DMAP-promoted domino annulation of β-ketothioamides with internal alkynes: a highly regioselective access to functionalized 1,3-thiazolidin-4-ones at room temperature

Abstract

DMAP-mediated rapid and efficient one-pot regioselective access to functionalized 1,3-thiazolidin-4-ones via annulation of β-ketothioamides with internal alkynes has been achieved under mild reaction conditions. The merit of this straightforward domino protocol is highlighted by its operational simplicity, short reaction time, tolerance of a large variety of functional groups, and efficiency of producing two new bonds (C–S and C–N) and one thiazolidine ring.

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Introduction

In recent years, the development of new domino strategies¹ that provide synthetic efficiency and atom economy has been an important goal of synthetic chemistry, and continues to be a great challenge. 1,3-Thiazolidin-4-ones are an important group of heterocyclic compounds that appear in a plethora of natural products as well as in various pharmaceutical compounds.² They are not only synthetically important scaffolds but also possess a wide range of promising biological activities such as anti-inflammatory,^3a antimicrobial,^3b,c anti-tubercular,^3d anti-HIV^3e,f and anti-viral.^3g Some derivatives proved to be highly effective in inhibiting HIV-1 replication at nanomolar concentrations with minimal cytotoxicity, thereby acting as non-nucleoside HIV-1 RT inhibitors (NNRTIs).^3h Furthermore, thiazolidine based compounds were found to be a prominent heat shock protein 70 (Hsp 70) inhibitor^4a and cyclooxygenase (COX-2) inhibitor.^4b,c Moreover, some thiazolidin-4-one derivatives are also used for the synthesis of highly efficient dyes sensitized solar cells.⁵ The utility of 1,3-thiazolidin-4-ones as synthons for various applications has given impetus to these studies.

In general, 1,3-thiazolidin-4-ones have been prepared by the condensation of aldehyde, amine (or Schiff's base) with mercaptoacetic acid.⁶ Bolognese et al.^7a employing microwave technique and Lingampalle and co-workers^7b utilizing ionic liquid synthesized thazoilidin-4-one derivatives. Borisevich et al.^8a synthesized 1,3-thiazolidin-4-ones using thioamide and alkyne or maleic anhydride. The use of Baker's yeast,^8b and Bi (SCH₂COOH)₃,^8c as a catalyst have also been reported for the preparation of 1,3-thiazolidin-4-ones. Although the aforesaid methods enriched approaches to thiazolidin-4-one derivatives, most of them suffer from one or more limitations, such as poor yield, harsh reaction conditions, limited functional group tolerability, highly toxic reagents and tedious work-up procedure. Therefore, the exploration of more general, efficient, rapid, and viable routes for the construction of functionalized 1,3-thiazolidin-4-one scaffolds, particularly those with wide general applicability to achieve more flexible substitution patterns from readily available precursors is quite desirable.

In continuation of our ongoing research for the development of new methodologies for various heterocyclic systems by exploiting reactions of β-oxodithioesters,⁹ herein, we report the first example of highly regioselective synthesis of functionalized 1,3-thiazolidin-4-ones via cascade annulation of β-ketothioamides with internal alkynes promoted by DMAP at room temperature (Scheme 1).

Scheme 1 Regioselectivity of the reaction.

Results and discussion

A rapidly increasing recognition of the rich and fascinating chemistry of β-oxodithioesters for the synthesis of various important heterocycles has been brought out recently by our group.⁹ β-Ketothioamides (KTAs) as polyfunctional scaffolds bearing several reactive sites with general structure 1 (Fig. 1), are shown to exhibit intriguing multinucleophilic reactivities and have proven to be important building blocks in the construction of various heterocyclic systems.¹⁰ In recent years, extensive work in this area has been done on the reactivities of the nucleophilic sites (N & C, O & S, and C & S atoms) of KTAs with dielectrophilic groups. However, the coupling reaction by means of N and S nucleophilic sites of KTAs with internal alkynes promoted by DMAP to form highly substituted 1,3-thiazolidin-4-ones has not been disclosed to date.

Fig. 1 Reactivity profile of β-ketothioamide.

The reactions of β-ketothioamides 1 with internal alkynes 2 might occur in two directions, as shown in Scheme 1. The nucleophilic attack of thiocarbonyl sulfur of 1e on internal alkyne 2a generates intermediate A₁. Intermediate A₁ could probably undergo intramolecular N-cyclization via its two possible rotamers A₁ and A₂ through two pathways to furnish 2,3,5-trisubstituted thiazolidin-4-one 3ea and 1,3-thiazine-4-one-6-carboxylate 4, respectively (Scheme 1, pathways I and II). When the equimolar amount of 1e and 2a was stirred in 5 mL of DCM at room temperature only one product isomer was obtained. However, the common characterization involving IR, ¹H and ¹³C NMR, and HRMS analyses could not sufficiently identify the structure of the product as 3ea or 4. Fortunately, we obtained a single crystal of the product 3ea, and the X-ray diffraction analysis of 3ea (Fig. 2) revealed that the obtained product was a thiazolidin-4-one derivative making the protocol highly regioselective.

Fig. 2 ORTEP diagrams of 3ea and 3qb.

In this paper, we report a facile and efficient one-pot highly regioselective synthesis of 2,3,5-trisubstituted thiazolidin-4-ones 3 via cascade annulation of β-ketothioamides 1 with doubly activated internal alkynes 2 mediated by 4-dimethylaminopyridine (DMAP) at room temperature (Scheme 2). DMAP is a frequently used least expensive and less toxic organic Lewis base, which finds widespread application in a number of chemical reactions.¹¹ Our literature survey at this stage revealed that there is no report on the use of DMAP as a catalyst in the synthesis of 1,3-thiazolidin-4-ones from β-ketothioamides. Our main strategy in this work is to develop a new methodology which is easy, fast and cleaner than conventional reactions.

Scheme 2 Synthesis of 2,3,5-trisubstituted thiazolidin-4-ones.

