Access to Fully Substituted Thiazoles and 2,3-Dihydrothiazoles via

Sep 25, 2017 - Access to Fully Substituted Thiazoles and 2,3-Dihydrothiazoles via Copper-Catalyzed [4 + 1] Heterocyclization of ...
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Cite This: J. Org. Chem. 2017, 82, 10846-10854

Access to Fully Substituted Thiazoles and 2,3-Dihydrothiazoles via Copper-Catalyzed [4 + 1] Heterocyclization of α‑(N‑Hydroxy/ aryl)imino-β-oxodithioesters with α‑Diazocarbonyls Abhijeet Srivastava, Gaurav Shukla, Dhananjay Yadav, and Maya Shankar Singh* Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India S Supporting Information *

ABSTRACT: An efficient chemoselective practical route to fully substituted thiazoles and 2,3-dihydrothiazoles has been devised by [4 + 1] heterocyclization of α-(N-hydroxy/aryl)imino-β-oxodithioesters with in situ generated Cu-carbenoids of diazocarbonyls. The α-(N-hydroxy/aryl)imino-β-oxodithioesters are readily accessible by the reaction of β-oxodithioesters with nitrous acid/nitrosoarenes. The overall transformation involves sequential N−O/C−N bonds cleavage followed by cascade C−N/C−S bonds formation in one-pot. This new strategy allows full control over the introduction of various sensitive functional groups at different positions of the thiazole ring, broadening the arsenal of synthetic methods to obtain such scaffolds.

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INTRODUCTION Among the N,S-heterocyclic frameworks, thiazoles are versatile privileged scaffolds present in many natural products1 and biologically active compounds.2 Some of the accepted drugs are dasatinib (anticancer), ritonavir (anti-HIV), nizatidine (antiulcer), and fentiazac (anti-inflammatory) among several medicines.2e Natural products containing thiazole moieties include apratoxins, firefly luciferin, dolastatin E, mirabazole, tantazoles, piscibactin, etc. These molecules show a wide range of biological activities such as anticancer, antimicrobial, antimalarial, antituberculosis, and neurotoxic properties.2f Further, they have been utilized as synthetic intermediates/ ligands, 3 and find extensive applications in functional materials.4a−d The less explored 2,3-dihydrothiazole core is also found to be active in various biological evaluations.4e−g Moreover, some thiazole derivatives have found application as liquid crystals for ferroelectric display5a,b and as cosmetic sunscreens.5c Therefore, many methods have been developed for the synthesis of thiazole derivatives.6,7 Direct synthetic protocols based on starting materials such as ketones,8a−d α,βunsaturated ketones8e and alkenes8f via the intermediary αhaloketones are reported for aminothiazoles. Recently, Jiang and co-workers9a developed a strategy for 2-aminothiazoles via copper-catalyzed coupling of oxime acetates with isothiocyanates. 2,4,5-Trisubstituted thiazoles are synthesized by the reaction of α-nitroepoxides with thioureas under strongly basic conditions.9b Although the reported approaches are useful tools © 2017 American Chemical Society

toward the construction of thiazole frameworks, most of them are associated with certain disadvantages with respect to their practical execution in the laboratory. Therefore, exploring more efficient and flexible strategies and improvement in the existing synthetic methods of thiazoles are highly desired and could be recognized as strategic issues. We recently reported the α-functionalization of β-oxodithioesters (DTEs), which were further transformed to 1,2,3thiadiazoles.10a In this context, α-(N-hydroxy)imino-β-oxodithioester is a new underdeveloped class of precursor, which has not been much explored. As they could represent excellent synthetic precursors for N,S-heterocycles, herein we synthesized α-(N-hydroxy)imino-β-oxodithioesters10a (2) and α-(Naryl)imino-β-oxodithioesters10b (4) by the reaction of βoxodithioesters (1) with nitrous acid and nitrosoarenes (3), respectively (Scheme 1a). In particular, the α-functionalization of β-oxodithioesters enables their site-specific derivatization to provide valuable materials. Diazocarbonyls play an important role as a coupling partner, and are versatile intermediates used as precursors of metal carbenoids for various synthetic transformations.11 To the best of our knowledge, diazocarbonyls have not been used as coupling partner with α-imino dithioesters until yet. Consequently, we treated α-imino dithioesters 2 and 4, Received: June 28, 2017 Published: September 25, 2017 10846

DOI: 10.1021/acs.joc.7b01601 J. Org. Chem. 2017, 82, 10846−10854

Article

The Journal of Organic Chemistry Scheme 1. α-Functionalization of β-Oxodithioesters (1) to 2 and 4 and Further Transformation to Fully Substituted Thiazoles (6) and 2,3-Dihydrothiazoles (7)

replacing aqueous HCl with HCl in CH3OH. This process reduced the reaction time significantly and enhanced the yield of oximes up to 98% (Scheme 1a, 2a−d, 2f, 2i, 2j, 2p). The α(N-hydroxy/aryl)imino-β-oxodithioesters display not only high stability, but also intriguing reactivity. However, to date, α-Narylimino-β-oxodithioesters have never been exploited as a stable substrate for the synthesis of heterocycles. Herein, we disclose the first chemoselective one-pot synthesis of fully substituted thiazoles and 2,3-dihydrothiazoles by Cu-catalyzed [4 + 1] heterocyclization of α-imino-β-oxodithioesters with diazocarbonyls in good to high yields.

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RESULTS AND DISCUSSION Heterocyclization using transition-metals15 and addition of carbon nucleophile to nitroso compounds has been a topic of continued interest during recent years.16 Utilizing nucleophilic propensity of α-carbon of β-oxodithioester toward α-functionalization, treatment of nitrosoarenes with dithioesters provided α-imino-dithioesters concurrently introducing aryl group on nitrogen of imines (Scheme 1a). The chemical properties of αimino-β-oxodithioesters (2 and 4) can be featured by the presence of three electrophilic and three nucleophilic (N, O and S) centers. Initially, the dithioester 1b and nitrosoarene 3b were taken as model substrates to optimize the reaction conditions for the synthesis of α-arylimino-β-oxodithioesters 4. In this context, we first carried out the above test reaction without any solvent and base at room temperature. Reaction failed to provide any product even after 24 h of stirring (Table 1, entry 1). After this failure, we performed the test reaction in methanol at room temperature, which triggered the reaction forming the desired product in a trace amount, and most of the reactants remained unreacted (Table 1, entry 2). Elevating the reaction temperature to 50 °C afforded the very complex TLC pattern of several overlapping spots within 15 min, which could not be isolated (Table 1, entry 3). It seems that at higher temperature decomposition of the substrates occurred. Therefore, next we performed the reaction at lower temperature, which increased the yield of desired compound 4bb to 10−15% (Table 1, entries 4 and 5). To increase the efficiency of the reaction, next we used 5 mol % of triethylamine at −20 °C and −30 °C, separately. To our pleasure, the yield of the desired product 4bb was increased to 55% and 65%, respectively within

separately with diazocarbonyls (5) in the presence of catalytic Cu(I) species, which afforded fully substituted thiazoles (6) and 2,3-dihydrothiazoles (7) in excellent yields (Scheme 1b). Ila and co-workers12 reported synthesis of 1-aroyl(or acyl)-2aryl-(or ethoxycarbonyl)-4-thioalkyl thiazoles from ketoketene N,S-acetals via multistep processes. Our group has a longstanding interest in exploiting the reactivity of β-oxodithioesters, which are potential platform for the construction of diverse important heterocycles.13 However, none of the above protocols involve the diazocarbonyl-based coupling reactions. In continuation of our recent interest toward the αfunctionalization of β-oxodithioesters, which were further transformed into promising heterocycles,14 herein, we modified our previous procedure10a of α-functionalization of DTEs by

