Synthesis of highly functionalized thiazolo[3,2-a]pyridine derivatives via a five-component cascade reaction based on nitroketene N,S-acetal

A highly efficient and straightforward synthesis of N-fused heterocyclic compounds including 5-amino-7-(aryl)-8-nitro-N'-(1-(aryl)ethylidene)-3,7-dihydro-2H-thiazolo[3,2-a]pyridine-6-carbohydrazide derivatives is successfully achieved via a five-component cascade reaction utilizing cyanoacetohydrazide, various acetophenones, aromatic aldehydes, 1,1-bis(methylthio)-2-nitroethylene and cysteamine hydrochloride in ethanol at reflux conditions. The new approach involves domino N,S-acetal formation, Knoevenagel condensation, Michael reaction, imine–enamine tautomerization and N-cyclization sequences. The prominent advantages of this protocol include: facility of operation, available and economical starting materials, no need for toxic solvents, high yields and tolerance of a wide variety of functional groups.


Introduction
The thiazolopyridine moiety is found in a wide spectrum of biologically active compounds. Thiazolo [3,2-a]pyridines are an important category with notable antibacterial and antifungal activity 1 and other considerable bioactivities including as a beta-amyloid production inhibitor, 2 potent CDK2-cyclin A inhibitor, 3 potential uterus stimulant, 4 coronary dilator, antihypertensives, and muscle relaxant. 5 Also they are useful for chemotherapy of various cancers, such as leukemia, lung cancer, and melanoma. [6][7][8] Some biologically active compounds with this nucleus are presented in Fig. 1. [9][10][11] Obviously, the synthesis of new classes of thiazolo[3,2-a] pyridines may give a library of compounds as possible candidates for various biological activities.
Cyclic ketene N,S-acetal structures are as such used as drugs for the treatment of hypertension diseases and usually employed as probes for nucleic acids to study the interaction between G4 (G-quadruplex) and its ligands (Fig. 1, I-III). 12 It's interesting that the cyclic nitroketene N,S-acetal nithiazine IV was the rst reported compound of neonicotinoid insecticides 13 and is widely used as a common insecticide around the world (Fig. 1). Synthetically, the cyclic nitroketene N,S-acetals have a rigid structure and act as Michael donor 1,3-N,C dinucleophiles for the generation of nitrogen-containing heterocyclic compounds. The ethylene motif has a polarized pushpull type of alkene, therefore the one end expands an electrophilic character, whereas the other end develops a nucleophilic character. This feature of nitroketene N,S-acetals make them highly useful to apply in the Michael addition, annulation and multicomponent reactions. 14 Today, multicomponent reactions (MCRs) have become a prominent strategy and are selected over stepwise synthesis due to the following reasons: reduced synthetic time, labor and cost, minimal utilization of toxic and harmful chemicals, simple workup of products, high yields, straight forward and simplicity of experimental procedures and economic viability; therefore, MCRs are a powerful approach to promotion of green chemistry by reducing the formation of large quantities of waste. [15][16][17][18][19] The ve-membered cyclic nitroketene N,S-acetal and commercially available six-membered nithiazine have been remarkably explored in the literature and their reactions with different Michael acceptors are most expected. Here we report the some synthesis of thiazolo[3,2-a]pyridine compounds performed with cyclic ketene N,S-acetals (Scheme 1). In 2005, Chakrabarti et al. described the reactions between diverse cyclic N,S-and N,N-ketene acetals and itaconic anhydride (A). 20 In 2010, Yan et al. reported one-pot synthesis of functionalized bicyclic pyridines under solvent-and catalyst-free conditions with triethoxymethane, ethyl 4,4,4-triuoro-3-oxobutanoate and various ketene aminals (B). 21 In 2011, Altug et al. developed the synthesis of thiazolo[3,2-a]pyridines via a one-pot reaction between 2-(nitromethylene)thiazolidine, aromatic aldehydes and ethyl 2-cyanoacetate, malononitrile or 2-(phenylsulfonyl) acetonitrile (C). 22 In 2018, our research group synthesized fused thiazolo[3,2-a] pyridines utilizing the ve-membered cyclic nitroketene N,Sacetal, dimedone and different aromatic aldehydes (D). 