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Total synthesis of natural spiro-trisindole enantiomers similisines A, B and their stereoisomers
Tetrahedron Letters 58 (2017) 1934–1938
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Total synthesis of natural spiro-trisindole enantiomers similisines A, B and their stereoisomers Lunyong Shi a, Lingyu Li b, Jun Wang a, Bin Huang c, Kewu Zeng a, Hongwei Jin c, Qingying Zhang a,⇑, Yanxing Jia c,⇑ a State Key Laboratory of Natural and Biomimetic Drugs and Department of Natural Medicines, School of Pharmaceutical Sciences, Peking University Health Science Center, 38 Xueyuan Road, Beijing 100191, China b Institute of Medicinal Plant Development, Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing 100193, China c State Key Laboratory of Natural and Biomimetic Drugs, Peking University Health Science Center, 38 Xueyuan Road, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 14 February 2017 Revised 27 March 2017 Accepted 31 March 2017 Available online 2 April 2017 Keywords: Similisine Spiro-trisindole Total synthesis
a b s t r a c t The first total synthesis of similisines A and B, a pair of unprecedented polybrominated spiro-trisindole enantiomers isolated from Laurencia similis, and their all stereoisomers had been accomplished in 6 steps from the known 5,6-dibromoindole. The described synthesis avoided any protecting-group manipulations, and the key all-carbon spirocenters were constructed via an intramolecular Friedel-Crafts cyclization. In addition, the rotamers of similisines and cytotoxic and NO production inhibitory activities of synthetic compounds were also discussed. Ó 2017 Elsevier Ltd. All rights reserved.
Similisines A [(+)-1a] and B [()-1a] (Fig. 1), isolated by us from Laurencia similis in 2013, are the first example of polybrominated spiro-trisindole alkaloids fused through a five-member ring. Indeed, they exist as a racemic mixture in nature and are obtained in optically pure forms after separation by chiral HPLC column.1 Although simple indoles and bisindoles have been widely found in nature, trisindoles are rather scarce2 and similisines A and B belong to a new member of trisindole alkaloids that feature a unique polybrominated [5,5] spirooxindole moiety fused through a five-membered ring. Due to the structural novelty and varied and significant biological activities of the compounds with spirooxindole skeleton, spirooxindoles have been drawing widespread attentions from the chemists and biologists.3 However, biological studies on similisines A and B are limited because of the low amounts available naturally. Motivated by the unique chemical structures and potential bioactivities of similisines, we initiated the total synthesis of similisines A and B, anticipating to lay groundwork for spirooxindole synthesis and further bioactivities evaluation. Furthermore, all stereoisomers of similisines A and B were simultaneously synthesized in consideration of the possible differences in bioactivities. Herein we reported the total synthesis of similisines A, B and their stereoisomers. Moreover, the rotamers
⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Y. Jia). http://dx.doi.org/10.1016/j.tetlet.2017.03.086 0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
Br
Br H N
Br Br Br Br
Br
4″
NH
Br H N
HN
3 4′ 4
1
2
O Me
Br N H similisine A [(+)-1a]
O Me
Br Br Br
Br N Br H similisine B [(-)-1a]
Fig. 1. Structures of similisines A and B.
of similisines and cytotoxic and NO production inhibitory activities of synthetic compounds were also discussed. Our retrosynthetic analysis of similisines A, B and their stereoisomers (1) was illustrated in Scheme 1. We envisioned that similisines A, B and their stereoisomers could be generated from spirooxindole 2 via direct and regioselective C-2 bromination. The five-membered spirocyclic skeleton of 2 could be constructed from trisindole 3 by intramolecular Friedel-Crafts cyclization. In turn, trisindole 3 could be obtained by coupling the aldehyde 4 with two equivalents of the known 5,6-dibromoindole 5, which could be readily prepared from commercially available methyl indole-3-carboxylate.4 The aldehyde 4 could be prepared by aldol reaction of 5,6-dibromoisatin 6 and propionaldehyde. Isatin 6 could be generated by oxidation of the known indole 5.
1935
L. Shi et al. / Tetrahedron Letters 58 (2017) 1934–1938 Br
Br H N
Br
C-2 bromination
Br N H 1 similisines A, B and their stereisomers O
N H 2
Br
Intramolecular Friedel-Crafts reaction
Br
[O]
Br
Br
O
N H
Br
N H 6
+ Me
O Me
Br
Br
Br
NH
Br
O Me
Br
Br
H N
Br NH
Br
Br
Br
CHO
5
Br
+
Br
HO
Me
Br CHO
Br
O
O
N H 3
N H 4
Br
NH
HO
Me Br
H N
Scheme 1. Retrosynthetic analysis of similisines A, B and their stereoisomers.
