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DOI: 10.1055/a-2214-5299
Rhodium-Catalyzed Direct Vinylene Annulation of 2-Aryloxazolines and Cascade Ring-Opening Using a Vinyl Selenone
This research was supported by a Grant-in-Aid for Scientific Research from JSPS (Specially Promoted Research, Grant No. JP 17H06092).
Abstract
Over the past two decades, transition-metal-catalyzed C–H activation and the subsequent oxidative cyclization with alkynes or their surrogates has emerged as a powerful synthetic tool for fused heteroaromatics. We report a Rh(III)-catalyzed annulation and ring-opening cascade reaction with 2-aryloxazolines. By utilizing a vinyl selenone as an oxidizing acetylene surrogate, the target three-component coupling products were obtained in high yields without using a stoichiometric amount of external oxidant.
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Key words
C–H bond activation - oxazolines - annulation - rhodium catalysis - vinylselenones - cascade reactionDerivatives of oxazoline (also known as 4,5-dihydro-1,3-oxazole) are frequently found in natural products[1] and synthetic functional molecules such as chiral ligands,[2] polymers,[3] and glycosyl donors.[4] In the organic-chemistry field, oxazolines have been used as directing groups because of their rigid structure and chemical stability. In particular, oxazoline-directed lithiation of aromatic substrates has been a useful synthetic tool over the past 50 years.[5] Recently, a transition-metal-catalyzed C–H activation strategy has attracted significant research interest,[6] and numerous direct bond-forming reactions have been developed by adopting oxazoline directing groups.[7]
In 2013, the Cheng group reported an efficient method for synthesizing polycyclic pyridinium salts from a series of N-heterocycles through Rh(III)-catalyzed C–H activation and subsequent oxidative annulation with alkynes.[8] They demonstrated that 2-phenyloxazoline can be coupled with diphenylacetylene in the presence of Cu(BF4)2 oxidant while maintaining its oxazoline ring skeleton (Scheme [1a]). A contrasting outcome was obtained in a recent work by Cui, where opening of the oxazoline ring was achieved by nucleophilic attack of an acetate anion derived from Cu(OAc)2 (Scheme [1b]).[9] Later, a few rare examples of Rh(III)-catalyzed tandem annulation/ring-opening reactions were developed by using iodonium ylides or α-diazo carbonyl compounds as the coupling partners.[10]
Our group has recently focused on the development of catalytic C–H activation and direct vinylene annulation reactions.[11] Vinylene carbonates[12] and vinyl selenones[13] are particularly useful vinylene-transfer reagents, because their oxidized nature permits the avoidance of the use of stoichiometric external oxidants. In 2023, we reported that vinyl selenones exhibit a higher productivity in an isoquinoline synthesis by vinylene annulation of imine derivatives, probably because of their enhanced Michael acceptor character.[11e] In this work, we developed a Rh(III)-catalyzed three-component reaction of a 2-aryloxazoline, a vinyl selenone, and an external nucleophile through oxazoline ring-opening (Scheme [1c]). This transformation achieves a challenging formal acetylene annulation in the presence of 1.0 mol% of a Rh catalyst to construct isoquinolone scaffolds.
a Standard conditions: 1a (0.1 mmol), 2a (0.15 mmol), [Cp*Rh(MeCN)3][SbF6]2, PivOH, solvent (1.0 mL), heating, 18 h.
b Estimated by NMR analysis.
c tert-Amyl alcohol.
d NaOPiv
e Not detected.
As an initial study, we examined the vinylene annulation of 2-phenyloxazoline (1a) with phenyl vinyl selenone (2a) as a model reaction (Table [1]; see the Supplementary Information for additional details). The corresponding product 3aa was obtained in 24% yield by using 6.0 mol% of [Cp*Rh(MeCN)3][SbF6]2 catalyst (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) and 1.2 equivalents of pivalic acid (PivOH) in toluene solvent (Table [1], entry 1). PivOH acted as a nucleophile to convert the oxazoline ring into an isoquinolinone scaffold. The structure of 3aa was unambiguously confirmed by X-ray crystallographic analysis.[14] On screening the solvents (entries 2–6), CHCl3 was found to give the highest yield (44%). The productivity was further improved on increasing the temperature and the amount of PivOH (entry 7). The Rh catalyst loading could be decreased to 1.0 mol% without affecting the yield of 3aa (entry 8). NaOPiv was not a suitable promoter for this transformation (entry 9). The highest yield (74%) was obtained when the reaction was conducted at 100 °C (entries 10 and 11). It is notable that the present system does not require any external oxidant to achieve the formal oxidative annulation. We assume that the leaving Se(IV) fragment acts as a two-electron oxidant within the catalytic cycle; however, the resulting Se(II) species has not yet been identified.