Table 1. Optimization of Reaction Conditions for the Synthesis of 4

entry 1 2 3 4 5 6 7 8 9 10 a

1b 1 1 1 1 1 1 1 1 1 1

mmol mmol mmol mmol mmol mmol mmol mmol mmol mmol

1 1 1 1 1 1 1 1 1 1

3b

solvent

mmol mmol mmol mmol mmol mmol mmol mmol mmol mmol

− CH3OH CH3OH CH3OH CH3OH CH3OH CH3OH CH3OH CH3CN DCM

base (mol %) − − − − − Et3N Et3N Et3N Et3N Et3N

temp (°C)

time

yielda (%)

rt rt 50 −10 −20 −20 −30 −30 −30 −30

24 h 24 h 15 min 12 h 12 h 1h 1h 1h 30 min 15 min

n.r.b tracec

(5) (5) (10) (5) (5)

d

10 15 55 65 65 85 95

Isolated yield. bNo reaction. cComplex TLC pattern. dDecomposition of the reactants occurred. 10847

DOI: 10.1021/acs.joc.7b01601 J. Org. Chem. 2017, 82, 10846−10854

Article

The Journal of Organic Chemistry

effective methods for the construction of carbon−sulfur bonds.17 The goal of α-imination of DTEs 1 is to realize the synthesis of N,S-heterocycles via cascade C−N/C−S bond formations in one-pot. Due to the presence of many active centers, the reactions of α-imino-β-oxodithioesters with various reagents may lead to the formation of diverse heterocyclic systems, where the sulfur atom could be present either in the ring system or as an external substituent. In view of great significance of N,S-heterocycles,18 our focus herein is to utilize diazocarbonyls as one of the reaction partner for the first time toward cyclization with α-imino-β-oxodithioesters that could generate N,S-heterocyclic frameworks. Most of the cycloaddition reaction of diazocarbonyls as carbenoid coupling partner has been achieved with extrusion of nitrogen in the presence of metals.11a,19 Other dimensions of metal carbenoids include X−H insertion,20a 1,2-hydride shift,20b,c and 1,2-C → C or 1,2-X → C bond migration (e.g., X = O, N, S, Si).20d−f The α-imino-β-oxodithioesters 2b and 4bb, and diazocarbonyl 5a were chosen as model substrates to optimize the reaction conditions (Table 2, top). To test the feasibility of our hypothesis, first we used Rh(OAc)2, the most common metal catalyst used for metal carbenoids reaction. Treatment of 1 mmol of 2b (or 4bb) with diazocarbonyl 5a (1.5 mmol) in the presence of 20 mol % of Rh(OAc)2 in THF at room temperature could give the target products 6ba and 7bba in 22% and 17% yield, respectively (Table 2, entry 1). Inspired by the above result, next we performed the above test reaction in different solvents such as dioxane, CH3CN, CH3OH, DMSO, DMF, and DCM (Table 2, entries 2−7). Among above all screened solvents, DCM was found to be a solvent of choice for this transformation, in which the desired products 6ba and 7bba were formed in 37% and 35% yield, respectively at room temperature (Table 2, entry 7). To increase the efficacy of the reaction, we increased the temperature from room temperature to 50 °C. Subsequently, the test reaction was carried out at 50 °C in DCM. Work up of the reaction (2b with 5a) provided 6ba in 47% yield, while the reaction between 4bb and 5a at 50 °C failed with no trace of 7bba (Table 2, entry 8). It could be due to the decomposition of α-N-arylimino-β-oxodithioester at higher temperature. Therefore, we decided to perform the test reaction at lower temperatures. Reaction at −20 °C in DCM could not provide the better result (Table 2, entry 9). Thus, the optimized reaction temperature for the formation of 6ba was found to be 50 °C and for 7bba at room temperature in DCM. After screening the catalytic role of RhII for this heterocyclization, we next screened [Rh(COD)Cl]2 and RhCl(PPh3)3 but no improvement in the result was observed (Table 2, entries 10 and 11). Among the various employed metals, copper is one of the most favorable metal for C−S bond forming reactions due to its low cost and low toxicity.21 Therefore, we next performed the above model reaction in the presence of different copper catalysts like Cu(OTf)2, CuCl, CuBr, CuI, Cu(CH3CN)4BF4, and Cu(CH3CN)4PF622 separately. Notably, copper catalysts facilitated the formation of desired products in improved yields under standard conditions (Table 2, entries 12−17). To our delight, Cu(CH3CN)4PF6 furnished the desired thiazoles 6ba and 7bba in 90% and 88% yield, respectively within 30 min (Table 2, entry 17). Further to check the generality of the reaction, we checked some other metal-catalysts such as Pd(OAc)2, AgOTf, AgOAc and AgSbF6 (Table 2, entries 18− 21). Silver catalysts triggered the reaction well affording the desired products in good yields, but could not provide better

1 h (Table 1, entries 6 and 7). Further increase in the amount of triethylamine (10 mol %) could not improve the result (Table 1, entry 8). Next, solvents like acetonitrile and dichloromethane (DCM) were screened (Table 1, entries 9 and 10). DCM superseded over acetonitrile and proved itself as a best solvent for this base catalyzed α-imination of βoxodithioesters with nitrosoarenes (Table 1, entry 10). Hence the optimized reaction conditions for the synthesis of 4bb is 1:1 stoichiometric ratio of β-oxodithioester 1b and nitrosoarene 3b in DCM at −30 °C in the presence of 5 mol % of triethylamine under inert atmosphere. Under the optimized reaction conditions, the scope of α-(Naryl)imino-β-oxodithioesters 4 was explored (Scheme 2). A Scheme 2. Synthesis of α-(N-Aryl)imino-β-oxodithioesters 4

variety of DTEs (1) bearing different substituents such as alkyl, aryl, hetaryl and 2-naphthyl at R1 moiety, and methyl, ethyl, propyl and 2-methallyl at R2 moiety proved to be the suitable substrates for this base-catalyzed α-imination of β-oxodithioesters with nitrosoarenes (3), which is functionally embedded with methyl benzoate, phenyl and 2,3,4,5,6-pentafluorophenyl groups. No obvious steric/electronic effects were observed for this transformation. The bulky 2-naphthyl group at R1 moiety of dithioester 1m and 2,3,4,5,6-pentafluorophenyl group of nitrosoarene 3 did not show any steric/electronic effect, and the reaction underwent smoothly to give the corresponding products 4ma and 4nc in 98% yield within 5 min. It is clear that the substrates are widely substituent tolerant. The recent advances in cross-coupling reactions using transition-metal catalysis have led to the development of 10848

DOI: 10.1021/acs.joc.7b01601 J. Org. Chem. 2017, 82, 10846−10854

Article

The Journal of Organic Chemistry Table 2. Optimization of Reaction Conditionsa for the Synthesis of Thiazoles 6 and 7

temp (°C) b

entry

catalyst

1 2 3 4 5 6 7 8 9 10 11 12 13 14g 15 16g 17g 18 19 20 21 22

Rh(OAc)2 Rh(OAc)2 Rh(OAc)2 Rh(OAc)2 Rh(OAc)2 Rh(OAc)2 Rh(OAc)2 Rh(OAc)2 Rh(OAc)2 [Rh(COD)Cl]2 RhCl(PPh3)3 Cu(OTf)2 CuCl CuBr CuI Cu(CH3CN)4BF4 Cu(CH3CN)4PF6 Pd(OAc)2 AgOTf AgOAc AgSbF6 no catalyst

yieldd (%)

time (h)

solvent

6ba

7bba

6ba

7bba

6ba

7bba

THF 1,4-Dioxane CH3CN CH3OH DMSO DMF DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM

rt rt rt rt rt rt rt 50c −20 50c 50c 50c 50c 50c 50c 50c 50c 50c 50c 50c 50c 50c

rt rt rt rt rt rt rt 50 −20 rt rt rt rt rt rt rt rt rt rt rt rt rt

12 10 15 24 24 22 9 5 20 24 24 8 1.5 1 1.5 0.7 0.7 18 2 8.5 7 48

7 5 10 24 24 15 3 2 16 24 24 6 1 0.75 1 0.5 0.5 16 1.5 8 6 48

22 24 13 −e −e 11 37 47 32 −e −e 71 70 80 65 83 90 55 89 75 86 n.r.h

17 20 10 −e −e 14 35 −f 28 12 8 66 64 77 60 80 88 52 83 73 84 n.r.h

a

Reaction was carried out with 0.2 mmol of 2b/4bb and 0.3 mmol of 5a (added in dropwise fashion within 5 min), DCM (0.5 mL). bThe reactions were performed with 20 mol % catalyst unless otherwise stated. cFor the formation of 6ba, after dropwise addition of 5a into 2b at rt, the temperature was raised to 50 or 60 °C. dIsolated yield. eTrace amount. fDecomposition occurred, complex TLC pattern. g10 mol % of catalyst. hNo reaction.

15−30 min (Scheme 4). Our studies revealed that precursors 2 and 4 bearing different substituents such as methyl, ethyl, and methallyl at R2 moiety were also tolerated well under the standard reaction conditions. Variation of the alkyl moieties to methyl, ethyl and methallyl in α-(N-hydroxy)imino DTEs 2 did not obviously alter the reaction efficiency, leading to dihydrothiazoles 7 up to 96% yield. However, when the bulkiness of the α-imino-dithioester 4 was increased by introducing 2-naphthyl group at R1 moiety, methallyl group at R 2 moiety and pentafluorophenyl group as N-aryl substituent, the product yields were dropped to 75% (7nca), and 77% (7lba). In contrast to most of the known substituted thiazoles, the present trisubstituted thiazoles 6 and tetrasubstituted dihydrothiazoles 7 bear three readily convertible functional groups, i.e., ester/carbonyl, carbonyl, and alkythio at the 2-, 4-, and 5-positions of the thiazole backbone. This structural feature is highly desired for thiazole derivatives to be used as organic synthons. The α-imino-β-oxodithioesters 2b and 4bb were readily synthesized in one step from the corresponding β-oxo/α-enolic dithioesters and nitroso compounds by dehydrative coupling

result than Cu(CH3CN)4PF6 catalyst (Table 2, entry 17). Finally, we performed the test reaction without any catalyst under standard conditions. No trace of the desired product was formed even after 48 h (Table 2, entry 22). Thus, the best yield, cleanest reaction, and most facile workup were achieved employing 1:1.5 stoichiometric ratio of 2 (or 4) and 5 in DCM in the presence of 10 mol % of Cu(CH3CN)4PF6 at 50 °C for thiazole 6ba and at room temperature for dihydrothiazole 7bba. Next, the protocol generality was investigated by performing the reactions of various α-(N-hydroxy)imino-β-oxodithioesters 2 with α-diazocarbonyls 5a−c (Scheme 3). It is noteworthy that methyl, phenyl, 4-methoxyphenyl, 3,4-methylenedioxy phenyl, 4-trifluoromethylphenyl and 3-bromophenyl groups at R1 moiety of α-(N-hydroxy)imino DTEs 2 were well tolerated affording the corresponding thiazoles 6 in 72−93% yields. After the successful utilization of α-(N-hydroxy)imino-βoxodithioesters 2, we next explored reaction scope of α-(Naryl)imino-β-oxodithioesters 4 with α-diazocarbonyls 5a−c. Reaction of 4 with 5 under standard conditions afforded the corresponding 2,3-dihydrothiazoles 7 in 74−96% yields within 10849

DOI: 10.1021/acs.joc.7b01601 J. Org. Chem. 2017, 82, 10846−10854

Article

The Journal of Organic Chemistry Scheme 3. Synthesis of 2,4,5-Trisubstituted Thiazoles 6

Scheme 5. Preliminary Results and Proposed Pathway

Scheme 4. Synthesis of 2,3-Dihydrothiazoles 7

cyclization can be explained as chemoselective nucleophilic attack by thiocarbonyl sulfur (over carbonyl oxygen) of αimino-β-oxodithioester 2b/4bb to metal carbenoid A to generate an adduct sulfur ylide intermediate C. Intermediate C further reacts with diazoacetate 5a to form zwitterionic intermediate D, regenerating Cu-carbenoid A, thus completing the catalytic cycle. Finally, intermediate D undergoes intramolecular N-cyclization affording 2,3-dihydrothiazole 7bba. Formation of thiazole 6ba involves moderate heating to 50 °C that facilitates dehydration leading to aromaticity (Scheme 5b). The overall transformation involves the formation of two new C−N bonds and one new C−S bond by both intermolecular and intramolecular reactions.

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CONCLUSION In summary, we have reported an efficient regioselective route to α-(N-hydroxy/aryl)imino-β-oxodithioesters (2 and 4) in high yields by the reaction of β-oxodithioesters 1 with nitrous acid and nitrosoarenes 3, respectively. Further these α-imino-βoxodithioesters are utilized as 4 atoms synthons toward Cucatalyzed [4 + 1] heterocyclization with diazocarbonyl as one carbon synthon to develop a straightforward general approach for diversity oriented synthesis of 2,4,5-trisubstituted thiazoles 6 and 2,3,4,5-tetrasubstituted 2,3-dihydrothiazoles 7. The chemoselective concise one-pot strategy reported herein allows a novel entry to fully substituted thiazoles and 2,3-dihydrothiazoles with full control over substitution at the various ring

via intermediate B and isolated before being used in the reaction (Scheme 5a). Alternatively, the α-imino-β-oxodithioesters 2b and 4bb could also be generated in situ via a base promoted coupling, enabling the entire process to be performed in one-pot. Mechanistically this [4 + 1] hetero10850

DOI: 10.1021/acs.joc.7b01601 J. Org. Chem. 2017, 82, 10846−10854

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The Journal of Organic Chemistry

Aqueous work up of reaction mixture was performed while using ethyl acetate as organic phase. Organic phase was separated and dried over Na2SO4. Step 8: The reaction mixture was chromatographed (silica gel, hexane/ethyl acetate = 6:1) to afford thiazoles, Rf = 0.45 (hexane/ ethyl acetate = 70/30). General Procedure for the Synthesis of 2,3-Dihydrothiazoles 7. Similar procedure as for the synthesis of 6, except step 6 in which whole set up was left for stipulated period of time at room temperature. The reaction mixture was chromatographed (silica gel, hexane/ethyl acetate = 9:1) to afford 2,3-dihydrothiazoles, Rf = 0.60 (hexane/ethyl acetate = 80/20). For 2a−d, 2f and 2j, see reference 10a. Methyl 3-(3-bromophenyl)-2-(N-hydroxyimino)-3-oxopropanedithioate (2i). The product was obtained as red solid; (0.31 g, 98%); mp 124−126 °C; 1H NMR (500 MHz, CDCl3) δ 12.46 (br, 1H), 7.95 (s, 1H), 7.73 (d, J = 7.6 Hz, 1H), 7.68 (d, J = 8.15 Hz, 1H), 7.35−7.32 (m, 1H), 2.64 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 217.9, 191.0, 157.3, 136.8, 136.7, 131.6, 130.5, 127.8, 123.0, 17.9. HRMS (ESI/QTOF) m/z [M + Na]+ Calcd. for C10H8BrNNaO2S2 339.9072, found 339.9073. Ethyl 3-(4-trifluoromethylphenyl)-2-(N-hydroxyimino)-3-oxopropanedithioate (2p). The product was obtained as red solid; (0.29 g, 90%); mp 138−140 °C; 1H NMR (500 MHz, CDCl3) δ 7.84 (d, J = 7.85 Hz, 2H), 7.72 (d, J = 7.90 Hz, 2H), 3.18 (q, J = 7.46 Hz, 2H), 1.24 (t, J = 6.92 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 218.4, 191.5, 157.5, 138.0, 134.9, 134.7, 134.4, 134.1, 129.1, 125.75, 125.72. HRMS (ESI/Q-TOF) m/z [M + Na]+ Calcd. for C12H10F3NNaO2S2 343.9997, found 343.9997. Methyl 2-[N-{methylbenzoate-4-yl}imino]-3-oxobutanedithioate (4ab). The product was obtained as red sticky solid; (0.26 g, 88%). 1H NMR (500 MHz, CDCl3) δ 7.88 (d, J = 8.5 Hz, 2H), 6.84 (d, J = 8.5 Hz, 2H), 3.80 (s, 3H), 2.54 (s, 3H), 2.48 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 224.2, 196.6, 166.5, 163.4, 151.5, 130.8, 130.3, 127.0, 119.6, 118.9, 52.1, 26.0, 18.8. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C13H14NO3S2 296.0410, found 296.0417. Methyl 2-[N-{methylbenzoate-4-yl}imino]-3-phenyl-3-oxopropanedithioate (4bb). The product was obtained as red sticky solid; (0.34 g, 95%). 1H NMR (500 MHz, CDCl3) δ 6.60 (d, J = 8.45 Hz, 2H), 6.42 (d, J = 7.30 Hz, 2H), 6.23−6.20 (m, 1H), 6.10−6.07 (m, 2H), 5.73 (d, J = 8.50 Hz, 2H), 2.56 (s, 3H), 1.44 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 222.9, 193.5, 166.4, 163.5, 150.9, 134.5, 134.5, 130.5, 129.1, 128.9, 127.6, 125.4, 122.6, 119.9, 52.1, 18.7. HRMS (ESI/QTOF) m/z [M + H]+ Calcd. for C18H16NO3S2 358.0566, found 358.0573. Ethyl 2-[N-{phenyl}imino]-3-phenyl-3-oxopropanedithioate (4ca). The product was obtained as red sticky solid; (0.30 g, 94%). 1 H NMR (300 MHz, CDCl3) δ 7.69 (d, J = 7.20 Hz, 2H), 7.48−7.43 (m, 1H), 7.35−7.30 (m, 2H), 7.19−7.14 (m, 2H), 7.04−6.96 (m, 3H), 3.28 (q, J = 7.45 Hz, 2H), 1.41 (t, J = 7.42 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 223.4, 195.0, 163.2, 146.9, 135.0, 134.2, 129.2, 129.0, 128.9, 126.6, 120.8, 29.4, 11.9. HRMS (ESI/Q-TOF) m/z [M + Na]+ Calcd. for C17H15NOS2Na 336.0487, found 336.0493. Propyl 2-[N-{phenyl}imino]-3-(4-tolyl)-3-oxopropanedithioate (4ea). The product was obtained as red sticky solid; (0.30 g, 85%). 1 H NMR (300 MHz, CDCl3) δ 7.59 (d, J = 8.04 Hz, 2H), 7.19−7.11 (m, 4H), 7.04−6.97 (m, 3H), 3.25 (t, J = 7.50 Hz, 2H), 2.32 (s, 3H), 1.87−1.75 (m, 2H), 1.09 (t, J = 7.50 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 223.6, 194.6, 163.5, 147.0, 145.4, 132.6, 129.7, 129.3, 129.0, 126.5, 120.9, 37.2, 21.8, 20.6, 13.8. HRMS (ESI/Q-TOF) m/z [M + Na]+ Calcd. for C19H19NOS2Na 364.0800, found 364.0801. Methyl 2-[N-{phenyl}imino]-3-(4-methoxyphenyl)-3-oxopropanedithioate (4fa). The product was obtained as red sticky solid; (0.30 g, 90%). 1H NMR (300 MHz, CDCl3) δ 7.66 (d, J = 8.76 Hz, 2H), 7.21−7.16 (m, 2H), 7.06−6.99 (m, 3H), 6.80 (d, J = 8.73 Hz, 2H), 3.79 (s, 3H), 2.68 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 224.3, 193.3, 164.5, 163.3, 146.9, 131.7, 129.0, 128.1, 126.6, 120.9, 114.2, 55.5, 18.7. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C17H16NO2S2 330.0617, found 330.0628. Ethyl 2-[N-{phenyl}imino]-3-(4-methoxyphenyl)-3-oxopropanedithioate (4ga). The product was obtained as red sticky solid; (0.31 g, 90%). 1H NMR (300 MHz, CDCl3) δ 7.66 (d, J = 8.79 Hz, 2H),

positions, which would otherwise be more difficult to prepare by alternative routes.

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EXPERIMENTAL SECTION

General Methods. All the reactions were carried out in a flame or oven-dried glass wares under an argon or nitrogen atmosphere with freshly distilled dry solvents under anhydrous conditions unless otherwise indicated. Chromatograms were visualized by fluorescence quenching with UV light at 254 nm. NMR spectra were recorded at room temperature on 300 MHz, 500 MHz spectrometers. The residual solvent signals were taken as the reference 7.26 ppm for 1H NMR spectra and 77.0 ppm for 13C NMR spectra in CDCl3. Sometimes the TMS signal at 0.0 ppm was used an internal standard for 1H NMR spectra. Chemical shift (δ) is reported in ppm, coupling constants (J) are given in Hz. Melting points are uncorrected. Materials. All solvents were distilled under argon and dried before use. Catalysts used in this protocol Rh(OAc)2, [Rh(COD)Cl]2, RhCl(PPh3)3, CuCl, CuBr, CuI, Cu(OTf)2, Pd(OAc)2, AgOTf, AgOAc and AgSbF6 were purchased from Sigma-Aldrich Chemicals Pvt. Ltd. Cu(CH3CN)4BF4 and Cu(CH3CN)4PF6 were prepared following the literature procedure.23 Modified Procedure for the Synthesis of α-(N-hydroxy)imino-β-oxodithioesters10a (2a−d, 2i, 2j). Step 1: In a roundbottom flask, β-oxodithioester 1 (1.0 mmol) was taken with 10 mL of dry methanol. Step 2: Flask was immersed into ice-bath. Step 3: To this 1.0 mL of “4 M HCl in methanol” was added slowly. Pause: The reaction mixture become turbid when the acid was added; wait until homogeneity of reaction mixture. Step 4: Portion wise addition of NaNO2 (3.0 mmol) for about 5 min. Pause: A careful slow addition of sodium nitrite resulted into redness of reaction mixture. Reaction mixture was stirred for stipulated period of time. NaCl started to form and settled down to bottom. Completion of reaction was monitored through TLC. Step 5: Whole reaction mixture was filtered on Buckner funnel. Step 6: Filtrate was collected and poured into ice cold water and filtered it again on Buckner funnel. Step 7: Filtered material was dried and recrystallized from DCM-Methanol mixture (1:1). Note: In case of synthesis of 2i, during step 6, poured reaction mixture in ice cold water, and was kept as such in freeze at 5 °C for 2 h. General Procedure for the Synthesis of α-(N-aryl)imino-βOxodithioesters 4. Step 1: Maintain temperature of methanol bath of JULABO FT902 (JULABO Gmbh, Germany) at either −30 °C (except 4kc, 4nc) or −50 °C (for 4kc and 4nc). Step 2: In an ovendried two-neck round-bottom flask, 1.0 mmol of 1, and 1.0 mmol of 3 was taken and sealed it with double channel balloon adopter followed by flushing with argon for 4 to 5 times with the help of vacuum pump. Step 3: 5 mL of dry dichloromethane was added into round-bottom flask of step 2, through the help of cannula. Step 4: This setup of step 3, was immersed into cold methanol bath of JULABO. Step 5: 5 mol % of Et3N was added in dropwise fashion to it through syringe. Pause: A careful slow addition of triethylamine resulted into dark redness of reaction mixture immediately. Reaction mixture was stirred for stipulated period of time. Completion of reaction was monitored through TLC. Step 6: No need of workup for the resulted reaction mixture of step 5. The reaction mixture was chromatographed (silica gel, hexane/ethyl acetate = 19:1) to afford α-(N-aryl)imino-βoxodithioesters as sticky solids, Rf = 0.50 (hexane/ethyl acetate = 90/10). General Procedure for the Synthesis of Thiazoles 6 (6ba for Example). Step 1: Two two-neck round-bottom flasks were coded as A and B. Step 2: In A, 10 mol % of Cu(CH3CN)4PF6 were taken and sealed with double channel balloon adopter and flushed with argon for 4 to 5 times with the help of vacuum pump. Step 3: In B, 0.2 mmol of 2b was taken, sealed with double channel balloon adopter and flushed with argon for 4 to 5 times with the help of vacuum pump. In this 0.5 mL of dry DCM was added. Step 4: The whole reaction mixture of B was transferred into A through the help of cannula. Step 5: In A, 0.3 mmol of 5a was added in dropwise fashion in 5 min. Step 6: The whole setup of A was put into preheated oil bath at 50 °C and left for stipulated period of time. Pause: A careful addition of 5a resulted into pale yellow coloration of reaction mixture after its completion. Step 7: 10851

DOI: 10.1021/acs.joc.7b01601 J. Org. Chem. 2017, 82, 10846−10854

Article

The Journal of Organic Chemistry

tert-Butyl 4-acetyl-5-(methylthio)thiazole-2-carboxylate (6ab). The product was obtained as yellow oil; (0.04 g, 72%). 1H NMR (500 MHz, CDCl3) δ 2.68 (s, 3H), 2.61 (s, 3H), 1.63 (s, 9H). 13C NMR (125 MHz, CDCl3) δ 194.3, 158.1, 155.8, 152.7, 147.0, 84.4, 28.1, 22.7, 14.1. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C11H16NO3S2 274.0566, found 274.0576. Ethyl 4-benzoyl-5-(methylthio)thiazole-2-carboxylate (6ba). The product was obtained as yellow solid; (0.06 g, 90%); mp 110−112 °C; 1 H NMR (300 MHz, CDCl3) δ 8.32 (d, J = 7.74 Hz, 2H), 7.58−7.45 (m, 3H), 4.46 (q, J = 7.06 Hz, 2H), 2.66 (s, 3H), 1.43 (t, J = 7.08 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 187.0, 159.8, 159.5, 151.4, 146.9, 137.2, 132.8, 130.8, 128.3, 62.8, 20.7, 14.2. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C14H14NO3S2 308.0410, found 308.0410. Ethyl 4-benzoyl-5-(ethylthio)thiazole-2-carboxylate (6ca). The product was obtained as yellow solid; (0.06 g, 88%); mp 107−109 °C; 1 H NMR (300 MHz, CDCl3) δ 8.31 (d, J = 7.26 Hz, 2H), 7.56−7.45 (m, 3H), 4.47 (q, J = 7.00 Hz, 2H), 3.10 (q, J = 7.31 Hz, 2H), 1.51 (t, J = 7.35 Hz, 3H), 1.44 (t, J = 7.05 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 187.1, 159.6, 157.7, 151.6, 147.3, 137.3, 132.9, 130.9, 128.3, 62.8, 31.9, 14.2, 13.2. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C15H16NO3S2 322.0566, found 322.0572. Ethyl 4-benzoyl-5-[(2-methylallyl)thio]thiazole-2-carboxylate (6da). The product was obtained as yellow solid; (0.06 g, 82%); mp 121−123 °C; 1H NMR (300 MHz, CDCl3) δ 8.29 (d, J = 7.59 Hz, 2H), 7.59−7.45 (m, 3H), 5.18 (s, 1H), 5.07 (s, 1H), 4.47 (q, J = 7.08 Hz, 2H), 3.71 (s, 2H), 1.90 (s, 3H), 1.43 (t, J = 7.09 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 187.2, 159.7, 152.0, 148.1, 138.3, 137.2, 133.0, 130.9, 128.4, 116.8, 62.8, 44.9, 44.8, 44.7, 21.5, 14.2. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C17H18NO3S2 348.0723, found 348.0723. Ethyl 4-(p-bromobenzoyl)-5-(methylthio)thiazole-2-carboxylate (6ia). The product was obtained as yellow solid; (0.07 g, 93%); mp 162−164 °C; 1H NMR (300 MHz, CDCl3) δ 8.48 (s, 1H), 8.30 (d, J = 7.89 Hz, 1H), 7.70−7.66 (m, 1H), 7.39−7.34 (m,1H), 4.47 (q, J = 7.11 Hz, 2H), 2.68 (s, 3H), 1.45 (t, J = 7.12 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 185.3, 160.7, 159.4, 151.6, 146.2, 138.9, 135.6, 133.7, 129.9, 129.4, 122.5, 62.9, 20.8, 14.1. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C14H13BrNO3S2 385.9515, found 385.9515. Ethyl 4-[(3′,4′-methylendioxy)benzoyl]-5-(methylthio)thiazole-2carboxylate (6ja). The product was obtained as yellow solid; (0.05 g, 77%); mp 127−129 °C; 1H NMR (300 MHz, CDCl3) δ 8.12 (d, J = 8.16 Hz,1H) 7.84 (s, 1H), 6.89 (d, J = 8.22 Hz, 1H), 6.04 (s, 2H), 4.47 (q, J = 7.02 Hz, 2H), 2.66 (s, 3H), 1.44 (t, J = 7.02 H, 3H). 13C NMR (75 MHz, CDCl3) δ 184.8, 159.5, 159.3, 151.8, 151.3, 147.8, 147.0, 131.5, 127.7, 110.6, 108.0, 101.8, 62.8, 20.7, 14.2. HRMS (ESI/QTOF) m/z [M + H]+ Calcd. for C15H14NO5S2 352.0308, found 352.0308. 2-Benzoyl-4-(4-methoxybenzoyl)-5-methylthiothiazole (6fc). The product was obtained as yellow solid; (0.31 g, 85%); mp 122−124 °C; 1 H NMR (500 MHz, CDCl3) δ 8.30−8.26 (m, 2H), 7.51−7.46 (m, 3H), 7.15−7.12 (m, 1H), 7.01 (d, J = 8.60 Hz, 2H), 6.64 (d, J = 8.55 Hz, 1H), 3.68 (s, 3H), 2.65 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 188.8, 186.8, 160.1, 159.6, 143.5, 142.4, 137.5, 136.1, 134.8, 132.9, 131.9, 131.3, 131.0, 130.3, 129.3, 128.6, 127.8, 125.4, 113.8, 55.2, 19.2. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C19H16NO3S2 370.0566, found 370.0566. 2-Benzoyl-4-(4-triflluoromethylbenzoyl)-5-ethylthiothiazole (6pc). The product was obtained as yellow solid; (0.38 g, 90%); mp 134−136 °C; 1H NMR (500 MHz, CDCl3) δ 8.10 (d, J = 7.75 Hz, 2H), 7.53 (d, J = 7.45 Hz, 2H), 7.46−7.43 (m, 1H), 7.40 (d, J = 8.05 Hz, 1H), 7.27 (d, J = 8.35 Hz, 1H), 7.19−7.7.16 (m, 2H), 3.06 (q, J = 7.31 Hz, 2H), 1.42 (t, J = 7.22 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 188.2, 186.8, 160.2, 137.4, 134.9, 134.8, 133.5, 132.7, 132.4, 131.2, 129.9, 129.2, 128.7, 128.6, 128.5, 128.0, 126.0, 125.131.2, 14.7. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C20H15F3NO2S2 422.0491, found 422.0491. tert-Butyl 4-acetyl-3-(methylbenzoate-4-yl)-5-(methylthio)-2,3dihydrothiazole-2-carboxylate (7abb). The product was obtained as yellow sticky solid; (0.07 g, 80%). 1H NMR (500 MHz, CDCl3) δ 7.98 (d, J = 8.55 Hz, 2H), 6.94 (d, J = 8.6 Hz, 2H), 5.49 (s, 1H), 3.89

7.21−7.16 (m, 2H), 7.06−6.98 (m, 3H), 6.80 (d, J = 8.79 Hz, 2H), 3.79 (s, 3H), 3.27 (q, J = 7.44 Hz, 2H), 1.42 (t, J = 7.44 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 223.6, 193.3, 164.5, 163.5, 147.0, 131.7, 129.0, 128.2, 126.5, 120.9, 114.2, 55.6, 55.5, 29.5, 11.9. HRMS (ESI/ Q-TOF) m/z [M + H]+ Calcd. for C18H18NO2S2 344.0773, found 344.0773. Methyl 2-[N-{phenyl}imino]-3-(4-chlorophenyl)-3-oxopropanedithioate (4ha). The product was obtained as red sticky solid; (0.31 g, 92%). 1H NMR (300 MHz, CDCl3) δ 7.61 (d, J = 8.46 Hz, 2H), 7.31−7.25 (m, 2H), 7.22−7.17 (m, 2H), 7.08−7.03 (m, 1H), 6.97 (d, J = 7.53 Hz, 2H), 2.69 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 224.0, 194.0, 162.5, 146.6, 140.8, 133.3, 130.4, 129.4, 129.1, 126.9, 120.9, 93.5, 18.6. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C16H13ClNOS2 334.0122, found 334.0128. Methyl 2-[N-{methylbenzoate-4-yl}imino]-3-(2-bromophenyl)-3oxopropanedithioate (4ib). The product was obtained as red sticky solid; (0.43 g, 98%). 1H NMR (500 MHz, CDCl3) δ 7.88 (d, J = 8.50 Hz, 2H), 7.65 (dd, J1 = 7.50 Hz, J2 = 1.80 Hz, 1H), 7.51 (d, J = 8.15 Hz, 1H), 7.27 (d, J = 7.55 Hz, 1H), 7.25 (s, 1H), 3.85 (s, 3H), 2.72 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 223.0, 191.6, 166.4, 162.4, 150.9, 135.0, 134.2, 132.6, 130.5, 130.3, 127.6, 122.4, 119.5, 118.7, 52.1, 18.7. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C18H15BrNO3S2 435.9671, found 435.9677. E/Z-Ethyl 2-[N-{2,3,4,5,6-pentafluorophenyl}imino]-3-(2-furoyl)3-oxopropanedithioate (4kc). The product was obtained as red sticky solid; (0.39 g, 98%). 1H NMR (500 MHz, CDCl3) δ 7.64 (s, 1H), 7.25 (d, J = 5.7 Hz, 1H), 6.58−6.57 (m, 1H), 2.92 (q, J = 7.30 Hz, 2H), 2.82 (q, J = 7.31 Hz, 2H), 1.37 (t, J = 7.35 Hz, 3H), 1.24 (t, J = 7.12 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 177.1, 151.9, 147.8, 147.4, 147.3, 120.3, 112.8, 112.7, 112.2, 32.5, 27.9, 15.1, 14.5. 19F NMR (470 MHz, CDCl3) δ −148.54 (d, J = 22 Hz, 2F), −158.88 (t, J = 22.17 Hz, 1F), −162.13 to −162.31 (m, 2F). HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C15H9F5NO2S2 393.9989, found 393.9999. Methyl 2-[N-{phenyl}imino]-3-(2-naphthoyl)-3-oxopropanedithioate (4ma). The product was obtained as red sticky solid; (0.34 g, 98%). 1H NMR (300 MHz, CDCl3) δ 8.16 (s, 1H), 7.86−7.76 (m, 4H), 7.57−7.46 (m, 2H), 7.14 (d, J = 7.63 Hz, 2H), 7.04 (d, J = 7.53 Hz, 2H), 6.98 (d, J = 7.18 Hz, 1H), 2.70 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 224.1, 195.1, 163.0, 146.8, 136.1, 132.5, 132.3, 132.1, 129.9, 129.3, 128.0, 127.1, 126.7, 123.6, 120.9, 18.6. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C20H16NOS2 350.0668, found 350.0676. (2-Methyl)allyl 2-[N-{2,3,4,5,6-pentafluorophenyl}imino]-3-(2naphthoyl)-3-oxopropanedithioate (4nc). The product was obtained as red sticky solid; (0.47 g, 98%). 1H NMR (500 MHz, CDCl3) δ 8.26 (s, 1H), 7.92 (d, J = 8.30 Hz, 1H), 7.87−7.84 (m, 2H), 7.81−7.79 (m, 1H), 7.64−7.61 (m, 1H), 7.58−7.55 (m, 1H), 5.13 (s, 1H), 5.02 (s, 1H), 3.96 (s, 2H), 1.89 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 219.8, 190.8, 169.4, 137.8, 136.4, 132.3, 132.0, 130.9, 130.4, 130.0, 129.7, 129.2, 128.9, 128.1, 127.4, 124.7, 123.3, 116.5, 42.9, 22.2, 21.5, 21.1. 19F NMR (470 MHz, CDCl3) δ −148.25 (s, 2F), −158.73 (s, 1F), −161.60 (s, 2F). HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C23H15F5NOS2 480.0510, found 480.0476. Ethyl 2-[N-{4-bromophenyl}imino]-3-(3,4-methylenedioxyphenyl)-3-oxopropanedithioate (4od). The product was obtained as red sticky solid; (0.41 g, 93%). 1H NMR (500 MHz, CDCl3) δ 7.32 (d, J = 8.55 Hz, 2H), 7.21 (d, J = 9.30 Hz, 2H), 7.19 (s, 1H), 6.87 (d, J = 8.60 Hz, 2H), 6.73 (d, J = 7.75 Hz, 1H), 6.01 (s, 2H), 3.27 (q, J = 7.35 Hz, 2H), 1.41 (t, J = 7.50 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 222.5, 192.2, 163.4, 153.0, 148.5, 145.8, 132.0, 129.6, 126.9, 122.5, 120.0, 108.3, 107.7, 102.2, 29.6, 11.9. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C18H14BrNNaO3S2 457.9491, found 457.9491. Ethyl 2-[N-{methylbenzoate-4-yl}imino]-3-(4-trifluoromethylyphenyl)-3-oxopropanedithioate (4pb). The product was obtained as red sticky solid; (0.39 g, 90%). 1H NMR (500 MHz, CDCl3) δ 7.88 (d, J = 8.50 Hz, 2H), 7.78 (d, J = 8.10 Hz, 2H), 7.62 (d, J = 8.15 Hz, 2H), 6.97 (d, J = 8.50 Hz, 2H), 3.84 (s, 3H), 3.30 (q, J = 7.38 Hz, 2H), 1.43 (t, J = 7.30 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 221.8, 192.8, 166.2, 162.7, 150.4, 137.1, 135.7, 135.4, 135.1, 134.9, 130.6, 129.1, 127.9, 126.0, 119.8, 52.1, 29.5, 11.8. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C20H16F3NNaO3S2 462.0416, found 462.0416. 10852

DOI: 10.1021/acs.joc.7b01601 J. Org. Chem. 2017, 82, 10846−10854

Article

The Journal of Organic Chemistry (s, 3H), 2.54 (s, 3H), 2.04 (s, 3H), 1.54 (s, 9H). 13C NMR (125 MHz, CDCl3) δ 190.0, 167.4, 166.5, 151.2, 146.7, 131.4, 131.0, 125.0, 117.5, 83.6, 73.3, 52.1, 27.9, 27.6, 19.5. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C19H24NO5S2 410.1090, found 410.1091. Ethyl 4-benzoyl-3-(methylbenzoate-4-yl)-5-(methylthio)-2,3-dihydrothiazole-2-carboxylate (7bba). The product was obtained as yellow powder; (0.08 g, 91%); mp 117−119 °C; 1H NMR (500 MHz, CDCl3) δ 8.15 (d, J = 7.85 Hz, 2H), 7.77−7.75 (m, 2H), 7.31−7.28 (m, 1H), 7.22−7.19 (m, 2H), 6.82 (d, J = 9.00 Hz, 2H), 5.67 (s, 1H), 4.44 (tt, J1 = 14.27 Hz, J2 = 3.77 Hz, J3 = 6.97 Hz, 2H), 3.80 (s, 3H), 2.58 (s, 3H), 1.43 (t, J = 7.07 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 184.4, 169.2, 166.4, 151.1, 147.3, 137.3, 132.3, 131.0, 130.6, 129.0, 127.9, 124.8, 118.0, 72.4, 62.9, 51.9, 19.8, 14.2. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C22H22NO5S2 444.0934, found 444.0937. tert-Butyl 4-benzoyl-3-(methylbenzoate-4-yl)-5-(methylthio)-2,3dihydrothiazole-2-carboxylate (7bbb). The product was obtained as yellow powder; (0.08 g, 88%); mp 147−149 °C; 1H NMR (500 MHz, CDCl3) δ 8.16−8.14 (m, 2H), 7.75 (d, J = 8.6 Hz, 2H), 7.31−7.27(m, 1H), 7.22−7.19 (m, 2H), 6.82 (d, J = 8.5 Hz, 2H), 5.57 (s, 1H), 3.80 (s, 3H), 2.58 (s, 3H), 1.62 (s, 9H). 13C NMR (125 MHz, CDCl3) δ 184.3, 167.9, 166.5, 151.3, 147.3, 137.3, 132.3, 131.0, 130.8, 129.1, 128.0, 124.7, 118.0, 84.0, 73.3, 52.0, 28.1, 19.9. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C24H26NO5S2 472.1247, found 472.1255. Ethyl 4-(4-methoxybenzoyl)-5-(methylthio)-3-phenyl-2,3-dihydrothiazole-2-carboxylate (7faa). The product was obtained as yellow powder; (0.06 g, 74%); mp 161−163 °C; 1H NMR (300 MHz, CDCl3) δ 8.27 (d, J = 8.73 Hz, 2H), 7.15−7.09 (m, 2H), 6.93−6.86 (m, 3H), 6.72 (d, J = 8.70 Hz, 2H), 5.59 (s, 1H), 4.41 (q, J = 3.60 Hz, 2H), 3.74 (s, 3H), 2.54 (s, 3H), 1.41 (t, J = 7.06 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 170.0, 163.0, 147.6, 132.9, 131.8, 130.4, 129.5, 124.0, 119.9, 113.2, 73.4, 62.6, 55.2, 20.0, 14.2. HRMS (ESI/Q-TOF) m/z [M] Calcd. for C21H21NO4S2 415.0912, found 415.0907. tert-Butyl 4-(2-bromobenzoyl)-3-(methylbenzoate-4-yl)-5-(methylthio)-2,3-dihydrothiazole-2-carboxylate (7ibb). The product was obtained as yellow powder; (0.11 g, 96%); mp 176−178 °C. 1H NMR (500 MHz, CDCl3) δ 7.99 (d, J = 7.50 Hz, 1H), 7.80 (d, J = 8.55 Hz, 2H), 7.33 (d, J = 7.55 Hz, 1H), 7.14−7.11 (m, 1H), 7.03−6.99 (m, 1H), 6.84 (d, J = 8.65 Hz, 2H), 5.47 (s, 1H), 3.83 (s, 3H), 2.61 (s, 3H), 1.61 (s, 9H). 13C NMR (125 MHz, CDCl3) δ 183.5, 167.4, 166.5, 151.1, 150.8, 138.4, 133.5, 131.4, 130.8, 129.8, 126.2, 124.8, 121.2, 118.1, 83.8, 73.3, 52.0, 28.0, 19.6. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C24H25BrNO5S2 550.0352, found 550.0352. Ethyl 4-(2-furoyl)-3-(2,3,4,5,6-pentafluorophenyl)-5-(ethylthio)2,3-dihydrothiazole-2-carboxylate (7 kca). The product was obtained as yellow sticky solid; (0.08 g, 81%). 1H NMR (500 MHz, CDCl3) δ 7.76 (d, J = 3.65 Hz, 1H), 7.54 (s, 1H), 6.45−6.44 (m, 1H), 5.38 (s, 1H), 4.34 (ABX3 pattern: qdd, J1 = 20.20 Hz, J2 = 10.65 Hz, J3 = 3.30 Hz, 2H), 3.04 (dq, J1 = 20.25 Hz, J2 = 7.28 Hz, 1H), 2.86 (dq, J1 = 20.10 Hz, J2 = 7.28 Hz, 1H), 1.34 (td, J1 = 7.22 Hz, J2 = 3.31 Hz, 6H). 13C NMR (125 MHz, CDCl3) δ 169.6, 168.1, 151.3, 147.2, 141.0, 131.9, 119.6, 112.2, 70.9, 62.7, 31.1, 15.0, 14.1. 19F NMR (470 MHz, CDCl3) δ −148.54 (d, J = 23.15 Hz, 1F), −148.85 (d, J = 23.05 Hz, 1F), −158.79 to −158.92 (m, 1F), −161.81 to −161.95 (m, 1F), −162.13 to −162.26 (m, 1F). HRMS (ESI/Q-TOF) m/z [M + Na]+ Calcd. for C19H14F5NNaO4S2 502.0177, found 502.0189. Methyl 4-(p-phenylbenzoyl)-3-phenyl-5-(methylthio)-2,3-dihydrothiazole-2-carboxylate (7lab). The product was obtained as yellow powder; (0.07 g, 77%); mp 143−145 °C; 1H NMR (300 MHz, CDCl3) δ 8.93 (s, 1H), 8.13 (d, J = 8.61 Hz, 1H), 7.80 (d, J = 6.81 Hz, 2H), 7.65−7.58 (m, 2H), 7.42−7.33 (m, 3H), 7.01−6.96 (m, 2H), 6.82 (d, J = 7.83 Hz, 2H), 6.75 (t, J = 7.18 Hz, 1H) 5.57 (s, 1H), 4.40 (q, J = 4.53 Hz, 2H), 2.51 (s, 3H), 1.38 (t, J = 7.08 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 183.6, 169.8, 147.5, 138.4, 136.0, 132.0, 130.9, 129.6, 129.1, 128.7, 128.2, 124.3, 119.9, 73.6, 62.7, 20.0, 14.1. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C26H24NO3S2 462.1192, found 462.1201. Ethyl 4-(2-naphthoyl)-3-(2,3,4,5,6-pentafluorophenyl)-5-(2-methylallylthio)-2,3-dihydrothiazole-2-carboxylate (7nca). The product was obtained as yellow sticky solid; (0.08 g, 75%). 1H NMR (500 MHz, CDCl3) δ 8.84 (s, 1H), 8.06 (d, J = 8.55 Hz, 1H), 7.91 (d, J =

8.00 Hz, 1H), 7.78 (dd, J1 = 11.85, J2 = 8.35 Hz, 2H), 7.56−7.48 (m, 2H), 5.46 (s, 1H), 5.03 (s, 1H), 4.92 (s, 1H), 4.41 (ABX3 pattern: qdd, J1 = 20.31 Hz, J2 = 10.83 Hz, J3 = 3.55 Hz, 2H), 3.60 (d, J = 13.65 Hz, 1H), 3.44 (d, J = 13.50 Hz, 1H), 1.75 (s, 3H), 1.41 (t, J = 7.15 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 183.0, 168.2, 140.3, 137.0, 135.3, 134.7, 134.5, 132.4, 131.1, 129.9, 129.6, 128.4, 128.0, 127.7, 126.6, 124.8, 115.5, 70.0, 62.6, 44.0, 29.8, 21.2, 14.2. 19F NMR (470 MHz, CDCl3) δ −148.31 (br, 2F), −158.10 (t, J = 22.85 Hz, 1F), −161.61 (t, J = 21.65 Hz, 2F). HRMS (ESI/Q-TOF) m/z [M + Na]+ Calcd. for C27H20F5NNaO3S2 588.0697, found 588.0712. 2-Benzoyl-3-(N-{4-bromophenyl}imino)-5-ethylthio-4-(3,4-methylenedioxyphenyl)-2,3-dihydrothioazole (7odc). The product was obtained as yellow sticky solid; (0.45 g, 81%). 1H NMR (500 MHz, CDCl3) δ 8.55 (d, J = 8.25 Hz, 1H), 7.98 (d, J = 7.35 Hz, 2H), 7.78 (d, J = 8.55 Hz, 2H), 7.72−7.69 (m, 2H), 7.60−7.57 (m, 2H), 7.53 (d, J = 8.25 H, 1H), 7.26 (s, 2H), 6.81 (d, J = 8.65 Hz, 2H), 6.51 (s, 1H), 2.98 (dq, J1 = 13.70 Hz, J2 = 6.70 Hz, 1H), 2.82 (dq, J1 = 12.60 Hz, J2 = 7.25 Hz, 1H), 1.43 (t, J = 7.07 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 188.7, 183.8, 166.3, 151.1, 134.3, 132.4, 131.2, 129.7, 129.3, 128.7, 125.3, 125.07, 125.04, 118.5, 74.4, 31.2, 14.8. HRMS (ESI/Q-TOF) m/z [M + Na]+ Calcd. for C26H20BrNO4S2Na 575.9909, found 575.9909. 2-Benzoyl-3-(N-{methyl benzoate-4-yl}imino)-5-ethylthio-4-(4trifluoromethylphenyl)-2,3-dihydrothiazole (7pbc). The product was obtained as yellow sticky solid; (0.53 g, 95%). 1H NMR (500 MHz, CDCl3) δ 8.55 (d, J = 8.25 Hz, 2H), 7.98 (d, J = 7.75 Hz, 2H), 7.78 (d, J = 9.00 Hz, 2H), 7.72−7.69 (m, 1H), 7.60−7.57 (m, 2H), 7.53 (d, J = 8.25 H, 2H), 6.81 (d, J = 8.55 Hz, 2H), 6.53 (s, 1H), 3.81 (s, 3H), 2.98 (dq, J1 = 12.50 Hz, J2 = 7.31 Hz, 1H), 2.82 (dq, J1 = 12.80 Hz, J2 = 7.30 Hz, 1H), 1.26 (t, J = 7.22 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 189.2, 184.2, 166.7, 151.5, 144.9, 140.5, 134.7, 131.6, 130.1, 129.7, 129.1, 125.6, 125.4, 125.4, 118.9, 74.8, 52.4, 31.6, 15.2. HRMS (ESI/Q-TOF) m/z [M + H]+ Calcd. for C28H23F3NO4S2 558.1015, found 558.1015.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b01601. Synthesis of starting materials, copies of 1H, 19F and 13C NMR spectra of all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-542-2368127. ORCID

Abhijeet Srivastava: 0000-0001-5118-7539 Maya Shankar Singh: 0000-0002-3199-0823 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the Science and Engineering Research Board (SERB/EMR/2015/002482), New Delhi and the Council of Scientific and Industrial Research (02(0263)/16/EMR-II), New Delhi, India. The authors (G.S., A.S. & D.Y.) are thankful to CSIR, New Delhi for research fellowship.

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REFERENCES

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