- Email: [email protected]
Transition-metal-free synthesis of aromatic amines via the reaction of benzynes with isocyanates
Tetrahedron Letters 59 (2018) 671–674
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Transition-metal-free synthesis of aromatic amines via the reaction of benzynes with isocyanates Jeong Hoon Seo, Haye Min Ko ⇑ Department of Bio-Nano Chemistry, Wonkwang University, 460 Iksandae-ro, Iksan, Jeonbuk 54538, Republic of Korea
a r t i c l e
i n f o
Article history: Received 13 November 2017 Revised 28 December 2017 Accepted 5 January 2018 Available online 9 January 2018
a b s t r a c t An unexpected reaction between benzynes and isocyanates to generate aromatic amines has been developed under transition-metal-free conditions. The in situ prepared anions formed through cleavage of the NAC bond in isocyanates, reacted with aryne precursors to afford various aniline derivatives in moderate to excellent yield and tolerated various substituents on the o-silyl aryl triflate and the isocyanate. Ó 2018 Elsevier Ltd. All rights reserved.
Keywords: Aniline derivatives Transition-metal-free Benzynes Isocyanates Fluoride
Introduction Aromatic amines are important motifs in natural products, materials, pharmaceuticals and dyes.1 Therefore, efficient and concise synthetic strategies towards this class of compounds is of significant interest. Traditional methods to access these useful building blocks typically proceed via the formation of carbonnitrogen bonds. In particular, the palladium-catalyzed carbonnitrogen bond formation reaction (Buchwald-Hartwig amination), Ullmann reaction, and Chan-Lam coupling reaction in the presence of Cu are well represented in the literature.2 An amination reaction using carbon-hydrogen activation has been reported as an alternative methodology to construct CarylAN bonds.3 Despite several advantages, the requirement for expensive transition metals and directing groups is an obstacle for the utility of these reactions. Over the past decades, benzyne chemistry has been extremely well developed. The reactivity of benzyne intermediates is attributed to a lowered LUMO arising from a strained p-bond, which facilitates several coupling reactions with neutral nucleophiles such as ureas4 and imines.5 The in situ generated benzyne intermediates formed from o-silyl aryl triflates under the action of fluoride reported by Kobayashi and co-workers6 have allowed functionalization at the 1- and 2-positions of benzene. Such mild reaction conditions help overcome the limitations of transition metal catalyzed cross-coupling amination reactions,
such as the use of strong bases or high temperature. Interestingly, with respect to benzynes, mono N-arylation7 has not been studied in detail. A representative mono N-arylation reaction utilizing primary or secondary amines has been demonstrated by Larock and co-workers.7d Due to the difficulty in reacting tertiary amines with arynes according to the formation of an ammonium salt, only one successful reaction has been reported by Biju and co-workers
⇑ Corresponding author. E-mail address: [email protected] (H.M. Ko). https://doi.org/10.1016/j.tetlet.2018.01.022 0040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
Fig. 1. Synthetic Methods for Constructing CarylAN Bonds utilizing benzyne.