Encouraged by the above result, we focussed on exploring the optimal reaction conditions for the synthesis of thiazolidin-4-one derivatives 3. β-Ketothioamide 1e and internal alkyne 2a were selected as the test substrates to optimize the reaction conditions. The various attempts are summarized in Table 1. Initially, the equimolar amount of 1e and 2a in EtOH (5 mL) or DCM (5 mL) was stirred at room temperature without any catalyst, the target compound 3ea was obtained in 12% and 22% yields respectively, and most of the 1e remained unconsumed (Table 1, entries 1 and 2). Next, different organic bases such as DMAP, DABCO, DBU, DBN, pyridine and Et₃N were employed as the catalyst. Delightedly, 5 mol% of DMAP gave the desired product 3ea in 92% yield within 3 min in DCM at room temperature (Table 1, entry 3). Other organic bases also promoted the reaction towards the formation of desired compound 3ea but their catalytic efficiency was inferior to that of DMAP (Table 1, entries 4–8). With DMAP Lewis base as a good catalyst in hand, next we intended to optimize its loading, and it was found that the use of 5 mol% of DMAP provided the best result (Table 1, entry 3). Increasing the DMAP loading did not give the significant change (Table 1, entry 9). Reducing the DMAP loading to 3 mol% reduced the yield and prolonged the reaction time significantly (Table 1, entry 10). Further, use of other polar protic and aprotic solvents such as EtOH, MeOH and CH₃CN could not improve the results (Table 1, entries 11–13). The use of H₂O as the solvent shut down the reaction because of its poor ability to dissolve the substrates (Table 1, entry 14). Next, to have the green conditions, the above optimized reaction was performed under solvent-free conditions, which gave the desired product 3ea in 56% yield after 35 min (Table 1, entry 15). Obviously, screening of the solvents revealed that DCM turned out to be an appropriate solvent, as it not only resulted in a shorter reaction time but also provided a higher yield than the other examined solvents. Consequently, optimal conditions were identified as equimolar amounts of 1e and 2a, 5 mol% of DMAP in DCM (5 mL) at room temperature.

Table 1 Optimization of the reaction conditions^a

Entry	Catalyst (mol%)	Solvent (5 mL)	Time	Yield^b (%)
a Reaction conditions: the mixture of 1e (1 mmol), 2a (1 mmol) and solvent (5 mL) was stirred at room temperature.b Isolated pure yields.c 1e remained unconsumed.d No reaction.
1	None	EtOH	10 h	12^c
2	None	DCM	10 h	22^c
3	DMAP (5)	DCM	3 min	92
4	DABCO (5)	DCM	3 min	58
5	DBU (5)	DCM	3 min	53
6	DBN (5)	DCM	3 min	55
7	Pyridine (5)	DCM	8 min	72
8	Et3N (5)	DCM	3 min	78
9	DMAP (10)	DCM	3 min	92
10	DMAP (3)	DCM	7 min	84
11	DMAP (5)	EtOH	3 min	74
12	DMAP (5)	MeOH	3 min	68
13	DMAP (5)	CH3CN	3 min	76
14	DMAP (5)	H2O	3 h	NR^d
15	DMAP (5)	None	35 min	56

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With the optimal conditions in hand (Table 1, entry 3), we commenced exploring the substrate scope and generality of this strategy. The results are summarized in Table 2. As can be seen, a wide range of twenty three β-ketothioamides 1a–w derived from aromatic (containing both electron-donating and electron-withdrawing groups), heteroaromatic and aliphatic ketones were well-tolerated, and in all cases the reactions proceeded smoothly to afford the corresponding thiazolidin-4-ones 3 in good to excellent yields. To assess the generality and applicability of this methodology, attempts to expand the scope of the reaction proved successful and a wide range of R¹ groups (aromatic, heteroaromatic, and aliphatic) and R² (aromatic and aliphatic) were incorporated, which provided a functional handle for further manipulation.

Table 2 Substrate scope for the synthesis of thiazolidin-4-ones^a

Product	R^1	R^2	R^3	Time (min)	Yield^b (%)
a Reaction conditions: 1 (1 mmol), 2 (1 mmol), DCM (5 mL), stirring at rt.b Isolated pure yields.
3aa	C6H5	C6H5	Et	3	85
3ba	4-BrC6H4	C6H5	Et	4	83
3ca	4-MeC6H4	C6H5	Et	3	84
3da	2-ClC6H4	C6H5	Et	3	82
3ea	3,4-OCH2O–C6H3	C6H5	Et	3	92
3fa	4-Ph–C6H4	C6H5	Et	5	82
3ga	1-Naphthyl	C6H5	Et	5	80
3ha	2-Thienyl	C6H5	Et	3	90
3ia	2-Furyl	C6H5	Et	3	88
3ja	i-Propyl	C6H5	Et	4	78
3ka	t-Butyl	C6H5	Et	4	75
3la	4-BrC6H4	Me	Et	3	85
3ma	4-MeC6H4	Me	Et	3	87
3na	3,4-OCH2O–C6H3	Me	Et	4	88
3oa	2-Thienyl	Me	Et	3	90
3mb	4-MeC6H4	Me	Me	3	86
3lb	4-BrC6H4	Me	Me	3	84
3nb	3,4-OCH2O–C6H3	Me	Me	3	89
3ob	2-Thienyl	Me	Me	3	92
3pb	4-OMeC6H4	n-Butyl	Me	5	78
3qb	2-Thienyl	n-Butyl	Me	5	81
3ra	4-OMeC6H4	Cyclopropyl	Et	5	78
3rb	4-OMeC6H4	Cyclopropyl	Me	5	80
3sb	2-Thienyl	Cyclopropyl	Me	5	80
3tb	2-Thienyl	Cyclohexyl	Me	7	74
3ub	2-Thienyl	C6H5CH2	Me	8	78
3vb	1-Naphthyl	4-ClC6H4CH2	Me	10	76
3wb				10	71

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Even extremely electron-rich aromatic β-ketothioamides such as 1h, 1i, 1o and 1q proceeded smoothly affording the desired products in excellent yields (Table 2, 3ha, 3ia, 3oa and 3qb). However, β-ketothioamides (1s–u) bearing cyclopropyl, cyclohexyl and benzyl substituents as R² afforded comparatively lower yields (Table 2, 3sb, 3tb and 3ub) than those bearing phenyl as R². In addition to incorporation of aryl, heteroaryl, and aliphatic substituents at R¹, extended aromatics are also viable for this protocol (Table 2, 3fa, 3ga and 3vb). To amplify molecular diversity, we also used β-ketothioamide 1w derived from 5-methoxytetralone, which was tolerated well and afforded the desired product in 71% yield (Table 2, 3wb).

We also focused on employing internal alkynes bearing methyl and ethyl substituents (R³) in this protocol, which were tolerated well to give the desired products in good yields. However, when some unsymmetrical electron-deficient alkynes such as 3-phenylpropiolate and acetyl phenyl acetylene were used under the optimized reaction conditions, reaction did not occur even after 24 h of stirring, thus limiting the scope of this protocol to some extent.

The structures of all of the newly synthesized thiazolidine-4-one derivatives 3 were identified by their IR, ¹H NMR, ¹³C NMR, and HRMS spectra and unequivocally confirmed by the X-ray single crystal diffraction analysis of two representative compounds 3ea and 3qb (Fig. 2). From the crystallographic data for compounds 3ea and 3qb, a Z-stereochemistry was observed for the both exocyclic double bonds at 2- and 5-positions of thiazolidine ring.

