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Alstobrogaline, an unusual pentacyclic monoterpenoid indole alkaloid with aldimine and aldimine-N-oxide moieties from Alstonia scholaris
Tetrahedron Letters 60 (2019) 789–791
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Alstobrogaline, an unusual pentacyclic monoterpenoid indole alkaloid with aldimine and aldimine-N-oxide moieties from Alstonia scholaris Premanand Krishnan a, Chun-Wai Mai b,c, Kien-Thai Yong d, Yun-Yee Low e, Kuan-Hon Lim a,⇑ a
School of Pharmacy, University of Nottingham Malaysia, Jalan Broga, 43500 Semenyih, Selangor, Malaysia School of Pharmacy, International Medical University, Bukit Jalil, 57000 Kuala Lumpur, Malaysia c Center for Cancer and Stem Cell Research, International Medical University, Bukit Jalil, 57000 Kuala Lumpur, Malaysia d Institute of Biological Sciences, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia e Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 15 November 2018 Revised 19 December 2018 Accepted 8 February 2019 Available online 10 February 2019 Keywords: Monoterpenoid indole alkaloid Aldimine Alstonia Apocynaceae
a b s t r a c t Alstobrogaline (1), an unusual monoterpenoid indole alkaloid incorporating a third N atom and possessing two aldimine functions, with one being in the form of N-oxide, was isolated from the leaves of Alstonia scholaris. Its structure and relative configuration were determined by extensive NMR spectroscopic analysis, while its absolute configuration was established by X-ray diffraction analysis. A possible biogenetic pathway to 1 was proposed. Compound 1 displayed weak cytotoxic effects against MDA-MB-231 and MCF7 breast cancer cells. Ó 2019 Elsevier Ltd. All rights reserved.
Plants of the genus Alstonia are known to be prolific producers of monoterpenoid indole alkaloids with intriguing polycyclic molecular skeletons and useful biological activities. Alstonia scholaris, which is widely distributed in tropical Asia, is used in traditional medicine in China, India, and Southeast Asia for the treatment of various diseases [1–3]. Although various samples of A. scholaris collected from different regions have previously been investigated [1,4,5], there was only one report on the Malaysian sample, which was collected from the east coast of Peninsular Malaysia. In the present study, the leaf sample of a cultivated A. scholaris collected from the west coast of Peninsular Malaysia was investigated, which resulted in the discovery of alstobrogaline (1), an unprecedented pentacyclic monoterpenoid indole alkaloid incorporating a third N atom as well as featuring an aldimine and an aldimine-N-oxide function (Fig. 1). Herein, we report the isolation, structure elucidation, and biogenetic pathway of compound 1. Alstobrogaline (1) was initially isolated as a light yellowish oil and subsequently crystallized from CHCl3 as light orange block crystals, mp 187 °C (decomposed), [a]D = +93 (c 0.10, CHCl3). The IR spectrum showed NH and ester carbonyl absorption bands at 3249 and 1739 cm1, respectively, while the UV spectrum showed characteristic dihydroindole absorption maxima at 235 and ⇑ Corresponding author. E-mail address: [email protected] (K.-H. Lim). https://doi.org/10.1016/j.tetlet.2019.02.018 0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.