NBS t-BuOH, rt
Br N H
Br
Br Br
Br
O N H
Br
5
Br O N H
97% (2 steps) Br 6
5a
Br
Br
Br
85% (dr = 1:1)
Br
HO
H N
Br
Me CH3 CH2 CHO Et3N, EtOH, 0 °C
Our total synthesis of similisines A, B and their stereoisomers commenced with the preparation of trisindole 3 as illustrated in Scheme 2. Oxidation of 5,6-dibromoindole 5 with NBS in t-BuOH and subsequent hydrolysis of the corresponding 3,3,5,6-tetrabromooxindole 5a in aqueous MeOH afforded the desired 5,6-dibromoisatin 6 in 97% yield.5 Aldol reaction of 6 with propionaldehyde in the presence of Et3N provided the desired 3hydroxy-aldehyde 4 in 85% yield with a diastereomeric ratio of 1 to 1, which could not be separated via column chromatography.6 Condensation of two equivalents of indole 5 with one equivalent of aldehyde 4 in the presence of a catalytic amount of I2 in acetonitrile afforded the desired trisindole 3 in 78% yield with a diastereomeric ratio of 1 to 1, which should be carefully separated via column chromatography.7 With trisindole 3 in hand, the construction of key spirooxindole through acid-promoted intramolecular Friedel-Crafts cyclization
O
MeOH/H 2O reflux
CHO
Br
O N H
NH
HO
5, I2, MeCN, rt
Me
Br
78%
4
O
N H
Br
3
Scheme 2. Synthesis of trisindole 3.
Br
Br Br
HCl, MeOH, MW, 100 °C
H N
Br NH
Br
+
O Me
Br Br
Br
8%
3
Br
Br
H N
Br + O Me
Br N H (± )-2c 7%
H N
PyHBr3
H N
Br
7 10%
O Me
Br
Br O
Br NH
HN
Br
Br Br
(± )-2d
7
16%
10%
Br
81% Br
Br Br NH
PyHBr3 Br
N H (± )-1b
H N
O Me
Br N H
Br
85%
HN
O Me
Br
(± )-1a
NH
NH
Br
Br
N H
Br
Br
H N
Br NH
Br
Br
Br
Br
Br
Br Br
Br
O Me
Br
Br NH
HN
+
Br
Br
O
Br
N H (± )-2b
Br
NH
Br
Br
O Me
8%
Br p-TsOH DCE, reflux
+
Br
Br
HN NH
Br
Br
Br
H N
Br
N H (± )-2a
Br
Br
H N
Br
N H (± )-1c 85%
Scheme 3. Completing the total synthesis of similisines A, B and their stereoisomers.
NH O Me
Br
Br
Br Br
Br
Br
N H (± )-1d 83%
1936
L. Shi et al. / Tetrahedron Letters 58 (2017) 1934–1938
Fig. 2. Absolute configuration determination by comparison of the experimental and calculated ECD curves.
