Letter Cite This: Org. Lett. 2018, 20, 6591−6595
pubs.acs.org/OrgLett
Phosphine-Catalyzed [3 + 2] Annulation of 2‑Hydroxy-1,4naphthaquinones and Allenoate: An Allene−Alkene [3 + 2] Annulation Mechanism Involving Consecutive γ‑Addition−Aldol Reaction Leijie Zhou,† Chang Wang,† Chunhao Yuan,†,‡ Honglei Liu,† Cheng Zhang,† and Hongchao Guo*,†,§ Org. Lett. 2018.20:6591-6595. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/19/18. For personal use only.
†
Department of Applied Chemistry, China Agricultural University, Beijing 100193, P. R. China School of Chemical Engineering, Taishan Medical University, Taian, Shandong 271016, P. R. China § State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China ‡
S Supporting Information *
ABSTRACT: A diastereoselective construction of biologically important tetrahydrocyclopenta[b]naphthalene derivatives has been achieved via a phosphine-catalyzed [3 + 2] annulation of 2hydroxy-1,4-naphthaquinone derivatives and allenoate. Various tetrahydrocyclopenta[b]naphthalene derivatives containing contiguous quaternary carbon centers are obtained in good yields with excellent diastereoselectivities. The asymmetric version gave the chiral product in up to 57% ee under catalysis of Kwon chiral phosphine. The reaction undergoes a reaction mechanism involving consecutive γ-addition−aldol reaction.
Q
uinone-based skeletons widely exist in most organisms.1 Their unique structural framework is frequently a key component of numerous natural biologically active compounds.2 Owing to their significant biological activities, quinone-based systems are usually attractive synthetic targets.3 Among them, natural products containing a naphthaquinone− cyclopentene ring system exhibit highly biological activity extensively (Figure 1).4 For example, zeylanone was originally
cyclopentenes,6,7 which also inspired development of a large amount of various phosphine-catalyzed annulation reactions for construction of structurally diverse carbo- and heterocyclic compounds.8 During the past two decades, Lu’s classic [3 + 2] annulation6,7 and its mechanism9 have well been studied. Generally, Lu’s [3 + 2] annulation works via consecutive conjugate addition−α- or γ-addition (Scheme 1a).9 Several Scheme 1. Phosphine-Catalyzed [3 + 2] Annulation Reaction of Allenoates
Figure 1. Natural products containing the naphthaquinone−cyclopentene core.
isolated from the evergreen shrub Plumbago zeylanica,4a which shows antibacterial and antifungal activity while also exhibiting cytotoxic activity against cancer cell lines.4b Shikometabolins E and F show significant neuraminidase inhibitory activity.4c Phosphine-catalyzed [3 + 2] annulation of electron-deficient allenes (or alkynes) with olefins, first reported by Lu in 1995,5 represents a powerful tool for synthesis of functionalized © 2018 American Chemical Society
Received: September 14, 2018 Published: October 10, 2018 6591
DOI: 10.1021/acs.orglett.8b02947 Org. Lett. 2018, 20, 6591−6595
Letter
Organic Letters
With the optimal reaction conditions established, the scope of the substrates was then investigated. As shown in Table 2,
types of alkenes such as mono-, di-, or trisubstituted were extensively investigated, leading to numerous biologically significant cyclopentene derivatives (Scheme 1a).6,7 In contrast, fully substituted alkenes were rarely explored, and their [3 + 2] annulation reactions with electron-deficient allenes were not very successful due to their large steric hindrance.10,11 Only two types of fully substituted alkenes such as [60]fullerene10 and isatin-derived α,β-unsaturated diesters and methylenemalononitriles11 were explored for [3 + 2] annulation of allenoates. Construction of contiguous quaternary centers constitutes a central issue and formidable challenge in synthesis of natural products and pharmaceutical drugs because of the severe steric repulsion between the substituents.12 As part of our continuing efforts on phosphinecatalyzed annulations,13 herein we report the first phosphinecatalyzed [3 + 2] annulation of allenoate with 2-hydroxy-1,4naphthaquinone-based fully substituted alkenes to construct contiguous quaternary centers, which proceeds through consecutive γ-addition−aldol reaction (Scheme 1b). In our initial studies, we investigated the reaction of 2hydroxy-1,4-naphthaquinone 2a with allenoate 1a in the presence of 20 mol % of PPh3 at room temperature. To our delight, the reaction proceeded smoothly in dichloromethane to give the desired product 3aa in 95% yield with >20:1 dr and >20:1 rr (regioisomer ratio) (Table 1, entry 1). Shifting the