290 nm. The HR-DART-MS measurements showed an [M + H]+ peak at m/z 352.1674, which analyzed for C20H21N3O3 + H. The molecular formula of 1 reveals the presence of 12 degrees of unsaturation, in addition to the presence of a third N atom, which is rare among monoterpenoid indole alkaloids. The 1H NMR data (Table 1) showed the presence of signals due to four aromatic hydrogens (dH 6.73–7.10), an ester methyl at dH 3.73 (s), an indolic NH at dH 4.92 (br s), and an ethylidene side chain (dH 1.75, d, 3H; dH 5.84, q, 2H; J = 7.5 Hz). Additionally, two unusually deshielded signals at dH 7.29 (br s) and 7.69 (s) were also observed. Consistent with the molecular formula established by HRMS measurements, the 13C NMR spectrum (Table 1) indicated a total of 20 carbon resonances, while the HSQC spectrum revealed the presence of 11 downfield resonances (comprising seven sp2 methine carbons, one N-bearing sp2 tertiary carbon, two sp2 quaternary carbons, and one ester carbonyl carbon) and nine upfield resonances (comprising two methyl carbons, two sp3 methylene carbon, three sp3 methine carbons, one sp3 quaternary carbon, and one significantly deshielded non-H-bearing sp3 carbon at dC 98.8). The downfield resonances corresponding to the indolic benzene ring (dC 111.3, 120.5, 123.0, 129.0, 136.3, and 145.7) and ethylidene side chain (dC 130.0, 132.3, and 14.4) were readily assigned based on comparison with other indole alkaloids with a dihydroindole chromophore and an ethylidene unit [6], and these assignments were corroborated by HMBC and NOESY data (Figs. 2 and 3).
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P. Krishnan et al. / Tetrahedron Letters 60 (2019) 789–791
Fig. 1. Structure of alstobrogaline (1).
Table 1 H and 13C NMR spectroscopic data of 1 in CDCl3.
1
a
Position
d Ca
2 3 5 6
98.8 69.2 167.9 47.4
7 8 9 10 11 12 13 14 140 15 16 18 19 20 21 CO2Me CO2Me NH
50.6 136.3 123.0 120.5 129.0 111.3 145.7 28.8 27.5 52.6 14.4 132.3 130.0 138.1 171.9 51.9
dH (J in Hz)a 4.55 t (2.5) (eq) 7.69 br s 3.25 m 3.25 m
6.99 d (7.5) 6.75 t (7.5) 7.10 t (7.5) 6.73 d (7.5) 2.24 dt (13.6, 3.8) (eq) 2.46 dt (13.6, 2.1) (ax) 3.34 m (eq) 2.81 d (3.6) (ax) 1.75 d (7.5) 5.84 q (7.5) 7.29 s 3.73 s 4.92 br s
Recorded at 600 and 150 MHz.
The COSY spectrum revealed the presence of CHCHCHCH, NCHCH2CHCH, and = CHCH3 partial structures corresponding to the C-9–C-10–C-11–C-12, N–C–3–C-14–C-15–C-16, and C-18–C-19 fragments in 2, respectively (Fig. 2). The C-9–C-10–C-11–C-12 fragment was readily assigned to the four contiguous aromatic methines of the dihydroindole moiety, which was firmly established based on the HMBC three-bond correlations from H-9 to C-7 and C-13, from H-12 to C-8, and from NH to C-7 and C-8. The N–C–3–C-14–C-15–C-16 fragment was deduced to be attached to C-2 and C-7 based on the three-bond correlations observed from H-3 to C-7, from H-14 to C-2, and from H-16 to C-2, C-6, and C-8 (Fig. 2), thus completed the assembly of the six-membered ring D. The HMBC correlations from H-16 and OMe (d 3.73) to the carbonyl carbon at dC 171.9 indicated the presence of the CO2Me group and its attachment to C-16.
Fig. 2. COSY and selected HMBCs of 1.
Fig. 3. Selected NOEs of 1.
