Abstract
An improved method for the nickel-catalyzed Suzuki–Miyaura cross-coupling of alkenyl ethers is reported. This stereoconvergent protocol allows for the utilization of a wide range of alkenyl ethers and aryl boronic esters for the synthesis of variously substituted styrene derivatives. An olefinic mixture with respect to the alkenyl ethers can be employed, thereby circumventing the stereodefined synthesis of starting materials. Preliminary mechanistic investigations indicate a nickel-catalyzed olefin isomerization following initial stereoretentive cross-coupling.
Nickel-catalyzed activations of classically inert bonds have received considerable attention in the last decades.1 Pioneering studies by Wenkert and others2 have resulted in the development of practical protocols for the nickel-catalyzed Kumada cross-coupling of various aryl and alkenyl ethers (Scheme 1a–c).3 In all of these cross-coupling protocols, nickel(II) salts were successfully employed as precatalysts as sacrifical organomagnesium coupling partners led to the in situ formation of the catalytically active species via a transmetalation/reductive elimination. Since these initial findings, milder reaction conditions and broadened substrate scope led to significant improvements,4 such as the extension of coupling partners from highly reactive organomagnesium to organoboron compounds (Scheme 1d).5 Despite these contributions, several challenges remain to be addressed and as organoboron coupling partners only sluggishly undergo this cross-coupling reaction, and highly air- and moisture-sensitive Ni(COD)26 or laboriously prepared nickel(0) precatalysts have to be employed.7 Apart from operational challenges, control over double-bond geometry decisively influences the synthetic utility of the present method. Early studies by Chatani and Tobisu have demonstrated that under the reaction conditions the intermediately obtained olefinic mixture of styrenes isomerizes in favor of the thermodynamically more stable E-isomer.5a This important finding was subsequently briefly examined in the cross-coupling of cyclic alkenyl ethers albeit with limited success.5e The authors propose the intermediacy of a Ni–H species to cause olefin isomerization.
Scheme 1. Nickel-catalyzed Kumada and Suzuki-Miyaura cross-coupling of alkenyl ethers.
We became interested in this topic as part of our ongoing research program combining remote functionalization via chain-walking with various postfunctionalization processes.8 Recently, we and Mazet have independently reported an efficient combined metal-catalyzed chain-walking/nickel-catalyzed Kumada cross-coupling of alkenyl ether to access a variety of styrene products (Figure 1).9 During these studies, we realized that the major limitation of this transformation was that the moderate E/Z-selectivity of the enol ether, obtained by isomerization of the double bond, was retained during the cross-coupling event. Based on these findings, we decided to embark on a study and identify conditions that would address this shortcoming, ideally resulting in the development of a mild, stereoconvergent cross-coupling of alkenyl ethers.
Figure 1.
Previously reported tandem metal-catalyzed ’chain-walking’/nickel-catalyzed Kumada cross-coupling.
At the outset of this study, we wondered whether an in situ generated Ni(0) could successfully promote a stereoconvergent cross-coupling of our model enol ether 1a, keeping in mind that we were concerned by delineating a new protocol that would be efficient and easy to manipulate with air stable Ni species. As it has been demonstrated that treatment of Ni(acac)2 with DIBAL in the presence of suitable ligands could furnish a range of catalytically active LnNi species,10 we were interested in applying those conditions to our transformation. Building on Chatani and Murai’s finding of superior reactivity of neopentyl boronic esters,11 we chose 2a as the model coupling partner and toluene as the standard solvent (Table 1).