Table 2. Scope of 2-Hydroxy-1,4-naphthaquinones 2a
Table 1. Optimization of the Reaction Conditionsa
entry
R in 2
3
time (h)
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
C6H5 (2a) 2-FC6H4 (2b) 3-FC6H4 (2c) 4-FC6H4 (2d) 2-ClC6H4 (2e) 4-ClC6H4 (2f) 2-BrC6H4 (2g) 3-BrC6H4 (2h) 3-CF3C6H4 (2i) 2-MeC6H4 (2j) 4-MeC6H4 (2k) 2-OMeC6H4 (2l) 3-OMeC6H4 (2m) 4-OMeC6H4 (2n) 2-thienyl (2o) 2-naphthyl (2p) N-Boc-3-indolyl (2q) Et (2r) n-hexyl (2s) n-C11H23 (2t) cyclohexyl (2u)
3aa 3ab 3ac 3ad 3ae 3af 3ag 3ah 3ai 3aj 3ak 3al 3am 3an 3ao 3ap 3aq 3ar 3as 3at 3au
24 24 24 24 36 36 24 24 36 36 36 36 36 36 24 24 24 48 48 48 60
95 88 89 85 90 78 95 85 82 86 90 98 83 81 83 88 84 98 95 85 76
a
entry
PR3
solvent
time (h)
yieldb (%)
1 2 3 4 5 6 7 8 9 10c 11d
PPh3 PPh2Me PPhMe2 PBu3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 DCE CHCl3 toluene THF MeOH CH2Cl2 CH2Cl2
24 24 48 48 48 36 48 48 48 48 48
95 86 62 trace 84 93 NRe NR trace 82 28
Unless indicated otherwise, reactions of 1a (0.20 mmol) and 2 (0.10 mmol) were carried out at rt in the presence of PPh3 (0.02 mmol) in 1 mL of CH2Cl2. In all cases, the dr is >20:1, determined by 1H NMR analysis. bIsolated yield.
various functionalized 2-hydroxy-1,4-naphthaquinones 2 having diverse electron-donating or -withdrawing groups at different positions on the benzene ring of the substituent R performed well in the reaction to give the corresponding products in good to excellent yields with excellent diastereoselectivities (entries 1−14). The 2-thiophene-yl-, 2-naphthyl-, and N-Boc-3-indolyl-substituted 2-hydroxy-1,4-naphthaquinones also worked efficiently under the optimal reaction conditions, producing the corresponding product in 83%, 88%, and 84% yields, respectively (entries 15−17). It is worth mentioning that alkyl substituents were also well tolerated in the reaction, and the annulation products were produced in 76−98% yields with extremely excellent diastereoselectivities (entries 18−21). In addition, the relative configuration of the products was confirmed by single-crystal X-ray analysis of the product 3ak. To develop an asymmetric variant of the current reaction, various chiral phosphines were screened. As shown in Table 3, BINAP P1 and bifunctional phosphine P2 showed no catalytic activity (entries 1 and 2). Spirocyclic chiral phosphine P3 only led to 5% yield of the product 3aa but with excellent diastereoselectivity and moderate enantioselectivity (entry 3). Delightedly, using Kwon phosphine P4 as the chiral catalyst, the reaction worked efficiently at room temperature to give the product in 92% yield, albeit with low enantioselectivity and moderate diastereoselectivity (entry 4), while the endophos-
a
Unless indicated otherwise, reactions of 1a (0.20 mmol) and 2a (0.10 mmol) were carried out at rt in the presence of catalyst (0.02 mmol) in 1 mL of solvent. In all cases, the dr is >20:1 and the rr is >20:1, determined by 1H NMR analysis; bIsolated yield; c10 mol % of PPh3 was used; d5 mol % of PPh3 was used. eNo reaction.
catalyst to PPh2Me and PPhMe2, the yields of product 3aa dropped to 86% and 62%, respectively (entries 2 and 3). In the case of PBu3, the reaction became sluggish, and only a trace amount of product 3aa was detected (entry 4). Investigation on solvents indicated that 1,2-dichloroethane (DCE) and CHCl3 are also suitable for the reaction (entries 5 and 6), while toluene and THF were not compatible with the reaction (entries 7 and 8). When polar MeOH was employed as the solvent, only a trace amount of 3aa was observed (entry 9). Reducing the catalyst loading to 10 mol % still afforded 82% yield of product 3aa (entry 10), whereas 5 mol % of catalyst only gave 28% yield of product (entry 11). 6592
DOI: 10.1021/acs.orglett.8b02947 Org. Lett. 2018, 20, 6591−6595
Letter
Organic Letters Table 3. Investigation on Asymmetric [3 + 2] Annulationa
entry
Px
yieldb (%)
drc (%)
eec (%)
1 2 3 4 5 6 7 8 9
P1 P2 P3 P4 P5 P6 P7 P8 P9
trace trace 5 92 90 80 82 85 85
>99:1 65:35 64:36 85:15 79:21 30:70 97.5:2.5
47 19 43 25 57 51 48
Scheme 2. Reaction on Gram Scale and Further Transformations
Scheme 3. Plausible Mechanism
a
Unless indicated otherwise, reactions of 1a (0.20 mmol) and 2a (0.10 mmol) were carried out at rt in the presence of catalyst (0.02 mmol) in 1 mL of CH2Cl2. bIsolated yield. cDetermined by chiral HPLC analysis.
