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DOI: 10.1039/C5RA07216F (Communication) RSC Adv., 2015, 5, 51581-51585

Indium/Fe(III) – mediated regioselective β-cross-coupling aldol type addition reaction of activated alkenes with isatins/isatinimines in aqueous media†

A. Sanjeeva Kumara, Palakuri Ramesha, G. Santosh Kumara, Jagadeesh Babu Nanubolub, T. Prabhakar Raoc and H. M. Meshram*a
aMedicinal Chemistry and Pharmacology Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India. E-mail: hmmeshram@yahoo.com; Fax: +91-40-27160512; Tel: +91-27191640
bLaboratory of X-ray Crystallography, Indian Institute of Chemical Technology, Hyderabad 500 007, India
cCenter for NMR and SC, Indian Institute of Chemical Technology, Hyderabad 500 007, India

Received 21st April 2015 , Accepted 4th June 2015

First published on 5th June 2015

Abstract

A highly efficient and regioselective β-cross coupling aldol type addition reaction of activated alkenes with isatin/isatinimine derivatives in the presence of Indium/Fe(III) in THF/H2O at room temperature is described. Our synthetic protocol explores a broad substrate scope and smoothly proceeds under base-free conditions. The resulting products are obtained in a short reaction time with moderate to high yields. This is the first example of Mannich-type coupling of isatinimines with activated alkenes to afford the alkyl 3-(2-oxo-3-(phenylamino)indolin-3-yl)propanoates and 3-(3-oxoalkyl)-3-(phenylamino)indolin-2-ones.

Functionalized oxindoles bearing a 3,3′-disubstituted ligand with a hetero atom at the C3-position is a privileged structural motif that exists in a large family of bioactive natural products and a series of pharmaceutically active compounds.1 Among them, 3-amino and 3-hydroxy oxindole frameworks have been present in a huge number of naturally occurring alkaloids and medicinally significant compounds.2 For example, the CRTH2 antagonist with good oral bioavailability,3a SSR-149415 which is used for the treatment of anxiety and depression.3b In addition to this, convolutamydines,4a,b 3-hydroxyglucoisatisin,4c,d celogentin K4e,f and dioxibrassinin4g are representative examples of biologically active 3-substituted-3-hydroxy oxindoles (Fig. 1).
image file: c5ra07216f-f1.tif
Fig. 1 Some examples of biologically active quaternary 3-amino/hydroxyl oxindoles.

Inspired by these important scaffolds, a variety of synthetic methods have been developed for the synthesis of 3-substituted 3-hydoxy/amino oxindoles. For example, organo catalysed aldol reactions,5 arylation of isatins6 and direct hydroxylation of 3-alkyl-substituted oxindoles,7 nucleophilic addition to imines,8 alkylation of 3-amino oxindoles,9 Mannich reaction.10 However, to the best our knowledge only a single example, Cheng and co-workers has been reported the β-reductive aldol coupling reaction of isatin derivatives with electron-deficient alkenes.11 But this reaction requires longer reaction time and the use of ammonia gas cause inconvenience. Moreover, the substrate scope was also limited. In this regard it is desirable to develop efficient, convenient and practical method for the synthesis of 3-substituted 3-substituted-3-hydroxy/amino oxindole structural motifs.

The Baylis–Hillman reaction is a significant carbon–carbon bond-forming reaction.12 It involves the coupling of an activated alkene with isatin and results in Baylis–Hillman adduct with respect to development of a coupling reaction, the regioselective β-cross-coupling reaction is an attractive because it provides easy access for the synthesis of a variety of oxindole derivatives (Scheme 1).


image file: c5ra07216f-s1.tif
Scheme 1 The reactions of isatin with activated alkenes.