Table 1. Optimization of Nickel-Catalyzed Suzuki-Miyaura Cross-Coupling of Alkenyl Ether 1aa.
| entry | ligand (mol %) | T (°C) | 3ab (%) | E/Zc |
|---|---|---|---|---|
| 1 | dcype (20) | 105 | nr | nd |
| 2 | dcypf (20) | 105 | nr | nd |
| 3 | dppe (20) | 105 | nr | nd |
| 4 | Xantphos (20) | 105 | nr | nd |
| 5 | ICy·HBF4 (40) | 105 | nr | nd |
| 6 | 6,6′-Me2-2,2′-pyridine (20) | 105 | nr | nd |
| 7 | SPhos (40) | 105 | nr | nd |
| 8 | PPh3 (40) | 105 | 25 | 85:15 |
| 9 | PCy3 (40) | 105 | 86 | 96:04 |
| 10d | PCy3 (20) | 105 | 62 | 95:05 |
| 11 | PCy3 (40)e | 105 | 60 | 95:05 |
| 12 | PCy3 (30) | 105 | 90 | 95:05 |
| 13 | PCy3(30) | 85 | 92f | 97:03 |
| 14 | PCy3 (30) | 60 | 57 | 96:04 |
All reactions were carried out using 1a (0.27 mmol) and 2a (0.32 mmol, 120 mol %) in 1.4 mL of toluene.
Yields determined by analysis of the unpurified mixture of products by 1H NMR with an internal standard in chloroform-d.
Ratio determined by analysis of the unpurified mixture of products by 1H NMR.
Ni(acac)2 (5.0 mol %), DIBAL (10 mol %).
CsF (120 mol %).
Yields of isolated products after purification by column chromatography.
A range of mono- and bidentate ligands were tested under the reaction conditions (Table 1, entries 1–9). Bidentate phosphines, NHC, or bipyridine ligands failed to promote product formation along with sterically demanding Buchwald-type ligand (Table 1, entries 1–7). Only PPh3 and PCy3 furnished the desired product in low and high yield, respectively (Table 1, entries 8 and 9). More importantly, PCy3 provided the product with excellent levels of stereocontrol over the double-bond geometry (E/Z 96:04). Reducing the catalyst loading (Table 1, entry 10) or utilizing CsF as additive (Table 1, entry 11) results in diminished product formation. Some improvement was achieved by reducing the ligand to metal ratio (Table 1, entry 12). Ultimately, lowering the reaction temperature to 85 °C allowed the isolation of 3a in 92% yield with an excellent E/Z ratio (Table 1, entry 13). Further reduction of the temperature to 65 °C resulted in decreased reactivity (Table 1, entry 14).
With the optimal conditions in hand (Table 1, entry 13), we set out to explore the scope of boronic esters as coupling partners (Scheme 2). A wide range of aromatic and heteroaromatic boronic esters delivered the products in usually high yield with excellent control of the double-bond geometry. Electron-withdrawing (Scheme 2, 3d, 3l, and 3n) as well as electron-donating substituents are well-tolerated (Scheme 2, 3h, 3j, 3k, and 3m). Heteroaromatic moieties, i.e., indole and furan, efficiently undergo the cross-coupling reaction (Scheme 2, entries 3m and 3o) albeit in lower yield in the latter case, most probably due to insufficient stability under the reaction conditions. Interestingly, when various anisole derivatives were used as coupling partners, no subsequent cross-coupling products were observed on the aromatic ring (Scheme 2, 3h and 3i). Steric hindrance resulted in slightly diminished E/Z ratios (Scheme 2, 3c and 3g). Additional steric bulk resulted in no product formation (Scheme 2, reactant 2p) in addition to boronic esters possessing either a nitro, pyridine, or thiophene unit (Scheme 2, reactant 2q–t).
Scheme 2. Cross-Coupling of Alkenyl Ether 1a with Different Boronic Esters.
All reactions were carried out using 1a (0.27 mmol) and 2b–t (0.33 mmol, 120 mol %) in 1.4 mL of toluene. Yields were individually obtained after purification by column chromatography on silica gel. The E/Z ratio was determined by 1H NMR spectroscopic analysis of the unpurified mixture of products.
The reaction was carried out at 105 °C.
