 6, 7, 15 1, 2, 4, 5, 7, 10 2a, 2b, 15 6, 15 5, 9b, 11, 13 6, 8, 11 5, 6, 8, 11, 12, 13 8, 9a, 14 7, 9a, 9b 8, 9b 8, 9a 7, 12, 13 11 11 - 6, 7, 9, 10, 11 1, 7, 8, 10, 14 1, 7, 8, 10, 14 6, 7, 8, 12, 13 7, 11, 13 7, 11, 12 1, 9, 10 3, 4, 5 7, 9a, 14 9b, 8, 7, 14 9a, 6, 11, 12 6, 12, 13 9b, 11, 13 6, 11, 12 2b, 8, 7 3, 5 Most relevant NOE correlations. Correlation plots in the HMBC spectrum allowed the construction of the bicyclic skeleton. For instance, proton H8 geminated to the hydroxyl function correlates with C6, C7, C9 and the ethylenic quaternary carbon C10. Proton H6, located at the ring junction, correlates with C1, C2 and C10 on the one hand and with C4, C5 and C7 on the other hand. Otherwise, protons H14 and H15 correlated with sp2 quaternary carbons C10 and C4. Thus, the C14 and C15 methyl groups were linked to C10 and C4, respectively. It is possible to determine the structure of compound 56 as cadina-1(10),4-dien-8-ol. The relative stereochemistry of substituents of compound 56 was established through NOESY spatial correlations. Protons H6, H11, H12 and H13 correlated together indicating a cis stereochemistry of H6 and the isopropyl group. Similarly, H7 correlated with H8 leading to the cis stereochemistry of the isopropyl group and the hydroxyl function. Coupling constants of H6 (11.0 Hz) and H7 (11.0 and 4.3 Hz) are in agreement with NOESY correlations. Therefore, compound 56 is cadina-1(10), 4-dien-8β-ol (Figure 3). Figure 3. Structure of compound 56. δ δ – → – Molecules 2020, 25, 5613 8 of 14 This compound is an epimer of cadina-1(10),4-dien-8α-ol isolated by Weyerstahl et al. [19] from the essential oil of Iranian Pulicaria gnaphalodes (Vent.) Boiss. Differences in chemical shifts of both isomers agree with the axial/equatorial stereochemistry of the hydroxyl group, particularly the shielding of C6 due to the γ steric effect of the hydroxyl group. However, bibliographic investigations carried out in the literature have indicated that the occurrence of compound 56 has been already mentioned in two studies: In the first one, a compound has been identified in the aerial parts of Ferula flabelliloba on the basis of its mass spectrum as being cadina-1(10),4-dien-8β-ol [20]. However, the structure represented by the authors, drawing a cis stereochemistry of H6 (hydrogen of the ring junction) and the isopropyl group and a trans stereochemistry of the isopropyl and hydroxyl groups, was rather that of the 8α isomer (Figure 3). Moreover, it could be pointed out that the measured retention index (RI) (CP Sil 5 CB) = 1678 [20] fitted with RI (CP Sil 5 CB) = 1680 measured for the 8α isomer [21] and RI (DB1) measured for the 8β isomer (1676; this work). In the second study, cadina-1(10),4-dien-8β-ol was identified by the retention index (RI = 1663, CP Sil 5 CB) and mass spectrum in different organs of Erigeron annuus [22]. However, to the best of our knowledge, NMR data of cadina-1(10),4-dien-8β-ol were not found in the literature. Therefore, the present study is the first available structural elucidation of that compound. 2.2. Chemical Composition of Leaf Essential Oil from I. dewevrei The chemical composition of two essential oil samples (S1, S2) from wild I. dewevrei was determined by a combination of repetitive column chromatography (CC), GC(RI), GC-MS and 13 C-NMR. In total, fifty-seven components accounting for 95.5 and 97.1% of the composition of the whole oil sample were identified. Compounds 38, 52 and 53 are reported for the first time, whereas NMR data of 56 are described for the first time. The composition of the two leaf oil samples (S1 and S2) was largely dominated by oxygenated sesquiterpenes (44.1 and 44.9%, respectively) and hydrocarbon