molecules

Review Tetrabutylammonium Bromide (TBAB) Catalyzed Synthesis of Bioactive Heterocycles

Bimal Krishna Banik 1,*, Bubun Banerjee 2 , Gurpreet Kaur 2, Shivam Saroch 2 and Rajat Kumar 2

1 Department of Mathematics and Natural Sciences, College of Sciences and Human Studies, Deanship of Research Development, Prince Mohammad Bin Fahd University, Al Khobar 31952, Saudi Arabia 2 Department of Chemistry, Indus International University, V.P.O. Bathu, Distt. Una, Himachal Pradesh 174301, India; [email protected] or [email protected] (B.B.); [email protected] (G.K.); [email protected] (S.S.); [email protected] (R.K.) * Correspondence: [email protected] or [email protected]



Academic Editors: Toshifumi Dohi, Cheng-Pan Zhang and György Keglevich
 Received: 25 August 2020; Accepted: 23 September 2020; Published: 14 December 2020

Abstract: 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.

Keywords: tetrabutylammonium bromide; TBAB; phase-transfer catalyst; metal-free synthesis; bioactive heterocycles

1. Introduction Heterocyclic skeletons are very common in commercially available drug molecules (Figure1)[ 1]. Heterocycles are the main building blocks of many naturally occurring compounds [2]. Various synthetic heterocyclic scaffolds have been found to possess a wide range of biological efficacies, including anti-inflammatory [3], anti-malarial [4], anti-tubercular [5], anti-cancer [6], anti-asthmatic [7], anti-histaminic [8], anti-hypertensive [9], anti-depressant [10], anti-microbial [11], anti-rheumatic [12], anti-diabetic [13], anti-Alzheimer’s, anti-Parkinson’s, anti-Huntington’s disease [14], and many more activities [15,16]. 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 [18–20]. 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.

Molecules 2020, 25, 5918; doi:10.3390/molecules25245918 www.mdpi.com/journal/molecules Molecules 2020, 25, 5918 2 of 24

Molecules 2020, 25, x FOR PEER REVIEW 2 of 25

FigureFigure 1. Glimpse1. Glimpse of of marketed marketed drugs containing containing heterocycles. heterocycles.

Recently,For the tetrabutylammonium synthesis of diverse heterocyclic bromide (TBAB) entities has, the gained screening tremendous of suitable attention catalysts asplays an eanffi cient important role [17]. At present, scientists prefer metal-free organocatalysts in order to avoid metal homogeneous phase-transfer catalyst. TBAB is an environmentally benign, non-volatile, non-flammable, contamination in the synthesized products. As a result, during the last decade, various non-corrosive, low-cost, commercially available ammonium salt with high thermal and chemical organocatalysts have gained a great deal of attention in carrying out organic transformations under stabilityenvironmentally [22]. In TBAB, benign tetrabutylammonium conditions [18–20]. Among salt can many dissolve others, in metal-free both aqueous phase-transfer as well ascatalysts in organic solvents,are whichbeing helpswidely to used transport in various the water-soluble organic re anionicactions reactantsdue to their into theecofriendly, organic phase. mild, Moreover,and moltenbiocompatible TBAB was alsonature employed [21]. Various as an phase-transfer efficient ionic catalysts liquid to sh carryowed out immense organic activity transformations in reactions under solvent-freewhere a conditions reactant soluble [23–25 in]. the In someorganic reactions, phase needs it was to observedreact with that an anionic the addition reactant of asoluble catalytic in the amount of TBABaqueous as co-catalyst phase. enhanced the reaction rate as well as product yields [26–30]. The abovementioned unique capabilitiesRecently, tetrabutylammonium of TBAB make this catalyst bromide very (TBAB) attractive. has gained In many tremendous occasions, attention normal monophasic as an catalystsefficient either homogeneous failed to carry phase-transfer out such reactions catalyst. or TB affABorded is an poor environmentally yields. As a result, benign, the non-volatile, catalytic activity non-flammable, non-corrosive, low-cost, commercially available ammonium salt with high thermal of TBAB has been continuously explored for various reactions. It showed excellent catalytic efficacies and chemical stability [22]. In TBAB, tetrabutylammonium salt can dissolve in both aqueous as well for the synthesis of N-aryl amines [31] and 1-alkyl/aryl-2-(1-arylsulfonyl alkyl) benzimidazoles [32]. as in organic solvents, which helps to transport the water-soluble anionic reactants into the organic It wasphase. also employedMoreover, molten for the TBAB carbonylation-peroxidation was also employed as an of efficient styrene ionic derivatives liquid to [carry33], the out alkylation organic of aldehydestransformations or ketones under [34], solvent-free the S-alkylation conditions of 4-mercapto-6-methyl-2-pyrone [23–25]. In some reactions, it was [35 observed], the N- alkylationthat the of acridonesaddition [36 of], thea catalytic sulfonylation amount of TBABpara-quinone as co-catalyst methides enhanced [37], the Suzuki reaction cross-coupling rate as well as reactionproduct [38], Heckyields reaction [26–30]. [39], The and abovementioned Suzuki–Miyaura unique reaction capabilities [40]. of TBAB make this catalyst very attractive. InIn themany following occasions, sections, normal we monophasic will discuss catalysts various either TBAB-catalyzed failed to carry synthetic out such approaches reactions or which haveafforded been reported poor yields. for the As preparation a result, the ofcatalytic diverse acti biologicallyvity of TBAB relevant has been heterocycles continuously reported.explored for various reactions. It showed excellent catalytic efficacies for the synthesis of N-aryl amines [31] and 2. Applications1-alkyl/aryl-2-(1-arylsulfonyl of TBAB for the alkyl) Synthesis benzimidazoles of Bioactive [32].N-Heterocycles It was also employed for the carbonylation-peroxidation of styrene derivatives [33], the alkylation of aldehydes or ketones [34], 2.1. Synthesisthe S-alkylation of 1,4-Dihydropyridines 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], 1,4-Dihydropyridineand Suzuki–Miyaura reaction and related [40]. derivatives are found to possess a wide range of biological efficacies, includingIn anti-bacterial the following [sections,41], anti-diabetic we will discuss [42], vari anti-cancerous TBAB-catalyzed [43], anti-HIV synthetic [44], approaches anti-convulsant which [45], and anti-tubercularhave been reported [46 for] activities. the preparation A number of divers ofe methodsbiologically were relevant reported heterocycles for the reported. synthesis of these biologically significant scaffolds using various homogeneous as well as heterogeneous catalysts [47–50], 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 Molecules 2020, 25, 5918 3 of 24

(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 (Scheme1). 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.

2.2. 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 (Scheme2). 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 Scheme3. 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 in-situ generated complex can catalyze the formation of the desired 2-phenylimidazoline 5 from the intermediate I-2 by entering the organic phase. Molecules 2020, 25, x FOR PEER REVIEW 3 of 25 2.3. Synthesis of 2,4,5-Triaryl Imidazoles 2. Applications of TBAB for the Synthesis of Bioactive N-Heterocycles 2,4,5-Trisubstituted imidazoles have been found to possess immense biological activities [55–59]. 2,4,5-Triphenyl2.1. Synthesis of imidazole1,4-Dihydropyridines (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 1,4-Dihydropyridine and related derivatives are found to possess a wide range of biological ammonium acetate (3) as the source of ammonia in the presence of various Lewis acidic metal salts as efficacies, including anti-bacterial [41], anti-diabetic [42], anti-cancer [43], anti-HIV [44], anti- catalyst [61–67]. These reported methods suffered from many drawbacks, such as harsh conditions, convulsant [45], and anti-tubercular [46] activities. A number of methods were reported for the use of metal-containing catalysts, and lower yields. Starting from the same batch of reactants, in synthesis of these biologically significant scaffolds using various homogeneous as well as 2008, Chary et al. [68] developed a facile, efficient, and environmentally benign protocol for the heterogeneous catalysts [47–50], ionic liquids [51], and fluorinated solvents [52]. The use of metal- synthesis of 2,4,5-triaryl imidazoles (8) using a catalytic amount of TBAB isopropanol under reflux containing catalysts and toxic solvents are some of the major drawbacks of these reported protocols. conditions (Scheme4). After completion of the reaction, TBAB-containing reaction medium was In 2014, Kumar et al. [53] developed a facile method for the synthesis of a series of 1,4- reused for a further run. Aldehydes with both electron-donating as well as electron-withdrawing dihydropyridine derivatives via one-pot pseudo four-component Hantzsch reaction between one substituents afforded the desired products with excellent yields. All the reactions were completed equivalent of various aryl or heteroaryl aldehydes (1), two equivalents of ethyl acetoacetate (2), and within thirty minutes. A probable mechanistic approach is outlined in Scheme5. TBAB activated the one equivalent of ammonium acetate (3) using 10 mol% of TBAB as an efficient phase-transfer carbonyl group of benzil, which facilitated the formation of intermediate I-3 by the attack of ammonium catalyst in aqueous medium at 60 °C (Scheme 1). Structurally diverse aromatic aldehydes produced generated from ammonium acetate. On the other hand, corresponding Schiff bases (I-4) were also the desired products with excellent yields. Heteroaryl aldehydes also smoothly underwent the preparedreaction fromand theyielded reaction the betweendesired aromaticproducts. aldehydes Under the and same in-situ-generated optimized condition, ammonia inthe the reactions presence ofafforded TBAB as comparable catalyst. Now, yields the by combination using benzyltriethyl of I-3 and ammoniumI-4 produced chloride another as intermediate catalyst whereasI-5 which lower on furtheryields were dehydration obtained followed with cetyltrimethylammonium by aromatization yielded bromide the desired as catalyst. 2,4,5-triaryl imidazoles (8).