On the basis of the above experimental results together with the related reports, a plausible reaction scenario for this one-pot cycloaddition reaction is outlined in Scheme 3. The first step in the mechanism is believed to be the abstraction of the acidic proton of methylene of β-ketothioamide 1 by DMAP followed by nucleophilic attack of thiocarbonyl sulfur atom to the sp-hybridized carbon of 2 (thia-Michael type addition), to generate an open-chain intermediate α-oxoketene-N,S-acetal A. The intermediate N,S-acetal A undergoes intramolecular N-cyclization with the extrusion of R³OH to give the thiazolidin-4-one 3. This operationally simple and two-component cascade annulation concomitantly created two new bonds (C–S and C–N) leading to thiazolidine ring.

Scheme 3 Plausible mechanism for the formation of thiazolidin-4-ones 3.

Conclusion

We have successfully developed a straightforward, mild and efficient highly regioselective cascade annulation to synthesize densely functionalized thiazolidine-4-one derivatives utilizing KTAs and internal alkynes in DCM at room temperature promoted by DMAP within 3–10 min. The procedure can be considered as an ideal means for the synthesis of thiazolidine-4-ones because of the following features: (1) the simplicity of execution and rapid production of thiazolidine-4-ones by cascade annulation, which minimizes the generation of waste; (2) no need for the use of any transition-metal catalyst or other additives; (3) high yields with flexible substitution patterns; (4) formation of two new bonds (C–S and C–N) and one ring in a single operation; (5) high atom-economy and an ecologically benign process in which only one molecule of either ethanol or methanol is lost. Further studies to expand the scope of KTAs as versatile building blocks are in progress and will be reported in due course.

Experimental section

General experimental

All the commercially available reagents were purchased from Merck, Aldrich and Fluka, and were used as received. All ¹H and ¹³C NMR spectra were recorded on JEOL AL 300 FT-NMR spectrometer. Chemical shifts are given as δ value with reference to tetramethylsilane (TMS) as the internal standard. The IR spectra were recorded on PerkinElmer Spectrum Version 10.03.05 FT-IR spectrophotometer. Mass spectra were recorded on Agilent Q-TOF and Waters-Q-Tof Premier-HAB213 instrument. X-ray diffraction was measured on Xcalibur Oxford CCD Diffractometer. All the reactions were monitored by TLC using precoated sheets of silica gel G/UV-254 of 0.25 mm thickness (Merck 60F₂₅₄) using UV light for visualization. Melting points were determined with Büchi B-540 melting point apparatus and are uncorrected.

General procedure for the synthesis of thiazolidin-4-ones (3aa–wb)

To a mixture of β-ketothioamide (1.0 mmol) and dialkyl acetylenedicarboxylate (1.0 mmol) in dichloromethane (5 mL) was added DMAP (5 mol%), and the reaction mixture was stirred for the stipulated period of time (Table 2) at room temperature. After completion of the reaction (monitored by TLC), water (20 mL) was added to the reaction mixture followed by extraction with dichloromethane (2 × 10 mL). The combined organic layer was dried over anhyd. Na₂SO₄ and then evaporated in vacuo. The crude residue thus obtained was purified by column chromatography over silica gel using increasing percentage of ethyl acetate in hexane as eluent to afford pure thiazolidin-4-ones.

3aa: yellow solid; m.p. 210–212 °C; IR (KBr, cm⁻¹): ν 2923, 2852, 1724, 1617, 1516, 1348, 1313, 1210, 1023; ¹H-NMR (300 MHz, CDCl₃): δ 7.73 (d, J = 7.2 Hz, 2H); 7.63–7.58 (m, 3H); 7.49–7.33 (m, 5H); 6.97 (s, 1H); 6.45 (s, 1H); 4.36 (q, J = 7.2 Hz, 2H); 1.39 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 188.9, 165.8, 165.0, 156.0, 142.3, 138.1, 134.7, 132.8, 130.5, 128.7, 128.1, 127.9, 119.1, 99.3, 61.9, 14.5; HRMS [ESI] calcd for C₂₁H₁₇NO₄S (M + H)⁺: 380.0957, found: 380.0952.

3ba: yellow solid; m.p. 248–250 °C; IR (KBr, cm⁻¹): ν 2985, 1708, 1684, 1564, 1526, 1418, 1310, 1190, 1027, 1004; ¹H-NMR (300 MHz, CDCl₃): δ 7.60–7.58 (m, 5H); 7.52 (d, J = 8.4 Hz, 2H); 7.33 (d, J = 6.3 Hz, 2H); 6.98 (s, 1H); 6.38 (s, 1H); 4.36 (q, J = 7.2 Hz, 2H); 1.38 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 187.5, 165.5, 164.7, 156.5, 141.8, 136.6, 134.3, 131.8, 130.3, 129.1, 127.8, 127.7, 119.2, 98.6, 61.7, 14.2; HRMS [ESI] calcd for C₂₁H₁₆BrNO₄S (M + H)⁺/[(M + 2) + H]⁺: 458.0062/460.0041, found: 458.0068/460.0024.

3ca: yellow solid; m.p. 215–217 °C; IR (KBr, cm⁻¹): ν 2982, 1720, 1701, 1607, 1543, 1495, 1350, 1314, 1217, 1185, 1030; ¹H-NMR (300 MHz, CDCl₃): δ 7.64–7.62 (m, 5H); 7.33 (d, J = 6.9 Hz, 2H); 7.18 (d, J = 8.1 Hz, 2H); 6.96 (s, 1H); 6.44 (s, 1H); 4.36 (q, J = 7.2 Hz, 2H); 2.36 (s, 3H); 1.38 (t, J = 7.2 Hz, 3H). ¹³C-NMR (75 MHz, CDCl₃): δ 188.3, 165.5, 164.7, 155.3, 143.3, 142.2, 135.4, 134.5, 130.2, 130.0, 129.2, 127.8, 118.6, 99.2, 61.6, 21.5, 14.2; HRMS [ESI] calcd for C₂₂H₁₉NO₄S (M + H)⁺: 394.1113, found: 394.1114.

3da: yellow solid; m.p. 176–178 °C; IR (KBr, cm⁻¹): ν 3060, 1727, 1687, 1591, 1525, 1374, 1344, 1207, 1027; ¹H-NMR (300 MHz, CDCl₃): δ 7.58–7.45 (m, 4H); 7.31–7.26 (m, 5H); 6.98 (s, 1H); 6.26 (s, 1H); 4.36 (q, J = 7.2 Hz, 2H); 1.38 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 189.9, 165.5, 164.6, 155.0, 141.9, 139.1, 134.2, 131.7, 131.1, 130.3, 130.1, 130.0, 129.8, 127.7, 126.9, 119.1, 103.3, 61.7, 14.2; HRMS [ESI] calcd for C₂₁H₁₆ClNO₄S (M + H)⁺: 414.0567, found: 414.0566.