With these encouraging results in hand, we then explored the scope of alkenyl ethers as coupling partners with 2a as a model boronic ester. To this end, a variety of cyclic and acyclic alkenyl ethers were subjected to the previously optimized reaction conditions (Table 2). Simple acyclic alkenyl ethers cleanly furnished the products in good to high yield with excellent E/Z ratios (Table 2, entries 1–6). Additional degrees of unsaturation were well-tolerated given that the olefin is embedded within a trisubstituted olefin (Table 2, entry 4), but less-substituted olefins undergo a well-precedented nickel-catalyzed chain-walking, giving rise to a mixture of isomers along with partial reduction product (Table 2, entry 3). Nevertheless, the efficiency of the projected cross-coupling remained unaffected, and the products were obtained in excellent combined yield as virtually single geometrical isomers. The same phenomenon of partial isomerization of the double-bond was also observed in the case of the sensitive enol ether 1g (Table 2, entry 6), which provided a mixture of two styrene products. This result further advocates the notion of a nickel-mediated olefin isomerization following the cross-coupling event.
Table 2. Cross-Coupling of Boronic Ester 2a with Different Alkenyl Ethers 1b–m.
All reactions were carried out using 1b–m (0.27 mmol) and 2a (0.33 mmol, 120 mol %) in 1.4 mL of toluene. Yields were individually obtained after purification by column chromatography on silica gel. The E/Z ratio was determined by 1H NMR spectroscopic analysis of the unpurified mixture of products.
Reaction was carried out at 105 °C.
Reaction was carried out using Ni(acac)2 (20 mol %), DIBAL (40 mol %), and PCy3 (60 mol %) in 1.4 mL of toluene at 105 °C.
Cyclic enol ether 1h could be converted successfully into linear alkenol 3v in good yield with reasonable control over the double-bond geometry (Table 2, entry 7). Interestingly, the corresponding six-membered alkenyl ether failed to undergo the desired transformation (see the Supporting Information). Alkenyl ethers bearing an aromatic moiety finally provided access to variously substituted stilbenes in good yields and excellent E/Z ratios (Table 2, entries 8–11). As expected, trisubstituted olefin 1m only partially underwent isomerization, furnishing the product with a low E/Z ratio (Table 2, entry 12).
Having established a robust protocol for the transformation of a variety of alkenyl ethers and boronic esters into substituted styrenes and stilbenes, we decided to test the applicability of this method to the synthesis of biologically relevant molecules. At the outset, a scale-up experiment established our confidence in the robustness of the current method (Scheme 3a). Subsequently, the synthesis of DMU-212, a resveratrol analogue with potential anticancer properties for the treatment of human ovarian cancer,12 was successfully executed (Scheme 3b).
Scheme 3. Scale-up Experiment of 3a and Synthesis of Potential Anticancer Agent DMU-212 (3ab).
In order to gain additional insight into this intriguing double-bond isomerization process, a series of control experiments were designed (Table 3). To this end, Z-3p was independently synthesized and exposed to our established reaction conditions (Table 3, entry 1). It was found that under standard conditions, complete double-bond isomerization took place. The same result was obtained in the absence of boronic ester coupling partner 2a (Table 3, entry 2) and even without ligand (Table 3, entry 3). From the above presented results it appears reasonable to assume that a nickel hydride may intermediately be formed, which causes double-bond isomerization or migration (Table 2, entry 3). Indeed, in the absence of DIBAL, no olefin isomerization of Z-3p was observed and only starting material was retrieved (Table 3, entry 4). To rule out a thermal process, the reaction was conducted at 60 and 25 °C in the presence of DIBAL and Ni catalyst (Table 3, entries 5–7). In both cases, E-3p was obtained as a single geometrical isomer, therefore excluding a thermally induced process. It should be noted that the pure thermal treatment of Z-3p does not produce the E-isomer either. These results support the conclusion that olefin isomerization ensues the cross-coupling process to provide a stereoconvergent protocol. We are proposing the presence of a nickel hydride promoting the isomerization process through an addition–elimination sequence (see the graphic below Table 3) that is also in good agreement with the results described in Table 2, entry 3, where olefin isomerization was observed.