The C-18–C-19 fragment was deduced to be part of the ethylidene side chain based on the HMBC three-bond correlation from H-18 to C-20. On the other hand, C-15 and C-21 (dC 138.1) were deduced to be attached to C-20 based on the correlations from H-19 to C-15 and C-21 and from H-21 (dH 7.29) to C-15, C-19, and C-20. Furthermore, the connection between C-21 and C-3 via N-4 was inferred by the HMBC correlation from H-21 to C-3, thus giving rise to the six-membered ring E. The unusually deshielded CH-21 (dC 138.1; dH 7.29) indicated the presence of a rare aldimine-N-oxide function at the N-4 C-21 fragment [6]. Finally, ring C was constructed by linking C-6 to C-7, and C-5 to C-2 via an N atom as indicated by the HMBC correlations from H-6 to C-2, C-5, C-8, and C-16 and from H-5 to C-2, C-6, and C-7. The unusually deshielded CH-5 (dC 167.9; dH 7.69) supported it to be an aldimine carbon (CH-5 = N), whereas the chemical shift observed for C-2 (dC 98.8) is consistent with it being an aminal carbon. The resulting 2D structure, as shown in 1, is in full agreement with the HMBC data (Fig. 2). The relative configurations at the various stereocenters were deduced from the NOESY data (Fig. 3). The NOE observed for H16/H-140 indicated a 1,3-diaxial relationship for H-16 and H-140 , whereas the NOEs observed for H-3/NH and H-15/H-16 indicated that both H-3 and H-15 were equatorially oriented. These observations also inferred that ring D adopted a chair conformation, while the C-3–N-4 and C-15–C-20 bonds were axial. Furthermore, the NOE observed for H-6/H-21 required ring C and the N-4–C-21–C20 fragment to be located on the same face of ring D. Taken together, the configurations at C-2, C-3, C-7, C-15, and C-16 were determined to be rel-(2S,3S,7R,15R,16R). Finally, the geometry of the C-19–C-20 double bond was deduced to be E based on the
Fig. 4. X-ray crystal structure of 1 [Flack parameter, x = 0.02(2)].
P. Krishnan et al. / Tetrahedron Letters 60 (2019) 789–791
791
Scheme 1. Possible biogenetic pathway to 1.
NOEs observed for H-19/H-21 and H-18/H-15. Since suitable crystals of 1 were obtained, X-ray diffraction analysis was carried out, which confirmed the absolute configurations at all stereocenters as 2S,3S,7R,15R,16R (Fig. 4) [7]. Alstobrogaline (1) represents a novel and unusual monoterpenoid indole alkaloid incorporating a third N atom, and possessing two aldimine functions, with one being in the form of N-oxide. To the best of knowledge, following the isolation of two 4,5-seco-picrinine-type alkaloids (i.e., alschomine and isoalschomine) [6], compound 1 represents the third instance in which a monoterpenoid indole alkaloid incorporates an aldimine-N-oxide function. A possible biogenetic pathway to 1 is shown in Scheme 1, starting from an akuammiline-type precursor such as strictamine (2). Firstly, 2 undergoes an oxidation to the C-5–N-4 iminium ion 3, which following hydrolytic cleavage gives the amine-aldehyde 4. Subsequently, transamination of the aldehyde in 4 gives a primary amine, which then performs a nucleophilic addition onto the imine C-2 to give the pentacyclic aminal 5. Finally, oxidation of 5 gives the desired alkaloid, alstobrogaline (1), which incorporates an aldimine and an aldimine-N-oxide function at C-5 and C-21, respectively. Alstosbrogaline (1) was evaluated for its cytotoxicity against a panel of five breast cancer cell lines. Compound 1 was weakly cytotoxic against MDA-MB-231 and MCF7 cells (IC50 25.3 and 24.1 lM, respectively), but was not active against MDA-MB-468, SKBR3, and T47D cells (IC50 > 30 lM) [8]. Acknowledgment P.K. and K.H.L. thank University of Nottingham Malaysia for providing PhD scholarship and partial funding for research.