3ea: yellow solid; m.p. 280–282 °C; IR (KBr, cm⁻¹): ν 3075, 2922, 1720, 1697, 1604, 1535, 1493, 1437, 1359, 1254, 1206, 1187, 1094, 1036; ¹H-NMR (300 MHz, CDCl₃): δ 7.62–7.60 (m, 2H); 7.34–7.29 (m, 5H); 6.96 (s, 1H); 6.76 (d, J = 8.1 Hz, 1H); 6.36 (s, 1H); 6.01 (s, 2H); 4.36 (q, J = 7.2 Hz, 2H); 1.38 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 186.7, 165.6, 164.7, 155.3, 151.4, 148.1, 142.2, 134.5, 132.7, 130.2, 130.0, 127.8, 123.6, 118.6, 107.8, 101.8, 99.0, 61.6, 14.2; HRMS [ESI] calcd for C₂₂H₁₇NO₆S (M + H)⁺: 424.0855, found: 424.0854.

3fa: yellow solid; m.p. 220–222 °C; IR (KBr, cm⁻¹): ν 2924, 1713, 1633, 1605, 1559, 1537, 1495, 1352, 1214, 1186, 1051; ¹H-NMR (300 MHz, CDCl₃): δ 7.81 (d, J = 8.4 Hz, 2H); 7.62–7.56 (m, 7H); 7.46–7.34 (m, 5H); 6.98 (s, 1H); 6.49 (s, 1H); 4.37 (q, J = 7.2 Hz, 2H); 1.39 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃ + DMSO-d₆): δ 186.9, 164.5, 163.4, 154.6, 144.1, 141.4, 138.5, 135.6, 133.5, 129.2, 129.1, 127.9, 127.1, 126.9, 126.0, 117.1, 98.0, 60.5, 13.2; HRMS [ESI] calcd for C₂₇H₂₁NO₄S (M + H)⁺: 456.1270, found: 456.1264.

3ga: yellow solid; m.p. 230–232 °C; IR (KBr, cm⁻¹): ν 3056, 2923, 1718, 1694, 1633, 1616, 1544, 1496, 1311, 1192, 1123, 1039; ¹H-NMR (300 MHz, CDCl₃): δ 8.18 (s, 1H); 7.87–7.81 (m, 4H); 7.65–7.49 (m, 5H); 7.38 (d, J = 7.2 Hz, 2H); 6.98 (s, 1H); 6.60 (s, 1H); 4.37 (q, J = 6.9 Hz, 2H); 1.39 (t, J = 6.9 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 188.5, 165.5, 164.8, 155.8, 142.1, 135.3, 134.5, 132.4, 130.3, 130.1, 129.4, 128.8, 128.4, 128.3, 127.9, 127.7, 126.6, 123.7, 118.9, 99.3, 61.6, 14.2; HRMS [ESI] calcd for C₂₅H₁₉NO₄S (M + H)⁺: 430.1113, found: 430.1113.

3ha: yellow solid; m.p. 213–215 °C; IR (KBr, cm⁻¹): ν 3074, 2986, 1726, 1616, 1528, 1418, 1357, 1316, 1211, 1190; ¹H-NMR (300 MHz, CDCl₃): δ 7.60–7.56 (m, 4H); 7.40–7.32 (m, 3H); 7.03 (t, J = 4.2 Hz, 1H); 6.95 (s, 1H); 6.28 (s, 1H); 4.35 (q, J = 6.9 Hz, 2H); 1.38 (t, J = 6.9 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 181.1, 165.5, 164.6, 155.1, 145.2, 141.7, 134.3, 133.4, 130.5, 130.2, 130.1, 128.0, 127.8, 118.7, 99.2, 61.6, 14.2; HRMS [ESI] calcd for C₁₉H₁₅NO₄S₂ (M + H)⁺: 386.0521, found: 386.0522.

3ia: yellow solid; m.p. 216–218 °C; IR (KBr, cm⁻¹): ν 2986, 1727, 1693, 1632, 1572, 1533, 1474, 1376, 1313, 1230, 1187, 1053; ¹H-NMR (300 MHz, CDCl₃): δ 7.62–7.59 (m, 3H); 7.46 (s, 1H); 7.32 (d, J = 7.2 Hz, 2H); 7.10 (d, J = 3.0 Hz, 1H); 6.96 (s, 1H); 6.48 (s, 1H); 6.35 (s, 1H); 4.36 (q, J = 7.2 Hz, 2H); 1.38 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 177.4, 165.5, 164.6, 155.1, 153.3, 145.7, 141.8, 134.3, 130.2, 130.0, 127.8, 118.8, 116.2, 112.5, 99.1, 61.6, 14.2; HRMS [ESI] calcd for C₁₉H₁₅NO₅S (M + H)⁺: 370.0749, found: 370.0748.

3ja: yellow solid; m.p. 112–114 °C; IR (KBr, cm⁻¹): ν 2975, 2932, 1712, 1693, 1543, 1368, 1311, 1239, 1190, 1067, 1026; ¹H-NMR (300 MHz, CDCl₃): δ 7.59–7.54 (m, 2H); 7.28–7.25 (m, 3H); 6.92 (s, 1H); 5.74 (s, 1H); 4.33 (q, J = 7.2 Hz, 2H); 2.54–2.48 (m, 1H); 1.37 (t, J = 7.2 Hz, 3H); 1.04 (d, J = 6.6 Hz, 6H); ¹³C-NMR (75 MHz, CDCl₃): δ 202.8, 165.5, 153.6, 142.1, 134.4, 130.1, 129.9, 127.7, 118.3, 100.9, 61.5, 41.0, 18.3, 14.2; HRMS [ESI] calcd for C₁₈H₁₉NO₄S (M + H)⁺: 346.1113, found: 346.1115.

3ka: yellow solid; m.p. 141–143 °C; IR (KBr, cm⁻¹): ν 3.65, 2965, 1715, 1694, 1529, 1373, 1318, 1222, 1193, 1082, 1024; ¹H-NMR (300 MHz, CDCl₃): δ 7.56 (brs, 3H); 7.28–7.26 (m, 2H); 6.92 (s, 1H); 5.93 (s, 1H); 4.34 (q, J = 3.6 Hz, 2H); 1.37 (t, J = 3.6 Hz, 3H); 1.05 (s, 9H); ¹³C-NMR (75 MHz, CDCl₃): δ 204.3, 165.5, 164.7, 154.3, 142.2, 134.5, 130.1, 129.9, 127.7, 118.3, 98.5, 61.5, 43.0, 26.4, 14.2; HRMS [ESI] calcd for C₁₉H₂₁NO₄S (M + H)⁺: 360.1270, found: 360.1263.