was then investigated. We found that the transformation of 3 to 2 was surprisingly challenging (Scheme 3). A variety of Lewis acids (AlCl3, InCl3, FeCl3, I2)8 and Brønsted acids (H2SO4, H3PO4, TFA, TfOH, TMSOTf)9 were screened. However, no desired product was obtained and the starting material was decomposed as judged by TLC and 1H NMR spectroscopy. To our delight, treatment of 3 with HCl in MeOH under microwave irradiation (50 W, 100 °C, 5 min) provided a workable condition, furnishing two racemic isomers (±)-2a (8% yield) and (±)-2b (8% yield), accompanied with the undesired trisindole 7 in 10% yield. However, the other two desired racemic isomers (±)-2c and (±)-2d were observed only in a trace amount. The formation of 7 most likely proceeded through retro-aldol and retro-Michael condensation under acid conditions giving 5,6-dibromoindole 5 and 5,6-dibromoisatin 6, followed by direct nucleophilic addition of indole 5 to isatin 6 in the presence of acid. This result was further verified by direct reaction of indole 5 with isatin 6 under the same reaction conditions. More interestingly, when trisindole 3 was treated with p-TsOH in DCE under reflux, racemic isomers (±)-2c and (±)-2d were produced in 7% and 16% yield, respectively, accompanied with trace amounts of racemic (±)-2a and (±)-2b. In this case, the undesired trisindole 7 was also obtained in 10% yield. Thus, we did obtain all the racemic steroisomers (±)-2a–(±)-2d through these two different conditions. These results also indicated that trisindole 3 was sensitive to acid conditions and it was rather difficult to access the all-carbon spirocenter. With the key (±)-2a–(±)-2d in hand, the synthesis of similisines A, B and their stereoisomers were achieved as shown in Scheme 3. Initial attempts to 2-bromination of 2 with NBS in CH2Cl210 or CCl411 were unsuccessful and led to the decomposition of the starting material. Fortunately, bromination of (±)-2a–(±)-2d with pyridinium tribromide in THF and CHCl3 (1:1)12 occurred smoothly to
Br
Br
Br
4″
HH N
NH
Br Br Br Br
H
4
Br
O Me Br NH
(±) -1b
HH N
Br Br Br
4″
Br
NH
HH N Br
O Me Br NH
NH
Me
1
H
Br
Br
4″
H
Br Br
2 O
4
Br NH
Br
(±) -1c
(±) -1d
Fig. 3. The key NOE correlations of 1b–1d.
give the corresponding desired racemic compounds of (±)-1a (85%), (±)-1b (81%), (±)-1c (85%), and (±)-1d (83%), respectively, and (±)1a was elucidated to be the racemic mixture of natural similisines A and B by the identical spectral data of 1D and 2D NMR and MS.1 Chiral resolution of (±)-1a–(±)-1d by HPLC on a chiral column afforded the anticipated enantiomers, respectively, evidenced by the exactly opposite Cotton effects in their measured CD spectra, and (+)-1a and ()-1a were determined to be similisines A and B respectively by the CD spectra (Fig. 2). The relative configuration of (+)-1b–(+)-1d and ()-1b–()-1d were elucidated by detailed interpretation of the key NOESY correlations. The same as 1a, H1 and H-400 of 1c were definitely assigned to be on the same side of the five-membered ring based on the key NOE correlation of H-1/H-400 . In addition, the 2-CH3 and H-400 were deduced to be on the different side due to the absence of NOE correlation of 2CH3/H-400 . Thus 1c was determined to be the 2-epi-isomer of natural similisines A [(+)-1a] and B [()-1a], and the relative configuration of 1c should be characterized as 1S, 2R, 3S or 1R, 2S, 3R. For 1b and 1d, the NOE correlation of H-4/H-400 and the absence of NOE correlation of H-1/H-400 suggested that H-4 and H-40 0 were on the
L. Shi et al. / Tetrahedron Letters 58 (2017) 1934–1938
1937
Fig. 4. The partial 1H NMR (400 MHz, DMSO-d6) spectra of 1a recorded at different temperature.