O Ar O O O 10 mol% TBAB Ar-CHO + + CH COONH 3 4 EtO OEt Me OEt o H2O, 60 C, 5 h, 60 95% Me N Me H 1; 1 equiv. 2; 2 equiv. 3; 1 equiv. 4

Ar = C6H5,4-OMeC6H4,4-ClC6H4,4-OHC6H4, 3,4,5-triOMeC6H2,3,4-diOMeC6H3, 3-OH-4-OMeC6H3, 4-N(Me)2C6H4,2-NO2C6H4, 2,4-diClC6H3, 2-furyl, 2-thienyl

SchemeScheme 1.1. TBAB-catalyzedTBAB-catalyzed synthesis of 1,4-dihy 1,4-dihydropyridinesdropyridines in in water water at at 60 60 °C.◦C.

2.2. 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 H2O2 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.

CHO TBAB H H3PW12O40 N R + NH2 H2N o N H2O2,H2O, 80 C, 2 6h,57 89% R

15 6

R = H, 4-Me, 4-Cl, 2,4-diCl Scheme 2. TBAB co-catalyzed synthesis of 2-substituted imidazolines in water. Molecules 2020, 25, x FOR PEER REVIEW 3 of 25

2. Applications of TBAB for the Synthesis of Bioactive N-Heterocycles

2.1. 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 [47–50], 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.

O Ar O O O 10 mol% TBAB Ar-CHO + + CH COONH 3 4 EtO OEt Me OEt o H2O, 60 C, 5 h, 60 95% Me N Me H 1; 1 equiv. 2; 2 equiv. 3; 1 equiv. 4

Ar = C6H5,4-OMeC6H4,4-ClC6H4,4-OHC6H4, 3,4,5-triOMeC6H2,3,4-diOMeC6H3, 3-OH-4-OMeC6H3, 4-N(Me)2C6H4,2-NO2C6H4, 2,4-diClC6H3, 2-furyl, 2-thienyl Scheme 1. TBAB-catalyzed synthesis of 1,4-dihydropyridines in water at 60 °C.

2.2. 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 H2O2 yielded peroxo complex C-2. This in- Moleculessitu generated2020, 25, 5918complex can catalyze the formation of the desired 2-phenylimidazoline 5 from4 ofthe 24 intermediate I-2 by entering the organic phase.

CHO TBAB H H3PW12O40 N R + NH2 H2N o N H2O2,H2O, 80 C, 2 6h,57 89% R

15 6

R = H, 4-Me, 4-Cl, 2,4-diCl

Molecules 2020, 25,Scheme x FOR PEER 2. TBAB REVIEW co-catalyzed synthesis of 2-substituted2-substituted imidazolines in water. 4 of 25

SchemeScheme 3.3.Plausible Plausible role role of of tungstophosphoric tungstophosphoric acid acid and and TBAB TBAB as an as effi ancient efficient combined combined phase-transfer phase- catalytictransfer catalytic system for system the synthesis for the synthesi of 2-substituteds of 2-substituted imidazolines imidazolines in water. in water. Molecules 2020, 25, x FOR PEER REVIEW 5 of 25

2.3. Synthesis of 2,4,5-Triaryl Imidazoles 2,4,5-Trisubstituted imidazoles have been found to possess immense biological activities [55– 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 [61–67]. 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 mediumSchemeScheme was 4.4. TBAB-catalyzedTBAB-catalyzed reused for a synthesisfurther run. of 2,4,5-triaryl2,4,5-triary Aldehydesl imidazoles with both in in isopropanol isopropanol electron-donating at at 82 82 °C.◦C. as well as electron-withdrawing substituents afforded the desired products with excellent yields. All the CH COONH 3 4 + reactions were completed within thirty minutes. AN(n-Bu) probable4 mechanistic approach is outlined in 3 Scheme 5. TBAB activated the carbonyl group of benzil, which facilitated the formation of O HO H H O 82 oC N 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 :NH Ar 3 -H2O NH and in-situ-generatedO ammonia in the presence of TBAB as catalyst. Now, theNH combination of I-3 and I-4 produced another intermediate I-5 which on further dehydration followed: by aromatization + yielded theN(n-Bu) desired4 2,4,5-triaryl imidazoles (8). I-3 7 CH COONH 3 4 HO H N H 82 oC Ar Ar Ar :NH NH N + O 3 H H H + I-4 N(n-Bu)4 O I-5 1 2 -H N H N Ar Ar N N+ H H

8 Scheme 5. Plausible mechanism of TBAB-promoted synthesis of 2,4,5-triaryl imidazoles.

2.4. 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. Molecules 2020, 25, x FOR PEER REVIEW 5 of 25

Molecules 2020, 25, 5918 5 of 24 Scheme 4. TBAB-catalyzed synthesis of 2,4,5-triaryl imidazoles in isopropanol at 82 °C.

CH COONH 3 4 + N(n-Bu)4 3 O HO H H O 82 oC N + :NH Ar 3 -H2O NH O NH : + N(n-Bu)4 I-3 7 CH COONH 3 4 HO H N H 82 oC Ar Ar Ar :NH NH N + O 3 H H H + I-4 N(n-Bu)4 O I-5 1 2 -H N H N Ar Ar N N+ H H

8 SchemeScheme 5.5. PlausiblePlausible mechanismmechanism of TBAB-promoted synthesis of of 2,4,5-triaryl 2,4,5-triaryl imidazoles. imidazoles. Molecules 2020, 25, x FOR PEER REVIEW 6 of 25 2.4. 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.

Scheme 6.6.TBAB-catalyzed TBAB-catalyzed synthesis synthesis of 1,3-dihydrobenzimidazol-2-ones of 1,3-dihydrobenzimidazol-2-ones and 1H -imidazo[4,5-and 1H-imidazo[4,5-b]pyridin- 2(3b]pyridin-2(3H)-one underH)-one microwave-assisted under microwave-assisted conditions. conditions.

2.5. 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-1,3- dimethyluracil (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.

Scheme 7. TBAB-catalyzed synthesis of pyrrolo[2,3-d]pyrimidine derivatives in ethanol. Molecules 2020, 25, x FOR PEER REVIEW 6 of 25

Molecules 2020, 25, 5918 6 of 24

2.4. 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 (Scheme6). 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 Scheme 6. TBAB-catalyzed synthesis of 1,3-dihydrobenzimidazol-2-ones and 1H-imidazo[4,5- good yields. Although the exact mechanism was not discussed in the mother literature, we can assume b]pyridin-2(3H)-one under microwave-assisted conditions. that TBAB initiates the reaction by activating the carbonyl carbon of urea. 2.5. Synthesis of Pyrrolo[2,3-d]pyrimidine Derivatives 2.5. Synthesis of Pyrrolo[2,3-d]pyrimidine Derivatives A straightforward one-pot three-component reaction protocol was reported utilizing TBAB as 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 the catalyst. Using this protocol, a series of densely functionalized pyrrolo[2,3-d]pyrimidine derivatives derivatives (17) were successfully synthesized from the reactions of aryl glyoxals (14), 6-amino-1,3- (17) were successfully synthesized from the reactions of aryl glyoxals (14), 6-amino-1,3-dimethyluracil dimethyluracil (15), and barbituric acid (16) or thiobarbituric acid derivatives (16a) in ethanol at 50 (15), and barbituric acid (16) or thiobarbituric acid derivatives (16a) in ethanol at 50 C (Scheme7)[ 70]. °C (Scheme 7) [70]. Clean reaction profile, excellent yields, short reaction times, and◦ easy work-up Clean reaction profile, excellent yields, short reaction times, and easy work-up procedure are some of procedure are some of the major advantages of this protocol. The plausible mechanism of this the major advantages of this protocol. The plausible mechanism of this reaction is shown in Scheme8. reaction is shown in Scheme 8. TBAB initiated the reaction by activating the aldehydic carbon of TBABaryl glyoxals. initiated Reaction the reaction of aryl by glyoxals activating and the 6-amino-1,3-dimethyluracil aldehydic carbon of aryl (15 glyoxals.) yielded Reactionintermediates of aryl I- glyoxals6, which and then 6-amino-1,3-dimethyluracil further reacted with barbituric (15) yielded acid (16 intermediates) or thiobarbituricI-6, which acid thenderivatives further (16a reacted) to withproduce barbituric the desired acid (16 products) or thiobarbituric 17. acid derivatives (16a) to produce the desired products 17.

Molecules 2020SchemeScheme, 25, x FOR 7. 7. TBAB-catalyzedPEER TBAB-catalyzed REVIEW synthesissynthesis ofof pyrrolo[2,3-pyrrolo[2,3-d]pyrimidine]pyrimidine derivatives in in ethanol. ethanol. 7 of 25

Scheme 8.8.Plausible Plausible mechanism mechanism of TBAB-catalyzed of TBAB-catalyzed synthesis synthesis of pyrrolo[2,3- of dpyrrolo[2,3-]pyrimidined]pyrimidine derivatives. derivatives.

2.6. Synthesis of 2,3-Dihydroquinazolin-4(1H)-Ones Heterocycles with quinazolin-4-one skeleton have been found to possess a broad range of biological activities [71–74]. Davoodnia et al. [75] achieved the synthesis of a series of 2- arylquinazolin-4(3H)-ones (19) via the cyclocondensation reactions between 2-aminobenzamide (18) and aromatic aldehydes (1) in the presence of a catalytic amount of tetrabutylammonium bromide under microwave-assisted solvent-free conditions at 120 °C (Scheme 9). All the reactions were completed within just four minutes. The desired products were isolated with excellent yields.