3la: yellow solid; m.p. 224–226 °C; IR (KBr, cm⁻¹): ν 3060, 1707, 1684, 1634, 1586, 1524, 1418, 1346, 1310, 1227, 1190, 1069; ¹H-NMR (300 MHz, CDCl₃): δ 7.81 (d, J = 8.1 Hz, 2H); 7.60 (d, J = 8.1 Hz, 2H); 6.91 (s, 1H); 6.69 (s, 1H); 4.33 (q, J = 7.2 Hz, 2H); 3.43 (s, 3H); 1.37 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 187.3, 165.4, 164.9, 156.0, 141.9, 136.7, 131.9, 129.2, 127.7, 119.0, 96.6, 61.6, 30.1, 14.2; HRMS [ESI] calcd for C₁₆H₁₄BrNO₄S (M + H)⁺/[(M + 2) + H]⁺: 395.9905/397.9885, found: 395.9909/397.9891.

3ma: yellow solid; m.p. 186–188 °C; IR (KBr, cm⁻¹): ν 2984, 1706, 1688, 1637, 1606, 1531, 1419, 1343, 1309, 1233, 1180, 1059, 1027; ¹H-NMR (300 MHz, CDCl₃): δ 7.87 (d, J = 7.5 Hz, 2H); 7.29 (2H merged with CDCl₃); 6.90 (s, 1H); 6.77 (s, 1H); 4.33 (q, J = 7.2 Hz, 2H); 3.43 (s, 3H); 2.47 (s, 3H); 1.37 (t, J = 6.9 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 188.2, 165.5, 165.0, 154.8, 143.5, 142.3, 135.5, 129.3, 127.8, 118.6, 97.2, 61.5, 30.1, 21.6, 14.2; HRMS [ESI] calcd for C₁₇H₁₇NO₄S (M + H)⁺: 332.0957, found: 332.0959.

3na: yellow solid; m.p. 186–188 °C; IR (KBr, cm⁻¹): ν 3074, 2996, 1702, 1674, 1538, 1504, 1435, 1355, 1318, 1261, 1200, 1093, 1034; ¹H-NMR (300 MHz, CDCl₃): δ 7.55 (d, J = 8.1 Hz, 1H); 7.47 (s, 1H); 6.90 (s, 1H); 6.86 (s, 1H); 6.69 (s, 1H); 6.06 (s, 2H); 4.33 (q, J = 6.9 Hz, 2H); 3.43 (s, 3H); 1.36 (t, J = 6.9 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 186.7, 165.5, 154.7, 151.5, 148.3, 142.3, 132.9, 126.2, 123.6, 118.5, 109.9, 107.8, 101.9, 101.7, 97.0, 61.5, 30.1, 14.2; HRMS [ESI] calcd for C₁₇H₁₅NO₆S (M + H)⁺: 362.0698, found: 362.0691.

3oa: yellow solid; m.p. 182–184 °C; IR (KBr, cm⁻¹): ν 3115, 3099, 2983, 1699, 1614, 1537, 1425, 1360, 1317, 1242, 1195, 1028; ¹H-NMR (300 MHz, CDCl₃): δ 7.72 (d, J = 3.6 Hz, 1H); 7.62 (d, J = 4.5 Hz, 1H); 7.12 (t, J = 4.2 Hz, 1H); 6.87 (s, 1H); 6.59 (s, 1H); 4.32 (q, J = 7.2 Hz, 2H); 3.41 (s, 3H); 1.36 (t, J = 6.9 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 180.9, 165.4, 164.8, 154.7, 145.2, 141.9, 133.2, 130.4, 128.1, 118.5, 97.4, 61.5, 30.0, 14.2; HRMS [ESI] calcd for C₁₄H₁₃NO₄S₂ (M + H)⁺: 324.0364, found: 324.0364.

3mb: yellow solid; m.p. 226–228 °C; IR (KBr, cm⁻¹): ν 2954, 1719, 1694, 1639, 1534, 1425, 1323, 1281, 1183, 1042, 1015; ¹H-NMR (300 MHz, CDCl₃): δ 7.86 (d, J = 7.8 Hz, 2H); 7.26 (d, J = 6.9 Hz, 2H); 6.89 (s, 1H); 6.76 (s, 1H); 3.87 (s, 3H); 3.42 (s, 3H); 2.41 (s, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 188.1, 165.9, 164.9, 154.6, 143.6, 142.6, 135.3, 129.3, 127.8, 117.9, 97.2, 52.3, 30.1, 21.6; HRMS [ESI] calcd for C₁₆H₁₅NO₄S (M + H)⁺: 318.0800, found: 318.0800.

3lb: yellow solid; m.p. 239–241 °C; IR (KBr, cm⁻¹): ν 2954, 1720, 1691, 1636, 1564, 1534, 1424, 1321, 1204, 1184, 1042, 1067; ¹H-NMR (300 MHz, CDCl₃): δ 7.82 (d, J = 8.1 Hz, 2H); 7.62 (d, J = 8.4 Hz, 2H); 6.93 (s, 1H); 6.71 (s, 1H); 3.88 (s, 3H); 3.44 (s, 3H); ¹³C-NMR (75 MHz, CDCl₃ + DMSO-d₆): δ 186.3, 164.8, 163.5, 154.3, 141.6, 135.6, 130.7, 128.4, 126.4, 116.5, 96.2, 51.3, 29.2; HRMS [ESI] calcd for C₁₅H₁₂BrNO₄S (M + H)⁺/[(M + 2) + H]⁺: 381.9749/383.9728, found: 381.9749/383.9729.

3nb: yellow solid; m.p. 242–244 °C; IR (KBr, cm⁻¹): ν 2954, 1698, 1637, 1531, 1488, 1441, 1324, 1244, 1198, 1115, 1034; ¹H-NMR (300 MHz, CDCl₃): δ 7.55 (d, J = 8.1 Hz, 1H); 7.46 (s, 1H); 6.90 (s, 1H); 6.87 (d, J = 8.4 Hz, 1H); 6.69 (s, 1H); 6.05 (s, 2H); 3.87 (s, 3H); 3.42 (s, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 186.7, 161.2, 158.4, 157.5, 154.6, 151.6, 142.6, 133.0, 126.9, 123.6, 118.0, 107.9, 101.9, 97.1, 52.4, 30.1; HRMS [ESI] calcd for C₁₆H₁₃NO₆S (M + H)⁺: 348.0542, found: 348.0537.