Table 3. Control Experiments To Identify Isomerization-Active Speciesa.
| entry | [Ni] | DIBAL | PCy3 | 2a | T (°C) | time (h) | E/Z |
|---|---|---|---|---|---|---|---|
| 1 | √ | √ | √ | √ | 105 | 16 | 98:02 |
| 2 | √ | √ | √ | × | 105 | 16 | 96:04 |
| 3 | √ | √ | × | × | 105 | 16 | 95:05 |
| 4 | √ | × | × | × | 105 | 16 | n.r. |
| 5 | √ | √ | × | × | 60 | 16 | >95:05 |
| 6 | √ | √ | × | × | 25 | 48 | >95:05 |
All reactions were carried out using Z-3p (0.27 mmol) and, if applicable, 2a (0.33
mmol, 120 mol %) in 1.4 mL of toluene. The E/Z ratio was determined by 1H NMR spectroscopic
analysis of the crude mixture.
In conclusion, we have established a user-friendly, operationally simple protocol for the stereoconvergent nickel-catalyzed cross-coupling of olefinic mixtures of alkenyl ethers with aromatic boronic esters. The utilization of easy-to-handle Ni(acac)2 as a precatalyst along with DIBAL as reductant provides a simple way to access a highly active nickel catalyst in situ. A broad array of variously functionalized styrenes and stilbenes could be accessed in good to high yields with generally excellent control over the double-bond geometry. Various aromatic and heteroaromatic boronic esters could be successfully employed in this transformation. Both acyclic and cyclic alkenyl ethers participated well in this process comprising Lewis basic groups or additional degrees of unsaturation. Mechanistic investigations led us to propose that the olefin isomerization is subsequent to the cross-coupling reaction.
Acknowledgments
This project has received funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 786976. H.S. acknowledges the Max-Planck-Society for a Minerva Fellowship.
Supporting Information Available
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00946.
Experimental procedures and characterization data for all new compounds (PDF)
The authors declare no competing financial interest.
Supplementary Material
References
- a Rosen B. M.; Quasdorf K. W.; Wilson D. A.; Zhang N.; Resmerita A.-M.; Garg N. K.; Percec V. Nickel-Catalyzed Cross-Couplings Involving Carbon–Oxygen Bonds. Chem. Rev. 2011, 111, 1346–1416. 10.1021/cr100259t. [DOI] [PMC free article] [PubMed] [Google Scholar]; b Dander J. E.; Garg N. K. Breaking Amides using Nickel Catalysis. ACS Catal. 2017, 7, 1413–1423. 10.1021/acscatal.6b03277. [DOI] [PMC free article] [PubMed] [Google Scholar]
- a Wenkert E.; Michelotti E. L.; Swindell C. S. Nickel-Induced Conversion of Carbon-Oxygen into Carbon-Carbon Bonds. One-StepTransformations of Enol Ethers into Olefins and Aryl Ethers into Biaryls. J. Am. Chem. Soc. 1979, 101, 2246–2247. 10.1021/ja00502a074. [DOI] [Google Scholar]; b Hayashi T.; Katsuro Y.; Kumada M. Nickel-Catalyzed Cross-Coupling of Silyl Enol Ethers with Grignard Reagents. Regio- and Stereocontrolled Synthesis of Olefins. Tetrahedron Lett. 1980, 21, 3915–3918. 10.1016/0040-4039(80)80215-2. [DOI] [Google Scholar]; c Wenkert E.; Leftin M. H.; Michelotti E. L. A Synthesis of Conjugated Dienes from Aromatic, Five-Membered Heterocycles. J. Chem. Soc., Chem. Commun. 