Pd-catalyzed decarboxylative allylic coupling of acetates of Baylis–Hillman alcohols with propiolic acids: a highly regio- and stereoselective synthesis of 1,5-diarylpent-1-en-4-yne derivatives

Abstract

Pd-catalyzed decarboxylative allylic coupling of acetates of Baylis–Hillman alcohols with alkynyl carboxylic acids leading to the formation of an important class of 1,5-diarylpent-1-en-4-ynes in a highly regio- and stereoselective manner has been developed. Decarboxylative coupling happened via an exclusively S_N2′ pathway. Acetates of the Baylis–Hillman alcohols derived from alkyl acrylates, ethyl vinyl ketone and phenyl vinyl sulfone provided exclusively (E)-1,5-diarylpent-1-en-4-ynes while the acetates of the Baylis–Hillman alcohols derived from acrylonitrile provided exclusively (Z)-1,5-diarylpent-1-en-4-ynes.

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Due to the many advantages over traditional oxidative coupling reactions,^1,2 decarboxylative cross coupling reactions have become an important tool for the construction of carbon–carbon bonds in modern synthetic organic chemistry. Decarboxylative cross coupling reactions employ more stable and inexpensive carboxylic acids as key reaction partners while traditional coupling reactions have typically involved expensive and usually air-sensitive organometallic species as their reaction partners.^1,2 Recently alkynyl carboxylic acids have been inducted into decarboxylative coupling partners by providing aryl alkyne derivatives.³ In 2008, Lee et al. reported first decarboxylative coupling of alkynyl carboxylic acids with aryl halides.^3a Later Li et al. expanded the scope of substrates to benzyl chlorides, bromides, and acetate in this coupling by providing desired benzyl alkynes.^3e Tunge and co-workers independently reported the first intramolecular Pd-catalyzed decarboxylative allylic rearrangement of allyl alkynoates in 2005.⁴ The Baylis–Hillman reaction is another important atom-economical carbon–carbon bond forming reaction providing densely functionalized molecules. Applications of Baylis–Hillman adducts in many organic transformations have been well documented.⁵ In continuation of our interest in the decarboxylative coupling and studies in the application of Baylis–Hillman adducts in various synthetic organic transformations⁶ we herein report Pd-catalyzed decarboxylative S_N2′ allylic coupling of acetates of the Baylis–Hillman alcohols with the alkynyl carboxylic acids leading to the formation of important class of 1,5-diarylpent-1-en-4-ynes in a highly regio- and stereoselective manner. To the best our knowledge there is no study reported on the Pd-catalyzed decarboxylative allylic coupling of alkynyl carboxylic with acetates of Baylis–Hillman adducts.^7,8 Furthermore, S_N2′ Pd-catalyzed decarboxylative addition of alkynyl carboxylic on allylic acetate has also not been described.⁹ In addition, 1,5-diarylpent-1-en-4-yne is an important structural moiety present in the extracts of some medicinal plants.¹⁰ Rooperol (1a), hypoxoside (1b) and many other 1,5-diarylpent-4-yne-1,2-diol derivatives found in plant extracts which were reported to exhibit medicinal properties such as anticancer, anti-inflammatory, antibacterial, and antioxidant activity (Fig. 1).¹⁰

Fig. 1 Structures of rooperol (1a) and hypoxoside (1b).

Being interested in the decarboxylative coupling and the development of new synthetic methodologies particularly using Baylis–Hillman adducts, we studied the Pd-catalyzed decarboxylative allylic coupling of acetates of the Baylis–Hillman alcohols with the alkynyl carboxylic acids. Accordingly, we subjected allyl acetate 2a¹¹ and phenyl propiolic acid 3a to decarboxylative coupling conditions using of a catalyst combination of 5 mol% of Pd(OAc)₂ and 10 mol% of S-Phos at 70 °C. Work-up and silica gel column chromatography provided (E)-methyl-2-(4-methylbenzylidene)-5-phenylpent-4-ynoate (4a) as the exclusive isolated product in 88% yield.^12,13 Proton NMR analysis of the crude reaction mixture did not indicate the presence of other regioisomer 6 or the corresponding Z-isomer 5. Formation of exclusively E-isomer 4a suggests that decarboxylative coupling between allyl acetate 2a and phenyl propiolic acid 3a happened in a highly regio- and stereoselective manner via exclusively S_N2′ allylic pathway.