3ob: yellow solid; m.p. 218–220 °C; IR (KBr, cm⁻¹): ν 2951, 1721, 1629, 1532, 1419, 1355, 1287, 1186, 1019; ¹H-NMR (300 MHz, CDCl₃): δ 7.72 (d, J = 3.0 Hz, 1H); 7.62 (d, J = 3.9 Hz, 1H); 7.13 (t, J = 4.2 Hz, 1H); 6.87 (s, 1H); 6.59 (s, 1H); 3.86 (s, 3H); 3.42 (s, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 180.9, 165.8, 164.9, 154.6, 145.3, 142.2, 133.1, 130.5, 128.1, 118.0, 97.5, 52.2, 30.1; HRMS [ESI] calcd for C₁₃H₁₁NO₄S₂ (M + H)⁺: 310.0208, found: 310.0208.

3pb: yellow solid; m.p. 160–162 °C; IR (KBr, cm⁻¹): ν 2957, 1705, 1600, 1531, 1324, 1262, 1199, 1167, 1021; ¹H-NMR (300 MHz, CDCl₃): δ 7.93 (d, J = 8.4 Hz, 2H); 6.97 (d, J = 8.1 Hz, 2H); 6.88 (s, 1H); 6.76 (s, 1H); 3.92 (t, J = 7.5 Hz, 2H); 3.88 (s, 3H); 3.87 (s, 3H); 1.75–1.65 (m, 2H); 1.47–1.39 (m, 2H); 1.00 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 187.2, 165.9, 165.0, 163.2, 153.7, 142.7, 131.0, 129.9, 117.7, 113.8, 96.9, 55.5, 52.3, 43.5, 29.0, 20.0, 13.6; HRMS [ESI] calcd for C₁₉H₁₇NO₅S (M + H)⁺/(M + Na)⁺: 376.1219/398.1038, found: 376.1205/398.1024.

3qb: yellow solid; m.p. 160–162 °C; IR (KBr, cm⁻¹): ν 2939, 1698, 1626, 1534, 1368, 1326, 1205, 1064; ¹H-NMR (300 MHz, CDCl₃): δ 7.71 (d, J = 3.3 Hz, 1H); 7.63 (d, J = 4.8 Hz, 1H); 7.14 (t, J = 4.2 Hz, 1H); 6.87 (s, 1H); 6.62 (s, 1H); 3.91 (t, J = 7.2 Hz, 2H); 3.86 (s, 3H); 1.74–1.69 (m, 2H); 1.47–1.39 (m, 2H); 1.00 (t, J = 7.2 Hz, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 180.9, 165.8, 164.9, 154.0, 145.3, 142.2, 133.2, 130.3, 128.1, 117.8, 97.3, 52.4, 43.5, 29.0, 20.0, 13.6; HRMS [ESI] calcd for C₁₆H₁₇NO₄S₂ (M + H)⁺: 352.0677, found: 352.0677.

3ra: yellow solid; m.p. 168–170 °C; IR (KBr, cm⁻¹): ν 2925, 1725, 1688, 1690, 1599, 1519, 1460, 1354, 1313, 1268, 1169, 1023; ¹H-NMR (300 MHz, CDCl₃): δ 7.96 (d, J = 8.7 Hz, 2H); 7.19 (s, 1H); 6.97 (d, J = 8.7 Hz, 2H); 6.84 (s, 1H); 4.32 (q, J = 7.2 Hz, 2H); 3.88 (s, 3H); 2.78–2.77 (m, 1H); 1.36 (t, J = 7.2 Hz, 3H); 1.27 (d, J = 6.0 Hz, 2H); 1.04 (s, 2H); ¹³C-NMR (75 MHz, CDCl₃): δ 187.3, 165.5, 165.3, 163.3, 154.7, 142.7, 131.1, 130.0, 129.9, 118.0, 113.8, 98.4, 61.4, 55.4, 25.6, 14.2, 7.5; HRMS [ESI] calcd for C₁₉H₁₉NO₅S (M + H)⁺: 374.1062, found: 374.1063.

3rb: yellow solid; m.p. 228–230 °C; IR (KBr, cm⁻¹): ν 2927, 1714, 1693, 1594, 1533, 1428, 1357, 1324, 1223, 1195, 1165, 1027; ¹H-NMR (300 MHz, CDCl₃): δ 7.96 (d, J = 8.4 Hz, 2H); 7.19 (s, 1H); 6.97 (d, J = 8.4 Hz, 2H); 6.85 (s, 1H); 3.88 (s, 3H); 3.86 (s, 3H); 2.77 (br, 1H); 1.27 (d, J = 6.6 Hz, 2H); 1.04 (s, 2H); ¹³C-NMR (75 MHz, CDCl₃): δ 187.3, 165.9, 165.2, 163.3, 154.5, 143.0, 131.1, 130.0, 117.5, 113.8, 98.5, 55.5, 52.3, 25.6, 7.5; HRMS [ESI] calcd for C₁₈H₁₇NO₅S (M + H)⁺: 360.0906, found: 360.0903.

3sb: yellow solid; m.p. 232–234 °C; IR (KBr, cm⁻¹): ν 2955, 1700, 1622, 1598, 1517, 1421, 1361, 1315, 1225, 1190, 1066; ¹H-NMR (300 MHz, CDCl₃): δ 7.72 (d, J = 2.7 Hz, 1H); 7.61 (d, J = 4.5 Hz, 1H); 7.13 (s, 1H); 7.03 (s, 1H); 6.83 (s, 1H); 3.85 (s, 3H); 2.75 (br, 1H); 1.25 (d, J = 5.4 Hz, 2H); 1.04 (s, 2H); ¹³C-NMR (75 MHz, CDCl₃): δ 181.0, 165.8, 165.1, 154.9, 145.5, 142.5, 133.0, 130.4, 128.1, 117.7, 98.9, 52.3, 25.7, 7.4; HRMS [ESI] calcd for C₁₅H₁₃NO₄S₂ (M + H)⁺: 336.0364, found: 336.0367.

