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SHORT COMMUNICATION - Two New Compounds from Elaeagnus umbellata Thunb.

Byline: Fiaz Aziz Minhas, Habib-ur-Rehman, Muhammad Naeem Ahmed, Khawaja Ansar Yasin and Abdul Majeed Khan

Summary: This article presents the isolation of two new compounds namely 2-(1-hydroxy-2-methylpropyl)-3-(2-hydroxyethyl)-1-methyl-1H-indole-4,7-diol (1) related to the class of indole alkaloid and propyl 4-(3, 4-dihydroxyhexyl)benzoate (2) related to the class of aromatic ester from the chloroform and petroleum ether fraction of Elaeagnus umbellata, respectively. The chemical structures of compounds 1 and 2 were determined by using the powerful tool of different spectroscopic techniques including MS, UV, IR and NMR spectral data. Furthermore, the structures were further supported by two dimensional NMR techniques like COSY-45o, HMQC and HMBC.

Keywords: Elaeagnus umbellata, Isolation, New compounds, Indole alkaloid, Aromatic ester

Introduction

Different plants play a vital role in the drug development program around the globe. Most of the terrestrial and marine plants are the rich and potential sources of biologically interesting compounds. The current research showed that some of the plants also contain novel skeletons that can prove to be an ideal candidate for cure of different biological disorders. In this connection, the research in hand is focused on terrestrial plant of biological significance. The investigated plant namely Elaeagnus umbellata is belonging to Elaeagnaceae family comprises of three genera including Elaeagnus (70 species), Hippophae (07 species), and Shepherdia (03 species). The plants of genus Elaeagnus are distributed widely in Asia, Europe and N. America [1]. These plants are well-known rich source of unique heterocyclic alkaloids with indole skeletons [2].

These alkaloids have got great interests from biological and therapeutic aspects, due to their anticancer, antimalarial, antihypertensive, antiarrhythmic activity and sedative properties [3-7]. Previous investigations have resulted in the isolation of different compounds [8-14].

Recent phytochemical investigations have also resulted in the isolation of [beta]-carbolin derivatives from various species of Elaeagnus [15]. Elaeagnus umbellate is a common shrub in the region of Azad Jammu and Kashmir as well as Himalayan regions of Pakistan [16-18]. The berry is rich source of various bioactive natural products [19-24]. The literature showed that its seeds are used in the treatment of respiratory disorders [19]. Previous investigations on this plant also led to the isolation of different compounds [25, 26].

Experimental

Pretreatment: The plant material was collected from the village Gali Khaiter Bheri, situated at the elevation of 7121 feet (N34Adeg 32.987; E73Adeg 34.837) above sea level in District Muzaffarabad, AJK, Pakistan. The powder form (6 Kg) of plant was soaked in commercial methanol (20 liters) for 12 days. The methanol extract was separated from the residue by decantation that yielded 390 g of methanol extract which was further separated by vacuum liquid chromatography (VLC) petroleum ether, ethyl acetate, chloroform and methanol (4 liters each) that afforded four fractions (F1-F4). These fractions were separately concentrated by rotary evaporator and further proceed for the isolation of compounds.

2-(1-Hydroxy-2-methylpropyl)-3-(2-hydroxyethyl)-1-methyl-1H-indole-4, 7-diol (1): The chloroform fraction (F3) was loaded on to column chromatography for the separation of its components. The column was run with ethyl acetate and petroleum ether mixture (1: 1) resulting different fractions and their final purification with the help of thin layer chromatography that produced pure white amorphous compound 1 (32 mg), which gave orange colored precipitate with Dragendorff's reagent and reddish brown color with Wagner's reagent. White amorphous solid, Rf: 0.31 (ethyl acetate-petroleum ether, 2: 8).IR (CHCl3): 3429 cm-1 (O-H), 3026 cm-1(aromatic C-H), 2876 cm-1 (aliphatic C-H), 1610 cm-1(C=C).UV/Vis I>>max (MeOH) nm: 226, 279, 300.