Indium, by virtue of its unique properties, has emerged as a powerful metal to mediate organic reactions in aqueous media.13 Indium metal is water or air – tolerant and in contrast to zinc it does not require any prior-to-use activation. Moreover, it can be handled safely without any apparent toxicity. In addition, Indium has a very low first ionization energy compared to zinc, accordingly it could be a potential reducing agent.14

In continuation of our efforts in the synthesis of 3- substituted oxindoles,15 herein we report an efficient β-reductive aldol type addition of activated alkenes with isatin and isatinimines using In/FeCl3 in aqueous media (Scheme 2). In this method, we synthesized a range of 3-substituted-3-hydroxy/amino oxindoles through the regioselective β-cross-coupling aldol type addition.


image file: c5ra07216f-s2.tif
Scheme 2 In/FeCl3-mediated regioselective ene-carbonyl cross coupling.

Results and discussions

Our initial studies focused on the effect of different metallic co-reductants and solvents for the regioselective β-cross-coupling aldol type addition reaction of isatin 1a and methyl vinyl ketone (MVK) 2a at 25 °C and the results are depicted in Table 1. As shown in Table 1, a sequence of metals, such as Mo, Al, Mg, Fe, Sn and In were tested for the reaction. Among them In was found the most suitable metal for this coupling reaction (Table 1, entries 1–7 and 14–16). In order to improve the yield of the product, several Lewis acids were screened as promoters (Table 1, entries 8–13) and it was observed that the FeCl3 provided impressive improvement in yield (Table 1, entry 18). It is noteworthy to mention that the reaction did not proceeded either with alone metal or Lewis acid (Table 1, entries 1–3 and 24). However, the use of combination of metal and Lewis acid gave desired product. Later, solvent effect was also investigated for this method and it was revealed that the media had a remarkable effect and significant enhancement of the reaction was observed by the addition of water to THF. Among the different tested solvent systems THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) was found to be an effective system in terms of yields (Table 1) and other solvent systems were found to be inefficient (Table 1, entries 19, 20 and 22 and 23). By increasing the time of the reaction did not lead to any further improvements in the yield (Table 1, entry 17).
Table 1 Optimization of reaction conditions for the regioselective ene-carbonyl couplinga

image file: c5ra07216f-u1.tif
Entry Metal Catalyst Solvent Time (h) Yield (%)b
a Reaction conditions: isatin 1a (1 mmol), methyl vinyl ketone 2a (1.2 mmol), metal (2 mmol), catalyst (5 mol%) in 5 mL of solvent at 25 °C.b Isolated yield.
1 Mo — THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 No reaction
2 Al — THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 No reaction
3 Fe — THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 No reaction
4 Sn Ni(OAc)2·4H2O THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 10
5 Fe InCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 15
6 Fe ZrCl4 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 12 55
7 In ZnCl2 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 10 65
8 In CuCl2·2H2O THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 15
9 In Bi(OTf)3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 5
10 In SnCl2·2H2O THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 10 50
11 In Sc(OTf)3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 20 5
12 In InCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 15
13 In Ni(OAc)2·4H2O THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 25
14 Mg FeCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 20 10
15 Sn FeCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 15 40
16 Fe FeCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 15
17 In FeCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 88
18 In FeCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9:1) 0.5 88
19 In FeCl3 DCM[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 1 5
20 In FeCl3 MeCN[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 0.5 50
21 In FeCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 1 50
22 In FeCl3 THF 1 20
23 In FeCl3 H2O 24 No reaction
24 — FeCl3 THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 No reaction
25 In — THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 No reaction

After achieving the optimized reaction conditions, the scope of aliphatic enones and isatins were explored (Scheme 3). The reaction of methyl vinyl ketone (MVK) 2a and ethyl vinyl ketone (EVK) 2b with simple isatin gave corresponding products in high yields (Table 2, 4a and 4i). Isatins bearing different substituents on the nitrogen atom and on the aromatic ring could also be coupled effectively under this condition. For example 5-halo isatins reacted with 2a/2b and resulted into moderate yields of coupling adducts (Table 2, 4b, 4c and 4j–4m). Other 5-substituted (–OCF3) isatins reacted smoothly with MVK and EVK to furnish desired product in high yield (Table 2, 4d and 4n). Similarly 4-substituted, 4,5-disubstituted and 4,7-disubstituted also afforded comparatively less yield of desired products (Table 2, 4f–4h and 4p). The substituents on nitrogen atom were found to have no effect on the reactivity (Table 2, 4e and 4o).