The E-stereochemistry was assigned based on the literature precedence of E/Z-stereochemical assignments of 3-aryl-(2-substituted)-prop-2-enoates.¹⁴ The E-stereochemistry of 4a was further ascertained by 2D NOESY. A strong NOE cross peak between the CH₂ (allylic methylene proton) and Ar-H_ortho (ortho proton of the benzene ring) was observed, while there was no (or a weak) NOE correlation between the CH₂ (allylic methylene) and CH (olefinic proton of the double bond). To provide the generality of the palladium-catalyzed allylic decarboxylative coupling, other acetates 2b–e were treated with propiolic acids 3a–c in the presence of 5 mol% of Pd(OAc)₂ and 10 mol% of S-Phos following the standard reaction procedure.¹² The crude products, obtained after usual work-up, were purified by silica gel chromatography to furnish pure (E)-1,5-diarylpent-1-en-4-yne derivatives 4b–i in high yields (Table 1).

Table 1 Regio- and stereo-selective Pd-catalyzed allylic decarboxylative coupling of methyl acrylate derived acetates 2b–e leading to the formation of (E)-1,5-diarylpent-1-en-4-ynes 4a–i

Entry	Acetate^a	Propiolic acid	Product(s)^b	Yield^c (%)
a Acetates of Baylis–Hillman alcohol 2b–e were prepared by following the typical procedure described in ref. 11.b Reactions were carried out following the typical procedure described in ref. 12.c Isolated yields after chromatographic purification on silica gel.
1				86
2		3a		81
3				86
4	2b	3b		82
5	2d	3b		79
6	2c			85
7		3a		78
8	2e	3c		83

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Encouraged by these results, we extended our studies to the acetates of Baylis–Hillman adducts derived from acrylonitrile. Accordingly, we subjected acetate 7a (derived from Baylis–Hillman alcohol of benzaldehyde and acrylonitrile)¹⁵ to decarboxylative coupling with phenyl propiolic acid 3a using a catalyst combination of 5 mol% of Pd(OAc)₂ and 10 mol% of S-Phos following the standard reaction procedure.¹⁶ Work-up and silica gel column chromatography provided exclusively (Z)-2-(4-methoxybenzylidene)-5-phenylpent-4-ynenitrile (8a) in 90% yield.¹⁶ (Scheme 2). Proton NMR analysis of the crude reaction mixture did not indicate the presence of other regioisomer 10 or the corresponding E-isomer 9. Again, formation of exclusively Z-isomer 8a clearly suggests that, in acrylonitrile derived acetate 7a case too, decarboxylative coupling happened regioselectively via exclusively S_N2′ pathway. However, in contrast to the methyl acrylate derived acetates (2a–e, Scheme 1 and Table 1), in this case (acrylonitrile derived acetate 7a), the isolated product 8a was formed with a shift in the stereoselectively, i.e., from E to Z (Scheme 2). The Z-stereochemistry in the case of 8a was ascertained by 2D NOESY experiments where a strong NOE cross peak was observed between the CH₂ (allylic methylene proton) and CH (olefinic proton of the double bond), while there was no (or a weak) NOE correlation between the CH₂ (allylic methylene proton) and Ar-H_ortho (ortho proton of the benzene ring). To expand the substrate scope, we treated other acrylonitrile derived acetates 7a–f¹² to stereoselective S_N2^′ allylic decarboxylative coupling with aryl propiolic acids 3a–c (Table 2). Upon standard work-up and silica gel chromatographic purification, pure (Z)-1,5-diarylpent-1-en-4-yne derivatives 8b–k were isolated in high yields with exclusively Z-stereochemistry (Table 2).¹⁶

Scheme 1 Regio- and stereo-selective Pd-catalyzed allylic decarboxylative coupling of methyl acrylate derived acetate 2a leading to the formation of exclusively E-isomer 4a.¹²

Scheme 2 Regio- and stereo-selective Pd-catalyzed allylic decarboxylative coupling of acrylonitrile derived acetate 7a leading to the formation of exclusively Z-isomer 8a.¹⁶

Table 2 Regio- and stereo-selective Pd-catalyzed allylic decarboxylative coupling of acrylonitrile derived acetates 7a–f leading to the formation of (Z)-1,5-diarylpent-1-en-4-ynes 8a–k

Entry	Acetate^a	Propiolic acid	Product(s)^b	Yield^c (%)
a Acetates of Baylis–Hillman alcohol 7a–f were prepared by following the typical procedure described in ref. 15.b Reactions were carried out following the typical procedure described in ref. 16.c Isolated yields after chromatographic purification on silica gel.
1				79
2		3a		83
3		3a		87
4	7b			80
5	7d	3b		82
6	7c			81
7		3a		78
8	7e	3c		80
9		3a		82
10	7f	3c		84