Scheme 9. TBAB-catalyzed synthesis of 2,3-dihydroquinazolin-4(1H)-ones under microwave- irradiated solvent-free conditions.

2.7. 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 BF3OEt [77], Ag3PW12O40 [78], PPA-SiO2 [79], zinc montmorillonite [80], Yb(OTf)3 [81], MgO-POCl3 [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) Molecules 2020, 25, x FOR PEER REVIEW 7 of 25

MoleculesScheme2020, 258., 5918 Plausible mechanism of TBAB-catalyzed synthesis of pyrrolo[2,3-d]pyrimidine7 of 24 derivatives.

2.6. Synthesis of 2,3-Dihydroquinazolin-4(1H)-Ones Heterocycles withwith quinazolin-4-one quinazolin-4-one skeleton skeleton have have been been found found to possess to possess a broad rangea broad of biologicalrange of activitiesbiological [71 activities–74]. Davoodnia [71–74]. et Davoodnia al. [75] achieved et al. the synthesis[75] achieved of a series the ofsynthesis 2-arylquinazolin-4(3 of a seriesH )-onesof 2- (arylquinazolin-4(319) via the cyclocondensationH)-ones (19) via reactions the cyclocondensation between 2-aminobenzamide reactions between (18) and 2-aminobenzamide aromatic aldehydes (18) (and1) in aromatic the presence aldehydes of a catalytic (1) in the amount presence of tetrabutylammonium of a catalytic amount bromide of tetrabutylammonium under microwave-assisted bromide solvent-freeunder microwave-assisted conditions at 120 solvent-free◦C (Scheme 9conditions). All the reactions at 120 were°C (Scheme completed 9). withinAll the just reactions four minutes. were Thecompleted desired within products just were four isolatedminutes. with The excellentdesired products yields. were isolated with excellent yields.

Scheme 9.9.TBAB-catalyzed TBAB-catalyzed synthesis synthesis of 2,3-dihydroquinazolin-4(1 of 2,3-dihydroquinazolin-4(1H)-onesH under)-ones microwave-irradiated under microwave- solvent-freeirradiated solvent-free conditions. conditions.

2.7. Synthesis of 1,5-Benzodiazepine Derivatives Benzodiazepine and and related related derivatives derivatives showed showed potent potent pharmaceutical pharmaceutical activities activities [76]. Among[76]. Among many others,many 2,3-dihydro-1others, 2,3-dihydro-1H-1,5-benzodiazepinesH-1,5-benzodiazepines (21) gained (21 much) gained attention. much A attention. number of A methods number were of reportedmethods for were the synthesis reported of thesefor the biologically synthesis promising of these scaffolds biologically utilizing promising numerous homogeneousscaffolds utilizing as well asnumerous heterogeneous homogeneous catalysts, suchas well as BF as3 OEtheterogeneous [77], Ag3PW 12catalysts,O40 [78], such PPA-SiO as BF2 [793OEt], zinc [77], montmorillonite Ag3PW12O40 [78], [80], Yb(OTf)PPA-SiO3 2[ 81[79],], MgO-POCl zinc montmori3 [82], Amberlystllonite [80], [83 ],Yb(OTf) and superacid3 [81], sulphatedMgO-POCl zirconia3 [82], [Amberlyst84]. The use [83], of a toxicand andsuperacid costly catalystsulphated is thezirconia common [84]. drawback The use of of a these toxic reported and costly methods. catalyst In is 2012, the common Baseer and drawback Khan [85 of] synthesizedthese reported a series methods. of structurally In 2012, Baseer diverse and 2,3-dihydro-1 Khan [85]H synthesized-1,5-benzodiazepines a series of (21 structurally) from the reactions diverse of2,3-dihydro-1 one equivalentH-1,5-benzodiazepines of o-phenylenediamine (21 (10) ) andfrom two the equivalents reactions of variousof one acyclic equivalent ketones (20 )of using o- TBAB as an efficient catalyst in ethanol at 60 C (Scheme 10). Under the same optimized conditions, they Moleculesphenylenediamine 2020, 25, x FOR ( PEER10) and REVIEW two equivalents◦ of various acyclic ketones (20) using TBAB as8 of an25 wereefficient also catalyst able to synthesizein ethanol aat series 60 °C of (Scheme spiro-benzodiazepine 10). Under the derivatives same optimized (23) using conditions, cyclic ketones they were (22) insteadalso able ofof acyclicto acyclic synthesize ketones ketones a (Scheme series(Scheme of11 ). spiro-benzodiazepine11). In allIn cases,all case excellents, excellent yieldsderivatives yields of the desiredof(23 the) using desired products cyclic products were ketones obtained were (22) obtainedthrough the through formation the formation of Schiff bases of Schiff followed bases by followed a cyclization by a pathway.cyclization pathway.

Scheme 10. TBAB-catalyzed synthesis of 1,5-benz 1,5-benzodiazepineodiazepine derivatives in ethanol.

2.8. 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 11. TBAB-catalyzed synthesis of spiro-benzodiazepine derivative.

2.8. 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 12. TBAB-promoted synthesis of 5-substituted 1H -tetrazoles.

3. Applications of TBAB for the Synthesis of Bioactive O-Heterocycles

3.1. 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 Molecules 2020, 25, x FOR PEER REVIEW 8 of 25 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.

Molecules 2020, 25, x FOR PEER REVIEW 8 of 25 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.

Scheme 10. TBAB-catalyzed synthesis of 1,5-benzodiazepine derivatives in ethanol.

Molecules 2020, 25, 5918 8 of 24 Scheme 10. TBAB-catalyzed synthesis of 1,5-benzodiazepine derivatives in ethanol.

Scheme 11. TBAB-catalyzed synthesis of spiro-benzodiazepine derivative.

2.8. 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 Schemeproduce 11.11. the TBAB-catalyzed desired products. synthesis Here, of spiro-benzodiazepinesp iro-benzodiazepineTBAB polarized derivative.derivative.the cyano group, which facilitated the attack by the azide ion. 2.8. 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 12. TBAB-promoted synthesis of 5-substituted 1 H -tetrazoles.

3. Applications Applications of of TBAB for the Synthesis of Bioactive O-Heterocycles 3.1. Synthesis of 3-Nitro-2H-Chromenes 3.1. Synthesis of 3-Nitro-2H-Chromenes Synthesis of 3-nitro-2H-chromenes (29) was achieved via the microwave-assisted reactions of Synthesis of 3-nitro-2H-chromenes (29) was achieved via the microwave-assisted reactions of substitutedMolecules 2020 salicylaldehydes, 25, x FOR PEER REVIEW (27) and 2-nitro ethanol (28) using anhydrous potassium carbonate as9 base of 25 substituted salicylaldehydes (27) and 2-nitro ethanol (28) using anhydrous potassium carbonate as in the presence of a catalytic amount of TBAB as catalyst under solvent-free conditions (Scheme 13)[87]. base in the presence of a catalytic amount of TBAB as catalyst under solvent-free conditions The presence of a baseScheme facilitated 12. TBAB-promoted the formation synthesis of carbanion of 5-substituted on the 1H carbon -tetrazoles. atom attached with a (Scheme 13) [87]. The presence of a base facilitated the formation of carbanion on the carbon atom nitro group. After being activated by TBAB, the aldehydic group underwent condensation with the attached with a nitro group. After being activated by TBAB, the aldehydic group underwent in-situ-generated3. Applications of carbanion TBAB for and the the Synthesis resulting of intermediate Bioactive O after-Heterocycles cyclization yielded the corresponding condensation with the in-situ-generated carbanion and the resulting intermediate after cyclization desired products 29. 3.1.yielded Synthesis the corresponding of 3-Nitro-2H-Chromenes desired products 29. 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

SchemeScheme 13.13.TBAB-catalyzed TBAB-catalyzed synthesis synthesis of 3-nitro-2 of 3-nitro-2H-chromenesH-chromenes under microwave-assisted under microwave-assisted conditions. conditions. 3.2. Synthesis of 3,4-Dihydropyrano[c]chromene 3.2. Synthesis3,4-Dihydropyrano[ of 3,4-Dihydropyrano[c]chromenec]chromenes and related derivatives have been found to possess a wide range of biological activities [88–90]. In 2009, Khurana and Kumar [91] reported a facile protocol for the 3,4-Dihydropyrano[c]chromenes and related derivatives have been found to possess a wide range of biological activities [88–90]. In 2009, Khurana and Kumar [91] reported a facile protocol for the synthesis of 3,4-dihydropyrano[c]chromenes (32) via one-pot three-component reactions of various aldehydes (1), malononitrile (30), and 4-hydroxycoumarin (31) in the presence of a catalytic amount of TBAB as an efficient catalyst in water at 100 °C (Scheme 14). The same batches of reactions also afforded excellent yields of the desired products under solvent-free conditions at 120 °C. Aldehydes (1) in the presence of TBAB underwent Knoevenagel condensation with malononitrile (30) and generated the corresponding intermediate I-7 which then further reacted with 4-hydroxycoumarin (31) to produce the desired product 32 (Scheme 15).

Scheme 14. TBAB-catalyzed synthesis of 3,4-dihydropyrano[c]chromenes in water or solvent-free conditions. Molecules 2020, 25, x FOR PEER REVIEW 9 of 25

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.

Scheme 13. TBAB-catalyzed synthesis of 3-nitro-2H-chromenes under microwave-assisted conditions.