23 Also we reported the synthesis of indenone-fused thiazolo [3,2-a] pyridines via a one-pot reaction between 2-(nitromethylene) thiazolidine, aromatic aldehydes and 1,3-indandione (E). 24 In addition, we were able to produce the desired products using cyanoacetamide, aromatic aldehydes and 2-(nitromethylene) thiazolidine (F). 25 Moreover, in 2018, the reaction of cyanoacetohydrazide with aromatic aldehydes and 2-(nitromethylene) thiazolidine/oxazolidine resulted in functionalized thiazolo/ oxazolo pyridine derivatives (G). 26 Following our efforts to synthesize the new heterocyclic compounds using cyanoacetohydrazide and based on previous works, we designed new reactions utilizing 2-(nitromethylene) thiazolidine as heterocyclic ketene aminal. In this article we report an efficient synthesis of highly functionalized 2H-thiazolo[3,2-a]pyridine-6-carbohydrazide compounds via a one-pot ve-component domino reaction. To the best of our knowledge, there is no report on the synthesis of these structures.
In general, due to the variable reactivity of cyanoacetohydrazide (based on its specic structure) and on the other hand due to the ve-component nature of the dened reactions, great efforts were made to obtain the desired products with high purity. At rst ethanol was examined and the experimental results showed when ethanol was used as solvent with triethylamine at reux conditions, the yield of desired product 6a was 93% (Table 1, entry 1). It should be noted that the catalyst used (NEt 3 ) is not working on the rate-limiting step. To prepare 2-(nitromethylene)thiazolidine solution (from 1,1bis(methylthio)-2-nitroethene and cysteamine hydrochloride, which is mentioned in the Experimental section), it is necessary to use triethylamine to separate cysteamine from its salt. 23,27 No reaction will occur without the use of triethylamine (entry 4). The use of other catalysts is related to the whole reaction.
In order to increase the reaction rate, two types of catalysts were used. With piperidine, the reaction efficiency decreased slightly (entry 2) and with acetic acid, the product did not form (entry 3). According to the investigations, it was determined that in basic and acidic medium, other products are formed (two-, three-and four-component products). The percentage of each was different for various derivatives. In general, it was found that the slightest change in the reaction conditions (even in ethanol amount) leads to a decrease in the efficiency of the desired product or oen its non-formation. In addition, we observed the formation of a four-component by-product in two cases, which are described in the general procedure section. However, we also studied the effect of other solvents. The use of water or acetonitrile did not result in the desired product (entry 5 and 7), and when the mixture of water and ethanol was used (overall 1 : 1, v/v), the efficiency decreased (entry 6). With chloroform, methanol and DMF, in reux conditions the desired products were not formed (entry 8, 9 and 10). With information obtained from optimization conditions table, we could synthesize target compounds (E)-5-amino-7-(aryl)-8-nitro-N'-(1-(aryl)ethylidene)-3,7-dihydro-2H-thiazolo[3,2a]pyridine-6-carbohydrazide 6a-p in good to high yields (70-95%) using cyanoacetohydrazide 1, acetophenone derivatives 2, various aromatic aldehydes 3, 1,1-bis(methylthio)-2-nitroethene 4 and cysteamine hydrochloride 5 as starting materials (Scheme 2).
The reactions were completed aer 24 h to afford the corresponding heterocyclic structures. The results are summarized in Table 2.

Scope and limitations
This reaction was performed with ortho derivatives of benzaldehyde (2-chloro, 2-hydroxy and 2-nitro) under the same conditions, which did not result in the product probably due to steric effects. Also the use of acetophenone and 4-methoxyacetophenone did not lead to the favorable products. The reaction was also used with aliphatic ketones instead of acetophenone derivatives and aliphatic aldehydes instead of aromatic aldehydes which resulted in no product formation.
It was found that the major by-product of this reaction is a fourcomponent structure that was previously synthesized using two equivalents of aldehyde 26 which will prevent its formation by performing the correct reaction steps (see Experimental section).