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Being fascinated by reversal in the stereochemistry when EWG (electron withdrawing group) was changed from methyl ester to cyano group and to understand the effect of other EWG on the stereochemical outcome in this Pd-catalyzed allylic decarboxylative coupling, we further extended our studies to the acetates of Baylis–Hillman adducts derived from other EWG (ketone, sulfone, other esters etc.). Accordingly, we subjected acetate 11 derived from Baylis–Hillman alcohol of benzaldehyde and ethyl vinyl ketone¹¹ to decarboxylative coupling with aryl propiolic acids 3a and 3c using standard reaction procedure.¹⁶ Work-up and silica gel column chromatography provided corresponding products 14 and 15 exclusively with (E)-stereoselectivity (Table 3). Proton NMR analysis of the crude reaction mixture did not indicate the presence of Z-isomer. Similarly, acetate 12 derived from Baylis–Hillman alcohol of 4-methylbenzaldehyde and ethyl acrylate¹¹ provided corresponding products 16 and 17 exclusively with (E)-stereoselectivity (Table 3). Again, proton NMR analysis of the crude reaction mixture did not indicate the presence of Z-isomer. Finally, we extended our studies to Baylis–Hillman acetates 13a–b derived from phenyl vinyl sulfone¹¹ to decarboxylative coupling with aryl propiolic acids 3a and 3c using standard reaction procedure.¹² Work-up and silica gel column chromatography provided corresponding products 18 and 19 exclusively with (E)-stereoselectivity (Table 3). Formation of exclusively E-isomers 14–19 clearly suggests that the decarboxylative coupling happened regioselectively via exclusively S_N2′ pathway in the case of electron withdrawing groups (EWG) such as ethyl vinyl ketone (11), ethyl acrylate (12) and phenyl vinyl sulfone (13). However, unlike the cyano group (Scheme 2, Table 2), the electron withdrawing group, ethyl vinyl ketone (11) and phenyl vinyl sulfone (13) and ethyl acrylate (12) provided similar E-stereoselectivity like the methyl acrylate (Scheme 1, Table 1).

Table 3 Regio- and stereoselective Pd-catalyzed allylic decarboxylative coupling of Baylis–Hillman acetates derived from other EWG; ethyl vinyl ketone (11), ethyl acrylate (12) and phenyl vinyl sulfone (13a, 13b)

Entry	Acetate^a	Propiolic acid	Product(s)^b	Yield^c (%)
a Acetates of Baylis–Hillman alcohol 11, 12, 13a and 13b were prepared by following the typical procedure described in ref. 11.b Reactions were carried out following the typical procedure described in ref. 12.c Isolated yields after chromatographic purification on silica gel.
1				78
2	11			80
3		3a		83
4	12	3c		85
5		3c		79
6		3c		85

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Transition state models and plausible explanation of the stereochemical switch between the acetates derived from methyl acrylate and acrylonitrile

The stereochemical reversal from an ester group to a cyano group is consistent with earlier reports on other synthetic transformations using the Baylis–Hillman adducts.^6d,7,18 and with formation of similar products using other strategies. The E-selectivity in the case of aryl propiolates of Baylis–Hillman alcohols derived from methyl acrylate can possibly be explained using transition state models TS-1a and TS-1b (Scheme 3, Path-I). Ester, ethyl ketone and phenyl sulfone groups are relatively bulkier and exert more steric effect on aryl group in TS-1a than the methylene group on aryl group in TS-1b. Therefore, product will form via more favoured transition state TS-1b leading to the formation of E-isomer. Similarly, The Z-selectivity in the case of aryl propiolates of Baylis–Hillman alcohols derived from acrylonitrile can be explained by transition state models TS-2a and TS-2b (Scheme 3, Path-II). Being a small linear group, CN group will have relatively lower steric effect on aryl group in TS-2a when compared to the methylene group on aryl group in TS-2b. Therefore, in this case (aryl propiolates of Baylis–Hillman alcohols derived from acrylonitrile) product will form via more favoured transition state TS-2a leading to the formation of Z-isomer.

Scheme 3 Plausible mechanism and transition state models leading E-selectivity in the case of acetates derived from alkyl acrylates (2a–e, 12) ethyl vinyl ketone (11) and phenyl vinyl sulfone and (13a–b) and Z-selectivity in the case of acrylonitrile derived acetates 7a–f.

