Letter Cite This: Org. Lett. 2018, 20, 4439−4443
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Dearomatization of Indole via Intramolecular [3 + 2] Cycloaddition: Access to the Pentacyclic Skeleton of Strychons Alkaloids Zhengshen Wang,∥,† Luxin Chen,∥,† Yuan Yao,† Zhigang Liu,† Jin-Ming Gao,† Xuegong She,‡ and Huaiji Zheng*,†,§ †
Org. Lett. 2018.20:4439-4443. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 08/04/18. For personal use only.
Shaanxi Key Laboratory of Natural Products and Chemical Biology, College of Chemistry and Pharmacy, Northwest A&F University, 3 Taicheng Road, Yangling 712100, China ‡ State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222 South Tianshui Road, Lanzhou 730000, China § Key Laboratory of Botanical Pesticide R&D in Shaanxi Province, Yangling 712100, China S Supporting Information *
ABSTRACT: An efficient method to build various multisubstituted polycyclic indoline-annulated normal to mediumsize rings through dearomatization of indole via a tandem 1,2acyloxy migration/intramolecular [3 + 2] cycloaddition process is described. The pentacyclic skeleton of strychnine could be synthesized via this tandem cycloaddition and a further Mannich reaction. This approach would provide a novel strategy to the synthesis of strychons alkaloids. Scheme 1. [3 + 2] Cycloaddition of Indole N-Tethered Propargylic Carboxylate and Related Natural Indolines
D
earomatization of indoles represents an attractive strategy providing biologically important indolenine or indoline alkaloids.1 In addition to methods using alkylations, arylations, and oxidations toward the functionalization of indoles, cycloadditions as powerful regio- and stereoselective controlled reactions have made significant progress in recent years. As an electron-rich subunit, the indolic C2−C3 double bond functions as a dipolarophile in a cycloaddition reaction with 1,3-dipoles, which are mainly derived from threemembered ring cycle openings2 or a carbonyl ylide.3 Recently, we have demonstrated a new type of 1,3-dipole in π-acid catalyzed isomerization of propargylic carboxylates.4,5 Lewis acidic complexes,6−9 such as gold, platinum, or gallium, are effective catalysts for the selective activation of π-bonds of alkynes10 and play an important role in modern catalysis for the construction of carbon−carbon or carbon−heteroatom bonds.11 We envisaged that indole10d,12 might serve as a πdonor to trap the metallic 1,3-dipole via a dearomatization process (Scheme 1, A → C). Further, the cyclized product C could be transformed into intermediate D via a Mannich type reaction13 for the synthesis of strychnos alkaloids such as rosibiline,14 strychnobaillonine,15 or caracurine V.16 This approach would provide a novel strategy to the synthesis of strychnine.17 Due to the biological and pharmaceutical importance of indolines,1f,18 in particular, indoline-annulated medium-size ring cycles, we wish to establish an intramolecular [3 + 2] cycloaddition reaction between indole and propargylic carboxylate. Initially, indole derivative 1a was selected to investigate the [3 + 2] cycloaddition. PtCl2 could successfully catalyze the reaction, and to our surprise, the cyclized product 2a containing an acid-sensitive group of the enol ketal was © 2018 American Chemical Society
isolated in high yield (86%, entry 1, Table 1). Compared to the hydrolysis product, the product of the enol ketal is more likely derived from the mechanism of the [3 + 2] cycloaddition. The ligand of PPh3 or 1,5-cyclooctadiene (COD) is detrimental for the reaction, and the catalyst of Au(I) gave a mixture of hydrolysis products in low yield. As shown in Table 2, the [3 + 2] cycloaddition process can be extended to various indoles, thus giving the corresponding cyclized products in moderate to high yields. Substituents (MeO, Me, and Br) on the aryl ring showed similar electronic effects on the reaction (entries 1−3). The cyclized product 2d with a methoxyl group on the side chain was stereospecifically synthesized via this transformation. Indoles with sterically Received: June 1, 2018 Published: July 17, 2018 4439
DOI: 10.1021/acs.orglett.8b01720 Org. Lett. 2018, 20, 4439−4443
Letter
Organic Letters Table 1. Studies on the Reaction Conditionsa
indolines 2g−2j were potential candidates for the syntheses of pharmaceutical tryptamine alkaloids. Compared to six-membered rings, the synthesis of mediumsize rings is often challenging.19,20 Further studies showed that the reaction scope can be expanded to the synthesis of medium-size rings (Figures 1 and 2, Scheme 2). Seven-
entry
catalyst
solvent
t/°C
time/h
yield/%b
1 2 3 4
PtCl2 cis-PtCl2(PPh3)2 PtCl2(COD) Au(IPr)Cl/AgNTf2
toluene toluene toluene CH2Cl2
40 40 40 rt
1 12 12 2
86 NR trace 63c
a
1a (0.25 mmol), catalyst (5 mol % for entry 1, 10 mol % for entries 2−4), and solvent (2.5 mL) under indicated reaction conditions. b Isolated yield. cA mixture of hydrolysis products.