same side of the five-membered ring. Furthermore, the absence of the key NOE correlation of 2-CH3/H-400 in 1b deduced the 1R, 2R, 3S or 1S, 2S, 3R configurations for 1b, whereas the occurrence of the key NOE correlation of 2-CH3/H-400 in 1d determined 1R, 2S, 3S or 1S, 2R, 3R configurations for 1d (Fig. 3). Then the absolute configurations of (+)-1b–(+)-1d and (–)-1b– (-)-1d were confirmed by comparison of the experimental and theoretically calculated electronic circular dichroism (ECD) curves of the two possible stereoisomers by using density functional theory B3LYP method of Gaussian 09 program. The experimental CD spectrum of (+)-1b and (+)-1d matched well with the ECD spectrum of isomers 1R, 2R, 3S and 1R, 2S, 3S, respectively, and thus deduced the absolute configuration of (+)-1b and ()-1b to be 1R, 2R, 3S and 1S, 2S, 3R, and (+)-1d and ()-1d to be 1R, 2S, 3S and 1S, 2R, 3R, respectively. The experimental CD spectrum of ()-1c matched well with the calculated ECD spectrum of 1S, 2R, 3S-isomer, and thus the absolute configuration of ()-1c and (+)-1c were characterized as 1S, 2R, 3S and 1R, 2S, 3R, respectively (Fig. 2). Furthermore, it was worth noting that a set of special smaller signals, initially overlooked in the 1H NMR spectra of the natural similisines A and B, existed in the 1H NMR spectra of (+)-1a and ()-1a and could not be removed by careful chiral separation. By further analyzing the structural conformation, we speculated that these small signals might be attributed to the rotational isomers (rotamer A and rotamer B) existing due to the steric hindrance of indole moiety at C-1 position. To confirm the speculation, the 1H NMR spectra of 1a at different temperatures were recorded and the smaller signals did decrease with the increasing of temperature (Fig. 4). Moreover, rotamer A was suggested to be the major conformer because of the found key NOE correlation between H-1 and H-4. The same phenomenon could also be found in the 1H NMR spectra of (+)-1b and ()-1b, which were the C-3 epimers of similisines A [(+)-1a] and B [()-1a]. The target compounds (+)-1a–(+)-1d and ()-1a–(-)-1d, as well as the other synthetic compounds, were tested for their cytotoxicity against four human cancer cell lines (Hep G2, A549, HCT116, and MCF-7) and inhibitory activity on NO production stimulated by lipopolysaccharide in BV-2 microglial cells. No compounds showed potent toxicity against all tested cancer cell lines at the concentration of 10 lM. Compound 3 exhibited the most significant inhibition on NO production with IC50 value of 6.88 ± 0.19 lM, while other tested compounds showed weak inhi-
bition on NO production at the concentration of 10 lM. Further bioactivity evaluation was in the progress. In summary, we achieved the first total synthesis of similisines A, B and their all stereoisomers in only 6 steps from the known 5,6dibromoindole 5 without using any protecting group. Evaluation of cytotoxic and NO production inhibitory activities revealed that compound 3 exhibited the most significant inhibition on NO production. The present synthesis provided a solid foundation for further chemical and bioactive approaches of similisine derivatives in the future. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (Nos. 21172008 and 21372015). A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2017.03. 086. References 1. Sun WS, Su S, Zhu RX, et al. Tetrahedron Lett. 2013;54:3617–3620. 2. (a) Shiri M, Zolfigol MA, Kruger HG, Tanbakouchian Z. Chem Rev. 2010;110:2250–2293; (b) Veale CGL, Davies-Coleman MT. In: Knölker HJ, ed. The Alkaloids, vol. 73. San Diego, CA: Academic Press; 2014:1–64; . For selected recent examples on total synthesis of bisindole alkaloids, see: (c) Blair LM, Sperry J. Chem Commun. 2016;52:800–802; (d) Boyd EM, Sperry J. Org Lett. 2015;17:1344–1346; (e) Zhang F, Wang B, Prasad P, Capon RJ, Jia Y. Org Lett. 2015;17:1529–1532; (f) Tian MQ, Yan M, Baran PS. J Am Chem Soc. 2016;138:14234–14237. 3. For selected recent reviews, see: (a) Singh GS, Desta ZY. Chem Rev. 2012;112:6104–6155; (b) Ball-Jones NR, Badillo JJ, Franz AK. Org Biomol Chem. 2012;10:5165–5181; (c) Cheng D, Ishihara Y, Tan B, Barbas III CF. ACS Catal. 2014;4:743–762. 4. Parsons TB, Ghellamallah C, Male L, Spencer N, Grainger RS. Org Biomol Chem. 2011;9:5021–5023. 5. (a) Parrick J, Yahya A, Ijaz AS, Jin Y. J Chem Soc Perkin Trans 1. 1989;2009–2015; (b) Parrick J, Yahya A, Jin Y. Tetrahedron Lett. 1984;25:3099–3100. 6. (a) Xue F, Zhang S, Liu L, Duan W, Wang W. Chem Asian J. 2009;4:1664–1667; (b) Guo Q, Zhao JC-G. Tetrahedron Lett. 2012;53:1768–1771. 7. (a) Bandgar BP, Shaikh KA. Tetrahedron Lett. 2003;44:1959–1961; (b) Ji SJ, Wang SY, Zhang Y, Loh TP. Tetrahedron. 2004;60:2051–2055.