We then examined several vinyl compounds under the standard reaction conditions. Neither the vinyl selenide 2b nor the vinyl sulfone 2c produced a coupling product (Schemes 2a and 2b). Vinylene carbonate (2d) was not an effective coupling partner in this case, affording 3aa in 21% yield (Scheme [2c]). These results highlight the significance of the selenonyl group in facilitating the annulation reaction after an oxazoline-directed C–H activation. Oxazoline 1a was fully recovered under the standard reaction conditions in the absence of selenone 2a (Scheme [2d]). This clearly shows that the oxazoline ring does not open simply on heating with PivOH, and we therefore assume that the corresponding pyridinium species forms before the ring-opening event (see below). In addition, a secondary amide 4 was converted into the annulation product 5, albeit in a low yield (Scheme [2e]).
Next, the scope and limitation of our established reaction system was evaluated (Scheme [3]).[15] With regards to the nucleophile, both aliphatic and aromatic acids were applicable, affording the corresponding products 3aa–ad in isolated yields of 60–73%. Interestingly, succinimide and phthalimide could be used as nitrogen nucleophiles, giving 3ae (36%) and 3af (22%), respectively. We also tested several other nitrogen (carboxamide, sulfonamide) and sulfur (thiol) nucleophiles, but these were not suitable reactants for this system (not shown).
A series of substituted 2-aryloxazolines were then examined with benzoic acid as the nucleophile (Scheme [4]). To our delight, chloro (3ba), bromo (3ca), and iodo (3da) functionalities remained intact during the reaction, which would be beneficial for post-functionalization of the isoquinolinone scaffold. Phenyl (1e), CF3 (1f), and OMe (1g) groups were tolerated, and the corresponding coupling products were obtained in yields of 54–64%, whereas a strongly electron-donating NMe2 substituent (1h) significantly retarded the reaction, and most of the starting material was recovered. 2-(1-naphthyl)oxazoline (1i) was also an applicable substrate with the developed catalytic system. For the reaction of 2-(2-naphthyl)oxazoline (1j), a sterically more accessible site was selectively annulated to produce 3ja as a single isomer. However, the m-Cl (1k) and m-Br (1l) oxazolines afforded mixtures of isomers in high total yields.
In accordance with reports in the literature,[8] [9] [16] we propose the mechanism for the coupling reaction of 1a with 2a and PivOH that is shown in Scheme [5]. A catalytically active Rh(III) species, assumed to be [Cp*Rh(OPiv)]+, undergoes directing-group-assisted C–H bond activation to form the rhodacycle intermediate A. Vinyl selenone 2a coordinates to the metal and inserts into the Rh–C bond (A→B). A subsequent nucleophilic substitution takes place at the carbon atom adjacent to Rh (B→C), liberating a molecule of benzeneseleninic acid (PhSeO2H). β-Hydrogen elimination produces an oxazolium salt intermediate, which is then converted into the ring-opening product 3aa by nucleophilic attack of a pivalate anion. The concurrently formed Rh(III) hydride species might undergo reductive elimination to give a Rh(I) species. We believe that this is oxidized by Se(IV) to regenerate the catalytically active Rh(III) complex. In our previous report, the resulting Se(II) was almost quantitatively recovered as diphenyl diselenide,[11e] but in the present reaction, the fate of the selenium content has not been determined.
In summary, we have developed a Rh(III)-catalyzed annulation and ring-opening cascade reaction with a 2-aryloxazoline, a vinyl selenone, and an external nucleophile. This transformation achieves a challenging formal acetylene annulation without requiring a stoichiometric amount of a metal salt as an external oxidant. The use of a vinyl selenone was important to facilitate the oxazoline ring-opening process and to obtain the target three-component coupling products in high yields. It is notable that the present reaction can involve nitrogen nucleophiles (such as succinimide or phthalimide), but further optimization needs to be conducted to improve its productivity.