3tb: yellow solid; m.p. 207–209 °C; IR (KBr, cm⁻¹): ν 2928, 2852 1712, 1622, 1608, 1517, 1421, 1317, 1200, 1174, 1058; ¹H-NMR (300 MHz, CDCl₃): δ 7.72 (s, 1H); 7.64 (d, J = 4.2 Hz, 1H); 7.17 (d, J = 4.2 Hz, 1H); 6.84 (s, 1H); 6.80 (s, 1H); 4.23 (br, 1H); 3.86 (s, 3H); 2.40–2.28 (m, 2H); 1.99–1.95 (m, 2H); 1.82–1.78 (m, 3H); 1.46–1.27 (m, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 181.0, 166.0, 133.1, 130.2, 128.2, 126.6, 117.4, 97.8, 52.4, 28.6, 26.1, 25.1; HRMS [ESI] calcd for C₁₈H₁₉NO₄S₂ (M + H)⁺: 378.0834, found: 378.0837.

3ub: yellow solid; m.p. 212–214 °C; IR (KBr, cm⁻¹): ν 2985, 1726, 1616, 1528, 1418, 1357, 1316, 1210, 1189, 1031; ¹H-NMR (300 MHz, CDCl₃): δ 7.56 (d, J = 4.5 Hz, 1H); 7.50 (d, J = 3.3 Hz, 1H); 7.38–7.30 (m, 5H); 7.09 (t, J = 4.2 Hz, 1H); 6.97 (s, 1H); 6.60 (s, 1H); 5.12 (s, 2H); 3.88 (s, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 180.8, 165.8, 165.2, 153.2, 145.1, 142.0, 134.0, 133.4, 130.5, 129.2, 128.2, 126.9, 118.2, 98.9, 52.4, 47.3; HRMS [ESI] calcd for C₁₉H₁₅NO₄S₂ (M + H)⁺/(M + Na)⁺: 386.0521/408.0340, found: 386.0510/408.0330.

3vb: yellow solid; m.p. 170–172 °C; IR (KBr, cm⁻¹): ν 2946, 1717, 1694, 1614, 1572, 1533, 1324, 1202, 1114, 1092; ¹H-NMR (300 MHz, CDCl₃): δ 8.23 (d, J = 8.1 Hz, 1H); 7.94 (d, J = 8.1 Hz, 1H); 7.86 (d, J = 7.8 Hz, 1H); 7.56–7.42 (m, 4H); 7.33 (d, J = 8.1 Hz, 2H); 7.17 (d, J = 8.1 Hz, 2H); 7.00 (s, 1H); 6.55 (s, 1H); 5.03 (s, 2H); 3.09 (s, 3H); ¹³C-NMR (75 MHz, CDCl₃): δ 192.0, 165.9, 165.2, 153.0, 142.1, 137.4, 134.2, 133.7, 132.4, 132.1, 129.9, 129.3, 128.4, 128.3, 127.5, 126.8, 126.4, 125.3, 124.6, 118.7, 102.8, 52.2, 46.4; HRMS [ESI] calcd for C₂₅H₁₈ClNO₄S (M + H)⁺: 464.0723, found: 464.0727.

3wb: yellow solid; m.p. 231–233 °C; IR (KBr, cm⁻¹): ν 2925, 1698, 1583, 1514, 1435, 1325, 1295, 1262, 1203, 1041; ¹H-NMR (300 MHz, CDCl₃): δ 7.65 (d, J = 8.1 Hz, 1H); 7.34–7.24 (m, 3H); 7.07 (d, J = 8.1 Hz, 2H); 6.97 (d, J = 8.4 Hz, 1H); 6.86 (s, 1H); 5.28 (s, 2H); 3.86 (s, 3H); 3.81 (s, 3H); 2.93 (t, J = 6.0 Hz, 2H); 2.68–2.65 (m, 2H); ¹³C-NMR (75 MHz, DMSO-d₆): δ 186.4, 165.9, 165.1, 155.3, 145.7, 142.9, 134.3, 133.6, 131.6, 130.1, 128.1, 127.0, 126.6, 118.5, 114.8, 114.4, 111.0, 55.4, 51.2, 48.5, 25.3, 20.1; HRMS [ESI] calcd for C₂₄H₂₀ClNO₅S (M + H)⁺: 470.0829, found: 470.0812.

Acknowledgements

We gratefully acknowledge the generous financial support from the Science and Engineering Research Board (Grant no. SB/S1/OC-30/2013) and the Council of Scientific and Industrial Research (Grant no. 02(0072)/12/EMR-II), New Delhi. Spectral help from Prof. Ganesh Pandey, CBMR, Lucknow and Prof. Manas Ghorai, IIT Kanpur is highly appreciated.