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8. (a) Dorbec M, Florent JC, Monneret C, Rager MN, Fosse C, Bertounesque E. Eur J Org Chem. 2008;1723–1731; (b) Kinthada LK, Ghosh S, Babu KN, Sharique M, Biswas S, Bisai A. Org Biomol Chem. 2014;12:8152–8173; (c) Yang Q, Wang L, Guo T, Yu Z. J Org Chem. 2012;77:8355–8361. 9. (a) Fukuyama T, Chen X. J Am Chem Soc. 1994;116:3125–3126; (b) Baran PS, Richter JM. J Am Chem Soc. 2004;126:7450–7451;
(c) Nicolaou KC, Chen DYK, Huang X, Ling T, Bella M, Snyder SA. J Am Chem Soc. 2004;126:12888–12896. 10. Dion I, Vincent-Rocan JF, Zhang L, et al. J Org Chem. 2013;78:12735–12749. 11. de Koning CB, Michael JP, Pathak R, van Otterlo WAL. Tetrahedron Lett. 2004;45:1117–1119. 12. (a) Luo S, Zificsak CA, Hsung RP. Org Lett. 2003;5:4709–4712; (b) Feng P, Fan Y, Xue F, Liu W, Li S, Shi Y. Org Lett. 2011;13:5827–5829.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Total synthesis of natural spiro-trisindole enantiomers similisines A, B and their stereoisomers Lunyong Shi a, Lingyu Li b, Jun Wang a, Bin Huang c, Kewu Zeng a, Hongwei Jin c, Qingying Zhang a,⇑, Yanxing Jia c,⇑ a State Key Laboratory of Natural and Biomimetic Drugs and Department of Natural Medicines, School of Pharmaceutical Sciences, Peking University Health Science Center, 38 Xueyuan Road, Beijing 100191, China b Institute of Medicinal Plant Development, Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing 100193, China c State Key Laboratory of Natural and Biomimetic Drugs, Peking University Health Science Center, 38 Xueyuan Road, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 14 February 2017 Revised 27 March 2017 Accepted 31 March 2017 Available online 2 April 2017 Keywords: Similisine Spiro-trisindole Total synthesis
a b s t r a c t The first total synthesis of similisines A and B, a pair of unprecedented polybrominated spiro-trisindole enantiomers isolated from Laurencia similis, and their all stereoisomers had been accomplished in 6 steps from the known 5,6-dibromoindole. The described synthesis avoided any protecting-group manipulations, and the key all-carbon spirocenters were constructed via an intramolecular Friedel-Crafts cyclization. In addition, the rotamers of similisines and cytotoxic and NO production inhibitory activities of synthetic compounds were also discussed. Ó 2017 Elsevier Ltd. All rights reserved.
Similisines A [(+)-1a] and B [()-1a] (Fig. 1), isolated by us from Laurencia similis in 2013, are the first example of polybrominated spiro-trisindole alkaloids fused through a five-member ring. Indeed, they exist as a racemic mixture in nature and are obtained in optically pure forms after separation by chiral HPLC column.1 Although simple indoles and bisindoles have been widely found in nature, trisindoles are rather scarce2 and similisines A and B belong to a new member of trisindole alkaloids that feature a unique polybrominated [5,5] spirooxindole moiety fused through a five-membered ring. Due to the structural novelty and varied and significant biological activities of the compounds with spirooxindole skeleton, spirooxindoles have been drawing widespread attentions from the chemists and biologists.3 However, biological studies on similisines A and B are limited because of the low amounts available naturally. Motivated by the unique chemical structures and potential bioactivities of similisines, we initiated the total synthesis of similisines A and B, anticipating to lay groundwork for spirooxindole synthesis and further bioactivities evaluation. Furthermore, all stereoisomers of similisines A and B were simultaneously synthesized in consideration of the possible differences in bioactivities. Herein we reported the total synthesis of similisines A, B and their stereoisomers. Moreover, the rotamers
⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Y. Jia). http://dx.doi.org/10.1016/j.tetlet.2017.03.086 0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
Br
Br H N
Br Br Br Br
Br
4″
NH
Br H N
HN
3 4′ 4
1
2
O Me
Br N H similisine A [(+)-1a]
O Me
Br Br Br
Br N Br H similisine B [(-)-1a]
Fig. 1. Structures of similisines A and B.
of similisines and cytotoxic and NO production inhibitory activities of synthetic compounds were also discussed. Our retrosynthetic analysis of similisines A, B and their stereoisomers (1) was illustrated in Scheme 1. We envisioned that similisines A, B and their stereoisomers could be generated from spirooxindole 2 via direct and regioselective C-2 bromination. The five-membered spirocyclic skeleton of 2 could be constructed from trisindole 3 by intramolecular Friedel-Crafts cyclization. In turn, trisindole 3 could be obtained by coupling the aldehyde 4 with two equivalents of the known 5,6-dibromoindole 5, which could be readily prepared from commercially available methyl indole-3-carboxylate.4 The aldehyde 4 could be prepared by aldol reaction of 5,6-dibromoisatin 6 and propionaldehyde. Isatin 6 could be generated by oxidation of the known indole 5.