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J.H. Seo, H.M. Ko / Tetrahedron Letters 59 (2018) 671–674
(Fig. 1).7a To the best of our knowledge, the use of substituted isocyanates as a coupling partner for mono N-arylation employing benzyne precursors has not been reported. Herein, we report an efficient transition-metal free mono N-arylation reaction between benzynes and various isocyanates. Results and discussion Initially, we investigated the reaction employing the model substrate 1a and the commercially available tosyl isocyanate 2a (1.2 equiv.) in presence of CsF in acetonitrile at room temperature (Table 1). Unexpectedly, the tosyl protected aniline 3a was obtained in 28% yield (Entry 1). Encouraged by this result, the temperature was increased to 100 °C, which improved the yield to 48% (Entry 2). Interestingly, when the amount of tosyl isocyanate 2a (2.4 equiv.) was increased, product 3a was isolated in excellent yield 94% (Entry 3). Decreasing the fluoride source amount or using THF instead of acetonitrile, resulted in a diminished yield for 3a (Entry 4). To examine the effect of additives, 18-crown-6, methanol and isopropyl alcohol were examined (Entries 5–7). Unfortunately, the crown ether or alcohol provided lower yield. Additionally, the use of TBAF failed to give product 3a (Entries 8–9). Upon using KF/18-crown-6 or K2CO3/18-crown-6 as the benzyne triggering reagent, 3a was observed in 28% or trace amounts, respectively (Entries 10–12). Finally, the use of tosyl isocyanate 2a (2.4 equiv.) in presence of CsF (3.0 equiv.) in acetonitrile at 100 °C was selected as the best reaction conditions. With the optimized conditions in hand, the scope of benzyne precursors was examined (Scheme 1). Both electron-donating and electron-withdrawing functional groups on the benzyne precursors were tolerated in this reaction. Alkyl groups such as tertbutyl, methyl, and dimethyl afforded the products (3b, 3d, 3e) in good to high yield, whereas methoxy groups gave 3c and 3f in 71% and 54% yields, respectively. In some cases involving tertbutyl, methoxy, dimethoxy groups, a regioisomeric mixture was observed (approximately 1:2 for 3b, 3f and 4:3 for 3e). However, only one regioisomer was detected for the desired products (3c, 3g, 3i, 3k). Polyaromatic 3g and compounds 3h, 3i, 3j were obtained in moderate yields.
Scheme 2. Substrate Scope of Different Isocyanates. aReagents and conditions: osilyl aryl triflate (0.1 mmol), tosyl isocyanate (0.24 mmol), CsF (0.3 mmol), acetonitrile (0.1 M), 100 °C, 14 h. bTosyl isocyanate (0.12 mmol), CsF (0.2 mmol). c Tosyl isocyanate (0.3 mmol), CsF (0.4 mmol).
Table 1 Optimization studies.
a
Entry c,e
1 2c 3 4 5 6 7 8d 9d 10 11g 12 a b c d e f g
F source
Solvent
CsF CsF CsF CsF CsF CsF CsF TBAF TBAF KF K2CO3 K2CO3
Acetonitrile Acetonitrile Acetonitrile THF Acetonitrile Acetonitrile Acetonitrile THF THF Acetonitrile Acetonitrile Acetonitrile
Additive (1 equiv)
18-crown-6f MeOH iPrOH MeOH 18-crown-6 18-crown-6 18-crown-6
Reagents and conditions: o-silyl aryl triflate (0.1 mmol), tosyl isocyanate (0.24 mmol), fluoride source (0.3 mmol), solvent (0.1 M), 100 °C, 14 h. Isolated yield. Tosyl isocyanate (0.12 mmol). Fluoride source (0.12 mmol). Room Temperature. Additive (3 equiv.). Potassium carbonate (0.24 mmol).
Yield 3a (%)b 28% 48% 94% 0% 60% 76% 48% 0% 0% 28% 5% 4%
J.H. Seo, H.M. Ko / Tetrahedron Letters 59 (2018) 671–674
673
Scheme 1. Substrate Scope of Various Benzyne Precursors. aReagents and conditions: o-silyl aryl triflate (0.1 mmol), tosyl isocyanate (0.24 mmol), CsF (0.3 mmol), acetonitrile (0.1 M), 100 °C, 14 h. bTosyl isocyanate (0.12 mmol), CsF (0.2 mmol). cTosyl isocyanate (0.5 mmol), CsF (0.6 mmol). dRatio of isomers was determined by 1H NMR spectroscopy.
