Formal [3 + 3] annulation of isatin-derived 2-bromoenals with 1,3-dicarbonyl compounds enabled by Lewis acid/N-heterocyclic carbene cooperative catalysis

Abstract

A series of novel isatin-derived 2-bromoenals were synthesized and applied in a formal [3 + 3] annulation with 1,3-dicarbonyl compounds enabled by a NHC/Lewis acid cooperative catalysis strategy. This newly developed methodology offers rapid access to functionalized spirooxindole δ-lactones. The newly synthesized isatin-derived 2-bromoenals may be further used as potential electrophilic 1,3-synthons for the diversity-oriented synthesis of spirooxindoles.

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Over the past two decades, the N-heterocyclic carbene (NHC)-catalyzed umpolung (polarity reversal) of aldehydes has opened up a new and unique area in the unconventional formation of carbon–carbon and carbon–heteroatom bonds via various reactive intermediates.¹ Among these intermediates, nucleophilic acyl anion intermediates² and homoenolate equivalents³ generated with NHC catalysis have attracted immense attention. Recently, α,β-unsaturated acyl azoliums I have been used as novel and versatile electrophilic 1,3-synthons for the synthesis of various heterocyclic compounds, such as dihydropyranones and dihydropyridinones, since their first discovery in 2006.⁴ To date, α,β-unsaturated acyl azoliums I have been commonly generated from six types of precursors, including ynals,⁵ enals under external oxidative conditions,^5f,6 2-haloenals,⁷ α,β-unsaturated esters⁸ or acyl fluorides,⁹ and in situ generated mixed α,β-unsaturated anhydrides from their corresponding carboxylic acids¹⁰ (Scheme 1).

Scheme 1 Generation of α,β-unsaturated acyl azoliums from various precursors.

Spirooxindoles have emerged as attractive privileged structures and have become important synthetic targets due to their prevalence in numerous natural products and synthetic compounds with diverse biological activities. However, the architecture of spirooxindole frameworks has always been a challenging endeavor for organic chemists because it often requires specific design strategies to install the carbocyclic or heterocyclic moieties at the C3 position of the oxindole core to form a spiro-quaternary carbon center. Therefore, the development of simple and versatile substrates or synthetic approaches for the diversity-oriented construction of spirooxindole skeletons is highly desirable.

As a continuation of our long-lasting goal to discover novel methodologies for rapid access to polycyclic indole derivatives,^{3m,5b,5f,6i,7g,11} we reasoned that isatin-derived α,β-unsaturated acyl azoliums II generated from diverse precursors under NHC catalysis could be utilized as versatile electrophilic 1,3-synthons to combine with different bisnucleophiles for the divergent synthesis of skeletally diverse spirooxindoles (Scheme 2). During our study on the generation and applications of isatin-derived α,β-unsaturated acyl azoliums II, Zhong and co-workers^6h very recently reported an oxidative NHC-catalyzed formal [3 + 3] annulation of imines with isatin-derived enals 3 for rapid access to spirooxindole 4 (Scheme 2). In this reaction, Z-isomers 3 were separated from the Z/E mixtures 2, which were prepared from substituted isatins 1 within 2 steps using known procedures. However, it is always difficult and time-consuming to separate Z-isomers 3 from their Z/E mixtures 2, which are obtained with low stereoselectivity in most cases. On the other hand, enals 3 are unstable in air, therefore they should be maintained under an inert atmosphere and be used as soon as possible. Because 2-bromoenals have been reported to be more stable than their corresponding enals and have been frequently used as α,β-unsaturated acyl azolium precursors, we tried to synthesize isatin-derived 2-bromoenals 5 via the electrophilic addition of Br₂ to Z/E mixtures 2, followed by elimination with a base in a one-pot procedure. Gratifyingly, introduction of the bromine atom to the α-position of the enals 2 highly enhances the stereoselectivity, which may be attributed to the steric effect of the more hindered bromine atom. E-isomers 5 were formed as the major products (E/Z = 9 : 1 to >20 : 1) and proved to be more stable in air (Scheme 2). The pure E-isomers 5 can be easily obtained by recrystallization of the crude products. The structure and stereochemistry of the E-isomers 5 were established by spectroscopic analysis and further confirmed by X-ray crystallography of 5d. To test the reactivity of the isatin-derived 2-bromoenals 5, 1,3-dicarbonyl compounds 6, good 1,3-bisnucleophiles well-established for formal [3 + 3] annulations with α,β-unsaturated acyl azoliums I, were used to react with substrates 5 under NHC/base conditions to deliver the desired spirooxindole δ-lactones 7 (Scheme 2). Herein, we wish to report the results.