1935
L. Shi et al. / Tetrahedron Letters 58 (2017) 1934–1938 Br
Br H N
Br
C-2 bromination
Br N H 1 similisines A, B and their stereisomers O
N H 2
Br
Intramolecular Friedel-Crafts reaction
Br
[O]
Br
Br
O
N H
Br
N H 6
+ Me
O Me
Br
Br
Br
NH
Br
O Me
Br
Br
H N
Br NH
Br
Br
Br
CHO
5
Br
+
Br
HO
Me
Br CHO
Br
O
O
N H 3
N H 4
Br
NH
HO
Me Br
H N
Scheme 1. Retrosynthetic analysis of similisines A, B and their stereoisomers.
NBS t-BuOH, rt
Br N H
Br
Br Br
Br
O N H
Br
5
Br O N H
97% (2 steps) Br 6
5a
Br
Br
Br
85% (dr = 1:1)
Br
HO
H N
Br
Me CH3 CH2 CHO Et3N, EtOH, 0 °C
Our total synthesis of similisines A, B and their stereoisomers commenced with the preparation of trisindole 3 as illustrated in Scheme 2. Oxidation of 5,6-dibromoindole 5 with NBS in t-BuOH and subsequent hydrolysis of the corresponding 3,3,5,6-tetrabromooxindole 5a in aqueous MeOH afforded the desired 5,6-dibromoisatin 6 in 97% yield.5 Aldol reaction of 6 with propionaldehyde in the presence of Et3N provided the desired 3hydroxy-aldehyde 4 in 85% yield with a diastereomeric ratio of 1 to 1, which could not be separated via column chromatography.6 Condensation of two equivalents of indole 5 with one equivalent of aldehyde 4 in the presence of a catalytic amount of I2 in acetonitrile afforded the desired trisindole 3 in 78% yield with a diastereomeric ratio of 1 to 1, which should be carefully separated via column chromatography.7 With trisindole 3 in hand, the construction of key spirooxindole through acid-promoted intramolecular Friedel-Crafts cyclization
O
MeOH/H 2O reflux
CHO
Br
O N H
NH
HO
5, I2, MeCN, rt
Me
Br
78%
4
O
N H
Br
3
Scheme 2. Synthesis of trisindole 3.
Br
Br Br
HCl, MeOH, MW, 100 °C
H N
Br NH
Br
+
O Me
Br Br
Br
8%
3
Br
Br
H N
Br + O Me
Br N H (± )-2c 7%
H N
PyHBr3
H N
Br
7 10%
O Me
Br
Br O
Br NH
HN
Br
Br Br
(± )-2d
7
16%
10%
Br
81% Br
Br Br NH
PyHBr3 Br
N H (± )-1b
H N
O Me
Br N H
Br
85%
HN
O Me
Br
(± )-1a
NH
NH
Br
Br
N H
Br
Br
H N
Br NH
Br
Br
Br
Br
Br
Br Br
Br
O Me
Br
Br NH
HN
+
Br
Br
O
Br
N H (± )-2b
Br
NH
Br
Br
O Me
8%
Br p-TsOH DCE, reflux
+
Br
Br
HN NH
Br
Br
Br
H N
Br
N H (± )-2a
Br
Br
H N
Br
N H (± )-1c 85%
Scheme 3. Completing the total synthesis of similisines A, B and their stereoisomers.
NH O Me
Br
Br
Br Br
Br
Br
N H (± )-1d 83%
1936
L. Shi et al. / Tetrahedron Letters 58 (2017) 1934–1938
Fig. 2. Absolute configuration determination by comparison of the experimental and calculated ECD curves.