1H-NMR (I', 400 MHz, CDCl3): 1.27(6 H, d, J = 6Hz), 1.51(1H, m), 3.47(3H, d, J = 5.0 Hz), 2.61(3H, s), 7.92 (1H, d, J = 8.3Hz), 6.98 (1H, d, J = 8.3 Hz), 1.23 (2H, t, J = 7.0 Hz), 3.34 (2H, qn, J = 7.0 Hz).13C-NMR (I', 100 MHz CDCl3): 32.10(-CH), 68.10(-CH-OH), 33.30(N-CH3), 138.60(2-C), 107.00(3-C), 143.00(4-C), 113.40(5-C), 115.30(6-C), 137.40(7-C), 130.10(8a-C), 118.30(8b-C), 19.40(11a-C), 19.40(11b-C), 26.20(12-C), 62.60(13-C). MS m/z (%) = 279(5.1), 246(100), 231(13.8), 213(24.6), 203(19.9), 185(14.9), 173(3), 167(24.2). HREIMS-: m/z [M +] 279.1465 (C15H21NO4, calcd. 279.1470).

Propyl 4-(3, 4-dihydroxyhexyl)benzoate (2): The petroleum ether fraction (F-1) was passed through the column chromatography eluted with different polarities of petroleum ether and ethyl acetate mixture (7: 3) that eluted more than 100 fractions. The fractions F1-64 were further purified by column chromatography which afforded sub-fractions with petroleum ether and ethyl acetate (7: 3) then one of the fraction afforded pure white amorphous compound 2 (28 mg).

White amorphous solid, Rf: 0.29, (petroleum ether-ethyl acetate, 8.5: 1.5); IR (CHCl3): 3074cm-1(aromatic C-H), 2985 cm-1 (alkyl C-H), 1725 cm-1 ([alpha], [beta]-unsaturated ester C=O); UV/Vis I>>max (MeOH) nm: 242, 275, 283; 1H-NMR (400 MHz, CDCl3): 7.51(2H, dd, J = 8.6, 3.3 Hz), 7.51(2H, dd, J = 8.6, 3.3 Hz), 7.68(2H, dd, J = 8.6, 3.3 Hz), 7.68(2H, dd, J = 8.6, 3.3 Hz), 1.67(2H, dd, J = 12.1, 6.0 Hz), 1.40(2H. m), 4.19(2H, m), 4.19(2H, m), 1.28(2H, m), 0.87(3H, t, J = 7.0 Hz), 4.23(2H, t, J = 6.0 Hz), 1.31(2H, m), 0.89(3H, t, J = 7.0 Hz). 13C-NMR (100 MHz CDCl3): 125.60(1-C), 127.10(2, 6-C), 127.60(3, 5-C), 132.00(4-C), 30.42(7-C), 25.30(8-C), 67.10(9-C), 68.20(10-C), 25.70(11-C), 10.20(12-C), 165.90(13-C), 66.35(14-C), 22.50(15-C), 10.30(16-C). MS m/z (%) = 280(10), 221(1.5), 219(2.6), 168(38), 150(100); HREIMS-: m/z [M +] 280.1669 (C16H24O4, calcd. 280.3593).

Results and Discussion

Our further phytochemical investigations on this plant have resulted in the isolation and characterization of a new indole alkaloid, 2-(1-hydroxy-2-methylpropyl)-3-(2-hydroxyethyl)-1-methyl-1H-indole-4,7-diol (1) and a new aromatic ester namely 4-(3,4-dihydroxyhexyl) propylbenzoate (2). The structures of both the compounds were determined by mass spectroscopy, IR, UV and finally by one dimensional and two dimensional NMR spectroscopy.

Structure Elucidation of Compound 1

UV, IR and MS: The UV spectrum (MeOH) of the compound 1 was characteristic of the substituted indole chromophore with I>>max absorption at 226, 279 and 300 nm. The IR spectrum (CHCl3) displayed absorptions at 3429, 3026, 2876 and 1610 cm-1, indicating the presence of O-H, aromatic C-H, aliphatic C-H and conjugated C=C respectively. The HRMS showed peak at m/z 279.1465 for molecular ion that corresponds to the molecular formula C15H21NO4 having six degrees of unsaturation in compound 1. Other prominent peaks observed at m/z 246 (C14H16NO3), indicating the simultaneous loss of terminal CH2OH and 2H from the molecule. This fragmented ion further loses methyl group (-CH3) to produce an ion at m/z 231(C13H13NO3). The peak at m/z 213 indicating the loss of water molecule from the ion at m/z 231.