Molecules3.2. Synthesis2020, 25 ,of 5918 3,4-Dihydropyrano[c]chromene 9 of 24 3,4-Dihydropyrano[c]chromenes and related derivatives have been found to possess a wide synthesisrange of ofbiological 3,4-dihydropyrano[ activities [88–90].c]chromenes In 2009, (Khurana32) via one-pot and Kumar three-component [91] reported reactionsa facile protocol of various for aldehydesthe synthesis (1), malononitrile of 3,4-dihydropyrano[ (30), and 4-hydroxycoumarinc]chromenes (32) via (31 )one-pot in the presence three-component of a catalytic reactions amount of of various aldehydes (1), malononitrile (30), and 4-hydroxycoumarin (31) in the presence of a catalytic TBAB as an efficient catalyst in water at 100 ◦C (Scheme 14). The same batches of reactions also afforded amount of TBAB as an efficient catalyst in water at 100 °C (Scheme 14). The same batches of excellent yields of the desired products under solvent-free conditions at 120 ◦C. Aldehydes (1) in the presencereactions of also TBAB afforded underwent excellent Knoevenagel yields of the condensation desired products with malononitrile under solvent-free (30) and conditions generated at 120 the corresponding°C. Aldehydes intermediate (1) in theI-7 presencewhich then of further TBAB reactedunderw withent 4-hydroxycoumarinKnoevenagel condensation (31) to produce with themalononitrile desired product (30) 32and(Scheme generated 15). the corresponding intermediate I-7 which then further reacted with 4-hydroxycoumarin (31) to produce the desired product 32 (Scheme 15).

Scheme 14. TBAB-catalyzed synthesis of 3,4-dihydropyrano[c]chromenes in water or solvent-free MoleculesScheme 2020 14., TBAB-catalyzed25, x FOR PEER REVIEW synthesis of 3,4-dihydropyrano[c]chromenes in water or solvent-free conditions.10 of 25 conditions.

CN n-Bu4NBr CN CN CN CN 30 R n-Bu4NBr CN

O n-Bu4NBr O I-7 R H R H 1

NH NH2 H CN CN O N O O R R CN H O O R O O O O

31 32 I-7 Scheme 15. PlausiblePlausible mechanism mechanism of TBAB-catalyzed synthesi synthesiss of 3,4-dihydropyrano[ c]chromenes. 3.3. Synthesis of 4-Phenyl-1,3-Dioxolan-2-One 3.3. Synthesis of 4-Phenyl-1,3-Dioxolan-2-One A combination of graphite carbon nitride and TBAB was used as an efficient catalytic system for A combination of graphite carbon nitride and TBAB was used as an efficient catalytic system the synthesis of 4-phenyl-1,3-dioxolan-2-one (34) from 2-phenyloxirane (33) under carbon-dioxide-filled for the synthesis of 4-phenyl-1,3-dioxolan-2-one (34) from 2-phenyloxirane (33) under carbon- reaction conditions at 105 C (Scheme 16)[92]. Although the reaction took twenty hours to complete, dioxide-filled reaction conditions◦ at 105 °C (Scheme 16) [92]. Although the reaction took twenty it produced 100% yield of the desired product. hours to complete, it produced 100% yield of the desired product.

Scheme 16. Combination of graphite carbon nitride and TBAB-catalyzed synthesis of 4-phenyl-1,3- dioxolan-2-one.

3.4. 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. Molecules 2020, 25, x FOR PEER REVIEW 10 of 25

CN n-Bu4NBr CN CN CN CN 30 R n-Bu4NBr CN

O n-Bu4NBr O I-7 R H R H 1

NH NH2 H CN CN O N O O R R CN H O O R O O O O

31 32 I-7 Scheme 15. Plausible mechanism of TBAB-catalyzed synthesis of 3,4-dihydropyrano[c]chromenes.

3.3. Synthesis of 4-Phenyl-1,3-Dioxolan-2-One A combination of graphite carbon nitride and TBAB was used as an efficient catalytic system for the synthesis of 4-phenyl-1,3-dioxolan-2-one (34) from 2-phenyloxirane (33) under carbon- Moleculesdioxide-filled2020, 25, 5918 reaction conditions at 105 °C (Scheme 16) [92]. Although the reaction took twenty10 of 24 hours to complete, it produced 100% yield of the desired product.

Scheme 16.SchemeCombination 16. Combination of graphite of graphite carbon nitridecarbon andnitride TBAB-catalyzed and TBAB-catalyzed synthesis synthesis of 4-phenyl-1,3-dioxolan-2-one. of 4-phenyl-1,3- dioxolan-2-one. 3.4. Synthesis of Isocoumarin-1-Imines and Isobenzofuran-1-Imines 3.4. Synthesis of Isocoumarin-1-Imines and Isobenzofuran-1-Imines A facile and regioselective TBAB-catalyzed efficient protocol was reported for the synthesis A facile and regioselective TBAB-catalyzed efficient protocol was reported for the synthesis of of a series of isocoumarin-1-imines (36) through the 6-endo-dig oxy-cyclization 2-alkynylbenzamide 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 (35) (Scheme 17) [93]. The reactions were carried out using two equivalents of Oxone as oxidizing agentagent in the in the presence presence of of potassium potassium carbonate carbonate as asbase base in THF-water in THF-water mixture mixture as solvent as solventat 80 °C. atThe 80 ◦C. The plausibleplausible mechanism of of the the transformation transformation is isshown shown in inScheme Scheme 18. 18Under. Under the same the same optimized optimized conditions,conditions, when whenN-phenyl N-phenyl 2-trimethylsilylethynylbenzamides 2-trimethylsilylethynylbenzamides (37) were(37) usedwere asused starting as components,starting the correspondingcomponents, the isobenzofuran-1-imines corresponding isobenzofuran-1-imines (38) were isolated (38) were with isolated excellent with yieldsexcellent (Scheme yields 19). The probable(Scheme 19). mechanistic The probable approach mechanistic of this approach transformation of this transformation is shown in Scheme is shown 20 in. Scheme 20. Molecules 2020, 25, x FOR PEER REVIEW 11 of 25

NHPh Ph 50 mol% TBAB N O 2equiv.Oxone R O R K CO ,THF:H O, 80 oC, 53 85% R1 R1 2 3 2 35 36

R = 5-Me, 5-OMe, 3-Me, 5-F, 5-Cl

1 R =4-CH3C6H4, 4-OMeC6H4,3-MeC6H4,4-FC6H4,4-ClC6H4, 2-thienyl, n-butyl, t-butyl, phenylpropanyl, nonyl

Scheme 17.SchemeTBAB-catalyzed 17. TBAB-catalyzed synthesis ofsynthesis isocoumarin-1-imines of isocoumarin-1-imines via 6-endo-dig via 6- oxyendo-dig-cyclization oxy-cyclization 2-alkynylbenzamide. 2- alkynylbenzamide. 3.5. Synthesis of Tetrahydrobenzo[b]pyran oxone oxone Many tetrahydrobenzo[b]pyransBr are being usedBr as anti-cancer,Br anti-coagulant,2 anti-anaphylactic, diuretic, and spasmolytic agents [94–96]. In 2010, Mobinikhaledi and Fard [97] developed a mild, efficient, and convenient protocol for the synthesis of a number of tetrahydrobenzo[b]pyran derivatives Ph (40) via one-pot three-componentNHPh reactions of substitutedNHPh benzaldehydes (1), malononitrileN (30), and dimedone (39) inO the presenceBr2 of a catalytic amount ofO TBAB as-HBr catalyst in aqueous mediumO under reflux conditionsR (Scheme 21). SubstitutedR benzaldehydes also proceededR smoothly and afforded R1 the desired products with1 excellent yields. Under the same optimized conditions, they were also R R1 H Br synthesized as35 a series of pyrano[2,3-d]pyrimidinoneBr derivatives (41) using barbituric acid (16) instead of dimedone (39) (Scheme 21). Excellent yields, mild reactionBr conditions, water as solvent, and easy HBr purification procedure are some of the majorstable advantages intermediate of this protocol.

Ph Ph N N -Br2 O O R R R1 1 H R Br Br 36

Scheme 18. Plausible mechanism of the formation of isocoumarin-1-imines via 6-endo-dig oxy- cyclization 2-alkynylbenzamide.

Scheme 19. TBAB-catalyzed synthesis of isobenzofuran-1-imines starting from N-phenyl 2- trimethylsilylethynylbenzamides. Molecules 2020, 25, x FOR PEER REVIEW 11 of 25

NHPh Ph 50 mol% TBAB N Molecules 2020, 25, x FORO PEER REVIEW 2equiv.Oxone 11 of 25 R O R NHPh Ph K CO ,THF:H50 mol%O, TBAB 80 oC, 53 85% R1 R1 2 3 2 N O 2equiv.Oxone R 35 O36 R K CO ,THF:H O, 80 oC, 53 85% R1 R = 5-Me, 5-OMe,R 3-Me,1 5-F, 5-Cl2 3 2 36 1 35 R =4-CH3C6H4, 4-OMeC6H4,3-MeC6H4,4-FC6H4,4-ClC6H4, 2-thienyl, n-butyl, R = 5-Me, 5-OMe, 3-Me, 5-F, 5-Cl t-butyl, phenylpropanyl, nonyl 1 SchemeR =4-CH17. TBAB-catalyzed3C6H4, 4-OMeC synthesis6H4,3-MeC of 6Hisocoumarin-1-imines4,4-FC6H4,4-ClC6H4 ,via 2-thienyl, 6-endo-dig n-butyl, oxy-cyclization 2- alkynylbenzamide.t-butyl, phenylpropanyl, nonyl Molecules 2020, 25, 5918 11 of 24 Scheme 17. TBAB-catalyzed synthesis of isocoumarin-1-imines via 6-endo-dig oxy-cyclization 2- alkynylbenzamide. oxone oxone Br Br Br2 oxone oxone Br Br Br2 Ph NHPh NHPh N NHPh Ph O Br2 NHPhO -HBr N O R R R O Br2 O -HBr O R R R R1 1 R R1 H RBr1 1 35 R Br R1 H Br 35 Br Br Br HBr stable intermediate HBr stable intermediate

Ph Ph Ph N N N Ph N -Br-Br2 2 O O OO R R R R 1 1 H R R R11 H R Br Br Br Br 36 36

Scheme 18.SchemePlausible 18. Plausible mechanism mechanism of the of formation the formation of isocoumarin-1-imines of isocoumarin-1-imines via via 6- endo-dig6-endo-dig oxyoxy-- cyclization Scheme 18. Plausible mechanism of the formation of isocoumarin-1-imines via 6-endo-dig oxy- 2-alkynylbenzamide.cyclization 2-alkynylbenzamide. cyclization 2-alkynylbenzamide.