Appendix A. Supplementary data Supplementary data (Experimental procedures, NMR and HRMS data of 1, crystal data and structure refinement parameters of 1) to this article can be found online at https://doi.org/10.1016/j.tetlet. 2019.02.018. These data include MOL files and InChiKeys of the most important compounds described in this article. References [1] M.S. Khyade, D.M. Kasote, N.P. Vaikos, J. Ethnopharmacol. 153 (2014) 1–18. [2] X.H. Cai, Z.Z. Du, X.D. Luo, Org. Lett. 9 (2007) 1817–1820. [3] T.S. Kam, K.T. Nyeoh, K.M. Sim, K. Yoganathan, Phytochemistry 45 (1997) 1303– 1305. [4] T. Yamauchi, F. Abe, R.F. Chen, G.I. Nonaka, T. Santisuk, W.G. Padolina, Phytochemistry 29 (1990) 3547–3552. [5] A.P.G. Macabeo, K. Krohn, D. Gehle, R.W. Read, J.J. Brophy, G.A. Cordell, S.G. Franzblau, A.M. Aguinaldo, Phytochemistry 66 (2005) 1158–1162. [6] F. Abe, R.F. Chen, T. Yamauchi, N. Marubayashi, I. Ueda, Chem. Pharm. Bull. 37 (1989) 887–890. [7] Crystal data for alstobrogaline (1): Light orange block crystals, C20H21N3O3. CHCl3, Mr = 470.77, orthorhombic, space group P212121, a = 9.3566(3) Å, b = 12.8522(5) Å, c = 17.2351(5) Å, V = 2072.57(12) Å3, Z = 4, Dcalcd = 1.509 gcm3, crystal size 0.20 0.13 0.10 mm3, F(000) = 976, Mo Ka radiation (k= 0.71073 Å), T = 169(2) K. A total of 17986 reflections were measured with 6305 independent reflections (Rint = 0.0279, Rsigma = 0.0369). The final R1 value was 0.0435 [I 2r (I)] and wR2 value was 0.1068 (all data). The absolute configuration was determined on the basis of the Flack parameter [x = 0.02(2)], refined using 2637 Friedel pairs. Crystallographic data for the structure of 1 have been deposited with the Cambridge Crystallographic Data Centre (deposition no. CCDC 1878436). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. [8] A.A.Q. Al-Khdhairawi, P. Krishnan, C.W. Mai, F.F.L. Chung, C.O. Leong, K.T. Yong, K.W. Chong, Y.Y. Low, T.S. Kam, K.H. Lim, J. Nat. Prod. 80 (2017) 2734–2740.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Alstobrogaline, an unusual pentacyclic monoterpenoid indole alkaloid with aldimine and aldimine-N-oxide moieties from Alstonia scholaris Premanand Krishnan a, Chun-Wai Mai b,c, Kien-Thai Yong d, Yun-Yee Low e, Kuan-Hon Lim a,⇑ a
School of Pharmacy, University of Nottingham Malaysia, Jalan Broga, 43500 Semenyih, Selangor, Malaysia School of Pharmacy, International Medical University, Bukit Jalil, 57000 Kuala Lumpur, Malaysia c Center for Cancer and Stem Cell Research, International Medical University, Bukit Jalil, 57000 Kuala Lumpur, Malaysia d Institute of Biological Sciences, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia e Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 15 November 2018 Revised 19 December 2018 Accepted 8 February 2019 Available online 10 February 2019 Keywords: Monoterpenoid indole alkaloid Aldimine Alstonia Apocynaceae
a b s t r a c t Alstobrogaline (1), an unusual monoterpenoid indole alkaloid incorporating a third N atom and possessing two aldimine functions, with one being in the form of N-oxide, was isolated from the leaves of Alstonia scholaris. Its structure and relative configuration were determined by extensive NMR spectroscopic analysis, while its absolute configuration was established by X-ray diffraction analysis. A possible biogenetic pathway to 1 was proposed. Compound 1 displayed weak cytotoxic effects against MDA-MB-231 and MCF7 breast cancer cells. Ó 2019 Elsevier Ltd. All rights reserved.