Scheme 2 Generation and applications of isatin-derived α,β-unsaturated acyl azoliums II.

We commenced the optimization studies using isatin-derived 2-bromoenal 5a and ethyl acetoacetate 6a as model substrates (Table 1). The initial screening of various carbene precursors A–F was discouraging as only the precatalyst F afforded the desired product 7a in less than 10% yield (entries 1–3). Further examination of a variety of bases and solvents using F as the precatalyst revealed that the yield was improved to 42% when the reaction was performed in 1,4-dioxane employing Cs₂CO₃ as a base (entry 12). In recent years, Lewis acids have been proven to coordinate with the substrates effectively and thus can facilitate the reactions, resulting in enhanced reaction yields as well as stereoselectivity.^6f,6m,11,12 Inspired by the findings, we tried several commonly used Lewis acids and found that LiCl could enhance the reaction yield to 63% (entry 15). Based on the abovementioned observations, the optimal reaction conditions were finally established as the reaction was carried out in 1,4-dioxane using Cs₂CO₃ (1.5 equiv.) as a base in the presence of 15 mol% of F and 1.1 equivalent of LiCl.

Table 1 Optimization of the reaction conditions^a

Entry	Catalyst	Base	Solvent	Additive	Yield^b
a All the reactions were performed in a sealed tube on a 0.2 mmol scale with 1.0 equivalent of 5a, 2.0 equivalents of 6a, 15 mol% of a carbene precursor, 1.5 equivalents of a base and 200 mg of 4 Å MS in an anhydrous solvent (3 mL) at 50 °C for 2 h under N2.b Isolated yields based on 5a.c 1.1 equivalents of a Lewis acid was used.d 20 mol% of Lewis acid was used. DBU = 1,8-diazabicyclo[5.4.0]-undec-7-ene; Mes = 2,4,6-(CH3)3C6H2; DIPEA = N,N-diisopropylethylamine.
1	A, B	DBU	THF	None	0
2	C–E	DBU	THF	None	Trace
3	F	DBU	THF	None	<10
4	F	K2CO3	THF	None	23
5	F	Et3N	THF	None	<10
6	F	DIPEA	THF	None	21
7	F	tBuOK	THF	None	16
8	F	Cs2CO3	THF	None	29
9	F	Cs2CO3	DCM	None	11
10	F	Cs2CO3	CH3CN	None	Trace
11	F	Cs2CO3	PhMe	None	<10
12	F	Cs2CO3	1,4-Dioxane	None	42
13	F	Cs2CO3	1,4-Dioxane	Ti(OPr)4^c	20
14	F	Cs2CO3	1,4-Dioxane	Yb(OTf)3^d	28
15	F	Cs2CO3	1,4-Dioxane	LiCl^c	63

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After establishing the optimal reaction conditions, we explored the scope of the reaction between various isatin-derived 2-bromoenals 5 with 1,3-dicarbonyl compounds 6 (Table 2). First, a variety of 1,3-dicarbonyl compounds were tested. The β-keto esters 6b–f with diverse substituted phenyl group underwent efficient annulation to afford products 7b–f in moderate to good yields (entries 2–6). The β-diketones 6g–i were also found to be applicable to this reaction (entries 7–9). However, cyclohexane-1,3-dione 6j did not work for this reaction (entry 10). Subsequently, isatin-derived 2-bromoenals 5 with diverse substituents on the phenyl ring were further examined using 1-phenylbutane-1,3-dione 6h as the model substrate (entries 11–15). 2-Bromoenals 5b–f with electron-donating and electron-withdrawing groups on the phenyl ring were well tolerated towards the reaction. In particular, substrate 5e with a 5-methyl group afforded the product 7m in a 95% yield (entry 14).