Scheme 3. Discovery of Unexpected Product 3e0
In order to further expand the reaction scope, different substituted isocyanates were also explored (Scheme 2). For the benzenesulfonyl isocyanates, para hydrogen as a neutral substituent or a para electron-withdrawing substituent such as F on the aryl group influenced the product yields (Scheme 2, 3aa-3ab). However, a variety of phenyl isocyanates bearing an aryl group instead of benzenesulfonyl group gave the desired products in 23–75% yield (Scheme 2, 3ac-3al). Introducing para methyl, methoxy and tertiary butyl groups onto the phenyl group did not hamper the Narylation reaction, affording 3ac, 3ad, 3ae in 77%, 75%, 51% yield, respectively. Additionally, electron-poor phenyl isocyanates with para-NO2, Cl, F, CF3 substituents reacted with benzyne 1a in moderate yields compared with benzenesulfonyl isocyanates. Interestingly, when 4-(6-methyl-2-benzothiazolyl)phenyl isocyanate was used, 3aj was afforded in 56% yield. Isocyanates with naphthyl and dimethylphenyl groups gave the corresponding products 3ak and 3al in 36%, 30% yields, respectively. Unfortu-
nately, when alkyl isocyanates were utilized instead of phenyl isocyanate, the reaction proceeded sluggishly and the desired product was not observed (3am, Scheme 2). During the investigation of several substrates, we observed the unexpected product 3e0 which had a biphenyl backbone with a tosyl amine in 45% yield (Scheme 3). This structure was confirmed by 1H NMR, 13C NMR, and high-resolution mass spectroscopy. The biphenyl scaffold is useful to develop new ligands, or synthesize materials, pharmaceuticals, and natural products. Reagents and conditions: o-silyl aryl triflate (0.1 mmol), tosyl isocyanate (0.24 mmol), CsF (0.3 mmol), acetonitrile (0.1 M), 100 °C, 14 h Conclusion An efficient transition-metal-free method for the preparation of aromatic amines using benzynes and isocyanates has been
674
J.H. Seo, H.M. Ko / Tetrahedron Letters 59 (2018) 671–674
developed. The reaction tolerates various functional groups such as alkyl, alkoxy, halogens, and polyaromatic groups. These mild reaction conditions display good yields and represent a novel synthetic strategy for the synthesis of substituted aniline compounds. A preliminary outcome to obtain diphenyl moiety is also described and a detailed examination regarding the scope of biphenyl formation is currently ongoing.
2.
3.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, FP7 Information and Communication Technologies & Future Planning (NRF-2015R1C1A2A01051829).
4.
A. Supplementary data
6. 7.
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.tetlet.2018.01.022. References 1. (a) Lawrence SA, ed. Amines: synthesis applications. Cambridge: Cambridge University Press; 2004;
properties
and
5.
(b) Rappoport Z, ed. The chemistry of anilines. Weinheim: Wiley VCH; 2007; (c) Ricci A, ed. Amino group chemistry: from synthesis to the life sciences. Weinheim: Wiley VCH; 2008. (a) Wolfe JP, Wagaw S, Marcoux J-F, Buchwald SL. Acc Chem Res. 1998;31:805–818; (b) Hartwig JF. Acc Chem Res. 2008;41:1534–1544. See also for Chan-Evans-Lam coupling; (c) Qiao JX, Lam PYS. Synthesis. 2011;829–856. (a) Barker TJ, Jarvo ER. Synthesis. 2011;3954–3964; (b) Cho SH, Kim JY, Kwak J, Chang S. Chem Soc Rev. 2011;40:5068–5083; (c) Zhang M, Zhang A. Synthesis. 2012;1–14; (d) Kuhl N, Hopkinson MN, Wencel-Delord J, Glorius F. Angew Chem Int Ed. 2012;51:10236–10254. (a) Yoshida H, Shirakawa E, Honda Y, Hiyama T. Angew Chem Int Ed. 2002;41:3247–3249; (b) Biswas K, Greaney MF. Org Lett. 2011;13:4946–4949. (a) Yoshida H, Fukushima H, Ohshita J, Kunai A. J Am Chem Soc. 2006;128:11040–11041; (b) Yoshida S, Yano T, Misawa Y, et al. J Am Chem Soc. 2015;137:14071–14074; (c) Swain SP, Shih Y-C, Tsay S-C, et al. Angew Chem Int Ed. 2015;54:9926–9930. Himeshima Y, Sonoda T, Kobayashi H. Chem Lett. 1983;1211–1214. (a) Bhojgude SS, Kaicharla T, Biju AT. Org Lett. 2013;15:5452–5455; (b) Hirsch M, Dhara S, Diesendruck CE. Org Lett. 2016;18:980–983; (c) Zhang J, Chen Z-X, Du T, Li B, Gu Y, Tian S-K. Org Lett. 2016;18:4872–4875; (d) Liu Z, Larock RC. Org Lett. 2003;5:4673–4675; (e) Zhang L, Geng Y, Jin Z. J Org Chem. 2016;81:3542–3552; (f) Lee Y-H, Chen Y-C, Hsieh J-C. Eur J Org Chem. 2012;2:247–250.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Transition-metal-free synthesis of aromatic amines via the reaction of benzynes with isocyanates Jeong Hoon Seo, Haye Min Ko ⇑ Department of Bio-Nano Chemistry, Wonkwang University, 460 Iksandae-ro, Iksan, Jeonbuk 54538, Republic of Korea
a r t i c l e
i n f o
Article history: Received 13 November 2017 Revised 28 December 2017 Accepted 5 January 2018 Available online 9 January 2018
a b s t r a c t An unexpected reaction between benzynes and isocyanates to generate aromatic amines has been developed under transition-metal-free conditions. The in situ prepared anions formed through cleavage of the NAC bond in isocyanates, reacted with aryne precursors to afford various aniline derivatives in moderate to excellent yield and tolerated various substituents on the o-silyl aryl triflate and the isocyanate. Ó 2018 Elsevier Ltd. All rights reserved.