Plants of the genus Alstonia are known to be prolific producers of monoterpenoid indole alkaloids with intriguing polycyclic molecular skeletons and useful biological activities. Alstonia scholaris, which is widely distributed in tropical Asia, is used in traditional medicine in China, India, and Southeast Asia for the treatment of various diseases [1–3]. Although various samples of A. scholaris collected from different regions have previously been investigated [1,4,5], there was only one report on the Malaysian sample, which was collected from the east coast of Peninsular Malaysia. In the present study, the leaf sample of a cultivated A. scholaris collected from the west coast of Peninsular Malaysia was investigated, which resulted in the discovery of alstobrogaline (1), an unprecedented pentacyclic monoterpenoid indole alkaloid incorporating a third N atom as well as featuring an aldimine and an aldimine-N-oxide function (Fig. 1). Herein, we report the isolation, structure elucidation, and biogenetic pathway of compound 1. Alstobrogaline (1) was initially isolated as a light yellowish oil and subsequently crystallized from CHCl3 as light orange block crystals, mp 187 °C (decomposed), [a]D = +93 (c 0.10, CHCl3). The IR spectrum showed NH and ester carbonyl absorption bands at 3249 and 1739 cm1, respectively, while the UV spectrum showed characteristic dihydroindole absorption maxima at 235 and ⇑ Corresponding author. E-mail address: [email protected] (K.-H. Lim). https://doi.org/10.1016/j.tetlet.2019.02.018 0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.
290 nm. The HR-DART-MS measurements showed an [M + H]+ peak at m/z 352.1674, which analyzed for C20H21N3O3 + H. The molecular formula of 1 reveals the presence of 12 degrees of unsaturation, in addition to the presence of a third N atom, which is rare among monoterpenoid indole alkaloids. The 1H NMR data (Table 1) showed the presence of signals due to four aromatic hydrogens (dH 6.73–7.10), an ester methyl at dH 3.73 (s), an indolic NH at dH 4.92 (br s), and an ethylidene side chain (dH 1.75, d, 3H; dH 5.84, q, 2H; J = 7.5 Hz). Additionally, two unusually deshielded signals at dH 7.29 (br s) and 7.69 (s) were also observed. Consistent with the molecular formula established by HRMS measurements, the 13C NMR spectrum (Table 1) indicated a total of 20 carbon resonances, while the HSQC spectrum revealed the presence of 11 downfield resonances (comprising seven sp2 methine carbons, one N-bearing sp2 tertiary carbon, two sp2 quaternary carbons, and one ester carbonyl carbon) and nine upfield resonances (comprising two methyl carbons, two sp3 methylene carbon, three sp3 methine carbons, one sp3 quaternary carbon, and one significantly deshielded non-H-bearing sp3 carbon at dC 98.8). The downfield resonances corresponding to the indolic benzene ring (dC 111.3, 120.5, 123.0, 129.0, 136.3, and 145.7) and ethylidene side chain (dC 130.0, 132.3, and 14.4) were readily assigned based on comparison with other indole alkaloids with a dihydroindole chromophore and an ethylidene unit [6], and these assignments were corroborated by HMBC and NOESY data (Figs. 2 and 3).
790
P. Krishnan et al. / Tetrahedron Letters 60 (2019) 789–791
Fig. 1. Structure of alstobrogaline (1).
Table 1 H and 13C NMR spectroscopic data of 1 in CDCl3.
1
a
Position
d Ca
2 3 5 6
98.8 69.2 167.9 47.4
7 8 9 10 11 12 13 14 140 15 16 18 19 20 21 CO2Me CO2Me NH
50.6 136.3 123.0 120.5 129.0 111.3 145.7 28.8 27.5 52.6 14.4 132.3 130.0 138.1 171.9 51.9
dH (J in Hz)a 4.55 t (2.5) (eq) 7.69 br s 3.25 m 3.25 m
6.99 d (7.5) 6.75 t (7.5) 7.10 t (7.5) 6.73 d (7.5) 2.24 dt (13.6, 3.8) (eq) 2.46 dt (13.6, 2.1) (ax) 3.34 m (eq) 2.81 d (3.6) (ax) 1.75 d (7.5) 5.84 q (7.5) 7.29 s 3.73 s 4.92 br s
Recorded at 600 and 150 MHz.