Table 2 The reaction scope^a

Entry	R^1, 5	R^2, R^3, 6	Products	Yield^b (%)
a All the reactions were performed in a sealed tube on a 0.2 mmol scale with 1.0 equivalent of 5, 2.0 equivalents of 6, 15 mol% of F, 1.5 equivalents of Cs2CO3, 1.1 equiv. of LiCl and 200 mg of 4 Å MS in an anhydrous 1,4-dioxane (3 mL) at 50 °C for 2 h under N2.b Isolated yields based on 5a.
1	H, 5a	Me, OEt, 6a	7a	63
2	H, 5a	Ph, OEt, 6b	7b	57
3	H, 5a	(4-OMe)Ph, OEt, 6c	7c	58
4	H, 5a	Ph, OMe, 6d	7d	80
5	H, 5a	(4-Me)Ph, OMe, 6e	7e	63
6	H, 5a	(4-Cl)Ph, OMe, 6f	7f	67
7	H, 5a	Me, Me, 6g	7g	54
8	H, 5a	Ph, Me, 6h	7h	80
9	H, 5a	Ph, Ph, 6i	7i	68
10	H, 5a	Cyclohexane-1,3-dione, 6j		0
11	5-F, 5b	Ph, Me, 6h	7j	62
12	5-Cl, 5c	Ph, Me, 6h	7k	68
13	5-Br, 5d	Ph, Me, 6h	7l	45
14	5-Me, 5e	Ph, Me, 6h	7m	95
15	7-Cl, 5f	Ph, Me, 6h	7n	45

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Because β-naphthol^5g and enolizable carbonyl compounds^7c have been successfully applied to the formal [3 + 3] annulations with α,β-unsaturated acyl azoliums I, we tested the feasibility of the annulation between 5a with β-naphthol and 3-oxo-3-phenylpropanenitrile (Scheme 3). Unfortunately, neither of the substrates was found to be applicable to this methodology.

Scheme 3 The reactions of 2-bromoenal 5a with β-naphthol or 3-oxo-3-phenylpropanenitrile.

A preliminary enantioselective study of the reaction was then carried out using chiral carbene precursors G and H. However, the desired product 7a was obtained in only moderate yields and had a 0% ee value in both cases (Scheme 4).

Scheme 4 Preliminary enantioselective studies.

The ring-opening reaction of the spirooxindole δ-lactones was also carried out (Scheme 5). Interestingly, when compound 7k was heated in MeOH, the ring-opening product 8 was obtained in a 93% yield with the loss of an acetyl group.

Scheme 5 The ring-opening reaction of the product 7k.

A plausible catalytic cycle is proposed in Scheme 6. The combination of isatin-derived 2-bromoenal 5a with NHC F′ generated upon the deprotonation of carbene precursor F with a base affords the Breslow intermediate 9. The a³–d³ umpolung of 9 induces the formation of intermediate 10, which is tautomerized to bromoacyl azolium 11. The subsequent leaving of the bromide obtains the more stable (E)-isatin-derived α,β-unsaturated acyl azolium 12. The stereochemistry of 12 was determined by trapping it with ethanol to only afford the (E)-α,β-unsaturated ester 13 in a 78% yield as a known compound.¹³ It can be noted that even if a E/Z (9 : 1) mixture of 5a was applied to this reaction, only E-ester 13 was obtained in similar yield. The Michael addition of carbon-centered nucleophile 14 derived from 6a under base/LiCl conditions to acyl azolium 12 generates intermediate 15, followed by intramolecular cyclization to afford spirooxindole δ-lactones 7a and regenerate NHC F′.