Structure determination
The structures of all new compounds 6a-p were supported by means of IR, 1 H NMR, 13 C NMR spectroscopic and mass spectrometric data (see the ESI †).
The 1 H NMR spectrum of 6a showed NH group at d 9.35 ppm. The NH 2 group appeared at d 8.15 ppm. The proton of CH at pyridine ring was seen at d 5.66 ppm. Four protons of two methylene groups appeared at d 4.22 to 4.38 ppm as two multiplets. The signal at d 2.11 ppm was related to methyl group.
The 1 H-decoupled 13 C NMR spectrum of 6a indicated 18 distinct resonances in accordance to desired structure. The characteristic signals of four aliphatic carbons (CH 3 , CH and two CH 2 groups) were seen at d 13.8, 37.7, 27.6 and 50.8 ppm respectively. Characteristic signal at d 81.8 ppm was related to C]C-CO. The carbonyl group appeared at d 165.7 ppm (Fig. 2).
The IR spectrum of 6a showed absorption bands at 3141 and 3284 cm À1 due to NH and NH 2 groups, strong absorption of carbonyl group at 1626 and C-N band at 1237 cm À1 . Two absorption bands due to nitro group appeared at 1514 and 1302 cm À1 .

Mechanism
A general plausible mechanism for the formation of thiazolo [3,2-a]pyridine carbohydrazides is shown in Scheme 3. The condensation of cyanoacetohydrazide 1 with acetophenone 2 leads to the hydrazide-hydrazone structures 7. On the basis of well-established chemistry of 1,1-bis(methylthio)-2-nitroethene, on the other hand, addition of cysteamine hydrochloride 5 to 1,1-bis(methylthio)-2-nitroethene 4 leads to the formation of ketene N,S-acetal 9. 23,27 The formation of b-nitrothiazolidine 9 occurs in the presence of an equivalent amount of triethylamine base for releasing cysteamine salt. Further, with adding aldehyde 3, the Knoevenagel condensation affords intermediate 8.

6-carbohydrazide derivatives
A mixture of cysteamine hydrochloride (0.113 g, 1 mmol), 1,1bis(methylthio)-2-nitroethylene (0.165 g, 1 mmol), Et 3 N (140 mL, 1 mmol) and 10 mL EtOH in a 50 mL ask was reuxed for 5 hours. In another 50 mL ask the stoichiometric mixture of cyanoacetohydrazide (1 mmol, 0.099 g) and acetophenone derivative (1 mmol) in EtOH (10 mL) was reuxed for 3-5 hours depending on the type of acetophenone. Aer these times, TLC shows the consumption of the starting components. Then, aromatic aldehyde (1 mmol) and the rst solution (HKA), were added to the second mixture simultaneously. The progress of the reaction was monitored by TLC using ethyl acetate/n-hexane (1 : 1). Aer completion of the reaction (24 hours), without the need for chromatography or recrystallization, the precipitated product was collected by ltration and washed with warm ethanol to give the pure products 6a-p in excellent yield. To achieve the pure products, it was necessary to complete the reaction of cyanoacetohydrazide and acetophenone derivatives in ethanol at reux conditions in sufficient time (3 hours for 4-nitroacetophenone and 5 hours for 4-chloro and 4-bromoacetophenone), then with no need for product separation, nitroenamine solution and aromatic aldehyde were added to two-component hydrazone mixture at the same time. We found two distinct cases (6g and 6h) that led to a mixture of two products: the desired product and the product without participation of acetophenone derivative. 26 (E)-5-Amino-7-(4-chlorophenyl)-N'-(1-(4-chlorophenyl)ethylidene)-8-nitro-3,7-dihydro-2H-thiazolo[3,2-a]pyridine-6carbohydrazide (6a). Yellow solid; yield: 0.468 g (93%); mp: 252-  Scheme 3 Proposed mechanism for the formation of products 6.