image file: c5ra07216f-s3.tif
Scheme 3 Putative mechanistic way for the formation of regioselective reductive aldol product 4a.
Table 2 Scope of regioselective β-reductive coupling of aliphatic enones with isatinsa,b

image file: c5ra07216f-u2.tif
a Reaction conditions: isatin 1 (1 mmol), aliphatic enones 2 (1 mmol), indium (2 mmol), and FeCl3 (5 mmol%) in 5 mL of THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) at 25 °C.b Isolated yield.
image file: c5ra07216f-u3.tif

Then we extended this method for the cyclic enone to different substituted isatins which gave good yields of the products (Table 3). For example, 5-halogenated isatins were reacted with cyclohex-2-ene-1-one in an identical condition and resulted in the expected products in high yields (Table 3, 4q–4t).

Table 3 Scope of regioselective β-reductive coupling of cyclic-enone with isatinsab

image file: c5ra07216f-u4.tif
a Reaction conditions: isatins 1 (1 mmol), cyclohexenone 2c (1 mmol), indium (2 mmol), and FeCl3 (5 mmol%) in 5 mL of THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) at 25 °C.b Isolated yield.
image file: c5ra07216f-u5.tif

The structure of one of the products, 4l was confirmed by a single-crystal X-ray diffraction analysis (Fig. 2). Compound 4d was further confirmed by NMR studies (DEPT Experiment, see ESI†). From the obtained yields of all the products, we observed the reactivity profile of α,β-unsaturated ketones is as following cyclohex-2-ene-1-one > EVK > MVK.


image file: c5ra07216f-f2.tif
Fig. 2 ORTEP diagram of the single-crystal X-ray structure of compound 4l.

Encouraged by the results obtained from the regioselective β-cross-coupling aldol type addition reaction of isatins, then the scope of the present protocol was further extended by treating a different isatinimines with activated alkenes, such as 2a/2b (methyl/ethyl vinyl ketone) and 2d/2e (methyl/ethyl acrylate) under standard reaction conditions and the results are presented in Table 4. Initially, the reaction of 2d with 3-(phenylimino) indolin-2-one worked well to afford the desired product in high yield (Tables 4 and 5g). The imines containing substituents on phenyl group with either multifarious electronic property were successfully applied in this Mannich-type of coupling. For example, 3-(3-halophenylimino) indolin-2-ones were reacted efficiently with 2b, 2d and 2e to form the β-reductive Mannich products in good to high yields (Table 4, 5a, 5e, 5j, 5l and 5p). Similarly 3-(3 (trifluoromethyl)phenylimino)indolin-2-one showed good reactivity with 2a, 2d and 2e, and furnished the resultant coupling products in high to excellent yields (Table 4, 5c, 5h and 5o). In the same way 4-substituted and 3, 5-disubstituted imines also took part in this reaction with 2d and 2e to give the Mannich adducts in satisfactory yields (Table 4, 5d, 5f, 5i, 5m and 5n).

Table 4 Scope of regioselective β-reductive coupling of acrylates and aliphatic enones with isatiniminesa,b

image file: c5ra07216f-u6.tif
a Reaction conditions: isatinimines 3 (1 mmol), acrylates or aliphatic enones 2 (1 mmol), Indium (2 mmol), and FeCl3 (5 mmol%) in 5 mL of THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (9[thin space (1/6-em)]:[thin space (1/6-em)]1) at 25 °C.b Isolated yield.
image file: c5ra07216f-u7.tif

A plausible mechanism is proposed based on previous reports16 as shown in Scheme 3. The reaction is initiated by a single-electron transfer from Indium to the methyl vinyl ketone 2a to form a radical enolate anion a. Brisk trapping of the oxygen-metal bond in the radical enolate anion a by FeCl3 gives the γ-Fe(III)-substituted allylic radical b. The radical b is further reduced by Indium to furnish the corresponding allylic Indium species c. Finally, coupling of the γ-Fe(III)-substituted allylic Indium species c with an isatin results in the formation of d and followed by the hydrolysis of d would lead to the desired product 4a.