Scheme 6 The proposed mechanism.

In summary, a cooperative NHC/Lewis acid-mediated formal [3 + 3] annulation of various isatin-derived 2-bromoenals 5 with 1,3-dicarbonyl compounds 6 is described. This protocol offers rapid access to functionalized spirooxindole δ-lactones 7. The newly synthesized isatin-derived 2-bromoenals 5 can be utilized as promising versatile electrophilic 1,3-synthons for the divergent construction of diverse spirooxindole skeletons. An enantioselective synthetic protocol from other precursors as well as further applications of isatin-derived 2-bromoenals to the synthesis of diverse spirooxindoles are currently undergoing study in our laboratory.

Experimental section

General procedure for the synthesis of 5 from 2

To the solution of compounds 2 (263 mg, 1.0 mmol) in CH₂Cl₂ (3–4 mL) was added Br₂ (62 μL, 1.2 mmol). The resulting mixture was stirred at 5–10 °C for 15 min, followed by the addition of Et₃N (235 μL, 1.7 mmol). The mixture was further stirred at 5–10 °C until completion of the reaction, as monitored by TLC. Then, saturated sodium carbonate aqueous solution (20 mL) was added and the resulting mixture was extracted with CH₂Cl₂ (3 × 10 mL). The organic phase was dried over anhydrous Na₂SO₄, and evaporated under reduced pressure. The residue was purified by chromatography on silica gel using hexane/EtOAc (3 : 1) as the eluent to afford products 5 as red solids.

General procedure for the synthesis of products 7

An oven-dried 15 mL glass cylindrical pressure vessel was charged with 2-bromoenals 5 (0.2 mmol), 1,3-dicarbonyl compounds 6 (0.4 mmol), carbene precursor F (8 mg, 0.03 mmol), Cs₂CO₃ (98 mg, 0.3 mmol), LiCl (9.0 mg, 0.22 mmol), and 200 mg of 4 Å MS under N₂ atmosphere. Then, anhydrous 1,4-dioxane (3 mL) was added and the vessel was immediately sealed tightly. The resulting mixture was stirred at 50 °C for 2 h. The mixture was cooled to room temperature. The solvent was evaporated under reduced pressure and the residue was purified by chromatography on silica gel using hexane/EtOAc (5 : 1) as the eluent afford products 7.

Acknowledgements

This study is funded by the National Natural Science Foundation of China (No. 21572270 and 81172933), the Jiangsu Provincial Natural Science Foundation of China (No. BK20131305), College Students Innovation Project for the R&D of Novel Drugs (J1030830), the National Undergraduate Training Program for Innovation and Entrepreneurship, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