Another peak observed at m/z 203 (C11H7NO3) was due to the loss of isopropyl (C3H7) group of the side chain of fragmented ion C14H16NO3 (m/z 246), which on further loss of CH2O yielded m/z 173 fragment ion corresponding to C10H5NO2. The fragment ion m/z 203 (C11H7NO3) also afforded a peak at m/z 185 by the loss of water molecule.

Another important peak at m/z 190 was observed due to simultaneous loss of the side chain and two H-atoms (M - C5H11O + 2H) from the molecular ion which revealed the attachment of C5H11O unit to the indole nucleus (Fig. 3).

1H-NMR Spectroscopy: The 1H-NMR of compound 1 (Table 1) showed a 3H singlet at I' 2.61, a characteristic peak for N-methyl (27). The 5-H aromatic proton appeared at I' 7.92 (1H, d, J = 8.3 Hz). The 6-H aromatic proton appeared at I' 6.98 (1H, d, J = 8.3 Hz). Both 5-H and 6-H showed ortho coupling indicating two substitutions at C-4 and C-7. Protons resonating at I' 3.47 (1H, d, J = 5.0 Hz) and a 2H quintet appeared at I' 3.34 (J = 7.0 Hz) were assigned to the 9-H and 13-H respectively, attached to the C-atoms bearing hydroxyl function. The 10-H appeared at I' 1.51 (1H, m). A 6H doublet at I' 1.27 (J = 7.0 Hz) was due to methyl groups attached to the C-10 and a 2H triplet at I' 1.23 (J = 7.0 Hz) was assigned to methylene group.

13C-NMR Spectroscopy: The 13C-NMR spectrum of compound 1 (Table-1) showed 15 carbon and the multiplicity of C-atoms was determined by the DEPT experiments at an angle of 45, 90 and 135 which showed four signals for methine, two of methylene and two of methyl carbons. Other seven signals in the broad band spectrum were assigned to the quaternary carbon atoms. Aliphatic methine carbon (C-9) and methylene carbon (C-13) appeared at I' 68.10 and I' 62.60, respectively, were consistent with the presence of -OH groups in the molecule. Aromatic carbons showed slightly downfield signals at I' 143.00 (4-C) and I' 137.40 (7-C) due to -OH functionalities. The aromatic CH carbons signal appeared at I' 113.4 and I' 115.3 were assigned to 5-C and 6-C, respectively. Signals at I' 130.10 (8a-C) and I' 118.30 (8b-C) were also in close agreement with the indole nucleus. Methyl carbon resonating at I' 33.30 was assigned to N-CH3.

Tertiary carbon atom (10-C) appeared at I' 32.10 and I' 19.40 was assigned to both methyl carbons (11a-C, 11b-C). Substituted aromatic C-atoms of pyrrole unit were assigned I' 138.60 (2-C) and I' 107.00 (3-C). The signal at I' 26.20 was assigned to methylene carbon (12-C).

HMQC and HMBC Interactions: The HMQC spectrum of compound 1 showed the interactions between 9-H (I' 3.47) and 9-C (I' 68.10) while the 10-H (I' 1.51) showed interaction with 10-C (I' 32.10). The 12-H (I' 1.23) showed interaction with 12-C (I' 26.20) whereas 13-H (I' 3.34) showed interaction with 13-C (I' 62.60) (Fig. 4). The structure of compound 1 was finally established by an HMBC spectrum (Fig. 5) which showed diagnostic cross peaks between 9-H (I' 3.47),12-H(I' 1.23) and 2-C (I' 138.60), revealing substitutions at 2-C and 3-C of the pyrrole moiety which was further confirmed by the cross peaks between 12-H (I' 1.23) with 3-C (I' 107.00) and 8b-C (I' 118.30) while the 5-H (I' 7.92) showed cross peak with 7-C (I' 137.40). Similarly, the 6-H (I' 6.98) showed cross peak with 4-C (I' 143.00) elucidating para substitution in benzene ring which was consistent with the strong ortho coupling constant (8.3 Hz) between aromatic protons (H-5 and H-6).

The substitution on N was corroborated by the correlation of -CH3 protons at I' 2.61 with C-2 and C-8b in the HMBC spectrum (Fig. 5). The spectroscopic data so discussed resulted in the identification of compound 1 as 2-(1-hydroxy-2-methylpropyl)-3-(2-hydroxyethyl)-1-methyl-1H-indole-4,7-diol.