We believe that the present method is amenable for the synthesis of novel compounds. In addition to this the method avoids the use of harsh and hazardous conditions.

Conclusion

In summary, we have demonstrated a β–reductive aldol type addition of ativated alkenes with isatins/isatinimines promoted by In/FeCl3 in aqueous media. This strategy is base free, atom-economical and regioselective for the synthesis of wide variety of 3-hydroxy/amino oxindole derivatives. Moreover, 3-amino oxindole derivatives had never synthesized before by β– reductive aldol type reaction.

Acknowledgements

A. S. K., P. R. and G. S. K. thank the CSIR India for award of a fellowship and Dr A. Kamal, Outstanding scientist and Head of MCP Division, for his support and encouragement. The authors thank CSIR-India for financial support as part of XII five year plan programme under title ORIGIN CSC 0108.

References

  1. For selected examples, see: (a) A. P. Antonchick, R. C. Gerding, M. Catarinella, M. Schürmann, H. Preut, S. Ziegler, D. Rauh and H. Waldmann, Nat. Chem., 2010, 2, 735 CrossRef CAS PubMed; (b) K. C. Nicolaou, D. Y. K. Chen, X. Huang, T. Ling, M. Bella and S. A. Snyder, J. Am. Chem. Soc., 2004, 126, 12888 CrossRef CAS PubMed; (c) X.-C. Lin and S. M. Weinreb, Tetrahedron Lett., 2001, 42, 2631 CrossRef CAS; (d) D. L. Silverio, S. Torker, T. Pilyugina, E. M. Vieira, M. L. Snapper, F. Haeffner and A. H. Hoveyda, Nature, 2013, 494, 216 CrossRef CAS PubMed; (e) G. Bergonzini and P. Melchiorre, Angew. Chem., Int. Ed., 2012, 51, 971 CrossRef CAS PubMed; (f) B. M. Trost and M. K. Brennan, Synthesis, 2009, 3003 CrossRef CAS.
  2. For selected examples, see: (a) H. Kitamura, A. Kato and T. Esaki, Eur. J. Pharmacol., 2001, 418, 225 CrossRef CAS; (b) M. Ochi, K. Kawasaki, H. Kataoka and Y. Uchio, Biochem. Biophys. Res. Commun., 2001, 283, 1118 CrossRef CAS PubMed; (c) Y. Kamano, H.-P. Zhang, Y. Ichhara, H. Kizu, K. Komiyama, H. Itokawa and G. R. Pettit, Tetrahedron Lett., 1995, 36, 2783 CrossRef CAS; (d) J. Qiu, J.-X. Zhang, S. Liu and X.-J. Hao, Tetrahedron Lett., 2013, 54, 300 CrossRef CAS PubMed; (e) C. H. Wang, A. R. White, S. N. Schwartz, S. Alluri, T. M. Cattabiani, L. K. Zhang, A. V. Buevich, A. K. Ganguly and T. M. Chan, Tetrahedron, 2012, 68, 9750 CrossRef CAS PubMed.
  3. The introduction of a heteroatom at the C3 ’ position of an oxindole unit played a key role in improving the stability of human plasma and the oral bioavailability of a CRTH2 antagonist; see: (a) S. Crosignani, C. Jorand-Lebrun, P. Page, G. Campbell, V. Colovray, M. Missotten, Y. Humbert, C. Cleva, J.-F. Arrighi, M. Gaudet, Z. Johnson, P. Ferro and A. Chollet, ACS Med. Chem. Lett., 2011, 2, 644 CrossRef CAS PubMed; (b) K. Bernard, S. Bogliolo and J. Ehrenfeld, Br. J. Pharmacol., 2005, 144, 1037 CrossRef CAS PubMed.