1984, 617–618. 10.1039/c39840000617. [DOI] [Google Scholar]; d Wenkert E.; Michelotti E. L.; Swindell C. S.; Tingoli M. Transformation of Carbon-Oxygen into Carbon-Carbon Bonds Mediated by Low-Valent Nickel Species. J. Org. Chem. 1984, 49, 4894–4899. 10.1021/jo00199a030. [DOI] [Google Scholar]; e Wadman S.; Whitby R.; Yeates C.; Kocienski P.; Cooper K. An Efficient and Stereoselective Synthesisi of Homoallylic Alcohols via Nickel-Catalysed Coupling of 5-Alkyl-2,3-Dihydrofurans with Grignard Reagents. J. Chem. Soc., Chem. Commun. 1987, 241–243. 10.1039/c39870000241. [DOI] [Google Scholar]; f Kocieński P.; Dixon N. J.; Wadman S. A Stereoselective Synthesis of Tri-Substituted Alkenes. The Nickel-Catalysed Coupling of Grignard Reagents with 6-Alkyl-3,4-Dihydro-2H-Pyrans. Tetrahedron Lett. 1988, 29, 2353–2356. 10.1016/S0040-4039(00)86058-X. [DOI] [Google Scholar]; g Kocienski P.; Dixon N. J. Stereoselective Synthesis of Homoallylic Alcohols by Migratory Insertion Reactions of Higher-Order Cyanocuprates and Nickel-Catalysed Coupling Reactions Involving Enol Carbamates. Synlett 1989, 1989, 52–54. 10.1055/s-1989-20354. [DOI] [Google Scholar]; h Kocienski P.; Wadman S.; Cooper K. A Highly Stereoselective and Iterative Approach to Isoprenoid Chains: Synthesis of Homogeraniol, Homofarnesol, and Homogeranylgeraniol. J. Org. Chem. 1989, 54, 1215–1217. 10.1021/jo00266a047. [DOI] [Google Scholar]; i Kocieński P. J.; Love C. J.; Richard J W.; Costello G.; Roberts D. A. A Total Synthesis of (±)-Zoapatanol and Demethyl-ORF13811. Tetrahedron 1989, 45, 3839–3848. 10.1016/S0040-4020(01)89243-7. [DOI] [Google Scholar]
- a Cornella J.; Martin R. Ni-Catalyzed Stereoselective Arylation of Inert C–O bonds at Low Temperatures. Org. Lett. 2013, 15, 6298–6301. 10.1021/ol4031815. [DOI] [PubMed] [Google Scholar]; b Tobisu M.; Takahira T.; Chatani N. Nickel-Catalyzed Cross-Coupling of Anisoles with Alkyl Grignard Reagents via C–O Bond Cleavage. Org. Lett. 2015, 17, 4352–4355. 10.1021/acs.orglett.5b02200. [DOI] [PubMed] [Google Scholar]; c Tobisu M.; Takahira T.; Morioka T.; Chatani N. Nickel-Catalyzed Alkylative Cross-Coupling of Anisoles with Grignard Reagents via C–O Bond Activation. J. Am. Chem. Soc. 2016, 138, 6711–6714. 10.1021/jacs.6b03253. [DOI] [PubMed] [Google Scholar]; d Hostier T.; Neouchy Z.; Ferey V.; Gomez Pardo D.; Cossy J. Nickel-Catalyzed System for the Cross-Coupling of Alkenyl Methyl Ethers with Grignard Reagents under Mild Conditions. Org. Lett. 2018, 20, 1815–1818. 10.1021/acs.orglett.8b00313. [DOI] [PubMed] [Google Scholar]
- a Li B.-J.; Yu D.-G.; Sun C.-L.; Shi Z.-J. Activation of “Inert” Alkenyl/Aryl C–O Bond and Its Application in Cross-Coupling Reactions. Chem. - Eur. J. 2011, 17, 1728–1759. 10.1002/chem.201002273. [DOI] [PubMed] [Google Scholar]; b Cornella J.; Zarate C.; Martin R. Metal-Catalyzed Activation of Ethers via C–O Bond Cleavage: a New Strategy for Molecular Diversity. Chem. Soc. Rev. 2014, 43, 8081–8097. 10.1039/C4CS00206G. [DOI] [PubMed] [Google Scholar]; c Tobisu M.; Chatani N. Cross-Couplings Using Aryl Ethers via C–O Bond Activation Enabled by Nickel Catalysts. Acc. Chem. Res. 2015, 48, 1717–1726. 10.1021/acs.accounts.5b00051. [DOI] [PubMed] [Google Scholar]