The COSY spectrum revealed the presence of CHCHCHCH, NCHCH2CHCH, and = CHCH3 partial structures corresponding to the C-9–C-10–C-11–C-12, N–C–3–C-14–C-15–C-16, and C-18–C-19 fragments in 2, respectively (Fig. 2). The C-9–C-10–C-11–C-12 fragment was readily assigned to the four contiguous aromatic methines of the dihydroindole moiety, which was firmly established based on the HMBC three-bond correlations from H-9 to C-7 and C-13, from H-12 to C-8, and from NH to C-7 and C-8. The N–C–3–C-14–C-15–C-16 fragment was deduced to be attached to C-2 and C-7 based on the three-bond correlations observed from H-3 to C-7, from H-14 to C-2, and from H-16 to C-2, C-6, and C-8 (Fig. 2), thus completed the assembly of the six-membered ring D. The HMBC correlations from H-16 and OMe (d 3.73) to the carbonyl carbon at dC 171.9 indicated the presence of the CO2Me group and its attachment to C-16.
Fig. 2. COSY and selected HMBCs of 1.
Fig. 3. Selected NOEs of 1.
The C-18–C-19 fragment was deduced to be part of the ethylidene side chain based on the HMBC three-bond correlation from H-18 to C-20. On the other hand, C-15 and C-21 (dC 138.1) were deduced to be attached to C-20 based on the correlations from H-19 to C-15 and C-21 and from H-21 (dH 7.29) to C-15, C-19, and C-20. Furthermore, the connection between C-21 and C-3 via N-4 was inferred by the HMBC correlation from H-21 to C-3, thus giving rise to the six-membered ring E. The unusually deshielded CH-21 (dC 138.1; dH 7.29) indicated the presence of a rare aldimine-N-oxide function at the N-4 C-21 fragment [6]. Finally, ring C was constructed by linking C-6 to C-7, and C-5 to C-2 via an N atom as indicated by the HMBC correlations from H-6 to C-2, C-5, C-8, and C-16 and from H-5 to C-2, C-6, and C-7. The unusually deshielded CH-5 (dC 167.9; dH 7.69) supported it to be an aldimine carbon (CH-5 = N), whereas the chemical shift observed for C-2 (dC 98.8) is consistent with it being an aminal carbon. The resulting 2D structure, as shown in 1, is in full agreement with the HMBC data (Fig. 2). The relative configurations at the various stereocenters were deduced from the NOESY data (Fig. 3). The NOE observed for H16/H-140 indicated a 1,3-diaxial relationship for H-16 and H-140 , whereas the NOEs observed for H-3/NH and H-15/H-16 indicated that both H-3 and H-15 were equatorially oriented. These observations also inferred that ring D adopted a chair conformation, while the C-3–N-4 and C-15–C-20 bonds were axial. Furthermore, the NOE observed for H-6/H-21 required ring C and the N-4–C-21–C20 fragment to be located on the same face of ring D. Taken together, the configurations at C-2, C-3, C-7, C-15, and C-16 were determined to be rel-(2S,3S,7R,15R,16R). Finally, the geometry of the C-19–C-20 double bond was deduced to be E based on the
Fig. 4. X-ray crystal structure of 1 [Flack parameter, x = 0.02(2)].
P. Krishnan et al. / Tetrahedron Letters 60 (2019) 789–791
791
Scheme 1. Possible biogenetic pathway to 1.