Keywords: Aniline derivatives Transition-metal-free Benzynes Isocyanates Fluoride
Introduction Aromatic amines are important motifs in natural products, materials, pharmaceuticals and dyes.1 Therefore, efficient and concise synthetic strategies towards this class of compounds is of significant interest. Traditional methods to access these useful building blocks typically proceed via the formation of carbonnitrogen bonds. In particular, the palladium-catalyzed carbonnitrogen bond formation reaction (Buchwald-Hartwig amination), Ullmann reaction, and Chan-Lam coupling reaction in the presence of Cu are well represented in the literature.2 An amination reaction using carbon-hydrogen activation has been reported as an alternative methodology to construct CarylAN bonds.3 Despite several advantages, the requirement for expensive transition metals and directing groups is an obstacle for the utility of these reactions. Over the past decades, benzyne chemistry has been extremely well developed. The reactivity of benzyne intermediates is attributed to a lowered LUMO arising from a strained p-bond, which facilitates several coupling reactions with neutral nucleophiles such as ureas4 and imines.5 The in situ generated benzyne intermediates formed from o-silyl aryl triflates under the action of fluoride reported by Kobayashi and co-workers6 have allowed functionalization at the 1- and 2-positions of benzene. Such mild reaction conditions help overcome the limitations of transition metal catalyzed cross-coupling amination reactions,
such as the use of strong bases or high temperature. Interestingly, with respect to benzynes, mono N-arylation7 has not been studied in detail. A representative mono N-arylation reaction utilizing primary or secondary amines has been demonstrated by Larock and co-workers.7d Due to the difficulty in reacting tertiary amines with arynes according to the formation of an ammonium salt, only one successful reaction has been reported by Biju and co-workers
⇑ Corresponding author. E-mail address: [email protected] (H.M. Ko). https://doi.org/10.1016/j.tetlet.2018.01.022 0040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
Fig. 1. Synthetic Methods for Constructing CarylAN Bonds utilizing benzyne.
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J.H. Seo, H.M. Ko / Tetrahedron Letters 59 (2018) 671–674
(Fig. 1).7a To the best of our knowledge, the use of substituted isocyanates as a coupling partner for mono N-arylation employing benzyne precursors has not been reported. Herein, we report an efficient transition-metal free mono N-arylation reaction between benzynes and various isocyanates. Results and discussion Initially, we investigated the reaction employing the model substrate 1a and the commercially available tosyl isocyanate 2a (1.2 equiv.) in presence of CsF in acetonitrile at room temperature (Table 1). Unexpectedly, the tosyl protected aniline 3a was obtained in 28% yield (Entry 1). Encouraged by this result, the temperature was increased to 100 °C, which improved the yield to 48% (Entry 2). Interestingly, when the amount of tosyl isocyanate 2a (2.4 equiv.) was increased, product 3a was isolated in excellent yield 94% (Entry 3). Decreasing the fluoride source amount or using THF instead of acetonitrile, resulted in a diminished yield for 3a (Entry 4). To examine the effect of additives, 18-crown-6, methanol and isopropyl alcohol were examined (Entries 5–7). Unfortunately, the crown ether or alcohol provided lower yield. Additionally, the use of TBAF failed to give product 3a (Entries 8–9). Upon using KF/18-crown-6 or K2CO3/18-crown-6 as the benzyne triggering reagent, 3a was observed in 28% or trace amounts, respectively (Entries 10–12). Finally, the use of tosyl isocyanate 2a (2.4 equiv.) in presence of CsF (3.0 equiv.) in acetonitrile at 100 °C was selected as the best reaction conditions. With the optimized conditions in hand, the scope of benzyne precursors was examined (Scheme 1). Both electron-donating and electron-withdrawing functional groups on the benzyne precursors were tolerated in this reaction. Alkyl groups such as tertbutyl, methyl, and dimethyl afforded the products (3b, 3d, 3e) in good to high yield, whereas methoxy groups gave 3c and 3f in 71% and 54% yields, respectively. In some cases involving tertbutyl, methoxy, dimethoxy groups, a regioisomeric mixture was observed (approximately 1:2 for 3b, 3f and 4:3 for 3e). However, only one regioisomer was detected for the desired products (3c, 3g, 3i, 3k). Polyaromatic 3g and compounds 3h, 3i, 3j were obtained in moderate yields.