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Conflict of Interest
The authors declare no conflict of interest.
Supporting Information
- Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-2214-5299.
- Supporting Information
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References and Notes
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- 2a Satoh T, Miura M. Chem. Eur. J. 2010; 16: 11212
- 2b Colby DA, Bergman RG, Ellman JA. Chem. Rev. 2010; 110: 624
- 2c Kuhl N, Schröder N, Glorius F. Adv. Synth. Catal. 2014; 356: 1443
- 2d Boyarskiy VP, Ryabukhin DS, Bokach NA, Vasilyev AV. Chem. Rev. 2016; 116: 5894
- 2e Gulías M, Mascareñas JL. Angew. Chem. Int. Ed. 2016; 55: 11000
- 2f Yang Y, Li K, Cheng Y, Wan D, Li M, You J. Chem. Commun. 2016; 52: 2872
- 2g Yoshino T, Matsunaga S. Adv. Synth. Catal. 2017; 359: 1245
- 2h Sambiagio C, Schönbauer D, Blieck R, Dao-Huy T, Pototschnig G, Schaaf P, Wiesinger T, Zia MF, Wencel-Delord J, Besset T, Maes BU. W, Schnürch M. Chem. Soc. Rev. 2018; 47: 6603
- 2i Vásquez-Céspedes S, Wang X, Glorius F. ACS Catal. 2018; 8: 242
- 2j Santhoshkumar R, Cheng C.-H. Chem. Eur. J. 2019; 25: 9366
- 2k Kuang G, Liu G, Zhang X, Lu N, Peng Y, Xiao Q, Zhou Y. Synthesis 2020; 52: 993
- 2l Nishii Y, Miura M. ACS Catal. 2020; 10: 9747
- 2m Nishii Y, Miura M. ACS Catal. 2020; 10: 9747
- 2n Dhawa U, Kaplaneris N, Ackermann L. Org. Chem. Front. 2021; 8: 4886
- 3 Connon R, Roche B, Rokade BV, Guiry PJ. Chem. Rev. 2021; 121: 6373
- 4 Glassner M, Vergaelen M, Hoogenboom R. Polym. Int. 2018; 67: 32
- 5a Li C, Wang L.-X. Chem. Rev. 2018; 118: 8359
- 5b Fairbanks AJ. Beilstein J. Org. Chem. 2018; 14: 416
- 6 Gschwend HW, Hamdan A. J. Org. Chem. 1975; 40: 2008
- 7a Gong T.-J, Xiao B, Cheng W.-M, Su W, Xu J, Liu Z.-J, Liu L, Fu Y. J. Am. Chem. Soc. 2013; 135: 10630
- 7b Seki M. ACS Catal. 2014; 4: 4047
- 7c Mei R, Loup J, Ackermann L. ACS Catal. 2016; 6: 793
- 7d Kim H, Park G, Park J, Chang S. ACS Catal. 2016; 6: 5922
- 7e Hermann GN, Bolm C. ACS Catal. 2017; 7: 4592
- 7f Li Y.-G, Wang Z.-Y, Zou Y.-L, So C.-M, Kwong F.-Y, Qin H.-L, Kantchev EA. B. Synlett 2017; 28: 499
- 7g Liu S, Zhang S, Lin Q, Huang Y, Li B. Org. Lett. 2019; 21: 1134
- 7h Zhou C, Zhao J, Guo W, Jiang J, Wang J. Org. Lett. 2019; 21: 9315
- 7i Gou X.-Y, Li Y, Wang X.-G, Liu H.-C, Zhang B.-S, Zhao J.-H, Zhou Z.-Z, Liang Y.-M. Chem. Commun. 2019; 55: 5487
- 8 Luo C.-Z, Gandeepan P, Jayakumar J, Parthasarathy K, Chang Y.-W, Cheng C.-H. Chem. Eur. J. 2013; 19: 14181
- 9 Yang Z, Jie L, Yao Z, Yang Z, Cui X. Adv. Synth. Catal. 2019; 361: 214
- 10a Zhang X, Wang P, Zhu L, Chen B. Chin. Chem. Lett. 2021; 32: 695