was then investigated. We found that the transformation of 3 to 2 was surprisingly challenging (Scheme 3). A variety of Lewis acids (AlCl3, InCl3, FeCl3, I2)8 and Brønsted acids (H2SO4, H3PO4, TFA, TfOH, TMSOTf)9 were screened. However, no desired product was obtained and the starting material was decomposed as judged by TLC and 1H NMR spectroscopy. To our delight, treatment of 3 with HCl in MeOH under microwave irradiation (50 W, 100 °C, 5 min) provided a workable condition, furnishing two racemic isomers (±)-2a (8% yield) and (±)-2b (8% yield), accompanied with the undesired trisindole 7 in 10% yield. However, the other two desired racemic isomers (±)-2c and (±)-2d were observed only in a trace amount. The formation of 7 most likely proceeded through retro-aldol and retro-Michael condensation under acid conditions giving 5,6-dibromoindole 5 and 5,6-dibromoisatin 6, followed by direct nucleophilic addition of indole 5 to isatin 6 in the presence of acid. This result was further verified by direct reaction of indole 5 with isatin 6 under the same reaction conditions. More interestingly, when trisindole 3 was treated with p-TsOH in DCE under reflux, racemic isomers (±)-2c and (±)-2d were produced in 7% and 16% yield, respectively, accompanied with trace amounts of racemic (±)-2a and (±)-2b. In this case, the undesired trisindole 7 was also obtained in 10% yield. Thus, we did obtain all the racemic steroisomers (±)-2a–(±)-2d through these two different conditions. These results also indicated that trisindole 3 was sensitive to acid conditions and it was rather difficult to access the all-carbon spirocenter. With the key (±)-2a–(±)-2d in hand, the synthesis of similisines A, B and their stereoisomers were achieved as shown in Scheme 3. Initial attempts to 2-bromination of 2 with NBS in CH2Cl210 or CCl411 were unsuccessful and led to the decomposition of the starting material. Fortunately, bromination of (±)-2a–(±)-2d with pyridinium tribromide in THF and CHCl3 (1:1)12 occurred smoothly to
Br
Br
Br
4″
HH N
NH
Br Br Br Br
H
4
Br
O Me Br NH
(±) -1b
HH N
Br Br Br
4″
Br
NH
HH N Br
O Me Br NH
NH
Me
1
H
Br
Br
4″
H
Br Br
2 O
4
Br NH
Br
(±) -1c
(±) -1d
Fig. 3. The key NOE correlations of 1b–1d.
give the corresponding desired racemic compounds of (±)-1a (85%), (±)-1b (81%), (±)-1c (85%), and (±)-1d (83%), respectively, and (±)1a was elucidated to be the racemic mixture of natural similisines A and B by the identical spectral data of 1D and 2D NMR and MS.1 Chiral resolution of (±)-1a–(±)-1d by HPLC on a chiral column afforded the anticipated enantiomers, respectively, evidenced by the exactly opposite Cotton effects in their measured CD spectra, and (+)-1a and ()-1a were determined to be similisines A and B respectively by the CD spectra (Fig. 2). The relative configuration of (+)-1b–(+)-1d and ()-1b–()-1d were elucidated by detailed interpretation of the key NOESY correlations. The same as 1a, H1 and H-400 of 1c were definitely assigned to be on the same side of the five-membered ring based on the key NOE correlation of H-1/H-400 . In addition, the 2-CH3 and H-400 were deduced to be on the different side due to the absence of NOE correlation of 2CH3/H-400 . Thus 1c was determined to be the 2-epi-isomer of natural similisines A [(+)-1a] and B [()-1a], and the relative configuration of 1c should be characterized as 1S, 2R, 3S or 1R, 2S, 3R. For 1b and 1d, the NOE correlation of H-4/H-400 and the absence of NOE correlation of H-1/H-400 suggested that H-4 and H-40 0 were on the
L. Shi et al. / Tetrahedron Letters 58 (2017) 1934–1938
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Fig. 4. The partial 1H NMR (400 MHz, DMSO-d6) spectra of 1a recorded at different temperature.