Scheme 19. TBAB-catalyzed synthesis of isobenzofuran-1-imines starting from N-phenyl 2- trimethylsilylethynylbenzamides. SchemeScheme 19. 19.TBAB-catalyzed TBAB-catalyzed synthesissynthesis of isoben isobenzofuran-1-imineszofuran-1-imines starting starting from from N-phenylN-phenyl 2- 2-

trimethylsilylethynylbenzamides.Moleculestrimethylsilylethynylbenzamides. 2020, 25, x FOR PEER REVIEW 12 of 25

oxone oxone Br Br Br2

NHPh NHPh NHPh -TMS O O Br2 O R R R

TMS 37 Br Br stable intermediate

-HBr

Ph Ph Ph N N N

-Br2 HBr R O R O R O

H H 38 H Br Br H Br Scheme 20.SchemePlausible 20. Plausible mechanism mechanism of the of formationthe formation of of isobenzofuran-1-imines isobenzofuran-1-imines starting starting from from N- N-phenyl phenyl 2-trimethylsilylethynylbenzamides. 2-trimethylsilylethynylbenzamides. 3.5. Synthesis of Tetrahydrobenzo[b]pyran Many tetrahydrobenzo[b]pyrans are being used as anti-cancer, anti-coagulant, anti- anaphylactic, diuretic, and spasmolytic agents [94–96]. In 2010, Mobinikhaledi and Fard [97] developed a mild, efficient, and convenient protocol for the synthesis of a number of tetrahydrobenzo[b]pyran derivatives (40) via one-pot three-component reactions of substituted benzaldehydes (1), malononitrile (30), and dimedone (39) in the presence of a catalytic amount of TBAB as catalyst in aqueous medium under reflux conditions (Scheme 21). Substituted benzaldehydes also proceeded smoothly and afforded the desired products with excellent yields. Under the same optimized conditions, they were also synthesized as a series of pyrano[2,3- d]pyrimidinone derivatives (41) using barbituric acid (16) instead of dimedone (39) (Scheme 21). Excellent yields, mild reaction conditions, water as solvent, and easy purification procedure are some of the major advantages of this protocol. Molecules 2020, 25, 5918 12 of 24 Molecules 2020, 25, x FOR PEER REVIEW 13 of 25

O R Me O O Me CN 39 CN Me O NH 30 45 min, 89 95% Me O NH2 40

R = H, 4-Br, 3-Cl, 4-Cl, 2,3-diCl, 2,4-diCl, CHO 4-OH, 3-OH, 3NO2,4-NO2,2-NO2, CN 10 mol% TBAB 2 2 2 CN 10 mol% TBAB 4-Me, 4-N(Me)2, 4-OMe + 2

+ CN CN H2O, reflux R 2 O 30 1 30 HN R

O N O H H O 16 16 CN HN 25 35 min, 81 90% O N O NH2 H R = 4-Br, 3-Cl, 2,3-diCl, 2,4-diCl, 3-OH, 3NO2,4-NO2,4-CN 2 2 41 41 Scheme 21.21. TBAB-catalyzedTBAB-catalyzed synthesissynthesis ofof tetrahydrobenzo[tetrahydrobenzo[b]pyran]pyran andand pyrano[2,3-pyrano[2,3-dd]pyrimidinone]pyrimidinone derivatives in water. 3.6. Synthesis of Xanthones 3.6. Synthesis of Xanthones Rao et al. [98] developed a simple and straightforward aqueous-mediated protocol for the Rao et al. [98] developed a simple and straightforward aqueous-mediated protocol for the intramolecular annulations of 2-aryloxybenzaldehydes (42) which afforded the corresponding intramolecular annulations of 2-aryloxybenzaldehydes (42) which afforded the corresponding xanthones (43) with moderate to excellent yields using tert-butyl hydroperoxide (TBHP) as an 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 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 transformation is shown in Scheme 23. The reaction proceeded through the direct oxidative reactions of C-H bonds of aldehydes and aromatic C-H bonds. Under the same optimized reaction coupling reactions of C-H bonds of aldehydes and aromatic C-H bonds. Under the same optimized conditions, 2-aryloxybenzaldehydes with both electron-donating as well as electron-withdrawing reaction conditions, 2-aryloxybenzaldehydes with both electron-donating as well as electron- substituents showed good tolerance and yielded the corresponding desired products. withdrawing substituents showed good tolerance and yielded the corresponding desired products.

Scheme 22. TBAB-catalyzed synthesis of xanthones from 2-aryloxybenzaldehydes in water at 120 Scheme 22.22. TBAB-catalyzedTBAB-catalyzed synthesissynthesis of of xanthones xanthones from from 2-aryloxybenzaldehydes 2-aryloxybenzaldehydes in waterin water at 120 at 120◦C. °C. Molecules 20202020,, 2525,, 5918x FOR PEER REVIEW 1314 ofof 2425 Molecules 2020, 25, x FOR PEER REVIEW 14 of 25

OH + O O OH OH + O O OH CHO O CHO O O O O O O O Br 1/2 Br2 Br 1/2 Br2 H2O O H2O O OH OH O O H H O O H H O O O Scheme 23. Plausible mechanismO of the formation of xanthones from 2-aryloxybenzaldehydes using SchemeTBABScheme as 23. 23.catalyst. PlausiblePlausible mechanismmechanism ofof thethe formationformation ofof xanthonesxanthones fromfrom 2-aryloxybenzaldehydes2-aryloxybenzaldehydes usingusing TBABTBAB asas catalyst.catalyst. 3.7. Synthesis of Aryl-14H-Dibenzo[a.j]xanthenes 3.7. Synthesis of Aryl-14H-Dibenzo[a.j]xanthenes 3.7. SynthesisBenzoxanthene of Aryl-14H-Dibenzo[a.j]xanthenes and its congeners have shown various biological activities [99–102]. In 2008, KantevariBenzoxantheneBenzoxanthene et al. [103] andand reported itsits congeners a facile have method shown vari variousfor ousthe biological synthesis activities of a series [99–102]. [99– 102of ].aryl-14 In 2008, H- Kantevaridibenzo[Kantevaria et. j]xantheneset al. [al.103 ][103] reported ( 45reported) afrom facile the methoda facilereactions for method the of synthesis two for equivalentsthe of a seriessynthesis of aryl-14of βof-naphthol Ha -dibenzo[series (44ofa.j )]xanthenes aryl-14and oneH- (equivalentdibenzo[45) froma the.j ]xanthenesof reactionsvarious aromatic of(45 two) from equivalents aldehydes the reactions of (1β)-naphthol using of two10 (mol%44equivalents) and TBAB one equivalentasof catalystβ-naphthol ofunder various (44 solvent-free) aromaticand one aldehydesconditionsequivalent ( 1atof) using various125 10°C mol% (Schemearomatic TBAB 24).aldehydes as catalystAll the ( under1reac) usingtions solvent-free 10 were mol% completed conditions TBAB as at withincatalyst 125 ◦ Ceighty (Schemeunder minutes solvent-free 24). All and the reactionsaffordedconditions were81–96% at 125 completed yields.°C (Scheme withinUsing 24). eightyTBAB All minutes theas catalyst,reac andtions affordedthe were same completed 81–96% reactions yields. within when Using eighty carried TBAB minutes out as catalyst, under and themicrowave-irradiatedafforded same reactions81–96% whenyields. conditions carried Using out TBABproduced under as microwave-irradiated catalyst,78–95% yieldsthe same within reactions conditions just six when minutes. produced carried Products 78–95% out yieldsunder were withinisolatedmicrowave-irradiated just by six simple minutes. crystallization Productsconditions were inproduced ethanol. isolated 78–95% by simple yields crystallization within just in ethanol.six minutes. Products were isolated by simple crystallization in ethanol.