- 10b Yang Z, Liu J, Li Y, Ding J, Zheng L, Liu Z.-Q. J. Org. Chem. 2022; 87: 14809
- 10c Yue X, Gao Y, Huang J, Feng Y, Cui X. Org. Lett. 2023; 25: 2923
- 11a Ghosh K, Nishii Y, Miura M. ACS Catal. 2019; 9: 11455
- 11b Ghosh K, Nishii Y, Miura M. Org. Lett. 2020; 22: 3547
- 11c Mihara G, Ghosh K, Nishii Y, Miura M. Org. Lett. 2020; 22: 5706
- 11d Kitano J, Nishii Y, Miura M. Org. Lett. 2022; 24: 5679
- 11e Inami A, Nishii Y, Hirano K, Miura M. Org. Lett. 2023; 25: 3206
- 12a Li X, Huang T, Song Y, Qi Y, Li L, Li Y, Xiao Q, Zhang Y. Org. Lett. 2020; 22: 5925
- 12b Hu Y, Nan J, Yin J, Huang G, Ren X, Ma Y. Org. Lett. 2021; 23: 8527
- 12c Nan J, Yin J, Gong X, Hu Y, Ma Y. Org. Lett. 2021; 23: 8910
- 12d Li Y, Wang H, Li Y, Li Y, Sun Y, Xia C, Li Y. J. Org. Chem. 2021; 86: 18204
- 12e Kim S, Choi SB, Kang JY, An W, Lee SH, Oh H, Ghosh P, Mishra NK, Kim IS. Asian J. Org. Chem. 2021; 10: 3005
- 12f Huang X, Xu Y, Li J, Lai R, Luo Y, Wang Q, Yang Z, Wu Y. Chin. Chem. Lett. 2021; 32: 3518
- 12g Yu Y, Wang Y, Li B, Tan Y, Zhao H, Li Z, Zhang C, Ma W. Adv. Synth. Catal. 2022; 364: 838
- 12h Liu M, Yan K, Wen J, Liu W, Wang M, Wang L, Wang X. Adv. Synth. Catal. 2022; 364: 512
- 13a Palomba M, Coelho Dias IF, Rosati O, Marini F. Molecules 2021; 26: 3148
- 13b Buyck T, Wang Q, Zhu J. J. Am. Chem. Soc. 2014; 136: 11524
- 14 CCDC 2302556 contains the supplementary crystallographic data for compound 3aa. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures
- 15 Isoquinolinones 3aa–la; General Procedure An oven-dried 10 mL screw-top tube was charged with the appropriate oxazoline 1 (0.2 mmol), phenyl vinyl selenone (2a; 64.5 mg, 0.3 mmol), [Cp*Rh(MeCN3)][SbF6]2 (1.7 mg, 1.0 mol%), and the appropriate nucleophile (1.0 mmol). The tube was filled with N2, and CHCl3 (2.0 mL) was added from a syringe. The mixture was heated in an oil bath at 100 °C for 18 h. The resulting suspension was filtered through a pad of Celite, eluting with CHCl3. The filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel) and, if indicated, gel-permeation chromatography. 2-(1-Oxoisoquinolin-2(1H)-yl)ethyl Pivalate (3aa) Purified by column chromatography [silica gel, hexane–EtOAc (2:1)] as a yellow solid; yield: 33.2 mg (61%); mp 108.3–110.3 °C. 1H NMR (400 MHz, CDCl3): δ = 8.43 (dt, J = 0.6, 8.0 Hz, 1 H), 7.67–7.62 (m, 1 H), 7.53–7.47 (m, 2 H), 7.05 (d, J = 7.4 Hz, 1 H), 6.48 (d, J = 7.3 Hz, 1 H), 4.42 (t, J = 5.3 Hz, 2 H), 4.27 (t, J = 5.3 Hz, 2 H), 1.17 (s, 9 H). 13C NMR (100 MHz, CDCl3): δ = 178.2, 162.2, 137.1, 132.4, 132.3, 127.8, 126.9, 126.2, 125.9, 105.7, 62.3, 48.4, 38.8, 27.2. HRMS (APCI): m/z [M + H]+ calcd for C16H20NO3: 274.1438; found: 274.1434.