Notes and references

1. (a) Domino Reactions in Organic Synthesis, ed. L. F. Tietze, G. Brasche and K. M. Gericke, Wiley-VCH, Weinheim, Germany, 2006; (b) D. Roca-Lopez, D. Sadaba, I. Delso, R. P. Herrera, T. Tejero and P. Merino, Tetrahedron: Asymmetry, 2010, 21, 2561; (c) A. Grossmann and D. Enders, Angew. Chem., Int. Ed., 2012, 51, 314; (d) D. Bonne, T. Constantieux, Y. Coquerel and J. Rodriguez, Org. Biomol. Chem., 2012, 10, 3969; (e) H. Pellissier, Chem. Rev., 2013, 113, 442.
2. (a) F. C. Brown, Chem. Rev., 1961, 61, 463; (b) S. P. Singh, S. S. Parmar, K. Raman and V. I. Stenberg, Chem. Rev., 1981, 81, 175; (c) A. K. Jain, A. Vaidya, V. Ravichandran, S. K. Kashaw and R. K. Agrawal, Bioorg. Med. Chem., 2012, 20, 3378; (d) W. Cunico, C. R. B. Gomes and W. T. Vellasco Jr, Mini-Rev. Org. Chem., 2008, 5, 336.
3. (a) A. Kumar, C. S. Rajput and S. K. Bhati, Bioorg. Med. Chem., 2007, 15, 3089; (b) K. Omar, A. Geronikaki, P. Zoumpoulakis, C. Camoutsis, M. Sokovic, A. Ciric and J. Glamoclija, Bioorg. Med. Chem., 2010, 18, 426; (c) D. Prasad, A. Kumar, P. K. Shukla and M. Nath, Org. Med. Chem. Lett., 2011, 1, 1; (d) S. G. Kucukguzel, E. E. Oruc, S. Rollas, F. Sahin and A. Ozbek, Eur. J. Med. Chem., 2002, 37, 197; (e) H. Chen, Z. Guo, Q. Yin, X. Duan, Y. Gu and X. Li, Front. Chem. Sci. Eng., 2011, 5, 231; (f) R. K. Rawal, R. Tripathi, S. B. Katti, C. Pannecouque and E. De Clercq, Bioorg. Med. Chem., 2007, 15, 1725; (g) N. Terzioglu, N. Karalı, A. Gursoy, C. Pannecouque, P. Leysen, J. Paeshuyse, J. Neyts and E. De Clercq, ARKIVOC, 2006, i, 109; (h) M. L. Barreca, J. Balzarini, A. Chimirri, E. De Clercq, L. De Luca, H. D. Holtje, M. Holtje, A. M. Monforte, P. Monforte, C. Pannecouque, A. Rao and M. Zappala, J. Med. Chem., 2002, 45, 5410.
4. (a) Y. Miyata, X. Li, H.-F. Lee, U. K. Jinwal, S. R. Srinivasan, S. P. Seguin, Z. T. Young, J. L. Brodsky, C. A. Dickey, D. Sun and J. E. Gestwicki, ACS Chem. Neurosci., 2013, 4, 930; (b) A. Zarghi, L. Najafnia, B. Daraee, O. G. Dadrass and M. Hedayati, Bioorg. Med. Chem. Lett., 2007, 17, 5634; (c) T. Zebardast, A. Zarghi, B. Daraie, M. Hedayati and O. G. Dadrass, Bioorg. Med. Chem. Lett., 2009, 19, 3162.
5. (a) J. Mao, N. He, Z. Ning, Q. Zhang, F. Guo, L. Chen, W. Wu, J. Hua and H. Tian, Angew. Chem., Int. Ed., 2012, 51, 9873; (b) M. Matsui, T. Inoue, M. Ono, Y. Kubota, K. Funabiki, J. Jin, T. Yoshida, S. Higashijima and H. Miura, Dyes Pigm., 2013, 96, 614; (c) R. Lebeuf, I. Ferezou, J. Rossier, S. Arseniyadis and J. Cossy, Org. Lett., 2009, 11, 4822.
6. (a) N. Oecal, F. Aydogan, C. Yolacan and Z. Turgut, J. Heterocycl. Chem., 2003, 40, 721; (b) H. G. Haecker, P. W. Elsinghorst, S. Michels, J. Daniels, G. Schnakenburg and M. Guetschow, Synthesis, 2009, 1195.
7. (a) A. Bolognese, G. Correale, M. Manfra, A. Lavecchia, E. Novellino and V. Barone, Org. Biomol. Chem., 2004, 2, 2809; (b) D. Lingampalle, D. Jawale, R. Waghmare and R. Mane, Synth. Commun., 2010, 40, 2397.
8. (a) A. M. Borisevich, V. M. Britsun, V. V. Pirozhenko and M. O. Lozinskii, Ukr. Khim. Zh., 2006, 72, 50; (b) U. R. Pratap, D. V. Jawale, M. R. Bhosle and R. A. Mane, Tetrahedron Lett., 2011, 52, 1689; (c) N. Foroughifar and S. Ebrahimi, Chin. Chem. Lett., 2013, 24, 389.
9. Recent papers: (a) S. Chowdhury, T. Chanda, S. Koley, B. J. Ramulu, R. C. F. Jones and M. S. Singh, Org. Lett., 2013, 15, 5386; (b) S. Koley, S. Chowdhury, T. Chanda, B. J. Ramulu and M. S. Singh, Tetrahedron, 2013, 69, 8013; (c) S. Samai, T. Chanda, H. Ila and M. S. Singh, Eur. J. Org. Chem., 2013, 4026; (d) M. S. Singh, G. C. Nandi and T. Chanda, RSC Adv., 2013, 3, 14183; (e) G. Shukla, G. K. Verma, A. Nagaraju, R. K. Verma, K. Raghuvanshi and M. S. Singh, RSC Adv., 2013, 3, 13811; (f) M. S. Singh, A. Nagaraju, G. K. Verma, G. Shukla, R. K. Verma, A. Srivastava and K. Raghuvanshi, Green Chem., 2013, 15, 954; (g) G. C. Nandi, S. Samai and M. S. Singh, J. Org. Chem., 2010, 75, 7785; (h) S. Chowdhury, G. C. Nandi, S. Samai and M. S. Singh, Org. Lett., 2011, 13, 3762; (i) G. C. Nandi, M. S. Singh, H. Ila and H. Junjappa, Eur. J. Org. Chem., 2012, 967.
10. (a) T. S. Jagodzinski, Chem. Rev., 2003, 103, 197; (b) L.-R. Wen, Y.-J. Shi, G.-Y. Liu and M. Li, J. Org. Chem., 2012, 77, 4252; (c) L.-R. Wen, C. Ji, M. Li and H.-Y. Xie, Tetrahedron, 2009, 65, 1287; (d) M. Li, Y.-L. Hou, L.-R. Wen and F.-M. Gong, J. Org. Chem., 2010, 75, 8522; (e) R. Surmont, G. Verniest, M. D. Schrijver, J. W. Thuring, P. T. Holte, F. Deroose and N. D. Kimpe, J. Org. Chem., 2011, 76, 4105; (f) P. Mathew and C. V. Asokan, Tetrahedron, 2006, 62, 1708; (g) R. Samuel, P. Chandran, S. Retnamma, K. A. Sasikala, N. K. Sreedevi, E. R. Anabha and C. V. Asokan, Tetrahedron, 2008, 64, 5944; (h) L.-R. Wen, J.-H. Sun, M. Li, E.-T. Sun and S.-S. Zhang, J. Org. Chem., 2008, 73, 1852; (i) S. Peruncheralathan, A. K. Yadav, H. Ila and H. Junjappa, J. Org. Chem., 2005, 70, 9644; (j) L.-R. Wen, T. He, M.-C. Lan and M. Li, J. Org. Chem., 2013, 78, 10617.
11. (a) D. J. Berry, C. V. Digiovanna, S. S. Metrick and R. Murugan, ARKIVOC, 2001, i, 201; (b) R. K. Verma, G. K. Verma, G. Shukla, A. Nagaraju and M. S. Singh, ACS Comb. Sci., 2012, 14, 224; (c) G. C. Nandi, S. Samai and M. S. Singh, J. Org. Chem., 2011, 76, 8009; (d) A. Sakakura, K. Kawajiri, T. Ohkubo, Y. Kosugi and K. Ishihara, J. Am. Chem. Soc., 2007, 129, 14775; (e) U. Ragnarsson and L. Grehn, Acc. Chem. Res., 1998, 31, 494; (f) B. S. Navale and R. G. Bhat, RSC Adv., 2013, 3, 5220; (g) Y. Shang, C. Wang, X. He, K. Ju, M. Zhang, S. Yu and J. Wu, Tetrahedron, 2010, 66, 9629.

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Footnote

† Electronic supplementary information (ESI) available: Starting materials 1 and 2 are defined and NMR (¹H & ¹³C) spectra of all products. CCDC 972111 (3ea) and 961594 (3qb). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra00131a

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