Scheme 24.24.TBAB-catalyzed TBAB-catalyzed synthesis synthesis of aryl-14 of Haryl-14-dibenzo[H-dibenzo[a,j]xanthenesa,j]xanthenes under solvent-free under solvent-free conditions. conditions.Scheme 24. TBAB-catalyzed synthesis of aryl-14H-dibenzo[a,j]xanthenes under solvent-free 3.8. Synthesis of 1,8-Dioxo-Octahydroxanthenes conditions. 3.8. SynthesisRecently, of 1,8-dioxo-octahydroxanthenes 1,8-Dioxo-Octahydroxanthenes have gained considerable attention due to their potent pharmacological3.8. SynthesisRecently, of 1,8-dioxo-octahy 1,8-Dioxo-Octahydroxanthenes activities, suchdroxanthenes as anti-cancer, have anti-bacterial, gained considerable anti-microbial, attention and due enzyme to their inhibitory potent propertiespharmacologicalRecently, [104 ].1,8-dioxo-octahy Ezabadiactivities, et such al. [105droxanthenes as ]anti-cancer, synthesized have an ati-bacterial, series gained of considerable 1,8-dioxo-octahydroxanthene anti-microbial, attention and dueenzyme to their derivatives inhibitory potent (propertiespharmacological46) via one-pot [104]. pseudo activities, Ezabadi three-component suchet al.as anti-cancer,[105] reactionssynthe ansizedti-bacterial, between a series oneanti-microbial, equivalentof 1,8-dioxo-octahydroxanthene and of aromatic enzyme aldehydesinhibitory derivativesproperties ([104].46) via Ezabadi one-pot pseudoet al. three-component[105] synthesized reacti a seriesons between of 1,8-dioxo-octahydroxanthene one equivalent of aromatic derivatives (46) via one-pot pseudo three-component reactions between one equivalent of aromatic Molecules 2020, 25, x FOR PEER REVIEW 15 of 25 MoleculesMolecules2020 2020,,25 25,, 5918x FOR PEER REVIEW 1415 ofof 2425 aldehydes (1) and two equivalents of dimedone (39) using TBAB as catalyst under solvent-free aldehydes (1) and two equivalents of dimedone (39) using TBAB as catalyst under solvent-free conditions at 120 °C (Scheme 25). Various substituted aldehydes afforded the desired products with (conditions1) and two at equivalents 120 °C (Scheme of dimedone 25). Various (39) substitu using TBABted aldehydes as catalyst afforded under solvent-freethe desired products conditions with at excellent yields. After being activated by TBAB, aldehydes (1) underwent condensation with one 120excellent◦C (Scheme yields. 25 After). Various being substituted activated aldehydesby TBAB, aaldehydesfforded the (1 desired) underwent products condensation with excellent with yields. one molecule of dimedone (39) and produced the corresponding intermediate I-8 which on further Aftermolecule being of activated dimedone by TBAB,(39) and aldehydes produced (1) underwentthe corresponding condensation intermediate with one moleculeI-8 which of on dimedone further attack by the second molecule of dimedone (39) afforded the desired product 46 at 120 °C (Scheme (attack39) and by produced the second the molecule corresponding of dimedone intermediate (39) affordedI-8 which the on desired further attackproduct by 46 the at second 120 °C molecule (Scheme 26). of26). dimedone (39) afforded the desired product 46 at 120 ◦C (Scheme 26).

Scheme 25. TBAB-catalyzed synthesis of 1,8-dioxo-octahydroxanthenes1,8-dioxo-octahydroxanthenes under neat conditions. Scheme 25. TBAB-catalyzed synthesis of 1,8-dioxo-octahydroxanthenes under neat conditions.

Scheme 26. TBAB-catalyzed synthesis of 1,8-dioxo-octahydroxanthenes under neat conditions. SchemeScheme 26. 26.Plausible TBAB-catalyzed mechanism synthesis of TBAB-catalyzed of 1,8-dioxo-oc synthesistahydroxanthenes of 1,8-dioxo-octahydroxanthenes under neat conditions. under neat conditions. 4. Applications of TBAB for the Synthesis of Bioactive N- as well as O-Heterocycles 4. Applications of TBAB for the Synthesis of Bioactive N- as well as O-Heterocycles 4. Applications of TBAB for the Synthesis of Bioactive N- as well as O-Heterocycles 4.1. Synthesis of Oxazolo[4,5-c]pyridine 4.1.4.1. SynthesisSynthesis ofof Oxazolo[4,5-c]pyridineOxazolo[4,5-c]pyridine Tjosaas et al. [106] designed a microwave-assisted TBAB-catalyzed facile protocol for the Tjosaas et al. [106] designed a microwave-assisted TBAB-catalyzed facile protocol for the efficientTjosaas synthesis et al. of [106 oxazolopyridine] designed a microwave-assisted2-tert-butyl-oxazolo[4,5- TBAB-catalyzedc]pyridine (48 facile) via the protocol cyclization for the of efficient synthesis of oxazolopyridine 2-tert-butyl-oxazolo[4,5-c]pyridine (48) via the cyclization of e4-bromo-3-pivaloylaminopyridinefficient synthesis of oxazolopyridine (47) 2-intert the-butyl-oxazolo[4,5- presence of cesiumc]pyridine carbonate (48 as) viabase. the The cyclization reaction 4-bromo-3-pivaloylaminopyridine (47) in the presence of cesium carbonate as base. The reaction ofwas 4-bromo-3-pivaloylaminopyridine completed within just ten minutes (47 and) in afforded the presence 78% yield of cesium (Scheme carbonate 27). as base. The reaction waswas completedcompleted withinwithin justjust tenten minutesminutes andand aaffordedfforded 78% 78% yield yield (Scheme (Scheme 27 27).). Molecules 2020, 25, 5918 15 of 24 Molecules 2020, 25, x FOR PEER REVIEW 16 of 25 Molecules 2020, 25, x FOR PEER REVIEW 16 of 25

Br H Br TBAB, Cs CO O Me N H tBu 2 3 Me Me N tBu TBAB, Cs2CO3 O Me N O N N O DMSO, MW N Me Me N N DMSO, MW 47 10 min, 78% 48 48 47 10 min, 78% SchemeScheme 27.SchemeTBAB-catalyzed 27. 27.TBAB-catalyzed TBAB-catalyzed synthesis synthesis ofsynthesis oxazolo[4,5- ofof oxazolo[4,5-oxazolo[4,5-c]pyridinec]pyridinec derivatives]pyridine derivatives underderivatives microwave-assisted under under microwave- microwave- conditions. assistedassisted conditions. conditions. 4.2. Synthesis of Pyran-Fused Spirooxindoles 4.2. Synthesis of Pyran-Fused Spirooxindoles 4.2. SynthesisIn 2012, Mobinikhaledi of Pyran-Fused et Spirooxindoles al. [107] prepared a series of structurally diverse pyran-fused spirooxindole In 2012, Mobinikhaledi et al. [107] prepared a series of structurally diverse pyran-fused derivativesIn 2012, using Mobinikhaledi TBAB as catalyst et in al. water [107] under prepared reflux conditions a series atof 100 structurally◦C. They prepared diverse spiro[(4 pyran-fusedH)-5,6,7, spirooxindole derivatives using TBAB as catalyst in water under reflux conditions at 100 °C. They 8-tetrahydrochromene-4,3spirooxindole derivatives0-(3 using0H)-indol]-(1 TBAB 0asH )-2catalyst0-one derivatives in water under (50) via reflux one-pot conditions three-component at 100 °C. reactions They of isatinsprepared (49), malononitrilespiro[(4H)-5,6,7,8-tetrahydrochromene-4,3 (30) or ethyl 2-cyanoacetate′ (-(330a′H),)-indol]-(1 and dimedone′H)-2′-one (39 )derivatives (Scheme 28 ).(50 Synthesis) via of preparedone-pot spiro[(4 three-componentH)-5,6,7,8-tetrahydrochromene-4,3 reactions of isatins (49), malononitrile′-(3′H)-indol]-(1 (30) or′H )-2ethyl′-one 2-cyanoacetate derivatives ( 30a(50), ) via spiro[(3 H)-indol-3 ,4,4(H)-5,6,7,8-tetrahydropyrano(2,3-d)pyrimidine]-(1 H)-2 -one derivatives (51) was one-potand0 three-componentdimedone0 (39 reactions) (Scheme of isatins28). (49Synthesis), malononitrile of spiro[(3 (30) 0or′H ethyl)-indol-30 2-cyanoacetate′,4,4(H)-5,6,7,8- (30a), 16 16a 39 achievedand tetrahydropyrano(2,3-dimedone from the same(39 reactions)d )pyrimidine]-(1(Scheme by using ′H28).)-2 barbituric′ -oneSynthesis derivatives acid derivatives of(51 ) wasspiro[(3 ( ,achieved′)H instead)-indol-3 from of ′,4,4( dimedonethe Hsame)-5,6,7,8- ( ) (Schemetetrahydropyrano(2,3-reactions 28). by Under using thebarbituricd)pyrimidine]-(1 same acid optimized derivatives′H)-2′ reaction-one (16 ,16aderivatives) conditions,instead of ( dimedone51 they) was also (achieved39) prepared (Scheme from 28). a numberUnderthe same of spiro[(3reactionsthe0 Hsame by)-indol-3 usingoptimized0 ,4,4(barbituric Hreaction)-benzo( acid conditions,g )chromen]-(1derivatives they (also016H)-2,16a prepared0-ones) instead ( 53a )number andof dimedone spiro[(3 of spiro[(30H ()-indol-339′H) )-indol-3(Scheme0,4,4(′,4,4( 28).H)-pyranoH )-Under (2,3-the samec)chromen]-(1benzo( optimizedg)chromen]-(10H)-2 reaction0-one′H)-2 derivatives′-ones conditions, (53) and (54 th) spiro[(3 fromey also the′H prepared)-indol-3 reactions′,4,4( a of number isatinsH)-pyrano(2,3- (49of ),spiro[(3 malononitrilec)chromen]-(1′H)-indol-3 (30′H))-2′ or,4,4(′- ethylH)- 2-cyanoacetatebenzo(oneg)chromen]-(1 derivatives (30a), ( and54′H))-2 from 2-hydroxynaphthalene-1,4-dione′-ones the (reactions53) and spiro[(3of isatins′H ()-indol-349), malononitrile (52) or′,4,4( 4-hydroxycoumarineH)-pyrano(2,3- (30) or ethylc )chromen]-(12-cyanoacetate (31), respectively′H )-2′- (30a), and 2-hydroxynaphthalene-1,4-dione (52) or 4-hydroxycoumarine (31), respectively (Scheme (Schemeone derivatives 29). Almost (54) allfrom the the reactions reactions were of completedisatins (49 within), malononitrile one hour and(30) affordedor ethyl excellent2-cyanoacetate yields. 29). Almost all the reactions were completed within one hour and afforded excellent yields. Simple Simple(30a), and reaction 2-hydroxynapht profile, waterhalene-1,4-dione as solvent, high (52)atom or 4-hydroxycoumarine economy, one-pot three-component(31), respectively synthesis, (Scheme and environmentallyreaction profile, water benign as nature solvent, are high some atom of the economy, major advantages one-pot three-component of this developed synthesis, protocol. and 29). Almostenvironmentally all the reactions benign nature were are completed some of the within major oneadvantages hour and of this afforded developed excellent protocol. yields. Simple reaction profile, water as solvent, high atom economy, one-pot three-component synthesis, and environmentally benign nature are some of the major advantages of this developed protocol.