NOEs observed for H-19/H-21 and H-18/H-15. Since suitable crystals of 1 were obtained, X-ray diffraction analysis was carried out, which confirmed the absolute configurations at all stereocenters as 2S,3S,7R,15R,16R (Fig. 4) [7]. Alstobrogaline (1) represents a novel and unusual monoterpenoid indole alkaloid incorporating a third N atom, and possessing two aldimine functions, with one being in the form of N-oxide. To the best of knowledge, following the isolation of two 4,5-seco-picrinine-type alkaloids (i.e., alschomine and isoalschomine) [6], compound 1 represents the third instance in which a monoterpenoid indole alkaloid incorporates an aldimine-N-oxide function. A possible biogenetic pathway to 1 is shown in Scheme 1, starting from an akuammiline-type precursor such as strictamine (2). Firstly, 2 undergoes an oxidation to the C-5–N-4 iminium ion 3, which following hydrolytic cleavage gives the amine-aldehyde 4. Subsequently, transamination of the aldehyde in 4 gives a primary amine, which then performs a nucleophilic addition onto the imine C-2 to give the pentacyclic aminal 5. Finally, oxidation of 5 gives the desired alkaloid, alstobrogaline (1), which incorporates an aldimine and an aldimine-N-oxide function at C-5 and C-21, respectively. Alstosbrogaline (1) was evaluated for its cytotoxicity against a panel of five breast cancer cell lines. Compound 1 was weakly cytotoxic against MDA-MB-231 and MCF7 cells (IC50 25.3 and 24.1 lM, respectively), but was not active against MDA-MB-468, SKBR3, and T47D cells (IC50 > 30 lM) [8]. Acknowledgment P.K. and K.H.L. thank University of Nottingham Malaysia for providing PhD scholarship and partial funding for research.
Appendix A. Supplementary data Supplementary data (Experimental procedures, NMR and HRMS data of 1, crystal data and structure refinement parameters of 1) to this article can be found online at https://doi.org/10.1016/j.tetlet. 2019.02.018. These data include MOL files and InChiKeys of the most important compounds described in this article. References [1] M.S. Khyade, D.M. Kasote, N.P. Vaikos, J. Ethnopharmacol. 153 (2014) 1–18. [2] X.H. Cai, Z.Z. Du, X.D. Luo, Org. Lett. 9 (2007) 1817–1820. [3] T.S. Kam, K.T. Nyeoh, K.M. Sim, K. Yoganathan, Phytochemistry 45 (1997) 1303– 1305. [4] T. Yamauchi, F. Abe, R.F. Chen, G.I. Nonaka, T. Santisuk, W.G. Padolina, Phytochemistry 29 (1990) 3547–3552. [5] A.P.G. Macabeo, K. Krohn, D. Gehle, R.W. Read, J.J. Brophy, G.A. Cordell, S.G. Franzblau, A.M. Aguinaldo, Phytochemistry 66 (2005) 1158–1162. [6] F. Abe, R.F. Chen, T. Yamauchi, N. Marubayashi, I. Ueda, Chem. Pharm. Bull. 37 (1989) 887–890. [7] Crystal data for alstobrogaline (1): Light orange block crystals, C20H21N3O3. CHCl3, Mr = 470.77, orthorhombic, space group P212121, a = 9.3566(3) Å, b = 12.8522(5) Å, c = 17.2351(5) Å, V = 2072.57(12) Å3, Z = 4, Dcalcd = 1.509 gcm3, crystal size 0.20 0.13 0.10 mm3, F(000) = 976, Mo Ka radiation (k= 0.71073 Å), T = 169(2) K. A total of 17986 reflections were measured with 6305 independent reflections (Rint = 0.0279, Rsigma = 0.0369). The final R1 value was 0.0435 [I 2r (I)] and wR2 value was 0.1068 (all data). The absolute configuration was determined on the basis of the Flack parameter [x = 0.02(2)], refined using 2637 Friedel pairs. Crystallographic data for the structure of 1 have been deposited with the Cambridge Crystallographic Data Centre (deposition no. CCDC 1878436). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. [8] A.A.Q. Al-Khdhairawi, P. Krishnan, C.W. Mai, F.F.L. Chung, C.O. Leong, K.T. Yong, K.W. Chong, Y.Y. Low, T.S. Kam, K.H. Lim, J. Nat. Prod. 80 (2017) 2734–2740.