sesquiterpenes (41.2 and 40.2%, respectively), the sesquiterpene fraction accounting for 85.3 and 85.1%, respectively (Table 4). Table 4. Chemical composition of leaf essential oil from Isolona dewevrei. N◦ Compounds RIa RIp RFF S1 (%) S2 (%) Identification 1 2 3 4 5 6 7 β 9 10 11 12 13 14 15 16 17 18 19 α-Thujene α-Pinene Sabinene β-Pinene Myrcene α-Terpinene p-Cymene Limonene (Z)-β-Ocimene (E)-β-Ocimene γ-Terpinene Linalool allo-Ocimene Terpinen-4-ol Geraniol Geranial δ-Elemene α-Cubebene α-Copaene 923 931 965 970 981 1009 1012 1021 1025 1036 1048 1083 1117 1161 1233 1244 1334 1347 1374 1016 1013 1120 1109 1158 1178 1268 1199 1230 1247 1242 1543 1370 1597 1843 1740 1464 1452 1485 0.765 0.765 0.765 0.765 0.765 0.765 0.698 0.765 0.765 0.765 0.765 0.869 0.765 0.869 0.869 0.887 0.751 0.751 0.751 tr 0.1 0.1 0.1 0.3 0.1 tr 1.1 3.4 4.5 0.2 tr 0.1 0.1 0.1 tr 0.1 0.9 0.1 0.1 0.4 0.2 0.3 0.1 0.1 1.1 4.5 4.2 0.2 0.1 0.2 0.1 0.2 0.1 0.5 0.1 0.7 RI, MS RI, MS RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS RI, MS, 13 C-NMR RI, MS RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS RI, MS, 13 C-NMR Molecules 2020, 25, 5613 9 of 14 Table 4. Cont. 0.751 0.751 0.751 0.751 0.751 0.751 0.707 0.751 0.751 0.751 0.751 0.751 0.751 0.751 0.751 0.819 0.751 0.819 S1 (%) 1.6 5.3 0.1 tr 0.1 1.7 tr 0.3 23.6 tr 0.1 1.8 0.2 0.2 2.5 0.9 1.4 tr S2 (%) 1.7 5.7 0.3 0.5 0.1 1.3 tr 0.3 20.5 0.2 tr 1.6 0.2 0.2 2.4 0.7 1.5 0.1 1853 0.830 7.3 8.7 N◦ Compounds RIa RIp RFF 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 β-Elemene (E)-β-Caryophyllene * α-Santalene * γ-Elemene # (E)-β-Farnesene α-Humulene α-Curcumene γ-Muurolene Germacrene D trans-β-Bergamotene β-Selinene Bicyclogermacrene α-Selinene β-Bisabolene δ-Cadinene cis-Lanceol (Z)-γ-Bisabolene trans-Sesquisabinene hydrate 1385 1416 1416 1426 1446 1448 1469 1471 1474 1478 1484 1489 1493 1500 1512 1517 1521 1530 1583 1589 1565 1630 1660 1662 1766 1683 1700 1676 1710 1721 1717 1719 1753 2087 1721 1984 1534 38 (10βH)-1β,8β-Oxido-cadin-4-ene 39 40 41 42 43 44 45 46 47 48 49 50 51 β-Elemol (E)-Nerolidol Germacrene B # cis-Sesquisabinene hydrate Caryophyllene oxide Germacrene D-8-one Humulene oxide II Alismol γ-Eudesmol δ-Cadinol Muurola-4,10(14)-dien-8β-ol α-Cadinol β-Bisabolol 1536 1547 1549 1562 1567 1584 1597 1609 1620 1626 1629 1634 1653 2077 2034 1818 2079 1973 2066 2042 2245 2172 2174 2186 2231 2144 0.819 0.819 0.751 0.819 0.830 0.841 0.830 0.830 0.819 0.819 0.830 0.819 0.819 tr 0.5 1.3 0.3 0.1 8.9 0.4 0.1 1.2 0.1 3.2 0.6 0.2 0.2 1.1 2.4 0.2 0.2 8.7 0.2 0.3 1.2 0.2 2.9 0.6 0.2 52 (7αH)-Germacrene D-8β-ol * 1657 2355 0.819 7.8 7.4 53 (7αH)-Germacrene D-αβ-ol * 1657 2355 0.819 2.6 2.5 54 55 α-Bisabolol epi-α-Bisabolol 1664 1667 2208 2214 0.819 0.819 1.4 0.1 1.5 tr 56 Cadina-1(10),4-dien-8β-ol 1676 2276 0.819 7.6 7.2 57 Cadina-4,10(14)-dien-8β-ol 1678 2280 0.830 0.8 0.8 10.0 0.2 41.2 44.1 95.5 11.5 0.5 40.2 44.9 97.1 Hydrocarbon monoterpenes Oxygenated monoterpenes Hydrocarbon sesquiterpenes Oxygenated sesquiterpenes Total Identification RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS QTOF-MS, 1D, 2D-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR QTOF-MS, 1D, 2D-NMR QTOF-MS, 1D, 2D-NMR RI, MS, 13 C-NMR RI, MS, 13 C-NMR QTOF-MS, 1D, 2D-NMR RI, MS, 13 C-NMR Order of elution and percentages are given on an apolar column (BP-1), except components with an asterisk (*), where percentages are taken on a polar column (BP-20). (#) Thermolabile compound, percentage evaluated by a combination of GC-FID and 13 C-NMR data [7]. RIa, RIp: retention indices measured on apolar and polar capillary column, respectively. RRF: relative response factors calculated using methyl octanoate as internal standard. The relative proportions of constituent are expressed in g/100 g. tr: traces level (<0.05%). 