Scheme 28. TBAB-catalyzed synthesis of spiro[(4H)-5,6,7,8-tetrahydrochromene-4,3′-(3′H)-indol]- Scheme 28. TBAB-catalyzed synthesis of spiro[(4H)-5,6,7,8-tetrahydrochromene-4,30-(30H)-indol]-(10H)-20-ones (1′H)-2′-ones and spiro[(3′H)-indol-3′,4,4(H)-5,6,7,8-tetrahydropyrano(2,3-d)pyrimidine]-(1′H)-2′- and spiro[(30H)-indol-30,4,4(H)-5,6,7,8-tetrahydropyrano(2,3-d)pyrimidine]-(10H)-20-ones. ones.

Scheme 28. TBAB-catalyzed synthesis of spiro[(4H)-5,6,7,8-tetrahydrochromene-4,3′-(3′H)-indol]- (1′H)-2′-ones and spiro[(3′H)-indol-3′,4,4(H)-5,6,7,8-tetrahydropyrano(2,3-d)pyrimidine]-(1′H)-2′- ones. Molecules 2020, 25, 5918 16 of 24 MoleculesMolecules 2020 2020, ,25 25, ,x x FOR FOR PEER PEER REVIEW REVIEW 1717 of of 25 25

OO

HOHO OO OO 5252 OO 1010 mol% mol% TBAB TBAB OO NHNH22 oo HH22O,O, 100 100 C,C, 55 55 9090 min min R'R' YY 66 entries, entries, 80 80 85%85% NN OO R'R' = = H, H, 5-Br 5-Br R"R" R"R" = = H, H, CH CH,CH,CHPhPh OO 33 22 5353 CNCN R'R' ++ OHOH NN OO ZZ R"R" OO 4949 3030;Z=CN;Z=CN OO OO OO 30a;Z=CO Et 30a;Z=CO22Et OO 3131 NHNH22 R'R' YY 1010 mol% mol% TBAB TBAB NN OO oo HH22O,O, 100 100 C,C, 40 40 7575 min. min. R"R" 1010 R'R' = = R" R" = = H H 5454 6entries,836entries,8390%90%

SchemeScheme 29.29. TBAB-catalyzed TBAB-catalyzed synthesissynthesis synthesis of ofof spiro[(3 spiro[(3spiro[(30H′H′H)-indol-3)-indol-3)-indol-30′,4,4(,4,4(′,4,4(HHH)-benzo()-benzo()-benzo(ggg)chromen]-(1)chromen]-(1)chromen]-(10′H′H)-2)-20′-ones-ones′-ones andand spiro[(3 spiro[(3′0H′HH)-indol-3)-indol-3)-indol-3′0,4,4(′,4,4(HH)-pyrano(2,3-)-pyrano(2,3-cc)chromen]-(1)chromen]-(1′H0′H)-2)-2′-ones.′0-ones.-ones. 4.3. Synthesis of 1-Trifluoromethyl-3-Alkylidene-1,3-Dihydrofuro[3,4-b]quinolines 4.3.4.3. Synthesis Synthesis of of 1-Trifluoromethyl-3-Alky 1-Trifluoromethyl-3-Alkylidene-1,3-Dihydrofuro[3,4-b]quinolineslidene-1,3-Dihydrofuro[3,4-b]quinolines A simple, facile, and efficient method was designed for the synthesis of 1-trifluoromethyl-3- AA simple,simple, facile,facile, andand efficientefficient methodmethod waswas desidesignedgned forfor thethe synthesissynthesis ofof 1-trifluoromethyl-3-1-trifluoromethyl-3- alkylidene-1,3-dihydrofuro[3,4-b]quinolines (57) via nucleophilic addition followed by cyclization alkylidene-1,3-dihydrofuro[3,4-alkylidene-1,3-dihydrofuro[3,4-bb]quinolines]quinolines ((5757)) viavia nucleophilicnucleophilic additionaddition followedfollowed byby cyclizationcyclization reactions of o-arylalkynylquinoline aldehydes (55) with trimethyl trifluoromethyl silane (56) using TBAB as reactionsreactions ofof oo-arylalkynylquinoline-arylalkynylquinoline aldehydesaldehydes ((5555)) withwith trimethyltrimethyl trifluoromethyltrifluoromethyl silanesilane ((5656)) usingusing catalyst and cesium fluoride as base in toluene at 0 C to ambient temperature (Scheme 30) [108]. All the TBABTBAB as as catalyst catalyst and and cesium cesium fluoride fluoride as as base base in in toluene toluene◦ at at 0 0 °C °C to to ambient ambient temperature temperature (Scheme (Scheme 30) 30) products were obtained in excellent yields. [108].[108]. All All the the products products were were obtained obtained in in excellent excellent yields. yields.

HH 11 HH CFCF RR11 RR 33 OO 1010 mol% mol% TBAB, TBAB, CsF CsF OO ++ TMSCFTMSCF33 RR22 NN 22 toluene,toluene, 0 0ooCC RTRT RR NN RR33 33 ArAr 22 4h,694h,6992%92% RR ArAr 5555 5656 5757 11 22 33 RR =H,Br,CH=H,Br,CH33,OCH,OCH33;R;R== H, H, Cl, Cl, CH CH33,OCH,OCH33;R;R=H,CH=H,CH33,C,C22HH55

ArAr = = C C66HH55,3,5-CF,3,5-CF33CC66HH33,4-FC,4-FC66HH44,, 2-MeC 2-MeC66HH44,, 3-MeC 3-MeC66HH44,, 4-MeC 4-MeC66HH44,2-OMeC,2-OMeC66HH44,, 4-OMeC4-OMeC66HH44,, 2-pyridyl, 2-pyridyl, 3-thienyl 3-thienyl SchemeScheme 30. 30.30.TBAB-catalyzed TBAB-catalyzedTBAB-catalyzed synthesis synthesissynthesis of 1-trifluoromethyl-3-alkylidene1,3-dihydrofuro[3,4- ofof 1-trifluoromethyl-3-alkylidene1,3-dihydrofuro[3,4-1-trifluoromethyl-3-alkylidene1,3-dihydrofuro[3,4-b]quinolines. bb]quinolines.]quinolines. 5. Applications of TBAB for the Synthesis of Bioactive N- as well as S-Heterocycles 5.5. Applications Applications of of TBAB TBAB for for the the Synthesis Synthesis of of Bioactive Bioactive N- N- as as well well as as S S-Heterocycles-Heterocycles Synthesis of 1,3-Thiazine Derivatives SynthesisSynthesisTBAB of of 1,3-Thiazine was1,3-Thiazine found toDerivatives Derivatives be an effi cient catalyst for the synthesis of thiazine derivatives. In 2018, Bankar and Dhankar [109] reported a facile and environmentally benign approach for the synthesis of TBABTBAB waswas foundfound toto bebe anan efficientefficient catalystcatalyst forfor thethe synthesissynthesis ofof thiazinethiazine derivatives.derivatives. InIn 2018,2018, BankarBankar and and Dhankar Dhankar [109] [109] reported reported a a facile facile and and environmentally environmentally benign benign approach approach for for the the synthesis synthesis ofof 5-(2-amino-6-aryl-5,6-dihydro-4 5-(2-amino-6-aryl-5,6-dihydro-4HH-1,3-thiazine-4-yl)-3,4-dihydropyrimidine-2(1-1,3-thiazine-4-yl)-3,4-dihydropyrimidine-2(1HH)-one)-one derivatives derivatives Molecules 2020, 25, 5918 17 of 24 Molecules 2020, 25, x FOR PEER REVIEW 18 of 25 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) 5-(2-amino-6-aryl-5,6-dihydro-4(59) from the reactions of 5-cinnamoyl-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H-1,3-thiazine-4-yl)-3,4-dihydropyrimidine-2(1H)-one derivativesH)-ones ( 5859) and excess thiourea (9a) in the presence of a catalytic amount of TBAB as catalyst in fromand theexcess reactions thiourea of 5-cinnamoyl-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1 (9a) in the presence of a catalytic amount of HTBAB)-ones as (58 )catalyst and excess in dichloromethane-water as biphasic solvent at 50 °C (Scheme 31). All the reactions were completed thioureadichloromethane-water (9a) in the presence as biphasic of a catalytic solvent amount at 50 °C of (Scheme TBAB as 31). catalyst All the in dichloromethane-waterreactions were completed as within just thirty minutes and afforded excellent yields. In the presence of TBAB, the ketonic carbon biphasicwithin just solvent thirty at minutes 50 ◦C (Scheme and affo 31rded). All excellent the reactions yields. were In the completed presence within of TBAB, just thirtythe ketonic minutes carbon and of 58 activated and formed the corresponding Schiff bases (I-9) with the reaction of thiourea (9a). aofff orded58 activated excellent and yields. formed In thethe presencecorresponding of TBAB, Schiff the ketonicbases (I-9 carbon) with of the58 activatedreaction of and thiourea formed (9a the). The in-situ-generated I-9 afforded the desired product by following the cyclization pathway shown correspondingThe in-situ-generated Schiff bases I-9 afforded (I-9) with the the desired reaction product of thiourea by following (9a). The the in-situ-generated cyclization pathwayI-9 aff shownorded in Scheme 32. thein Scheme desired 32. product by following the cyclization pathway shown in Scheme 32.