same side of the five-membered ring. Furthermore, the absence of the key NOE correlation of 2-CH3/H-400 in 1b deduced the 1R, 2R, 3S or 1S, 2S, 3R configurations for 1b, whereas the occurrence of the key NOE correlation of 2-CH3/H-400 in 1d determined 1R, 2S, 3S or 1S, 2R, 3R configurations for 1d (Fig. 3). Then the absolute configurations of (+)-1b–(+)-1d and (–)-1b– (-)-1d were confirmed by comparison of the experimental and theoretically calculated electronic circular dichroism (ECD) curves of the two possible stereoisomers by using density functional theory B3LYP method of Gaussian 09 program. The experimental CD spectrum of (+)-1b and (+)-1d matched well with the ECD spectrum of isomers 1R, 2R, 3S and 1R, 2S, 3S, respectively, and thus deduced the absolute configuration of (+)-1b and ()-1b to be 1R, 2R, 3S and 1S, 2S, 3R, and (+)-1d and ()-1d to be 1R, 2S, 3S and 1S, 2R, 3R, respectively. The experimental CD spectrum of ()-1c matched well with the calculated ECD spectrum of 1S, 2R, 3S-isomer, and thus the absolute configuration of ()-1c and (+)-1c were characterized as 1S, 2R, 3S and 1R, 2S, 3R, respectively (Fig. 2). Furthermore, it was worth noting that a set of special smaller signals, initially overlooked in the 1H NMR spectra of the natural similisines A and B, existed in the 1H NMR spectra of (+)-1a and ()-1a and could not be removed by careful chiral separation. By further analyzing the structural conformation, we speculated that these small signals might be attributed to the rotational isomers (rotamer A and rotamer B) existing due to the steric hindrance of indole moiety at C-1 position. To confirm the speculation, the 1H NMR spectra of 1a at different temperatures were recorded and the smaller signals did decrease with the increasing of temperature (Fig. 4). Moreover, rotamer A was suggested to be the major conformer because of the found key NOE correlation between H-1 and H-4. The same phenomenon could also be found in the 1H NMR spectra of (+)-1b and ()-1b, which were the C-3 epimers of similisines A [(+)-1a] and B [()-1a]. The target compounds (+)-1a–(+)-1d and ()-1a–(-)-1d, as well as the other synthetic compounds, were tested for their cytotoxicity against four human cancer cell lines (Hep G2, A549, HCT116, and MCF-7) and inhibitory activity on NO production stimulated by lipopolysaccharide in BV-2 microglial cells. No compounds showed potent toxicity against all tested cancer cell lines at the concentration of 10 lM. Compound 3 exhibited the most significant inhibition on NO production with IC50 value of 6.88 ± 0.19 lM, while other tested compounds showed weak inhi-
bition on NO production at the concentration of 10 lM. Further bioactivity evaluation was in the progress. In summary, we achieved the first total synthesis of similisines A, B and their all stereoisomers in only 6 steps from the known 5,6dibromoindole 5 without using any protecting group. Evaluation of cytotoxic and NO production inhibitory activities revealed that compound 3 exhibited the most significant inhibition on NO production. The present synthesis provided a solid foundation for further chemical and bioactive approaches of similisine derivatives in the future. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (Nos. 21172008 and 21372015). A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2017.03. 086. References 1. Sun WS, Su S, Zhu RX, et al. Tetrahedron Lett. 2013;54:3617–3620. 2. (a) Shiri M, Zolfigol MA, Kruger HG, Tanbakouchian Z. Chem Rev. 2010;110:2250–2293; (b) Veale CGL, Davies-Coleman MT. In: Knölker HJ, ed. The Alkaloids, vol. 73. San Diego, CA: Academic Press; 2014:1–64; . For selected recent examples on total synthesis of bisindole alkaloids, see: (c) Blair LM, Sperry J. Chem Commun. 2016;52:800–802; (d) Boyd EM, Sperry J. Org Lett. 2015;17:1344–1346; (e) Zhang F, Wang B, Prasad P, Capon RJ, Jia Y. Org Lett. 2015;17:1529–1532; (f) Tian MQ, Yan M, Baran PS. J Am Chem Soc. 2016;138:14234–14237. 3. For selected recent reviews, see: (a) Singh GS, Desta ZY. Chem Rev. 2012;112:6104–6155; (b) Ball-Jones NR, Badillo JJ, Franz AK. Org Biomol Chem. 2012;10:5165–5181; (c) Cheng D, Ishihara Y, Tan B, Barbas III CF. ACS Catal. 2014;4:743–762. 4. Parsons TB, Ghellamallah C, Male L, Spencer N, Grainger RS. Org Biomol Chem. 2011;9:5021–5023. 5. (a) Parrick J, Yahya A, Ijaz AS, Jin Y. J Chem Soc Perkin Trans 1. 1989;2009–2015; (b) Parrick J, Yahya A, Jin Y. Tetrahedron Lett. 1984;25:3099–3100. 6. (a) Xue F, Zhang S, Liu L, Duan W, Wang W. Chem Asian J. 2009;4:1664–1667; (b) Guo Q, Zhao JC-G. Tetrahedron Lett. 2012;53:1768–1771. 7. (a) Bandgar BP, Shaikh KA. Tetrahedron Lett. 2003;44:1959–1961; (b) Ji SJ, Wang SY, Zhang Y, Loh TP. Tetrahedron. 2004;60:2051–2055.
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