References

1. For selected reviews, see: (a) D. M. Flanigan, F. Romanov-Michailidis, N. A. White and T. Rovis, Chem. Rev., 2015, 115, 9307; (b) J. Mahatthananchai and J. W. Bode, Acc. Chem. Res., 2014, 47, 696; (c) H. U. Vora and T. Rovis, Aldrichimica Acta, 2011, 44, 3; (d) N. Marion, S. Diez-Gonzalez and S. P. Nolan, Angew. Chem., Int. Ed., 2007, 46, 2988; (e) A. Grossmann and D. Enders, Angew. Chem., Int. Ed., 2012, 51, 314; (f) X. Bugaut and F. Glorius, Chem. Soc. Rev., 2012, 41, 3511; (g) P. Dominguez de Maria and S. Shanmuganathan, Curr. Org. Chem., 2011, 15, 2083; (h) D. C. M. Albanese and N. Gaggero, Eur. J. Org. Chem., 2014, 5631; (i) M. N. Hopkinson, C. Richter, M. Schedler and F. Glorius, Nature, 2014, 510, 485; (j) J. Douglas, G. Churchill and A. D. Smith, Synthesis, 2012, 44, 2295; (k) J. Izquierdo, G. E. Hutson, D. T. Cohen and K. A. Scheidt, Angew. Chem., Int. Ed., 2012, 51, 11686; (l) D. Enders and T. Balensiefer, Acc. Chem. Res., 2004, 37, 534; (m) D. Enders, O. Niemeier and A. Henseler, Chem. Rev., 2007, 107, 5606.
2. For selected examples, see: (a) J. Xu, C. Mou, T. Zhu, B.-A. Song and Y. R. Chi, Org. Lett., 2014, 16, 3272; (b) T. Liu, S.-M. Han, L.-L. Han, L. Wang, X.-Y. Cui, C.-Y. Du and S. Bi, Org. Biomol. Chem., 2015, 13, 3654; (c) Z. Rafiński, A. Kozakiewicz and K. Rafińska, Tetrahedron, 2014, 70, 5739; (d) S. M. Langdon, C. Y. Legault and M. Gravel, J. Org. Chem., 2015, 80, 3597; (e) C. G. Goodman and J. S. Johnson, J. Am. Chem. Soc., 2014, 136, 7539; (f) M.-Q. Jia and S.-L. You, ACS Catal., 2013, 3, 622; (g) L.-H. Sun, Z.-Q. Liang, W.-Q. Jia and S. Ye, Angew. Chem., Int. Ed., 2013, 52, 5803; (h) Z. Rafiński, A. Kozakiewicz and K. Rafińska, ACS Catal., 2014, 4, 1404; (i) A. Patra, A. Bhunia and A. T. Biju, Org. Lett., 2014, 16, 4798; (j) J. Zhang, C. Xing, B. Tiwari and Y. R. Chi, J. Am. Chem. Soc., 2013, 135, 8113; (k) K. R. Law and C. S. P. McErlean, Chem.–Eur. J., 2013, 19, 15852; (l) P. M. Lalli, T. S. Rodrigues, A. M. Arouca, M. N. Eberlin and B. A. D. Neto, RSC Adv., 2012, 2, 3201.
3. For selected examples, see: (a) C. Burstein and F. Glorius, Angew. Chem., Int. Ed., 2004, 43, 6205; (b) S. S. Sohn, E. L. Rosen and J. W. Bode, J. Am. Chem. Soc., 2004, 126, 14370; (c) M. He and J. W. Bode, Org. Lett., 2005, 7, 3131; (d) A. Chan and K. A. Scheidt, J. Am. Chem. Soc., 2007, 129, 5334; (e) A. Chan and K. A. Scheidt, J. Am. Chem. Soc., 2008, 130, 2740; (f) Y. Li, Z.-A. Zhao, H. He and S.-L. You, Adv. Synth. Catal., 2008, 350, 1885; (g) E. M. Phillips, T. E. Reynolds and K. A. Scheidt, J. Am. Chem. Soc., 2008, 130, 2416; (h) M. Rommel, T. Fukuzumi and J. W. Bode, J. Am. Chem. Soc., 2008, 130, 17266; (i) J.-L. Li, B. Sahoo, C.