Conclusions

In conclusion, we have developed a highly regio- and stereoselective palladium-catalyzed S_N2′ allylic decarboxylative coupling between acetates of the Baylis–Hillman alcohols and aryl propiolic acids leading to the formation of important classes of 1,5-diarylpent-1-en-4-ynes with exclusively E- or Z-selectivity. Acetates of the Baylis–Hillman alcohols derived from alkyl acrylates, ethyl vinyl ketone and phenyl vinyl sulfone provide exclusively (E)-1,5-diarylpent-1-en-4-ynes and the acetates obtained from acrylonitrile provide exclusively (Z)-1,5-diarylpent-1-en-4-ynes.

Acknowledgements

We sincerely appreciate the financial support from Albany Molecular Research Inc. The authors also thank Dr Anjan Chakrabarti for his support during the preparation of this manuscript.

Notes and references

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7. Very recently our work on a Palladium-catalyzed tandem intramolecular decarboxylative coupling via the allylic rearrangement of aryl propiolates derived from the Baylis–Hillman adducts leading to the formation important class of 1,5-diarylpent-1-en-4-ynes highly regio- and stereoselective manner has accepted T. Satyanarayana, V. Dhanunjaya Naidu, M. Parthasarathy and N. Shashidhar, Tetrahedron Lett., 2015, 56, 2008–2013.
8. During their original study of intramolecular Pd-catalyzed decarboxylative allylic rearrangement of allyl alkynoates, Tunge and co-workers also reported two isolated examples of Pd-catalyzed decarboxylative allylic S_N2 type substitution between two allyl acetates and phenyl propiolic acid.⁴ In their case, no any coupling involving S_N2′-type Pd-catalyzed decarboxylative allylic substitution was observed.
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9. Pd-catalyzed allylic substitution involving the attack of various nucleophiles on an allylic Pd intermediate, providing S_N2 or S_N2′ substitution products, has been investigated with great intensity.¹⁷.
10. 1,5-Diarylpent-1-en-4-yne is an important core present in the extracts of the African potato (Hypoxis hemerocallidea) which are widely used for a variety of medical conditions. For details, see: (a) P. M. O. Owira and J. A. O. Ojewole, Phytother. Res., 2009, 23, 147–296; (b) S. E. Drewes and F. S. Khan, S. Afr. J. Sci., 2004, 100, 425–430; (c) G. B. M. Bettolo, M. Patamia, M. Nicoletti, C. Galeffi and I. Messana, Tetrahedron, 1982, 38, 1683–1687; (d) S. E. Drewes, A. J. Hall, R. A. Learmonth and U. J. Upfold, Phytochemistry, 1984, 23, 1313–1316; (e) C. Galeffi, G. Multari, V. Y. De, I. Messana, M. Nicoletti and G. B. Marini-Bettolo, Planta Med., 1989, 55, 318–320; (f) W.-L. Chang, M.-J. Su and S.-S. Lee, J. Nat. Prod., 1997, 60, 76–80; (g) W. L. Chang, C. H. Chen and S. S. Lee, J. Nat. Prod., 1999, 62, 734–739; (h) K. J. Wang, N. Li and H. Wang, Molecules, 2008, 13, 1696–1701; (i) G. J. Boukes, B. B. Daniels, C. F. Albrecht and M. Van de Venter, Oncol. Res., 2009, 18, 365–376; (j) M. J. Vandermerwe, K. Jenkins, E. Theron and B. Vanderwalt, Biochem. Pharmacol., 1993, 45, 303–311; (k) M. M. Kabanda, Chem. Res. Toxicol., 2012, 25, 2153–2166; (l) J. Li, T. S. Kaoud, J. LeVieux, B. Gilbreath, S. Moharana, K. N. Dalby and S. M. Kerwin, ChemBioChem, 2013, 14, 66–71.