Scheme 31. TBAB-catalyzed synthesis of 1,3-thiazine-4-yl-3,4-dihydropyrimidine-2(1H)-one Schemederivatives. 31.31.TBAB-catalyzed TBAB-catalyzed synthesis synthesis of 1,3-thiazine-4-yl-3,4-dihydropyrimidine-2(1 of 1,3-thiazine-4-yl-3,4-dihydropyrimidine-2(1H)-one derivatives.H)-one derivatives. R1 R1 R1 R1 R 1 R1 R R S R R S R R H2N NH2 H N 9aNH 2 2 S HN 9a S O S HN HN N S HN N O NH NH2 O N CH3 2 H HN N HN N O N CH NH O N CH3 NH2 O N CH3 3 2 H H H 58 O N CH O N CH3 n-Bu4NBr I-9 3 58 H H 59 n-Bu NBr I-9 4 59 SchemeScheme 32. 32.Plausible Plausible mechanism mechanism of TBAB-catalyzed of TBAB-catalyzed synthesis of 1,3-thiazine-4-yl-3,4-dihydropyrimidine-synthesis of 1,3-thiazine-4-yl-3,4- 2(1Schemedihydropyrimidine-2(1H)-one derivatives.32. PlausibleH )-onemechanism derivatives. of TBAB-catalyzed synthesis of 1,3-thiazine-4-yl-3,4- dihydropyrimidine-2(1H)-one derivatives. InIn the the same same year, year, Khurana Khurana andand hishis groupgroup [[110]110] reportedreported anotheranother TBAB-catalyzed facile facile and and convenientconvenientIn the protocolsame protocol year, for for theKhurana the effi efficientcient and synthesis synthesis his group of of a seriesa[110] series ofreported of novel novel benzo[ another benzo[e][1,3]thiazinese ][1,3]thiazinesTBAB-catalyzed (65 ()65 via )facile via one-pot one- and pseudoconvenientpot pseudo four-component protocol four-component for reactionsthe efficient reactions between synthesis between one of equivalent a oneseries equivalent of of novel substituted benzo[of substituted anilinese][1,3]thiazines (60 anilines), two (65 equivalents ()60 via), twoone- ofpotequivalents formaldehyde pseudo four-componentof formaldehyde (61), and one reactions (61 equivalent), and between one of equivalent thiophenols one equivalent of (62thiophenols) underof substituted solvent-free (62) under anilines conditionssolvent-free (60), two at 100equivalentsconditions◦C (Scheme atof 100 33formaldehyde ).°C Under (Scheme the 33). same(61 ),Under optimizedand theone same equivalent conditions, optimized of a numberconditions,thiophenols of naphtho[1,2- a number(62) under ofe ][1,3]thiazinenaphtho[1,2- solvent-free derivativesconditionse][1,3]thiazine (at66 100) werederivatives °C also(Scheme synthesized (66 33).) were Under from also the the synthesized same reactions optimized between from conditions,the anilines reactions (60 a), numberbetween formaldehyde of anilines naphtho[1,2- (61 ),(60 and), 2-thionaphthole][1,3]thiazineformaldehyde derivatives ((6361),), whereasand 2-thionaphthol (66 reactions) were also between (63 ),synthesized whereas anilines reactions (from60), formaldehyde thebetween reactions anilines ( 61between), ( and60), 1-thionaphtholformaldehyde anilines (60), (formaldehyde64(61)), a ffandorded 1-thionaphthol the (61), corresponding and 2-thionaphthol (64) afforded naphtho[2,1- the (63 corresponding), wherease][1,3]thiazine reactions naphtho[2,1- derivatives betweene][1,3]thiazine anilines (67) with (60 ), excellentderivatives formaldehyde yields (67) (Scheme(61with), and excellent 331-thionaphthol). Use yields of a metal-free(Scheme (64) afforded 33 catalyst,). Use the ofcorresponding excellent a metal-free yields, naphtho[2,1-catalyst, broad substrateexcellente][1,3]thiazine yields, scope, andbroad derivatives solvent-free substrate (67 ) conditionswithscope, excellent and are solvent-free someyields of(Scheme the conditions major 33). advantages Use are ofsome a metal-free ofof this theprotocol. major catalyst, advantages TBAB excellent facilitated of yields, this theprotocol.broad formation substrate TBAB of Schiscope,facilitatedff basesand the fromsolvent-free formation the reactions conditionsof Schiff of anilinesbases are fromsome (60 )the andof reactionsthe formaldehyde major of anilinesadvantages (61 ),(60 which) ofand this formaldehyde on furtherprotocol. reaction TBAB (61), withfacilitatedwhich thiophenol on thefurther formation yielded reaction of the with Schiff intermediate thiophenol bases from I-10yielded the. Thereactions the formation intermediate of anilines of anotherI-10 (60. The) and intermediate formation formaldehyde of I-11another (was61), accomplishedwhichintermediate on further viaI-11 thereaction was reaction accomplished with of I-10thiophenolwith via the yielded secondthe reaction the molecule intermediate of ofI-10 formaldehyde. with I-10 . theThe secondformation Cyclization molecule of followed another of byintermediateformaldehyde. aromatization I-11 Cyclization of wasI-11 yieldedaccomplished followed the desired byvia aromatization productsthe reaction65 (Scheme of of I-11 I-10 34yielded ).with the desiredsecond productsmolecule 65of (Scheme 34). formaldehyde. Cyclization followed by aromatization of I-11 yielded the desired products 65 (Scheme 34). Molecules 2020, 25, x FOR PEER REVIEW 19 of 25 Molecules 2020, 25, 5918 18 of 24

Molecules 2020, 25, x FOR PEER REVIEW 19 of 25

Scheme 33. TBAB-catalyzed synthesis of benzo[e][1,3]thiazines, naphtho[1,2-e][1,3]thiazines and SchemeScheme 33. 33. TBAB-catalyzedTBAB-catalyzed synthesissynthesis ofof benzo[benzo[ee][1,3]thiazines,][1,3]thiazines, naphtho[1,2-naphtho[1,2-e][1,3]thiazines][1,3]thiazines and and naphtho[2,1-e][1,3]thiazine derivatives. naphtho[2,1-naphtho[2,1-e][1,3]thiazinee][1,3]thiazine derivatives. derivatives.

SH SH n-Bu4NBr R NH2 N n-Bu4NBr R NH S NH2 O N 62 NH S + O 62 + H H -H2O Nucleophilic attack H R R and isomerisation H H -H2O Nucleophilic attack I-10 H R R and isomerisation 60 61 I-10

60 61 -H2O HCHO 61 -H2O HCHO 61 Cyclization R N S N S Cyclization R N S H N S

I-11 65 H Scheme 34. Plausible mechanism of TBAB-catalyzed synthesis of benzo[e][1,3]thiazines, 65 I-11 naphtho[1,2-e][1,3]thiazines and naphtho[2,1-e][1,3]thiazine derivatives. SchemeScheme 34. 34.Plausible Plausible mechanism mechanism of TBAB-catalyzed of TBAB-catalyzed synthesis ofsy benzo[nthesise][1,3]thiazines, of benzo[e][1,3]thiazines, naphtho[1,2-e] [1,3]thiazinesnaphtho[1,2- ande][1,3]thiazines naphtho[2,1- ande][1,3]thiazine naphtho[2,1- derivatives.e][1,3]thiazine derivatives. Molecules 2020, 25, 5918 19 of 24

6. Conclusions Recently,TBAB has been regarded as an efficient, environmentally benign, and versatile phase-transfer catalyst. As a result, a huge number of TBAB-promoted protocols are available in the literature. In this review, we have highlighted the catalytic efficiency of this fascinating catalyst for the synthesis of various biologically promising heterocycles. It was evident the synthesis of 1,4-dihydropyridines, 2-substituted imidazolines, 2,4,5-triaryl imidazoles, 1,5-benzodiazepine derivatives, 4-phenyl-1,3-dioxolan-2-one, 5-substituted 1H-tetrazoles, 1,3-dihydrobenzimidazol-2-ones, 3,4-dihydropyrano[c]chromene, 3-nitro-2H-chromenes, 1,3-thiazines, and many other heterocyclic scaffolds can be achieved under mild reaction conditions by using TBAB as catalyst.

Funding: This research received no external funding. Acknowledgments: B.K.B. is grateful to Prince Mohammed Bin Fahd University, Kingdom of Saudi Arabia, for encouragement. B.B. is grateful to Indus International University, Himachal Pradesh, India, as well as Kartha Education Society, Mumbai, India. Conflicts of Interest: The authors declare no conflict of interest.

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