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Corresponding Authors
Publication History
Received: 25 October 2023
Accepted after revision: 20 November 2023
Accepted Manuscript online:
20 November 2023
Article published online:
14 December 2023
© 2023. Thieme. All rights reserved
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References and Notes
- 1 Tilvi S, Singh KS. Curr. Org. Chem. 2016; 20: 898
- 2a Satoh T, Miura M. Chem. Eur. J. 2010; 16: 11212
- 2b Colby DA, Bergman RG, Ellman JA. Chem. Rev. 2010; 110: 624
- 2c Kuhl N, Schröder N, Glorius F. Adv. Synth. Catal. 2014; 356: 1443
- 2d Boyarskiy VP, Ryabukhin DS, Bokach NA, Vasilyev AV. Chem. Rev. 2016; 116: 5894
- 2e Gulías M, Mascareñas JL. Angew. Chem. Int. Ed. 2016; 55: 11000
- 2f Yang Y, Li K, Cheng Y, Wan D, Li M, You J. Chem. Commun. 2016; 52: 2872
- 2g Yoshino T, Matsunaga S. Adv. Synth. Catal. 2017; 359: 1245
- 2h Sambiagio C, Schönbauer D, Blieck R, Dao-Huy T, Pototschnig G, Schaaf P, Wiesinger T, Zia MF, Wencel-Delord J, Besset T, Maes BU. W, Schnürch M. Chem. Soc. Rev. 2018; 47: 6603
- 2i Vásquez-Céspedes S, Wang X, Glorius F. ACS Catal. 2018; 8: 242
- 2j Santhoshkumar R, Cheng C.-H. Chem. Eur. J. 2019; 25: 9366
- 2k Kuang G, Liu G, Zhang X, Lu N, Peng Y, Xiao Q, Zhou Y. Synthesis 2020; 52: 993
- 2l Nishii Y, Miura M. ACS Catal. 2020; 10: 9747
- 2m Nishii Y, Miura M. ACS Catal. 2020; 10: 9747
- 2n Dhawa U, Kaplaneris N, Ackermann L. Org. Chem. Front. 2021; 8: 4886
- 3 Connon R, Roche B, Rokade BV, Guiry PJ. Chem. Rev. 2021; 121: 6373
- 4 Glassner M, Vergaelen M, Hoogenboom R. Polym. Int. 2018; 67: 32
- 5a Li C, Wang L.-X. Chem. Rev. 2018; 118: 8359
- 5b Fairbanks AJ. Beilstein J. Org. Chem. 2018; 14: 416
- 6 Gschwend HW, Hamdan A. J. Org. Chem. 1975; 40: 2008
- 7a Gong T.-J, Xiao B, Cheng W.-M, Su W, Xu J, Liu Z.-J, Liu L, Fu Y. J. Am. Chem. Soc. 2013; 135: 10630
- 7b Seki M. ACS Catal. 2014; 4: 4047
- 7c Mei R, Loup J, Ackermann L. ACS Catal. 2016; 6: 793
- 7d Kim H, Park G, Park J, Chang S. ACS Catal. 2016; 6: 5922
- 7e Hermann GN, Bolm C. ACS Catal. 2017; 7: 4592
- 7f Li Y.-G, Wang Z.-Y, Zou Y.-L, So C.-M, Kwong F.-Y, Qin H.-L, Kantchev EA. B. Synlett 2017; 28: 499
- 7g Liu S, Zhang S, Lin Q, Huang Y, Li B. Org. Lett. 2019; 21: 1134
- 7h Zhou C, Zhao J, Guo W, Jiang J, Wang J. Org. Lett. 2019; 21: 9315
- 7i Gou X.-Y, Li Y, Wang X.-G, Liu H.-C, Zhang B.-S, Zhao J.-H, Zhou Z.-Z, Liang Y.-M. Chem. Commun. 2019; 55: 5487
- 8 Luo C.-Z, Gandeepan P, Jayakumar J, Parthasarathy K, Chang Y.-W, Cheng C.-H. Chem. Eur. J. 2013; 19: 14181
- 9 Yang Z, Jie L, Yao Z, Yang Z, Cui X. Adv. Synth. Catal. 2019; 361: 214
- 10a Zhang X, Wang P, Zhu L, Chen B. Chin. Chem. Lett. 2021; 32: 695
- 10b Yang Z, Liu J, Li Y, Ding J, Zheng L, Liu Z.-Q. J. Org. Chem. 2022; 87: 14809