13 C-NMR: compounds identified by NMR in the essential oil samples and obvious in at least one fraction of chromatography; 13 C-NMR (italic): compounds identified by NMR in fractions of chromatography. Molecules 2020, 25, 5613 10 of 14 Essential oil samples S1 and S2 displayed close chemical compositions, dominated by germacrene D (23.6 and 20.5%, respectively), followed by germacrene d-8-one (8.9 and 8.7%), (10βH)-1β,8β-oxido-cadin-4-ene (38) (7.3 and 8.7%), (7αH)-germacrene d-8β-ol (52) (7.8 and 7.4%) and cadina-1(10),4-dien-8β-ol (56) (7.6 and 7.2%). Other compounds were also present in both samples at appreciable contents: (E)-β-caryophyllene (5.3 and 5.7%), (E)-β-ocimene (4.5 and 4.2%) and (Z)-β-ocimene (3.4 and 4.5%). Investigations carried out on I. dewevrei leaf essential oil, using a combination of chromatographic (CC, GC(RI)) and spectroscopic techniques (GC-MS, 13 C-NMR), led to the identification of fifty-seven constituents accounting for 95.51 and 97.1% of the whole oil samples’ compositions. The two samples were characterized by a similar chemical composition dominated by germacrene D (23.6 and 20.5%, respectively), followed by germacrene d-8-one (8.9 and 8.7%), (10βH)-1β,8β-epoxy-cadina-4-ene (38) (7.3 and 8.7%), (7αH)-germacrene d-8β-ol (52) (7.8 and 7.4%) and cadina-1(10),4-dien-8β-ol (56) (7.6 and 7.2%). Compounds 38, 52 as well as (7αH)-germacrene D-8α-ol 53 are new natural sesquiterpenes, isolated from sample S2 and fully characterized by QTOF-MS, 1D and 2D-NMR. In addition, cadina-1(10),4-dien-8β-ol (56) was also isolated from this oil sample and its NMR data are reported for the first time. Leaves from I. dewevrei produced a sesquiterpene-rich essential oil with an original chemical composition, displaying various compounds that are reported for the first time. The previously reported chemical composition of leaf oil from this species was dominated by germacrene B, (5αH,10βMe)-6,12-oxido-elema-1,3,6,11(12)-tetraene, germacrene D, (Z)-β-ocimene, γ-elemene and (E)-β-caryophyllene [14]. Thus, qualitative and quantitative differences appeared between the compositions of the previous and the present study. Hence, the chemical variability of the leaf essential oil of I. dewevrei should be evaluated by investigating a larger number of oil samples. 3. Materials and Methods 3.1. Plant Material The fresh leaves samples (2210 and 2412 g, respectively) were collected on individual I. dewevrei trees, which were growing in different ecological conditions, in the Bossématié forest (region of Abengourou, Eastern Côte d’Ivoire, geographical coordinates: 6◦ 26′ 57.9′′ N and 3◦ 28′ 47.5′′ O) in April 2016. Plant material was authenticated by botanists from Centre Suisse de Recherches Scientifiques (CSRS) and Centre National de Floristique (CNF) Abidjan, Côte d’Ivoire. A voucher specimen was deposited at the herbarium of CNF, Abidjan, with the reference LAA 12874. 3.2. Essential Oil Isolation and Fractionation The essential oil samples (S1 and S2) were obtained by hydrodistillation of fresh leaves for 3 h using a Clevenger-type apparatus. Yields were calculated from fresh material (w/w). The oil sample S2 (2.9 g) was repeatedly fractionated by column chromatography (CC) as shown on Scheme 1, using a gradient of solvents, n-pentane: diethyl ether of increasing polarity. Silica gel (200–500 µm, 90 g) was used to afford the first eight fractions. Fractions F4 and F5 were again fractionated with silica gel (60–200 µm, 20 g each). Sub-fractions F4.2, F4.3 and F5.2 were then fractionated with silica gel (35–70 µm, 10, 6 and 10 g, respectively). Lastly, sub-fraction F5.3 was submitted to a Sephadex LH-20 column (3.0 g) using chloroform. Compound 38 (98.7%) was the main constituent of sub-fraction F4.3.1. Sub-fraction F5.3.1 contained compound 56 (98.3%) and sub-fraction F5.3.3 contained compounds 52 and 53, both accounting for 99.4% (ratio 7/3). –200 μm, 20 g each μm, 10, 6 and 10 g – Molecules 2020, 25, 5613 11 of 14 Scheme 1. Fractionation process of leaf essential oil sample S2. 3.3. Gas Chromatography Analyses were performed on a Clarus 500 PerkinElmer Chromatograph (PerkinElmer, Courtaboeuf, France), equipped with flame ionization detector (FID) and two fused-silica capillary columns (50 m × 0.22 mm, film thickness 0.25 µm), BP-1 (polydimethylsiloxane) and BP-20 (polyethylene glycol). The oven temperature was programmed from 60 ◦ C to 220 ◦ C at 2 ◦ C/min and then held isothermal at 220 ◦ C for 20 min; injector temperature: 250 ◦ C; detector temperature: 250 ◦ C; carrier gas: helium (0.8 mL/min); split: 1/60; injected volume: 0.5 µL. Retention indices (RI) were determined relative to the retention times of a series of n-alkanes (C8–C29) with linear interpolation (« Target Compounds » – software from PerkinElmer). The relative response factor (RFF) of each compound was calculated according to the IOFI recommended practice for the use of predicted relative response factors for the rapid quantification of volatile flavoring compounds by GC(FID) [23]. Methyl octanoate was used as an internal reference and the relative proportion of each constituent (expressed in g/100 g) was calculated using the weight of essential oil and reference, peak area and relative response factors (RRF). 3.4. Gas Chromatography–Mass Spectrometry in Electron Impact Mode The essential oil samples and all fractions of chromatography were analyzed with a PerkinElmer – TurboMass detector (quadrupole), directly coupled with a PerkinElmer Autosystem XL (PerkinElmer, Courtaboeuf, France), equipped with a Rtx-1 (polydimethylsiloxane) fused-silica capillary column (60 m × 0.22 mm i.d., film thickness 0.25 µm). The oven temperature was programmed from 60 to 230 ◦ C at 2◦ /min and then held isothermal for 45 min; injector temperature, 250 ◦ C; ion-source temperature, 250 ◦ C; carrier gas, He (1 mL/min); split ratio, 1:80; injection volume, 0.2 µL; ionization energy, 70 eV. The electron ionization (EI) mass spectra were acquired over the mass range 35–350 Da. 3.5. Gas Chromatography–High Resolution Mass Spectrometry – High-resolution EI-mass spectra were recorded using an Agilent 7200 GC-QTOF system (Agilent, Santa Clara, CA, USA) equipped with an Agilent J&W, VF-waxMS capillary column (30 m × 0.25 mm; 0.25 µm film thickness). The mass spectrometer was operating at 70 eV with an acquisition rate of 2 GHz over a 35−450 m/z range, affording a resolution of ∼8000. Injection volume 1 µL; split ratio 1:20; inlet temperature 250 ◦ C, detector temperature 230 ◦ C; column flow (He) 1.2 mL/min; temperature program for oven 60 ◦ C (5 min isotherm) to 240 ◦ C at 5 ◦ C/min, then 10 min isotherm at 240 ◦ C. Molecules 2020, 25, 5613 12 of 14 3.6. Nuclear Magnetic Resonance All nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE 400 Fourier transform spectrometer (Bruker, Wissembourg, France) operating at 400.132 MHz for 1 H and 100.623 MHz for 13 C, equipped with a 5 mm probe, in CDCl3 , with all shifts referred to internal TMS. The 1 H-NMR spectra were recorded with the following parameters: pulse width (PW), 4.3 µs; relaxation delay 1 s and acquisition time 2.6 s for 32 K data table with a spectral width (SW) of 6000 Hz. 13 C-NMR spectra of the oil samples and fractions of CC were recorded with the following parameters: pulse width = 4 µs (flip angle 45◦ ); acquisition time = 2.7 s for 128 K data table with a spectral width of 25,000 Hz (250 ppm); CPD mode decoupling; digital resolution = 0.183 Hz/pt. Standard pulse sequences from Bruker TopspinTM (Bruker, Wissembourg, France) library were used for two-dimensional spectra. Gradient-enhanced sequences were used for the heteronuclear two-dimensional experiments. Spectra were processed via Mestrelab MestreNOVA software (version 12.0.0-20080). 3.7. Identification of Individual Components Identification of individual components was carried out: (i) by comparison of their GC retention indices (RI) on polar and apolar columns with those of reference compounds [24,25]; (ii) on computer matching against commercial mass spectral libraries [26,27]; (iii) on comparison of the signals in the 13 C-NMR spectra of the mixtures with those of reference spectra compiled in the laboratory spectral library, with the help of laboratory-made software [15,16]. This method allowed the identification of individual components of the essential oil at content as low as 0.4–0.5%. 3.8. Spectral Data (10βH)-1β,8β-Epoxy-cadina-4-ene (38): C15 H24 O; 1 H-NMR (CDCl3 , 400 MHz) and 13 C-NMR (CDCl3 , 100 MHz) data: see Table 1. HREIMS: m/z 220.1821 (calculated for C15 H24 O, 220.1822); EI-MS 70 eV, m/z (rel. int.): 220(13, M•+ ), 178(15), 177(100), 159(34), 149(43), 135(16), 131(10), 121(19), 119(20), 110(56), 109(11), 107(31), 105(32), 97(67), 95(34), 93(41), 91(41), 81(26), 79(28), 69(78), 67(15), 65(11), 55(46), 53(14), 42(27), 41(58). (7αH)-Germacrene d-8β-ol (52): C15 H24 O; 1 H-NMR (CDCl3 , 400 MHz) and 13 C-NMR (CDCl3 , 100 MHz) data: see Table 2. HREIMS: m/z 220.1823 (calculated for C15 H24 O, 220.1822); EI-MS 70 eV, m/z (rel. int.): 220(1, M•+ ), 202(34, M•+ − H2 O), 160(20), 159(100), 146(30), 145(30), 131(50), 121(25), 120(25), 119(50), 117(31), 109(21), 107(45), 105(68), 95(20), 93(65), 92(23), 91(88), 81(53), 80(20), 79(74), 77(47), 69(41), 67(35), 65(20), 55(43), 53(28), 43(61), 41(98). (7αH)-Germacrene d-α-ol (53): C15 H24 O; 1 H-NMR (CDCl3 , 400 MHz) and 13 C-NMR (CDCl3 , 100 MHz) data: see Table 2. HREIMS: m/z 220.1823 (calculated for C15 H24 O, 220.1822); EI-MS 70 eV, m/z (rel. int.): 220(1, M•+ ), 202(34, M•+ − H2 O), 160(20), 159(100), 146(30), 145(29), 131(48), 121(25), 120(24), 119(49), 117(30), 109(20), 107(44), 105(67), 95(20), 93(63), 92(21), 91(87), 81(51), 80(20), 79(72), 77(47), 69(38), 67(34), 65(19), 55(42), 53(26), 43(60). Cadina-1(10),4-dien-8β-ol (56): C15 H24 O; 1 H-NMR (CDCl3 , 400 MHz) and 13 C-NMR (CDCl3 , 100 MHz) data: see Table 3. HREIMS: m/z 220.1823 (calculated for C15 H24 O, 220.1822); EI-MS 70 eV, m/z (rel. int.): 220(1, M•+ ), 202(12, M•+ − H2 O), 187(28), 174(7), 160(13), 159(100), 146(6), 145(11), 144(7), 134(10), 131(15), 129(6), 121(6), 119(21), 117(8), 115(6), 107(6), 105(19), 93(9), 91(19), 79(8), 77(10), 55(7), 43(9), 41(15). Supplementary Materials: The following are available online, Figures S1–S24: 1D, 2D-NMR and EI-mass spectra of (10βH)-1β,8β-oxido-cadin-4-ene (38), (7αH)-Germacrene D-8β-ol (52), (7αH)-Germacrene D-8α-ol (53) and Cadina-1(10),4-dien-8β-ol (56). Author Contributions: Conceptualization, D.A.K., J.B.B. and F.T.; methodology, D.A.K., T.A.Y. and Z.A.O.; software, D.A.K.; validation, J.J.B., F.T. and A.B.; formal analysis, D.A.K.; investigation, D.A.K., T.A.Y. and Z.A.O.; writing—original draft preparation, D.A.K., J.B.B., F.T. and J.C.; writing—review and editing, D.A.K., J.B.B., F.T. and J.C.; visualization, D.A.K., J.J.B. and A.B.; supervision, J.B.B. and F.T. All authors have read and agreed to the published version of the manuscript. Molecules 2020, 25, 5613 13 of 14 Funding: This research received no external funding. 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