Scheme 2. Substrate Scope of Different Isocyanates. aReagents and conditions: osilyl aryl triflate (0.1 mmol), tosyl isocyanate (0.24 mmol), CsF (0.3 mmol), acetonitrile (0.1 M), 100 °C, 14 h. bTosyl isocyanate (0.12 mmol), CsF (0.2 mmol). c Tosyl isocyanate (0.3 mmol), CsF (0.4 mmol).
Table 1 Optimization studies.
a
Entry c,e
1 2c 3 4 5 6 7 8d 9d 10 11g 12 a b c d e f g
F source
Solvent
CsF CsF CsF CsF CsF CsF CsF TBAF TBAF KF K2CO3 K2CO3
Acetonitrile Acetonitrile Acetonitrile THF Acetonitrile Acetonitrile Acetonitrile THF THF Acetonitrile Acetonitrile Acetonitrile
Additive (1 equiv)
18-crown-6f MeOH iPrOH MeOH 18-crown-6 18-crown-6 18-crown-6
Reagents and conditions: o-silyl aryl triflate (0.1 mmol), tosyl isocyanate (0.24 mmol), fluoride source (0.3 mmol), solvent (0.1 M), 100 °C, 14 h. Isolated yield. Tosyl isocyanate (0.12 mmol). Fluoride source (0.12 mmol). Room Temperature. Additive (3 equiv.). Potassium carbonate (0.24 mmol).
Yield 3a (%)b 28% 48% 94% 0% 60% 76% 48% 0% 0% 28% 5% 4%
J.H. Seo, H.M. Ko / Tetrahedron Letters 59 (2018) 671–674
673
Scheme 1. Substrate Scope of Various Benzyne Precursors. aReagents and conditions: o-silyl aryl triflate (0.1 mmol), tosyl isocyanate (0.24 mmol), CsF (0.3 mmol), acetonitrile (0.1 M), 100 °C, 14 h. bTosyl isocyanate (0.12 mmol), CsF (0.2 mmol). cTosyl isocyanate (0.5 mmol), CsF (0.6 mmol). dRatio of isomers was determined by 1H NMR spectroscopy.