phine P5 afforded both moderate enantioselectivity and diastereoselectivity (entry 5). Next, differently substituted Kwon phosphines were screened (entries 6−9). Unfortunately, excellent enantioselectivity was not accomplished. p-Methoxysubstituted phosphine P6 showed better diastereoselective performance than P4 and P5 but bad enantioselective performance (entry 6). Up to 57% ee and 79:21 dr values was obtained when chiral phosphine P7 was employed (entry 7). When p-fluoro-substituted phosphine P8 was employed, a reverse diastereoselectivity was observed as 30:70 (entry 8). 1Naphthyl-substituted phosphine P9 gave a balanced performance (85% yield, 97.5:2.5 dr and 48% ee) (entry 9). The diastereoselectivity of the catalyst was obviously influenced by the size of the aryl substituent (entry 9 vs entries 4−7), but it is difficult to determine the reason for reverse diastereoselectivity caused by p-fluoro-substituted phosphine P8 (entry 8). As indicated in Scheme 2, the gram-scale reaction of allenoate 1a (1.12 g, 10 mmol) with naphthoquinone 2a (1.32 g, 5 mmol) was carried out under the optimal reaction conditions to give the dihydrocyclopenta[b]naphthalene-4,9dione derivative 3aa in 92% yield. With use of 1.0 equiv of PPh2Me, the product 3aa worked as an electron-deficient alkene to react with the allenoate 1a at 100 °C to provide the cycloadduct 4 bearing four continuous stereogenic centers in 75% yield with 4:1 dr. The [3 + 2] cycloaddition reaction of the compound 3aa with nitrilimines proceeded smoothly at room temperature, producing an interesting pyrazole derivative 6 in 95% yield with 10:1 dr. As shown in Scheme 3, a plausible mechanism was proposed. Treatment of the allenoate 1a with PPh3 leads to generation of phosphonium dienolate A, which acts as a base to deprotonate the substrate 2a to give the intermediates B and
C. The γ-addition of the intermediate B to C followed by a 1,2proton transfer affords the intermediate E. Subsequent intramolecular aldol reaction occurred to form the cyclization intermediate F. A proton transfer followed by β-elimination of the phosphine catalyst produced the cycloadduct 3aa with simultaneous regeneration of PPh3. The aldol reaction occurred on the same side of the six-membered ring with γaddition reaction because of the steric hindrance resulting from benzyl group, which is the key reason for excellent diastereoselective control. The current mechanism is very different from the mechanism of Lu’s allene−alkene [3 + 2] annulation, which usually involves consecutive conjugate addition−α- or γ-addition.5,9 In conclusion, we have developed phosphine-catalyzed [3 + 2] annulation of 2-hydroxy-1,4-naphthaquinone derivatives with allenoate, furnishing an array of biologically important dihydrocyclopenta[b]naphthalene-4,9-dione derivatives containing two contiguous quaternary carbon centers in good to excellent yields with extremely excellent diastereoselectivities. With the use of Kwon’s chiral phosphine P7 as chiral catalyst, up to 57% ee of the product 3aa was obtained. Furthermore, the electron-deficient double bond of the products 3 enables diverse transformations for multiring systems, showing their high potential for constructing different biologically important skeletons. Unlike traditional Lu’s [3 + 2] annulation reaction, this reaction undergoes an unusual annulation mechanism involving consecutive γ-addition−aldol reaction. 6593
DOI: 10.1021/acs.orglett.8b02947 Org. Lett. 2018, 20, 6591−6595
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Organic Letters
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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.8b02947. Experimental procedure, characterization data, and NMR spectra (PDF) Accession Codes
CCDC 1857261 contains 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.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Cheng Zhang: 0000-0002-8760-8152 Hongchao Guo: 0000-0002-7356-4283 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the NSFC (21372256 and 21572264), the State Key Laboratory of Chemical Resource Engineering, and the Program for Changjiang Scholars and Innovative Research Team Project IRT1042.
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REFERENCES
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