Table-1: 1H-NMR chemical shift (400 MHz, CDCl3) of compound 1.

Protons###Chemical Shift (I')###Multiplicity###J in Hz###Carbons###Chemical shift (I')###HMBC interactions

11-H,-CH3###1.27###6 H (d)###7.0###-CH###32.10###-

10-H,-CH###1.51###1 H (m)###-###-CH-OH###68.10###-

CH-OH###3.47###1 H (d)###5.0###N-CH3###33.30###2-C

N-CH3###2.61###3 H (s)###-###2-C###138.60###2-C, 8a-C

5-H###7.92###1 H (d)###8.3###3-C###107.00###7-C

6-H###6.98###1 H (d)###8.3###4-C###143.00###4-C

-CH2-###1.23###2 H (t)###7.0###5-C###113.40###2-C, 3-C, 8b-C

-CH2-OH###3.34###2 H (qn)###7.0###6-C###115.30###-

###7-C###137.40

###8a-C###130.10

###8b-C###118.30

###11a-C###19.40

###11b-C###19.40

###12-C###26.20

###13-C###62.60

Structure Elucidation of Compound 2

UV, IR and MS: The UV spectrum (MeOH) of the compound 2 was characteristic of substituted benzoate chromophore with I>>max at 242 nm, 275 nm and 283 nm. The IR spectrum (CHCl3) showed characteristic absorption peaks at 3435 cm-1 and 3074 cm-1 of O-H and aromatic C-H, respectively. Aliphatic saturated C-H and [alpha], [beta]-unsaturated ester C=O peaks were observed at 2985 and 1725 cm-1, respectively. The molecular ion peak was observed at m/z 280.1669 by HRMS revealing the molecular formula C16H24O4, indicating five degrees of unsaturation in the molecule. Mass fragmentation pattern showed a peak at m/z 221 due to the loss of propoxy group (C3H7O) from the molecular ion indicating the presence of propyl ester of substituted benzoic acid. Another prominent peak observed at m/z 168 was produced by the loss of CO and C2H2 units from the fragment ion m/z 221.

The base ion peak afforded at m/z 150 was observed due to loss of water molecule from the fragment ion m/z 168.A peak at m/z 119 was also observed due to the loss of propyl group (C3H7) and water molecule from the molecular ion (Fig. 6).

Table-2: 1H-NMR chemical shifts (400 MHz, CDCl3) of compound 2.

Protons###Chemical shifts (I')###Multiplicity###J values (Hz)###Carbons###Chemical shifts (I')###HMBC interactions

2-H###7.51###2 H (dd)###8.6 , 3.3###1-C###125.60###1-C, 13-C

6-H###-do-###-do-###-do-###2,6-C###127.10###-do-

3-H###7.68###2 H (dd)###8.6,3.3###3,5-C###127.60###2-C, 4-C, 7-C

5-H###-do-###-do-###-do-###4-C###132.00###6-C, 4-C, 7-C

7-H###1.67###2 H (dd)###12.1,6.0###7-C###30.42###-

8-H###1.40###2 H (m)###-###8-C###25.30###-

9-H###4.19###2H (m)###-###9-C###67.10###11-C

10-H###-do-###-do-###-###10-C###68.20###8-C, 12-C

11-H###1.28###2H (m)###-###11-C###25.70###-

12-H###0.87###3H (t)###7.0###12-C###10.20###-

14-H###4.23###2H (t)###6.0###13-C###165.90###13-C

15H###1.31###2H (m)###-###14-C###66.35###-

16H###0.89###3H (t)###7.0###15-C###22.50###-

###16-C###10.30

1H-NMR Spectroscopy: 1H-NMR spectrum of the compound 2 (Table-2) showed two proton resonances at I' 7.51 (2H, dd, J1 = 8.6 Hz, J2 = 3.3 Hz, 2-H, 6-H) and I' 7.68 (2H, dd, J1 = 8.6 Hz, J2 = 3.3 Hz, 3-H, 5-H), indicating substitutions at C-1 and C-4 of the benzene ring.