  4. (a) J. Kohno, Y. Koguchi, M. Nishio, K. Nakao, M. Kuroda, R. Shimizu, T. Ohnuki and S. Komatsubara, J. Org. Chem., 2000, 65, 990 CrossRef CAS PubMed; (b) K. Monde, K. Sasaki, A. Shirata and M. Tagusuki, Phytochemistry, 1991, 30, 2915 CrossRef CAS; (c) T. Kawasaki, M. Nagaoka, T. Satoh, A. Okamoto, R. Ukon and A. Ogawa, Tetrahedron, 2004, 60, 3493 CrossRef CAS PubMed; (d) A. Frechard, N. Fabre, C. Pean, S. Montaut, M. T. Fauvel, P. Rollin and I. Fouraste, Tetrahedron Lett., 2001, 42, 9015 CrossRef CAS; (e) H. Suzuki, H. Morita, M. Shiro and M. Kobayashi, Tetrahedron, 2004, 60, 2489 CrossRef CAS PubMed; (f) H. B. Rasmussen and J. K. MacLeod, J. Nat. Prod., 1997, 60, 1152 CrossRef CAS; (g) W. Balk-Bindseil, E. Helmke, H. Weyland and H. Laatsch, Liebigs Ann. Chem., 1995, 1291 CrossRef CAS PubMed.
  5. For selected examples, see: (a) S. Nakamura, N. Hara, H. Nakashima, K. Kubo, N. Shibata and T. Toru, Chem.–Eur. J., 2008, 14, 8079 CrossRef CAS PubMed; (b) F. Xue, S. Zhang, L. Liu, W. Duan and W. Wang, Chem.–Asian J., 2009, 4, 1664 CrossRef CAS PubMed; (c) T. Itoh, H. Ishikawa and Y. Hayashi, Org. Lett., 2009, 11, 3854 CrossRef CAS PubMed; (d) N. Hara, S. Nakamura, N. Shibata and T. Toru, Chem.–Eur. J., 2009, 15, 6790 CrossRef CAS PubMed; (e) Q. Guo, M. Bhanushali and C.-G. Zhao, Angew. Chem., Int. Ed., 2010, 49, 9460 CrossRef CAS PubMed.
  6. (a) R. Shintani, M. Inoue and T. Hayashi, Angew. Chem., Int. Ed., 2006, 45, 3353 CrossRef CAS PubMed; (b) P. Y. Toullec, R. B. C. Jagt, J. G. de Vries, B. L. Feringa and A. J. Minnaard, Org. Lett., 2006, 8, 2715 CrossRef CAS PubMed; (c) D. Tomita, K. Yamatsugu, M. Kanai and M. Shibasaki, J. Am. Chem. Soc., 2009, 131, 6946 CrossRef CAS PubMed; (d) H. Lai, Z. Huang, Q. Wu and Y. Qin, J. Org. Chem., 2009, 74, 283 CrossRef CAS PubMed; (e) N. V. Hanhan, A. H. Sahin, T. W. Chang, J. C. Fettinger and A. K. Franz, Angew. Chem., Int. Ed., 2010, 49, 744 CrossRef CAS PubMed.
  7. (a) D. Sano, K. Nagata and T. Itoh, Org. Lett., 2008, 10, 1593 CrossRef CAS PubMed; (b) T. Bui, N. R. Candeias and C. F. Barbas III, J. Am. Chem. Soc., 2010, 132, 5574 CrossRef CAS PubMed; (c) T. Ishimaru, N. Shibata, J. Nagai, S. Nakamura, T. Toru and S. Kanemasa, J. Am. Chem. Soc., 2006, 128, 16488 CrossRef CAS PubMed.