-G. Daniliuc and F. Glorius, Angew. Chem., Int. Ed., 2014, 53, 10515; (j) K. P. Jang, G. E. Hutson, R. C. Johnston, E. O. McCusker, P. H. Y. Cheong and K. A. Scheidt, J. Am. Chem. Soc., 2014, 136, 76; (k) C. Guo, B. Sahoo, C. G. Daniliuc and F. Glorius, J. Am. Chem. Soc., 2014, 136, 17402; (l) C. Guo, M. Schedler, C. G. Daniliuc and F. Glorius, Angew. Chem., Int. Ed., 2014, 53, 10232; (m) J. Xu, S. Hu, Y. Lu, Y. Dong, W. Tang, T. Lu and D. Du, Adv. Synth. Catal., 2015, 357, 923.
4. K. Zeitler, Org. Lett., 2006, 8, 637.
5. For selected examples, see: (a) Z.-Q. Zhu, X.-L. Zheng, N.-F. Jiang, X. Wan and J.-C. Xiao, Chem. Commun., 2011, 47, 8670; (b) D. Du, Z. Hu, J. Jin, Y. Lu, W. Tang, B. Wang and T. Lu, Org. Lett., 2012, 14, 1274; (c) J. Mahatthananchai, J. Kaeobamrung and J. W. Bode, ACS Catal., 2012, 2, 494; (d) F. Romanov-Michailidis, C. Besnard and A. Alexakis, Org. Lett., 2012, 14, 4906; (e) B. Zhou, Z. Luo and Y. Li, Chem.–Eur. J., 2013, 19, 4428; (f) Y. Lu, W. Tang, Y. Zhang, D. Du and T. Lu, Adv. Synth. Catal., 2013, 355, 321; (g) J. Kaeobamrung, J. Mahatthananchai, P. Zheng and J. W. Bode, J. Am. Chem. Soc., 2010, 132, 8810.
6. For selected examples, see: (a) S. D. Sarkar and A. Studer, Angew. Chem., Int. Ed., 2010, 49, 9266; (b) A. Biswas, S. D. Sarkar, R. Fröhlich and A. Studer, Org. Lett., 2011, 13, 4966; (c) Z.-Q. Rong, M.-Q. Jia and S.-L. You, Org. Lett., 2011, 13, 4080; (d) B. Wanner, J. Mahatthananchai and J. W. Bode, Org. Lett., 2011, 13, 5378; (e) A. G. Kravina, J. Mahatthananchai and J. W. Bode, Angew. Chem., Int. Ed., 2012, 51, 9433; (f) S. Bera, R. C. Samanta, C. G. Daniliuc and A. Studer, Angew. Chem., Int. Ed., 2014, 53, 9622; (g) Z.-Q. Liang, D.-L. Wang, H.-M. Zhang and S. Ye, Org. Lett., 2015, 17, 5140; (h) D. Xie, L. Yang, Y. Lin, Z. Zhang, D. Chen, X. Zeng and G. Zhong, Org. Lett., 2015, 17, 2318; (i) S. Hu, B. Wang, Y. Zhang, W. Tang, M. Fang, T. Lu and D. Du, Org. Biomol. Chem., 2015, 13, 4661; (j) S. R. Yetra, S. Mondal, S. Mukherjee, R. G. Gonnade and A. T. Biju, Angew. Chem., Int. Ed., 2016, 55, 268; (k) J.-H. Mao, Z.-T. Wang, Z.-Y. Wang and Y. Cheng, J. Org. Chem., 2015, 80, 6350; (l) H. Lu, J.-Y. Liu, C.-G. Li, J.-B. Lin, Y.-M. Liang and P. Xu, Chem. Commun., 2015, 51, 4473; (m) S. Bera, C. G. Daniliuc and A. Studer, Org. Lett., 2015, 17, 4940.