11. Typical procedure for the synthesis of methyl-2-(acetoxy(4-methoxyphenyl)methyl)acrylate (2a): To a solution of methyl-2-(hydroxy(4-methoxyphenyl)methyl)acrylate (0.45 mmol) and pyridine (0.90 mmol) in dichloromethane (5 mL), was added acetyl chloride (0.54 mmol) at 0 °C. Reaction mixture was allowed to slowly warm to room temperature while stirring was continued. Upon reaction completion as judged by TLC, the resulting mixture was diluted with dichloromethane (10 mL), washed with 2 N HCl solution (5 mL), satd NaHCO₃ solution (5 mL) and dried over Na₂SO₄. The organic layer was concentrated under reduced pressure and purified by column chromatography on silica (EtOAc–hexanes) to afford methyl-2-(acetoxy(4-methoxyphenyl)methyl)acrylate 2a (0.42 mmol, 95%). ¹H NMR (400 MHz, CDCl₃): δ 2.07 (s, 3H), 3.68 (s, 3H), 3.77 (s, 3H), 5.86–5.87 (m, 1H), 6.37 (s, 1H), 6.63 (s, 1H), 6.86 (d, J = 8.79 Hz, 2H), 7.28 (d, J = 8.67 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 21.0, 51.9, 55.2, 72.8, 113.8, 125.1, 129.1, 129.8, 139.7, 159.6, 165.4, 169.4; ESI-MS calc. for C₁₄H₁₇O₅ [M + H]⁺ = 265; found: m/z 265.
12. Typical procedure for the synthesis of (E)-methyl-2-(4-methoxybenzylidene)-5-phenylpent-4-ynoate (4a): To a stirred solution of methyl-2-(acetoxy(4-methoxyphenyl)methyl)acrylate 2a (0.42 mmol) in dry THF (15 mL), were added Pd(OAc)₂ (0.021 mmol), 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (0.042 mmol), Cs₂CO₃ (0.84 mmol) and 3-phenylpropiolic acid (0.50 mmol). The mixture was heated to 70 °C for 30 min. Upon reaction completion as judged by TLC, the resulting mixture was diluted with ethyl acetate (10 mL), washed with water (5 mL), brine (5 mL) and dried over Na₂SO₄. The organic layer was concentrated under reduced pressure and purified by column chromatography on silica (EtOAc–hexanes) to afford (E)-methyl-2-(4-methoxybenzylidene)-5-phenylpent-4-ynoate 4a (0.36 mmol, 85%). ¹H NMR (400 MHz, CDCl3): δ 3.62 (s, 2H), 3.85 (s, 3H), 3.86 (s, 3H), 6.97 (d, J = 8.83 Hz, 2H), 7.26–7.28 (m, 3H), 7.39–7.42 (m, 2H), 7.58 (d, J = 8.71 Hz, 2H), 7.76 (s, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 18.7, 52.2, 55.3, 80.8, 87.2, 114.1, 123.6, 125.4, 127.5, 127.7, 128.1, 131.7, 131.7, 140.5, 160.3, 168.0; ESI-MS calc. for C₁₉H₁₉O₃ [M + H]⁺: 307; found: m/z 307.
13. Similar compounds were synthesized using the organometallic species involving different synthetic strategies such as the addition of (a) alkynyl Grignard on acetates of the Baylis–Hillman alcohols^19a (b) alkynyl cuprate^19b and alkynyl indium^19c species on bromides derived from the Baylis–Hillman alcohols.
14. In 3-aryl-(2-substituted)-prop-2-enoates, the β-vinylic proton cis to the ester group of the E-isomers appear at δ ∼ 7.7 ppm while the δ-vinylic proton trans to the ester group of the Z-isomers appear at δ ∼ 6.8 ppm.^7,18.
15. Typical procedure for the synthesis of 2-cyano-1-(4-methoxyphenyl)allyl acetate (7a): To a stirred solution of 2-(hydroxy(4-methoxyphenyl)methyl)acrylonitrile (0.52 mmol), pyridine (1.04 mmol) in dichloromethane (5 mL), acetyl chloride (0.63 mmol) was added at 0 °C. Reaction mixture was allowed to slowly warm to room temperature while stirring was continued. Upon reaction completion as judged by TLC, the resulting mixture was diluted with dichloromethane (10 mL), washed with 2 N HCl solution (5 mL), satd. NaHCO₃ solution (5 mL) and dried over Na₂SO₄. The organic layer was concentrated under reduced pressure and purified by column chromatography on silica (EtOAc–hexanes) to afford 2-cyano-1-(4-methoxyphenyl)allyl acetate 7a (0.69 mmol, 91%). ¹H NMR (400 MHz, CDCl₃): δ 2.15 (s, 3H), 3.81 (s, 3H), 5.97 (d, J = 1.39 Hz, 1H), 6.05 (d, J = 1.07 Hz, 1H), 6.28 (s, 1H), 6.90–6.93 (d, J = 8.78 Hz, 2H), 7.32 (d, J = 8.53 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 20.9, 55.3, 74.0, 114.3, 116.3, 123.3, 127.7, 128.5, 131.5, 160.2, 169.2; ESI-MS calc. for C₁₃H₁₄NO₃ [M + H]⁺ = 232; found: m/z 232.