Figure 1. Seven-membered ring fused indolines synthesis.
hindered substituents on the C3 position (entries 4−9), even C3 and C2 substituted indoles (entries 10−15), gave the cyclized products in good yields. The more complex hexacyclic indolines with four contiguous quaternary carbon centers were obtained from cyclopenta[b]indoles (1l−1o) and cyclohepta[b]indole (1p). Due to steric effects, a longer reaction time was needed for the products of 2k−2p. Specifically, the pentacyclic
membered ring fused indolines were readily obtained from 7(1H-indol-1-yl) substituted propargyl benzoate under the standard conditions, as shown in Figure 1. These sevenmembered rings can be either an all-carbon chain (3a and 3c) or oxygen-tethered chain (3b and 3d−3f). The relative configurations of 3e and 3f were confirmed by their X-ray experiments.
Table 2. Substrate Scopea
All reactions were carried out with 1 (0.25 mmol), PtCl2 (5 mol %), and PhMe (2.5 mL) at 40 °C. bIsolated yield on neutral alumina. cReaction time in brackets. dThe reaction was carried out in gram scale (1b: 5.39 mmol).
a
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Organic Letters
subjected to the [3 + 2] cycloaddition using the catalyst Au(IPr)Cl/AgNTf2. The gold complex proved to be more effective than PtCl2 for the aliphatic ester. By treatment of the resulting cycloaddition product with TFA, N,O-acetal 13 was obtained in moderate yield. The final pentacyclic compound 14 was achieved in 65% yield via a Mannich reaction (Scheme 4). Scheme 4. Synthesis of the Pentacyclic Skeleton of Strychnine
Figure 2. Eight- and nine-membered ring synthesis.
Scheme 2. Synthesis of Ten-Membered Rings
In conclusion, a variety of indole N-tethered propargylic carboxylates have been cyclized by a 1,2-benzoyloxy migration/intramolecular [3 + 2] cycloaddition process giving various indoline-annulated normal to medium-size rings. A further Friedel−Crafts reaction of the resulting enol ketal was also tested giving a new polycyclic product. More importantly, the [6.5.6.6.5] skeleton core of strychnine was established via a Mannich reaction between methyl ketone and N,O-acetal. The total syntheses of strychnine and other strychnos alkaloids are in progress.
Different from the substrate of conjugated diene in the earlier study, indoline-annulated eight- and nine-membered rings were formed without the coordination by a Z-inner C−C double bond.4c The synthesis of eight-membered (4a−4c) and nine-membered rings (5a−5g) were conducted under the optimal conditions. However, a longer reaction time (24 h) and/or increased catalyst loading (10 mol %) were needed. Moreover, the successful synthesis of cyclic products 6a−6c with a highly strained ten-membered ring further exemplified this tandem protocol to be a powerful method (Scheme 2). The enol ketal group was easily converted to an oxonium cation and an enolate anion under acid conditions; hydrolysis thus occurred by adding water. If the oxonium cation was trapped by an aromatic nucleophile instead of water, a novel tandem reaction involving 1,2-acyloxy migration/[3 + 2] cycloaddition/Friedel−Crafts reaction would occur.21 When 1t was treated with Au(IPr)Cl/AgSbF6, polycyclic compound 7 was obtained in 63% yield (Scheme 3). The structure of 7 was confirmed by its X-ray experiment. An attempt to construct the [6.5.6.6.5] pentacyclic skeleton core of strychnine was thus conducted. Compound 8 was first converted to formate 12 in four steps. Subsequently, 12 was
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01720. Experimental procedures and physical properties of the compounds (PDF) Accession Codes
CCDC 1584873−1584877, 1584884, 1585425, 1585428− 1585429, and 1846688−1846689 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Scheme 3. Tandem [3 + 2] Cycloaddition/Friedel−Crafts Reaction
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jin-Ming Gao: 0000-0003-4801-6514 Xuegong She: 0000-0002-3002-2433 4441
DOI: 10.1021/acs.orglett.8b01720 Org. Lett. 2018, 20, 4439−4443
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Organic Letters
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Huaiji Zheng: 0000-0003-1674-7582 Author Contributions ∥
Z.W. and L.C. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for generous financial support from the National Natural Science Foundation of China (21772156 and 21502151 to H.Z., 21702168 to Z.W., and 21702167 to Z.L.), the Fundamental Research Funds for the Central Universities (2452017180 to H.Z.), the Natural Science Foundation of Shaanxi Province (S2016YFJQ0080 to Z.L.), and a Start-Up Grant from Northwest A&F University (Z111021405 to H.Z., Z109021711 to Z.W.).
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