7. For selected examples, see: (a) F.-G. Sun, L.-H. Sun and S. Ye, Adv. Synth. Catal., 2011, 353, 3134; (b) C. Yao, D. Wang, J. Lu, T. Li, W. Jiao and C. Yu, Chem.–Eur. J., 2012, 18, 1914; (c) S. R. Yetra, T. Kaicharla, S. S. Kunte, R. G. Gonnade and A. T. Biju, Org. Lett., 2013, 15, 5202; (d) H.-R. Zhang, Z.-W. Dong, Y.-J. Yang, P.-L. Wang and X.-P. Hui, Org. Lett., 2013, 15, 4750; (e) S. R. Yetra, T. Roy, A. Bhunia, D. Porwal and A. T. Biju, J. Org. Chem., 2014, 79, 4245; (f) H.-M. Zhang, W.-Q. Jia, Z.-Q. Liang and S. Ye, Asian J. Org. Chem., 2014, 3, 462; (g) D. Jiang, S. Dong, W. Tang, T. Lu and D. Du, J. Org. Chem., 2015, 80, 11593; (h) C.-L. Zhang, D.-L. Wang, K.-Q. Chen and S. Ye, Org. Biomol. Chem., 2015, 13, 11255.
8. For selected examples, see: (a) Z. Zhang, X. Zeng, D. Xie, D. Chen, L. Ding, A. Wang, L. Yang and G. Zhong, Org. Lett., 2015, 17, 5052; (b) S. J. Ryan, L. Candish and D. W. Lupton, J. Am. Chem. Soc., 2009, 131, 14176; (c) J. Cheng, Z. Huang and Y. R. Chi, Angew. Chem., Int. Ed., 2013, 52, 8592; (d) L. Candish, A. Levens and D. W. Lupton, J. Am. Chem. Soc., 2014, 136, 14397.
9. For selected examples, see: (a) S. J. Ryan, L. Candish and D. W. Lupton, J. Am. Chem. Soc., 2011, 133, 4694; (b) L. Candish and D. W. Lupton, J. Am. Chem. Soc., 2012, 135, 58; (c) S. Pandiancherri, S. J. Ryan and D. W. Lupton, Org. Biomol. Chem., 2012, 10, 7903; (d) S. J. Ryan, A. Stasch, M. N. Paddon-Row and D. W. Lupton, J. Org. Chem., 2012, 77, 1113; (e) L. Candish, C. M. Forsyth and D. W. Lupton, Angew. Chem., Int. Ed., 2013, 52, 9149.
10. X.-Y. Chen, Z.-H. Gao, C.-Y. Song, C.-L. Zhang, Z.-X. Wang and S. Ye, Angew. Chem., Int. Ed., 2014, 53, 11611.
11. Y. Zhang, Y. Lu, W. Tang, T. Lu and D. Du, Org. Biomol. Chem., 2014, 12, 3009.
12. For selected examples, see: (a) D. E. A. Raup, B. Cardinal-David, D. Holte and K. A. Scheidt, Nat. Chem., 2010, 2, 766; (b) D. T. Cohen and K. A. Scheidt, Chem. Sci., 2012, 3, 53; (c) D. T. Cohen, B. Cardinal-David and K. A. Scheidt, Angew. Chem., Int. Ed., 2011, 50, 1678; (d) B. Cardinal-David, D. E. A. Raup and K. A. Scheidt, J. Am. Chem. Soc., 2010, 132, 5345; (e) J. Dugal-Tessier, E. A. O'Bryan, T. B. H. Schroeder, D. T. Cohen and K. A. Scheidt, Angew. Chem., Int. Ed., 2012, 51, 4963; (f) A. M. ElSohly, D. A. Wespe, T. J. Poore and S. A. Snyder, Angew. Chem., Int. Ed., 2013, 52, 5789; (g) G.-T. Huang, T. Lankau and C.-H. Yu, Org. Biomol. Chem., 2014, 12, 7297; (h) J. Mo, X. Chen and Y. R. Chi, J. Am. Chem. Soc., 2012, 134, 8810; (i) J. Qi, X. Xie, R. Han, D. Ma, J. Yang and X. She, Chem.–Eur. J., 2013, 19, 4146; (j) Z. Xiao, C. Yu, T. Li, X.-S. Wang and C. Yao, Org. Lett., 2014, 16, 3632.
13. C. Palumbo, G. Mazzeo, A. Mazziotta, A. Gambacorta, M. A. Loreto, A. Migliorini, S. Superchi, D. Tofani and T. Gasperi, Org. Lett., 2011, 13, 6248.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1436250. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra27848a

‡ These authors contribute to this work equally.

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