Scheme 3. Discovery of Unexpected Product 3e0
In order to further expand the reaction scope, different substituted isocyanates were also explored (Scheme 2). For the benzenesulfonyl isocyanates, para hydrogen as a neutral substituent or a para electron-withdrawing substituent such as F on the aryl group influenced the product yields (Scheme 2, 3aa-3ab). However, a variety of phenyl isocyanates bearing an aryl group instead of benzenesulfonyl group gave the desired products in 23–75% yield (Scheme 2, 3ac-3al). Introducing para methyl, methoxy and tertiary butyl groups onto the phenyl group did not hamper the Narylation reaction, affording 3ac, 3ad, 3ae in 77%, 75%, 51% yield, respectively. Additionally, electron-poor phenyl isocyanates with para-NO2, Cl, F, CF3 substituents reacted with benzyne 1a in moderate yields compared with benzenesulfonyl isocyanates. Interestingly, when 4-(6-methyl-2-benzothiazolyl)phenyl isocyanate was used, 3aj was afforded in 56% yield. Isocyanates with naphthyl and dimethylphenyl groups gave the corresponding products 3ak and 3al in 36%, 30% yields, respectively. Unfortu-
nately, when alkyl isocyanates were utilized instead of phenyl isocyanate, the reaction proceeded sluggishly and the desired product was not observed (3am, Scheme 2). During the investigation of several substrates, we observed the unexpected product 3e0 which had a biphenyl backbone with a tosyl amine in 45% yield (Scheme 3). This structure was confirmed by 1H NMR, 13C NMR, and high-resolution mass spectroscopy. The biphenyl scaffold is useful to develop new ligands, or synthesize materials, pharmaceuticals, and natural products. Reagents and conditions: o-silyl aryl triflate (0.1 mmol), tosyl isocyanate (0.24 mmol), CsF (0.3 mmol), acetonitrile (0.1 M), 100 °C, 14 h Conclusion An efficient transition-metal-free method for the preparation of aromatic amines using benzynes and isocyanates has been
674
J.H. Seo, H.M. Ko / Tetrahedron Letters 59 (2018) 671–674
developed. The reaction tolerates various functional groups such as alkyl, alkoxy, halogens, and polyaromatic groups. These mild reaction conditions display good yields and represent a novel synthetic strategy for the synthesis of substituted aniline compounds. A preliminary outcome to obtain diphenyl moiety is also described and a detailed examination regarding the scope of biphenyl formation is currently ongoing.
2.
3.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, FP7 Information and Communication Technologies & Future Planning (NRF-2015R1C1A2A01051829).
4.
A. Supplementary data
6. 7.
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.tetlet.2018.01.022. References 1. (a) Lawrence SA, ed. Amines: synthesis applications. Cambridge: Cambridge University Press; 2004;
properties
and
5.
(b) Rappoport Z, ed. The chemistry of anilines. Weinheim: Wiley VCH; 2007; (c) Ricci A, ed. Amino group chemistry: from synthesis to the life sciences. Weinheim: Wiley VCH; 2008. (a) Wolfe JP, Wagaw S, Marcoux J-F, Buchwald SL. Acc Chem Res. 1998;31:805–818; (b) Hartwig JF. Acc Chem Res. 2008;41:1534–1544. See also for Chan-Evans-Lam coupling; (c) Qiao JX, Lam PYS. Synthesis. 2011;829–856. (a) Barker TJ, Jarvo ER. Synthesis. 2011;3954–3964; (b) Cho SH, Kim JY, Kwak J, Chang S. Chem Soc Rev. 2011;40:5068–5083; (c) Zhang M, Zhang A. Synthesis. 2012;1–14; (d) Kuhl N, Hopkinson MN, Wencel-Delord J, Glorius F. Angew Chem Int Ed. 2012;51:10236–10254. (a) Yoshida H, Shirakawa E, Honda Y, Hiyama T. Angew Chem Int Ed. 2002;41:3247–3249; (b) Biswas K, Greaney MF. Org Lett. 2011;13:4946–4949. (a) Yoshida H, Fukushima H, Ohshita J, Kunai A. J Am Chem Soc. 2006;128:11040–11041; (b) Yoshida S, Yano T, Misawa Y, et al. J Am Chem Soc. 2015;137:14071–14074; (c) Swain SP, Shih Y-C, Tsay S-C, et al. Angew Chem Int Ed. 2015;54:9926–9930. Himeshima Y, Sonoda T, Kobayashi H. Chem Lett. 1983;1211–1214. (a) Bhojgude SS, Kaicharla T, Biju AT. Org Lett. 2013;15:5452–5455; (b) Hirsch M, Dhara S, Diesendruck CE. Org Lett. 2016;18:980–983; (c) Zhang J, Chen Z-X, Du T, Li B, Gu Y, Tian S-K. Org Lett. 2016;18:4872–4875; (d) Liu Z, Larock RC. Org Lett. 2003;5:4673–4675; (e) Zhang L, Geng Y, Jin Z. J Org Chem. 2016;81:3542–3552; (f) Lee Y-H, Chen Y-C, Hsieh J-C. Eur J Org Chem. 2012;2:247–250.