16. Typical procedure for the synthesis of (Z)-2-(4-methoxybenzylidene)-5-phenylpent-4-ynenitrile (8a): To a stirred solution of 2-cyano-1-(4-methoxyphenyl)allyl acetate 7a (0.43 mmol) in dry THF (15 mL), were added Pd(OAc)₂ (0.021 mmol), 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (0.042 mmol), Cs₂CO₃ (0.86 mmol) and 3-phenylpropiolic acid (0.50 mmol). The mixture was heated to 70 °C for 30 min. Upon reaction completion as judged by TLC, the resulting mixture was diluted with ethyl acetate (10 mL), washed with water (5 mL), brine (5 mL) and dried over Na₂SO₄. The organic layer was concentrated under reduced pressure and purified by column chromatography on silica (EtOAc–hexanes) to afford (Z)-2-(4-methoxybenzylidene)-5-phenylpent-4-ynenitrile (8a) (0.38 mmol, 89%). ¹H NMR (300 MHz, CDCl₃): δ 3.56 (d, J = 3.0, Hz, 2H), 3.85 (s, 3H), 6.92–6.95 (m, 2H), 7.31–7.34 (m, 3H), 7.45–7.49 (m, 2H), 7.74–7.77 (m, 2H); ¹³C NMR (75 MHz, CDCl₃): δ 26.2, 55.4, 83.2, 85.5, 103.1, 114.2, 118.7, 122.8, 126.0, 128.3, 128.4, 130.6, 131.7, 143.8, 161.2; ESI-MS calc. for C₁₉H₁₆NO [M + H]⁺: 274; found: m/z 274.
17. For Pd-catalyzed allylic substitution of β-keto acids: (a) B. M. Trost and D. L. Van Vranken, Chem. Rev., 1996, 96, 395–422; (b) B. M. Trost, Acc. Chem. Res., 1996, 29, 355–364; (c) Q.-H. Fan, Y.-M. Li and A. S. C. Chan, Chem. Rev., 2002, 102, 3385–3466; (d) B. M. Trost and M. L. Crawley, Chem. Rev., 2003, 103, 2921–2944; (e) B. M. Trost, J. Org. Chem., 2004, 69, 5813–5837; (f) J. D. Weaver, A. Recio III, A. J. Grenning and J. A. Tunge, Chem. Rev., 2010, 111, 1846–1913; (g) Z. Lu and S. Ma, Angew. Chem., Int. Ed., 2008, 47, 258–297; (h) E. C. Burger and J. A. Tunge, Org. Lett., 2004, 6, 4113–4115; (i) J. A. Tunge and E. C. Burger, Eur. J. Org. Chem., 2005, 9, 1715–1726; (j) S. R. Mellegaard-Waetzig, D. K. Rayabarapu and J. A. Tunge, Synlett, 2005, 2759–2762; (k) M. Nakamura, A. Hajra, K. Endo and E. Nakamura, Angew. Chem., 2005, 117, 7414–7417 (Angew. Chem., Int. Ed., 2005, 44, 7248–7251); (l) B. M. Trost and J. Xu, J. Am. Chem. Soc., 2005, 127, 17180–17181; (m) J. T. Mohr, D. C. Behenna, A. M. Harned and B. M. Stoltz, Angew. Chem., 2005, 117, 7084–7087 (Angew.Chem. Int. Ed., 2005, 44, 6924–6927).
18. (a) D. Basavaiah and P. K. S. Sarma, Chem. Commun., 1992, 955–957; (b) D. Basavaiah, M. Krishnamacharyulu, R. S. Hyma and S. Pandiaraju, Tetrahedron Lett., 1997, 38, 2141–2144; (c) H. J. Lee, M. R. Seong and J. N. Kim, Tetrahedron Lett., 1998, 39, 6223.
19. (a) S. Gowrisankar, K. Y. Lee, C. G. Lee and J. N. Kim, Tetrahedron Lett., 2004, 45, 6141–6146; (b) J. W. Lim, K. H. Kim, S. H. Kim and J. N. Kim, Tetrahedron Lett., 2012, 53, 5449–5454; (c) J. S. Yadav, B. V. S. Reddy, N. N. Yadav, A. P. Singh, M. Choudhary and A. C. Kunwar, Tetrahedron Lett., 2008, 49, 6090–6094.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, ¹H NMR, 2D NOESY, ¹³C NMR and MS data of all the products. See DOI: 10.1039/c5ra06168g

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