- 10c Yue X, Gao Y, Huang J, Feng Y, Cui X. Org. Lett. 2023; 25: 2923
- 11a Ghosh K, Nishii Y, Miura M. ACS Catal. 2019; 9: 11455
- 11b Ghosh K, Nishii Y, Miura M. Org. Lett. 2020; 22: 3547
- 11c Mihara G, Ghosh K, Nishii Y, Miura M. Org. Lett. 2020; 22: 5706
- 11d Kitano J, Nishii Y, Miura M. Org. Lett. 2022; 24: 5679
- 11e Inami A, Nishii Y, Hirano K, Miura M. Org. Lett. 2023; 25: 3206
- 12a Li X, Huang T, Song Y, Qi Y, Li L, Li Y, Xiao Q, Zhang Y. Org. Lett. 2020; 22: 5925
- 12b Hu Y, Nan J, Yin J, Huang G, Ren X, Ma Y. Org. Lett. 2021; 23: 8527
- 12c Nan J, Yin J, Gong X, Hu Y, Ma Y. Org. Lett. 2021; 23: 8910
- 12d Li Y, Wang H, Li Y, Li Y, Sun Y, Xia C, Li Y. J. Org. Chem. 2021; 86: 18204
- 12e Kim S, Choi SB, Kang JY, An W, Lee SH, Oh H, Ghosh P, Mishra NK, Kim IS. Asian J. Org. Chem. 2021; 10: 3005
- 12f Huang X, Xu Y, Li J, Lai R, Luo Y, Wang Q, Yang Z, Wu Y. Chin. Chem. Lett. 2021; 32: 3518
- 12g Yu Y, Wang Y, Li B, Tan Y, Zhao H, Li Z, Zhang C, Ma W. Adv. Synth. Catal. 2022; 364: 838
- 12h Liu M, Yan K, Wen J, Liu W, Wang M, Wang L, Wang X. Adv. Synth. Catal. 2022; 364: 512
- 13a Palomba M, Coelho Dias IF, Rosati O, Marini F. Molecules 2021; 26: 3148
- 13b Buyck T, Wang Q, Zhu J. J. Am. Chem. Soc. 2014; 136: 11524
- 14 CCDC 2302556 contains the supplementary crystallographic data for compound 3aa. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures
- 15 Isoquinolinones 3aa–la; General Procedure An oven-dried 10 mL screw-top tube was charged with the appropriate oxazoline 1 (0.2 mmol), phenyl vinyl selenone (2a; 64.5 mg, 0.3 mmol), [Cp*Rh(MeCN3)][SbF6]2 (1.7 mg, 1.0 mol%), and the appropriate nucleophile (1.0 mmol). The tube was filled with N2, and CHCl3 (2.0 mL) was added from a syringe. The mixture was heated in an oil bath at 100 °C for 18 h. The resulting suspension was filtered through a pad of Celite, eluting with CHCl3. The filtrate was concentrated in vacuo, and the residue was purified by column chromatography (silica gel) and, if indicated, gel-permeation chromatography. 2-(1-Oxoisoquinolin-2(1H)-yl)ethyl Pivalate (3aa) Purified by column chromatography [silica gel, hexane–EtOAc (2:1)] as a yellow solid; yield: 33.2 mg (61%); mp 108.3–110.3 °C. 1H NMR (400 MHz, CDCl3): δ = 8.43 (dt, J = 0.6, 8.0 Hz, 1 H), 7.67–7.62 (m, 1 H), 7.53–7.47 (m, 2 H), 7.05 (d, J = 7.4 Hz, 1 H), 6.48 (d, J = 7.3 Hz, 1 H), 4.42 (t, J = 5.3 Hz, 2 H), 4.27 (t, J = 5.3 Hz, 2 H), 1.17 (s, 9 H). 13C NMR (100 MHz, CDCl3): δ = 178.2, 162.2, 137.1, 132.4, 132.3, 127.8, 126.9, 126.2, 125.9, 105.7, 62.3, 48.4, 38.8, 27.2. HRMS (APCI): m/z [M + H]+ calcd for C16H20NO3: 274.1438; found: 274.1434.
For selected reviews, see:
For selected recent reviews, see:
For selected recent examples, see:
For related examples, see:
