Tetrabutylammonium Bromide (TBAB) Catalyzed Synthesis of Bioactive Heterocycles

During the last two decades, tetrabutylammonium bromide (TBAB) has gained significant attention as an efficient metal-free homogeneous phase-transfer catalyst. A catalytic amount of TBAB is sufficient to catalyze various alkylation, oxidation, reduction, and esterification processes. It is also employed as an efficient co-catalyst for numerous coupling reactions. It has also acted as an efficient zwitterionic solvent in many organic transformations under molten conditions. In this review, we have summarized the recent developments on TBAB-catalyzed protocols for the efficient synthesis of various biologically promising heterocyclic scaffolds.

For the synthesis of diverse heterocyclic entities, the screening of suitable catalysts plays an important role [17]. At present, scientists prefer metal-free organocatalysts in order to avoid metal contamination in the synthesized products. As a result, during the last decade, various organocatalysts have gained a great deal of attention in carrying out organic transformations under environmentally benign conditions [18][19][20]. Among many others, metal-free phase-transfer catalysts are being widely used in various organic reactions due to their ecofriendly, mild, and biocompatible nature [21]. Various phase-transfer catalysts showed immense activity in reactions where a reactant soluble in the organic phase needs to react with an anionic reactant soluble in the aqueous phase. For the synthesis of diverse heterocyclic entities, the screening of suitable catalysts plays an important role [17]. At present, scientists prefer metal-free organocatalysts in order to avoid metal contamination in the synthesized products. As a result, during the last decade, various organocatalysts have gained a great deal of attention in carrying out organic transformations under environmentally benign conditions [18][19][20]. Among many others, metal-free phase-transfer catalysts are being widely used in various organic reactions due to their ecofriendly, mild, and biocompatible nature [21]. Various phase-transfer catalysts showed immense activity in reactions where a reactant soluble in the organic phase needs to react with an anionic reactant soluble in the aqueous phase.
Recently, tetrabutylammonium bromide (TBAB) has gained tremendous attention as an efficient homogeneous phase-transfer catalyst. TBAB is an environmentally benign, non-volatile, non-flammable, non-corrosive, low-cost, commercially available ammonium salt with high thermal and chemical stability [22]. In TBAB, tetrabutylammonium salt can dissolve in both aqueous as well as in organic solvents, which helps to transport the water-soluble anionic reactants into the organic phase. Moreover, molten TBAB was also employed as an efficient ionic liquid to carry out organic transformations under solvent-free conditions [23][24][25]. In some reactions, it was observed that the addition of a catalytic amount of TBAB as co-catalyst enhanced the reaction rate as well as product yields [26][27][28][29][30]. The abovementioned unique capabilities of TBAB make this catalyst very attractive. In many occasions, normal monophasic catalysts either failed to carry out such reactions or afforded poor yields. As a result, the catalytic activity of TBAB has been continuously explored for various reactions. It showed excellent catalytic efficacies for the synthesis of N-aryl amines [31] and 1-alkyl/aryl-2-(1-arylsulfonyl alkyl) benzimidazoles [32]. It was also employed for the carbonylation-peroxidation of styrene derivatives [33], the alkylation of aldehydes or ketones [34], the S-alkylation of 4-mercapto-6-methyl-2-pyrone [35], the N-alkylation of acridones [36], the sulfonylation of para-quinone methides [37], Suzuki cross-coupling reaction [38], Heck reaction [39], and Suzuki-Miyaura reaction [40].
In the following sections, we will discuss various TBAB-catalyzed synthetic approaches which have been reported for the preparation of diverse biologically relevant heterocycles reported. Recently, tetrabutylammonium bromide (TBAB) has gained tremendous attention as an efficient homogeneous phase-transfer catalyst. TBAB is an environmentally benign, non-volatile, non-flammable, non-corrosive, low-cost, commercially available ammonium salt with high thermal and chemical stability [22]. In TBAB, tetrabutylammonium salt can dissolve in both aqueous as well as in organic solvents, which helps to transport the water-soluble anionic reactants into the organic phase. Moreover, molten TBAB was also employed as an efficient ionic liquid to carry out organic transformations under solvent-free conditions [23][24][25]. In some reactions, it was observed that the addition of a catalytic amount of TBAB as co-catalyst enhanced the reaction rate as well as product yields [26][27][28][29][30]. The abovementioned unique capabilities of TBAB make this catalyst very attractive. In many occasions, normal monophasic catalysts either failed to carry out such reactions or afforded poor yields. As a result, the catalytic activity of TBAB has been continuously explored for various reactions. It showed excellent catalytic efficacies for the synthesis of N-aryl amines [31] and 1-alkyl/aryl-2-(1-arylsulfonyl alkyl) benzimidazoles [32]. It was also employed for the carbonylation-peroxidation of styrene derivatives [33], the alkylation of aldehydes or ketones [34], the S-alkylation of 4-mercapto-6-methyl-2-pyrone [35], the N-alkylation of acridones [36], the sulfonylation of para-quinone methides [37], Suzuki cross-coupling reaction [38], Heck reaction [39], and Suzuki-Miyaura reaction [40].
In the following sections, we will discuss various TBAB-catalyzed synthetic approaches which have been reported for the preparation of diverse biologically relevant heterocycles reported.

Synthesis of 1,4-Dihydropyridines
1,4-Dihydropyridine and related derivatives are found to possess a wide range of biological efficacies, including anti-bacterial [41], anti-diabetic [42], anti-cancer [43], anti-HIV [44], anti-convulsant [45], and anti-tubercular [46] activities. A number of methods were reported for the synthesis of these biologically significant scaffolds using various homogeneous as well as heterogeneous catalysts [47][48][49][50], ionic liquids [51], and fluorinated solvents [52]. The use of metal-containing catalysts and toxic solvents are some of the major drawbacks of these reported protocols. In 2014, Kumar et al. [53] developed a facile method for the synthesis of a series of 1,4-dihydropyridine derivatives via one-pot pseudo four-component Hantzsch reaction between one equivalent of various aryl or heteroaryl aldehydes (1), two equivalents of ethyl acetoacetate (2), and one equivalent of ammonium acetate (3) using 10 mol% of TBAB as an efficient phase-transfer catalyst in aqueous medium at 60 • C (Scheme 1). Structurally diverse aromatic aldehydes produced the desired products with excellent yields. Heteroaryl aldehydes also smoothly underwent the reaction and yielded the desired products. Under the same optimized condition, the reactions afforded comparable yields by using benzyltriethyl ammonium chloride as catalyst whereas lower yields were obtained with cetyltrimethylammonium bromide as catalyst.

Synthesis of 2-Substituted Imidazolines
Liu et al. [54] reported a facile and eco-friendly method for the efficient synthesis of 2-substituted imidazolines (6) starting from aromatic aldehydes (1) and ethylenediamine (5) in the presence of a catalytic amount of both tungstophosphoric acid as well as tetrabutylammonium bromide as an efficient phase-transfer catalytic system using hydrogen peroxide as oxidant in water at 80 • C (Scheme 2). During optimization, a lower yield was recorded in the absence of TBABI (i.e., using only tungstophosphoric acid as catalyst). A probable role of the dual catalysts is shown in Scheme 3. In the organic phase, the rapid condensation of benzaldehyde and ethylenediamine produced intermediate I-1, which after cyclization produced the second intermediate I-2. In the aqueous phase, the combination of a catalytic amount of TBAB and tungstophosphoric acid generated a novel complex C-1 which in the presence of H 2 O 2 yielded peroxo complex C-2. This in-situ generated complex can catalyze the formation of the desired 2-phenylimidazoline 5 from the intermediate I-2 by entering the organic phase.
2.3. Synthesis of 2,4,5-Triaryl Imidazoles 2,4,5-Trisubstituted imidazoles have been found to possess immense biological activities [55][56][57][58][59]. 2,4,5-Triphenyl imidazole (8) was first synthesized in 1882 from the reaction of aryl aldehyde (1) and benzyl (7) in alcoholic ammonia solution [60]. Later on, a number of methods were reported involving ammonium acetate (3) as the source of ammonia in the presence of various Lewis acidic metal salts as catalyst [61][62][63][64][65][66][67]. These reported methods suffered from many drawbacks, such as harsh conditions, use of metal-containing catalysts, and lower yields. Starting from the same batch of reactants, in 2008, Chary et al. [68] developed a facile, efficient, and environmentally benign protocol for the synthesis of 2,4,5-triaryl imidazoles (8) using a catalytic amount of TBAB isopropanol under reflux conditions (Scheme 4). After completion of the reaction, TBAB-containing reaction medium was reused for a further run. Aldehydes with both electron-donating as well as electron-withdrawing substituents afforded the desired products with excellent yields. All the reactions were completed within thirty minutes. A probable mechanistic approach is outlined in Scheme 5. TBAB activated the carbonyl group of benzil, which facilitated the formation of intermediate I-3 by the attack of ammonium generated from ammonium acetate. On the other hand, corresponding Schiff bases (I-4) were also prepared from the reaction between aromatic aldehydes and in-situ-generated ammonia in the presence of TBAB as catalyst. Now, the combination of I-3 and I-4 produced another intermediate I-5 which on further dehydration followed by aromatization yielded the desired 2,4,5-triaryl imidazoles (8).

Synthesis of 1,4-Dihydropyridines
1,4-Dihydropyridine and related derivatives are found to possess a wide range of biological efficacies, including anti-bacterial [41], anti-diabetic [42], anti-cancer [43], anti-HIV [44], anticonvulsant [45], and anti-tubercular [46] activities. A number of methods were reported for the synthesis of these biologically significant scaffolds using various homogeneous as well as heterogeneous catalysts [47][48][49][50], ionic liquids [51], and fluorinated solvents [52]. The use of metalcontaining catalysts and toxic solvents are some of the major drawbacks of these reported protocols. In 2014, Kumar et al. [53] developed a facile method for the synthesis of a series of 1,4dihydropyridine derivatives via one-pot pseudo four-component Hantzsch reaction between one equivalent of various aryl or heteroaryl aldehydes (1), two equivalents of ethyl acetoacetate (2), and one equivalent of ammonium acetate (3) using 10 mol% of TBAB as an efficient phase-transfer catalyst in aqueous medium at 60 °C (Scheme 1). Structurally diverse aromatic aldehydes produced the desired products with excellent yields. Heteroaryl aldehydes also smoothly underwent the reaction and yielded the desired products. Under the same optimized condition, the reactions afforded comparable yields by using benzyltriethyl ammonium chloride as catalyst whereas lower yields were obtained with cetyltrimethylammonium bromide as catalyst.

Synthesis of 2-Substituted Imidazolines
Liu et al. [54] reported a facile and eco-friendly method for the efficient synthesis of 2substituted imidazolines (6) starting from aromatic aldehydes (1) and ethylenediamine (5) in the presence of a catalytic amount of both tungstophosphoric acid as well as tetrabutylammonium bromide as an efficient phase-transfer catalytic system using hydrogen peroxide as oxidant in water at 80 °C (Scheme 2). During optimization, a lower yield was recorded in the absence of TBABI (i.e.,  Scheme 3. In the organic phase, the rapid condensation of benzaldehyde and ethylenediamine produced intermediate I-1, which after cyclization produced the second intermediate I-2. In the aqueous phase, the combination of a catalytic amount of TBAB and tungstophosphoric acid generated a novel complex C-1 which in the presence of H2O2 yielded peroxo complex C-2. This insitu generated complex can catalyze the formation of the desired 2-phenylimidazoline 5 from the intermediate I-2 by entering the organic phase.

Synthesis of 2,4,5-Triaryl Imidazoles
2,4,5-Trisubstituted imidazoles have been found to possess immense biological activities [55][56][57][58][59]. 2,4,5-Triphenyl imidazole (8) was first synthesized in 1882 from the reaction of aryl aldehyde (1) and benzyl (7) in alcoholic ammonia solution [60]. Later on, a number of methods were reported involving ammonium acetate (3) as the source of ammonia in the presence of various Lewis acidic metal salts as catalyst [61][62][63][64][65][66][67]. These reported methods suffered from many drawbacks, such as harsh conditions, use of metal-containing catalysts, and lower yields. Starting from the same batch of reactants, in 2008, Chary et al. [68] developed a facile, efficient, and environmentally benign protocol for the synthesis of 2,4,5-triaryl imidazoles (8) using a catalytic amount of TBAB isopropanol under reflux conditions (Scheme 4). After completion of the reaction, TBAB-containing reaction medium was reused for a further run. Aldehydes with both electron-donating as well as electron-withdrawing substituents afforded the desired products with excellent yields. All the reactions were completed within thirty minutes. A probable mechanistic approach is outlined in Scheme 5. TBAB activated the carbonyl group of benzil, which facilitated the formation of intermediate I-3 by the attack of ammonium generated from ammonium acetate. On the other hand, corresponding Schiff bases (I-4) were also prepared from the reaction between aromatic aldehydes

Synthesis of 1,3-Dihydrobenzimidazol-2-Ones
Aghapoor et al. [69] developed a simple, rapid, microwave-assisted protocol for the efficient synthesis of 1,3-dihydrobenzimidazol-2-ones (12) from the reactions of urea (9) and ophenylenediamines (10) using TBAB as a catalyst in ethanol. Under the optimized reaction conditions, pyridine-2,3-diamine (11) also reacted with urea (9) and afforded the corresponding 1Himidazo [4,5-b]pyridin-2(3H)-one (13) with 68% yield (Scheme 6). All the reactions were completed within just fifteen minutes. o-Phenylenediamines with both electron-donating as well as electronwithdrawing substituents produced the desired products with good yields. Although the exact mechanism was not discussed in the mother literature, we can assume that TBAB initiates the reaction by activating the carbonyl carbon of urea.

Synthesis of 1,3-Dihydrobenzimidazol-2-Ones
Aghapoor et al. [69] developed a simple, rapid, microwave-assisted protocol for the efficient synthesis of 1,3-dihydrobenzimidazol-2-ones (12) from the reactions of urea (9) and o-phenylenediamines (10) using TBAB as a catalyst in ethanol. Under the optimized reaction conditions, pyridine-2,3-diamine (11) also reacted with urea (9) and afforded the corresponding 1H-imidazo [4,5-b]pyridin-2(3H)-one (13) with 68% yield (Scheme 6). All the reactions were completed within just fifteen minutes. o-Phenylenediamines with both electron-donating as well as electron-withdrawing substituents produced the desired products with good yields. Although the exact mechanism was not discussed in the mother literature, we can assume that TBAB initiates the reaction by activating the carbonyl carbon of urea.

Synthesis of Pyrrolo[2,3-d]pyrimidine Derivatives
A straightforward one-pot three-component reaction protocol was reported utilizing TBAB as the catalyst. Using this protocol, a series of densely functionalized pyrrolo[2,3-d]pyrimidine derivatives (17) were successfully synthesized from the reactions of aryl glyoxals (14), 6-amino-1,3dimethyluracil (15), and barbituric acid (16) or thiobarbituric acid derivatives (16a) in ethanol at 50 °C (Scheme 7) [70]. Clean reaction profile, excellent yields, short reaction times, and easy work-up procedure are some of the major advantages of this protocol. The plausible mechanism of this reaction is shown in Scheme 8. TBAB initiated the reaction by activating the aldehydic carbon of aryl glyoxals. Reaction of aryl glyoxals and 6-amino-1,3-dimethyluracil (15) yielded intermediates I-6, which then further reacted with barbituric acid (16) or thiobarbituric acid derivatives (16a) to produce the desired products 17.

Synthesis of 1,5-Benzodiazepine Derivatives
Benzodiazepine and related derivatives showed potent pharmaceutical activities [76]. Among many others, 2,3-dihydro-1H-1,5-benzodiazepines (21) gained much attention. A number of methods were reported for the synthesis of these biologically promising scaffolds utilizing numerous homogeneous as well as heterogeneous catalysts, such as BF 3 OEt [77], Ag 3 PW 12 O 40 [78], PPA-SiO 2 [79], zinc montmorillonite [80], Yb(OTf) 3 [81], MgO-POCl 3 [82], Amberlyst [83], and superacid sulphated zirconia [84]. The use of a toxic and costly catalyst is the common drawback of these reported methods. In 2012, Baseer and Khan [85] synthesized a series of structurally diverse 2,3-dihydro-1H-1,5-benzodiazepines (21) from the reactions of one equivalent of o-phenylenediamine (10) and two equivalents of various acyclic ketones (20) using TBAB as an efficient catalyst in ethanol at 60 • C (Scheme 10). Under the same optimized conditions, they were also able to synthesize a series of spiro-benzodiazepine derivatives (23) using cyclic ketones (22) instead of acyclic ketones (Scheme 11). In all cases, excellent yields of the desired products were obtained through the formation of Schiff bases followed by a cyclization pathway. instead of acyclic ketones (Scheme 11). In all cases, excellent yields of the desired products were obtained through the formation of Schiff bases followed by a cyclization pathway.

Synthesis of 5-Substituted 1H-Tetrazoles
Xie et al. [86] reported a simple and efficient protocol for the synthesis of a series of 5-substituted 1H-tetrazoles (26) with excellent yields from the reactions of aryl nitrile (24) and sodium azide (25) in molten tetrabutylammonium bromide at 105 • C (Scheme 12). In this reaction, molten TBAB played a dual role, both as a solvent as well as a catalyst. Under this condition, aliphatic nitrile failed to produce the desired products. Here, TBAB polarized the cyano group, which facilitated the attack by the azide ion. Scheme 10. TBAB-catalyzed synthesis of 1,5-benzodiazepine derivatives in ethanol. Scheme 11. TBAB-catalyzed synthesis of spiro-benzodiazepine derivative.

Synthesis of 5-Substituted 1H-Tetrazoles
Xie et al. [86] reported a simple and efficient protocol for the synthesis of a series of 5substituted 1H-tetrazoles (26) with excellent yields from the reactions of aryl nitrile (24) and sodium azide (25) in molten tetrabutylammonium bromide at 105 °C (Scheme 12). In this reaction, molten TBAB played a dual role, both as a solvent as well as a catalyst. Under this condition, aliphatic nitrile failed to produce the desired products. Here, TBAB polarized the cyano group, which facilitated the attack by the azide ion. Scheme 12. TBAB-promoted synthesis of 5-substituted 1H -tetrazoles.

Synthesis of 5-Substituted 1H-Tetrazoles
Xie et al. [86] reported a simple and efficient protocol for the synthesis of a series of 5substituted 1H-tetrazoles (26) with excellent yields from the reactions of aryl nitrile (24) and sodium azide (25) in molten tetrabutylammonium bromide at 105 °C (Scheme 12). In this reaction, molten TBAB played a dual role, both as a solvent as well as a catalyst. Under this condition, aliphatic nitrile failed to produce the desired products. Here, TBAB polarized the cyano group, which facilitated the attack by the azide ion. Scheme 12. TBAB-promoted synthesis of 5-substituted 1H -tetrazoles.

Synthesis of 3-Nitro-2H-Chromenes
Synthesis of 3-nitro-2H-chromenes (29) was achieved via the microwave-assisted reactions of substituted salicylaldehydes (27) and 2-nitro ethanol (28) using anhydrous potassium carbonate as base in the presence of a catalytic amount of TBAB as catalyst under solvent-free conditions (Scheme 13) [87]. The presence of a base facilitated the formation of carbanion on the carbon atom attached with a nitro group. After being activated by TBAB, the aldehydic group underwent condensation with the in-situ-generated carbanion and the resulting intermediate after cyclization yielded the corresponding desired products 29.

Synthesis of Isocoumarin-1-Imines and Isobenzofuran-1-Imines
A facile and regioselective TBAB-catalyzed efficient protocol was reported for the synthesis of a series of isocoumarin-1-imines (36) through the 6-endo-dig oxy-cyclization 2-alkynylbenzamide (35) (Scheme 17) [93]. The reactions were carried out using two equivalents of Oxone as oxidizing agent in the presence of potassium carbonate as base in THF-water mixture as solvent at 80 °C. The plausible mechanism of the transformation is shown in Scheme 18. Under the same optimized conditions, when N-phenyl 2-trimethylsilylethynylbenzamides (37) were used as starting components, the corresponding isobenzofuran-1-imines (38) were isolated with excellent yields (Scheme 19). The probable mechanistic approach of this transformation is shown in Scheme 20.

Synthesis of Isocoumarin-1-Imines and Isobenzofuran-1-Imines
A facile and regioselective TBAB-catalyzed efficient protocol was reported for the synthesis of a series of isocoumarin-1-imines (36) through the 6-endo-dig oxy-cyclization 2-alkynylbenzamide (35) (Scheme 17) [93]. The reactions were carried out using two equivalents of Oxone as oxidizing agent in the presence of potassium carbonate as base in THF-water mixture as solvent at 80 • C. The plausible mechanism of the transformation is shown in Scheme 18. Under the same optimized conditions, when N-phenyl 2-trimethylsilylethynylbenzamides (37) were used as starting components, the corresponding isobenzofuran-1-imines (38)

Synthesis of Xanthones
Rao et al. [98] developed a simple and straightforward aqueous-mediated protocol for the intramolecular annulations of 2-aryloxybenzaldehydes (42) which afforded the corresponding xanthones (43) with moderate to excellent yields using tert-butyl hydroperoxide (TBHP) as an oxidant in the presence of TBAB as catalyst at 120 °C (Scheme 22). The plausible mechanism of this transformation is shown in Scheme 23. The reaction proceeded through the direct oxidative coupling reactions of C-H bonds of aldehydes and aromatic C-H bonds. Under the same optimized reaction conditions, 2-aryloxybenzaldehydes with both electron-donating as well as electronwithdrawing substituents showed good tolerance and yielded the corresponding desired products.

Synthesis of Xanthones
Rao et al. [98] developed a simple and straightforward aqueous-mediated protocol for the intramolecular annulations of 2-aryloxybenzaldehydes (42) which afforded the corresponding xanthones (43) with moderate to excellent yields using tert-butyl hydroperoxide (TBHP) as an oxidant in the presence of TBAB as catalyst at 120 • C (Scheme 22). The plausible mechanism of this transformation is shown in Scheme 23. The reaction proceeded through the direct oxidative coupling reactions of C-H bonds of aldehydes and aromatic C-H bonds. Under the same optimized reaction conditions, 2-aryloxybenzaldehydes with both electron-donating as well as electron-withdrawing substituents showed good tolerance and yielded the corresponding desired products.

Synthesis of Xanthones
Rao et al. [98] developed a simple and straightforward aqueous-mediated protocol for the intramolecular annulations of 2-aryloxybenzaldehydes (42) which afforded the corresponding xanthones (43) with moderate to excellent yields using tert-butyl hydroperoxide (TBHP) as an oxidant in the presence of TBAB as catalyst at 120 °C (Scheme 22). The plausible mechanism of this transformation is shown in Scheme 23. The reaction proceeded through the direct oxidative coupling reactions of C-H bonds of aldehydes and aromatic C-H bonds. Under the same optimized reaction conditions, 2-aryloxybenzaldehydes with both electron-donating as well as electronwithdrawing substituents showed good tolerance and yielded the corresponding desired products.

Synthesis of Aryl-14H-Dibenzo[a.j]xanthenes
Benzoxanthene and its congeners have shown various biological activities [99][100][101][102]. In 2008, Kantevari et al. [103] reported a facile method for the synthesis of a series of aryl-14Hdibenzo[a.j]xanthenes (45) from the reactions of two equivalents of β-naphthol (44) and one equivalent of various aromatic aldehydes (1) using 10 mol% TBAB as catalyst under solvent-free conditions at 125 °C (Scheme 24). All the reactions were completed within eighty minutes and afforded 81-96% yields. Using TBAB as catalyst, the same reactions when carried out under microwave-irradiated conditions produced 78-95% yields within just six minutes. Products were isolated by simple crystallization in ethanol.

Synthesis of Aryl-14H-Dibenzo[a.j]xanthenes
Benzoxanthene and its congeners have shown various biological activities [99][100][101][102]. In 2008, Kantevari et al. [103] reported a facile method for the synthesis of a series of aryl-14H-dibenzo[a.j]xanthenes (45) from the reactions of two equivalents of β-naphthol (44) and one equivalent of various aromatic aldehydes (1) using 10 mol% TBAB as catalyst under solvent-free conditions at 125 • C (Scheme 24). All the reactions were completed within eighty minutes and afforded 81-96% yields. Using TBAB as catalyst, the same reactions when carried out under microwave-irradiated conditions produced 78-95% yields within just six minutes. Products were isolated by simple crystallization in ethanol.

Synthesis of Aryl-14H-Dibenzo[a.j]xanthenes
Benzoxanthene and its congeners have shown various biological activities [99][100][101][102]. In 2008, Kantevari et al. [103] reported a facile method for the synthesis of a series of aryl-14Hdibenzo[a.j]xanthenes (45) from the reactions of two equivalents of β-naphthol (44) and one equivalent of various aromatic aldehydes (1) using 10 mol% TBAB as catalyst under solvent-free conditions at 125 °C (Scheme 24). All the reactions were completed within eighty minutes and afforded 81-96% yields. Using TBAB as catalyst, the same reactions when carried out under microwave-irradiated conditions produced 78-95% yields within just six minutes. Products were isolated by simple crystallization in ethanol.

Synthesis of Pyran-Fused Spirooxindoles
In 2012, Mobinikhaledi et al. [107] prepared a series of structurally diverse pyran-fused spirooxindole derivatives using TBAB as catalyst in water under reflux conditions at 100 °C. They

Synthesis of Pyran-Fused Spirooxindoles
In
Molecules 2020, 25, x FOR PEER REVIEW 18 of 25 (59) from the reactions of 5-cinnamoyl-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H)-ones (58) and excess thiourea (9a) in the presence of a catalytic amount of TBAB as catalyst in dichloromethane-water as biphasic solvent at 50 °C (Scheme 31). All the reactions were completed within just thirty minutes and afforded excellent yields. In the presence of TBAB, the ketonic carbon of 58 activated and formed the corresponding Schiff bases (I-9) with the reaction of thiourea (9a). The in-situ-generated I-9 afforded the desired product by following the cyclization pathway shown in Scheme 32.
In the same year, Khurana and his group [110] reported another TBAB-catalyzed facile and convenient protocol for the efficient synthesis of a series of novel benzo[e] [1,3]thiazines (65) (59) from the reactions of 5-cinnamoyl-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H)-ones (58) and excess thiourea (9a) in the presence of a catalytic amount of TBAB as catalyst in dichloromethane-water as biphasic solvent at 50 °C (Scheme 31). All the reactions were completed within just thirty minutes and afforded excellent yields. In the presence of TBAB, the ketonic carbon of 58 activated and formed the corresponding Schiff bases (I-9) with the reaction of thiourea (9a). The in-situ-generated I-9 afforded the desired product by following the cyclization pathway shown in Scheme 32.
In the same year, Khurana and his group [110] reported another TBAB-catalyzed facile and convenient protocol for the efficient synthesis of a series of novel benzo[e] [1,3]thiazines (65)
Funding: This research received no external funding.