13C-NMR Spectroscopy: The 13C-NMR spectrum of the compound 2 (Table-2) showed 13 carbon resonances in the molecule. The DEPT experiment showed signals of four aromatic, five methylene, two methyl and two methine carbon atoms. The remaining three signals in the broad band spectrum were assigned to the C-1, C-4 and the C-13 carbonyl carbon. Aromatic carbon showed slightly downfield signal at I'125.60 (1-C) due to the presence of ester functionality and more downfield signal of C-4 at I' 132.00 is also consistent with the para substituted benzoates. The carbonyl carbon of ester appeared at I' 165.90. The signals at I'10.20 and I'10.30 were assigned to C-12 and C-16 methyl carbons, respectively. The two symmetrical carbons, C-2 and C-6 appeared at I' 127.10 while the other two symmetrical carbons, C-3 and C-5 were appeared at I' 127.60. The C-8 and C-9 were appeared at I' 25.30 and I' 67.10, respectively. The down field signal of C-9 showed the presence of oxygen function on it.

The down field signal of C-10 appeared at I' 68.20 is also due to the oxygen function on it. The C-11 and C-15 appeared at I' 25.70 and I' 22.50, respectively.

HMQC and HMBC Interactions: The HMQC data of compound 2 demonstrate that 2-H, 6-H and 3-H, 5-H are attached to carbons at I' 127.10 and I' 127.60, respectively. The HMBC (Fig. 7) correlation observed between the aromatic hydrogens (2-H,6-H) at I' 7.51 and the carbons (C-1,C-13) at I' 125.60 and I' 165.90, besides the correlation between aromatic hydrogens (3-H,5-H) at I' 7.68 and carbons (C-2,C-4,C-7) at I' 127.10, I' 132.00, I' 30.42, further suggests the substituent at C-1 and C-4. A two proton methylene multiplets appeared at I'1.28 and I'1.31 were assigned to the 11-H and 15-H, respectively.

The 3H triplet at I' 0.87 (J = 6.0 Hz) and I' 0.89 (J = 7.0 Hz) were observed due to 12-H and 16-H methyl proton. A double duplet at I' 1.67 (2 H, J1 = 12.1 Hz, J2 = 6.0 Hz) and a 2H multiplet at I' 1.40 were assigned to 7-H and 8-H, respectively. The two protons of methylene group attached to O-atom of ester were appeared at I' 4.23 as a triplet that displayed interaction with carbonyl carbon at I' 165.90 of the ester functionality in HMBC spectrum. The 9-H and 10-H appeared at I' 4.19 as a multiplet and C-14 appeared at I' 66.35. The structure of the side chain was further verified by the HMBC spectrum of compound 2 which showed correlation between 10-H (I' 4.19) and 8-C (I' 25.30) as well as 12-C (I' 10.20) while the 9-H (I' 4.19) showed correlation with 11-C (I' 25.70). These observations showed that the two vicinal hydroxyl groups were attached at the C-9 and C-10 locations (Fig. 8). Thus, the structure of compound 2 was identified as 4-(3, 4-dihydroxyhexyl) propyl benzoate.

Stereochemistry of compound 1

Experimental value of J3 for H-9 and H-10 comes out as 5.0 Hz which shows that the two protons are having gauche interactions (as shown in yellow circle). Moreover, the methyl groups on N and of isopropyl are farthest apart. This is only possible if OH at C-9 is in alpha position as shown in left hand side of the figure given below and has (R) configuration.

The compound 2 has two (adjacent) stereocenters giving rise to two multiplets (2H) at 4.19. There are following four stereoisomers are possible for this compound.

Conclusions

The compounds isolated from terrestrial plants could lead to drug development program due to their unique structure activity relationship. The previous investigations on Elaeagnus umbellata Thunb showed that the plant under investigation is significantly active against different biological disorders. In addition, different classes of compounds which have been previously reported from this plant were also found to be biologically active. The previous research stimulated us to further investigate its chemical constituents. In this connection, two new compounds 1 and 2 having interesting skeleton were isolated from this plant. However, these compounds were not subjected to screening of biological activities due to their small quantities. Therefore, this research article presents only the structure elucidation of these compounds using different spectroscopic techniques.

Further detailed investigation on these compounds is required particularly their synthesis is very important to increase their quantity for biological screening against different biological tests.

Acknowledgements

The authors are greatly thankful to HEC, Pakistan for financial support.

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