  8. (a) Q.-X. Guo, Y.-W. Liu, X.-C. Li, L.-Z. Zhong and Y.-G. Peng, J. Org. Chem., 2012, 77, 3589 CrossRef CAS PubMed; (b) B. Alcaide, P. Almendros and C. Aragoncillo, Eur. J. Org. Chem., 2010, 2845 CrossRef CAS PubMed; (c) Y. H. Shi, Z. Wang, Y. Shi and W. P. Deng, Tetrahedron, 2012, 68, 3649 CrossRef CAS PubMed.
  9. T. Emura, T. Esaki, K. Tachibana and M. Shimizu, J. Org. Chem., 2006, 71, 8559 CrossRef CAS PubMed.
  10. T. Nishikawa, S. Kajii and M. Isobe, Chem. Lett., 2004, 440 CrossRef CAS.
  11. Y. Chien Hung, L. Yi Chuen, M. Subramaniyan, H. Kevin and C. Chien Hong, Adv. Synth. Catal., 2014, 356, 831 CrossRef PubMed.
  12. (a) Y. M. Chung, Y. J. Im and J. N. Kim, Bull. Korean Chem. Soc., 2002, 23, 1651 CrossRef CAS; (b) S. C. Kim, S. Gowrisankar and J. N. Kim, Tetrahedron Lett., 2006, 47, 3463 CrossRef CAS PubMed; (c) S. C. Kim, K. Y. Lee, S. Gowrisankar and J. N. Kim, Bull. Korean Chem. Soc., 2006, 27, 1133 CrossRef CAS.
  13. For reviews, see: (a) T. P. Loh and G. L. Chua, Chem. Commun., 2006, 2739 RSC; (b) P. Cintas, Synlett, 1995, 1087 CrossRef CAS PubMed; (c) J. Podlech and T. C. Maier, Synthesis, 2003, 633 CrossRef CAS; (d) V. Nair, S. Ros, C. N. Jayan and B. S. Pillai, Tetrahedron, 2004, 60, 1959 CrossRef CAS PubMed; (e) B. C. Ranu, Eur. J. Org. Chem., 2000, 2347 CrossRef CAS; (f) J. Auge, N. Lubin-Germain and J. Uziel, Synthesis, 2007, 1739 CrossRef CAS PubMed.
  14. M. R. Pitts, J. R. Harrision and C. J. Moody, J. Chem. Soc., Perkin Trans. 1, 2001, 955 RSC.
  15. (a) H. M. Meshram, D. A. Kumar, P. R. Goud and B. C. Reddy, Synth. Commun., 2010, 40, 39 CrossRef CAS PubMed; (b) H. M. Meshram, P. Ramesh, B. C. Reddy, B. Sridhar and J. S. Yadav, Tetrahedron, 2011, 67, 3150 CrossRef CAS PubMed; (c) H. M. Meshram, P. Ramesh, B. C. Reddy and G. S. Kumar, Chem. Lett., 2011, 4, 357 CrossRef; (d) H. M. Meshram, P. Ramesh, A. S. Kumar and A. Swetha, Tetrahedron Lett., 2011, 52, 5862 CrossRef CAS PubMed; (e) H. M. Meshram, N. N. Rao, L. C. Rao and N. S. Kumar, Tetrahedron Lett., 2012, 53, 3963 CrossRef CAS PubMed; (f) B. T. Pramod, K. Sirisha, A. V. S. Sarma, J. N. Babu and H. M. Meshram, Tetrahedron, 2013, 69, 6415 CrossRef PubMed; (g) B. T. Pramod and H. M. Meshram, RSC. Adv., 2014, 4, 6019 RSC; (h) B. T. Pramod and H. M. Meshram, RSC. Adv., 2014, 4, 5343 RSC.
  16. (a) C.-H. Yeh, R. P. Korivi and C.-H. Cheng, Angew. Chem., Int. Ed., 2008, 47, 4892 CrossRef CAS PubMed; (b) C.-H. Yeh, R. P. Korivi and C.-H. Cheng, Adv. Synth. Catal., 2013, 355, 1338 CrossRef CAS PubMed.

Footnote

† Electronic supplementary information (ESI) available. CCDC 950572. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra07216f
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