- a Shimasaki T.; Konno Y.; Tobisu M.; Chatani N. Nickel-Catalyzed Cross-Coupling Reaction of Alkenyl Methyl Ethers with Aryl Boronic Esters. Org. Lett. 2009, 11, 4890–4892. 10.1021/ol901978e. [DOI] [PubMed] [Google Scholar]; b Liu C.-F.; Xiong D.-C.; Ye X.-S. Ring Opening-Ring Closure” Strategy for the Synthesis of Aryl-C-Glycosides. J. Org. Chem. 2014, 79, 4676–4686. 10.1021/jo500730y. [DOI] [PubMed] [Google Scholar]; c Guo L.; Hsiao C.-C.; Yue H.; Liu X.; Rueping M. Nickel-Catalyzed Csp2–Csp3 Cross-Coupling via C–O Bond Activation. ACS Catal. 2016, 6, 4438–4442. 10.1021/acscatal.6b00801. [DOI] [Google Scholar]; d Guo L.; Liu X.; Baumann C.; Rueping M. Nickel-Catalyzed Alkoxy-Alkyl Interconversion with Alkylborane Reagents through C-O Bond Activation of Aryl and Enol Ethers. Angew. Chem., Int. Ed. 2016, 55, 15415–15419. 10.1002/anie.201607646. [DOI] [PubMed] [Google Scholar]; e Ohtsuki A.; Sakurai S.; Tobisu M.; Chatani N. Nickel-catalyzed Ring-opening Cross-coupling of Cyclic Alkenyl Ethers with Arylboronic Esters via Carbon–Oxygen Bond Cleavage. Chem. Lett. 2016, 45, 1277–1279. 10.1246/cl.160712. [DOI] [Google Scholar]
- Dander J. E.; Weires N. A.; Garg N. K. Benchtop Delivery of Ni(cod)2 using Paraffin Capsules. Org. Lett. 2016, 18, 3934–3936. 10.1021/acs.orglett.6b01758. [DOI] [PMC free article] [PubMed] [Google Scholar]
- a Standley E. A.; Jamison T. F. Simplifying Nickel(0) Catalysis: An Air-Stable Nickel Precatalyst for the Internally Selective Benzylation of Terminal Alkenes. J. Am. Chem. Soc. 2013, 135, 1585–1592. 10.1021/ja3116718. [DOI] [PMC free article] [PubMed] [Google Scholar]; b Magano J.; Monfette S. Development of an Air-Stable, Broadly Applicable Nickel Source for Nickel-Catalyzed Cross-Coupling. ACS Catal. 2015, 5, 3120–3123. 10.1021/acscatal.5b00498. [DOI] [Google Scholar]; c Shields J. D.; Gray E. E.; Doyle A. G. A Modular, Air-Stable Nickel Precatalyst. Org. Lett. 2015, 17, 2166–2169. 10.1021/acs.orglett.5b00766. [DOI] [PMC free article] [PubMed] [Google Scholar]
- a Chinkov N.; Majumdar S.; Marek I. New Approach to the Stereoselective Synthesis of Metalated Dienes via an Isomerization–Elimination Sequence. J. Am. Chem. Soc. 2002, 124, 10282–10283. 10.1021/ja027027y. [DOI] [PubMed] [Google Scholar]; b Chinkov N.; Majumdar S.; Marek I. Stereoselective Preparation of Dienyl Zirconocene Complexes via a Tandem Allylic C–H Bond Activation-Elimination Sequence. J. Am. Chem. Soc. 2003, 125, 13258–13264. 10.1021/ja036751t. [DOI] [PubMed] [Google Scholar]; c Chinkov N.; Levin A.; Marek I. Unsaturated Fatty Alcohol Derivatives as a Source of Substituted Allylzirconocene. Angew. Chem., Int. Ed. 2006, 45, 465–468. 10.1002/anie.200501946. [DOI] [PubMed] [Google Scholar]; d Marek I.; Chinkov N.; Levin A. A Zirconium Promenade - An Efficient Tool in Organic Synthesis. Synlett 2006, 2006, 0501–0514. 10.1055/s-2006-932483. [DOI] [Google Scholar]; e Masarwa A.; Didier D.; Zabrodski T.; Schinkel M.; Ackermann L.; Marek I. Merging Allylic Carbon–Hydrogen and Selective Carbon–Carbon Bond Activation. Nature 2014, 505, 199–203. 10.1038/nature12761. [DOI] [PubMed] [Google Scholar]; f Vasseur A.; Perrin L.; Eisenstein O.; Marek I. Remote Functionalization of Hydrocarbons with Reversibility Enhanced Stereocontrol. Chem. Sci. 2015, 6, 2770–2776. 10.1039/C5SC00445D. [DOI] [PMC free article] [PubMed] [Google Scholar]; g Vasseur A.; Bruffaerts J.; Marek I. Remote Functionalization Through Alkene Isomerization. Nat. Chem. 2016, 8, 209–219. 10.1038/nchem.2445. [DOI] [PubMed] [Google Scholar]; h Singh S.; Bruffaerts J.; Vasseur A.; Marek I. A Unique Pd-Catalysed Heck Arylation as a Remote Trigger for Cyclopropane Selective Ring-Opening. Nat. Commun. 2017, 8, 14200. 10.1038/ncomms14200. [DOI] [PMC free article] [PubMed] [Google Scholar]; i Sommer H.; Juliá-Hernández F.; Martin R.; Marek I. Walking Metals for Remote Functionalization. ACS Cent. Sci. 2018, 4, 153–165. 10.1021/acscentsci.8b00005. [DOI] [PMC free article] [PubMed] [Google Scholar]
- a Ho G.-M.; Judkele L.; Bruffaerts J.; Marek I. Metal-Catalyzed Remote Functionalization of ω-Ene Unsaturated Ethers: Towards Functionalized Vinyl Species. Angew. Chem., Int. Ed. 2018, 57, 8012–8016. 10.1002/anie.201802434. [DOI] [PubMed] [Google Scholar]; b Romano C.; Mazet C. Multicatalytic Stereoselective Synthesis of Highly Substituted Alkenes by Sequential Isomerization/Cross-Coupling Reactions. J. Am. Chem. Soc. 2018, 140, 4743–4750. 10.1021/jacs.8b02134. [DOI] [PubMed] [Google Scholar]
- a Yamago S.; Nakamura E. Synthesis of Propellanes by “Exocyclic” Transannular Cycloaddition of Olefinic Methylenecyclopropanes. Tetrahedron 1989, 45, 3081–3088. 10.1016/S0040-4020(01)80134-4. [DOI] [Google Scholar]; b Sato Y.; Ohashi K.; Mori M. Synthesis of Biaryls using Nickel-Catalyzed [2 + 2+2] Cocyclization. Tetrahedron Lett. 1999, 40, 5231–5234. 10.1016/S0040-4039(99)00945-4. [DOI] [Google Scholar]; c Montgomery J. Nickel-Catalyzed Reductive Cyclizations and Couplings. Angew. Chem., Int. Ed. 2004, 43, 3890–3908. 10.1002/anie.200300634. [DOI] [PubMed] [Google Scholar]
- Kakiuchi F.; Usui M.; Ueno S.; Chatani N.; Murai S. Ruthenium-Catalyzed Functionalization of Aryl Carbon-Oxygen Bonds in Aromatic Ethers with Organoboron Compounds. J. Am. Chem. Soc. 2004, 126, 2706–2707. 10.1021/ja0393170. [DOI] [PubMed] [Google Scholar]
- a Piotrowska H.; Myszkowski K.; Abraszek J.; Kwiatkowska-Borowczyk E.; Amarowicz R.; Murias M.; Wierzchowski M.; Jodynis-Liebert J. DMU-212 Inhibits Tumor Growth in Xenograft Model of Human Ovarian Cancer. Biomed. Pharmacother. 2014, 68, 397–400. 10.1016/j.biopha.2014.02.001. [DOI] [PubMed] [Google Scholar]; b Piotrowska H.; Kujawska M.; Nowicki M.; Petzke E.; Ignatowicz E.; Krajka-Kuźniak V.; Zawierucha P.; Wierzchowski M.; Murias M.; Jodynis-Liebert J. Effect of Resveratrol Analogue, DMU-212, on Antioxidant Status and Apoptosis-Related Genes in Rat Model of Hepatocarcinogenesis. Hum. Exp. Toxicol. 2017, 36, 160–175. 10.1177/0960327116641734. [DOI] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.