Catalyst-Free Synthesis of Polysubstituted 5-Acylamino-1,3-Thiazoles via Hantzsch Cyclization of α-Chloroglycinates

A catalyst-free heterocyclization reaction of α-chloroglycinates with thiobenzamides or thioureas leading to 2,4-disubstituted-5-acylamino-1,3-thiazoles has been developed. The methodology provides straightforward access to valuable building blocks for pharmaceutically relevant compounds.


Introduction
Heterocyclic compounds are an integral part of many biologically active small molecules. Indeed, many currently marketed drugs exhibit heterocycles as their core structures [1,2]. In particular, compounds based on a 1,3-thiazole display a wide range of activities [3]. Therefore, increasing attention has been devoted in recent years to the preparation of polysubstituted thiazoles, primarily for pharmaceutical applications [4][5][6][7], but also in connection with problems in material science [8]. Of special relevance in medicinal chemistry are aminothiazoles and their derivatives [9][10][11][12][13][14][15][16]. Such compounds show potential in oncology [17,18], in the treatment of inflammatory conditions [19,20] and neurological disorders [21]. Examples ( Figure 1) include compound 1, an experimental CDK5 inhibitor for the treatment of Alzheimer's disease [22], and avatrombopag 2, approved in 2018 the treatment of adult thrombocytopenia [23]. The research described herein finds its genesis in Dompé Farmaceutici's identification of novel thiazole derivatives such as 3 (Figure 2), with proven efficacy in the urology and pain areas [4,24]. As a consequence of this discovery, congeners of 3 incorporating alkylamino-or acylamino substituents, i.e., substances 4-5, became of special interest. Curiously, such thiazoles are scantly documented in The research described herein finds its genesis in Dompé Farmaceutici's identification of novel thiazole derivatives such as 3 ( Figure 2), with proven efficacy in the urology and pain areas [4,24]. As a consequence of this discovery, congeners of 3 incorporating alkylamino-or acylamino substituents, i.e., substances 4-5, became of special interest. Curiously, such thiazoles are scantly documented in the literature. For instance, the SciFinder database records only 47 compounds of the type 4, described in 11 publications as of this writing [25,26]. Substances of general structure 5 are even rarer (11 compounds, 6 publications) [27,28]. Furthermore, good synthetic procedures that lead directly to compounds 4-5 are lacking. Possibly for these reasons, such heterocycles are quite uncommon in medicinal chemistry. Figure 1. Examples of Biologically Active Compounds that Incorporate Acylamino-1,3-thiazoles.
The research described herein finds its genesis in Dompé Farmaceutici's identification of novel thiazole derivatives such as 3 ( Figure 2), with proven efficacy in the urology and pain areas [4,24]. As a consequence of this discovery, congeners of 3 incorporating alkylamino-or acylamino substituents, i.e., substances 4-5, became of special interest. Curiously, such thiazoles are scantly documented in the literature. For instance, the SciFinder database records only 47 compounds of the type 4, described in 11 publications as of this writing [25,26]. Substances of general structure 5 are even rarer (11 compounds, 6 publications) [27,28]. Furthermore, good synthetic procedures that lead directly to compounds 4-5 are lacking. Possibly for these reasons, such heterocycles are quite uncommon in medicinal chemistry. Our interest in developing general methods for the synthesis of pharmaceutically relevant heterocyclic compounds [29][30][31] induced us to launch a program aiming to establish widely applicable procedures for the direct synthesis of the desired thiazoles. In drug discovery, the chemical modifications of thiazole ring moieties could be a useful tool in the discovery of new ways to make variations on existing drugs. But this approach is limited for organic chemists because there are only so many changes that can be made to a complex heterocyclic compound. The cyclization of polyfunctionalized acyclic precursors is much more advantageous for medical and biotechnological applications [32]. Taking into account a potential industrial development of the methodology, it was essential to avoid harsh reaction conditions, issues of regioselectivity that may result in the formation of multiple products, the need for costly catalysts, elaborate reaction protocols, and complex purification procedures.

Results
Among the numerous methods for thiazole synthesis [33][34][35], the venerable Hantzsch reaction [36] and its variants [37,38], i.e., the cyclocondensation of α-halocarbonyl compounds with thioamides or thioureas (Scheme 1, Equation 1), remains especially popular. This transformation reliably produces 1,3-thiazoles having alkyl, aryl, or heterocyclic substituents in good to excellent yields. Furthermore, the reaction requires no metallic catalysts, expensive reagents, or stringent measures to exclude moisture and air: a significant advantage in terms of environmental impact and total cost of the synthetic procedure. It appeared that the target compounds 4-5 could be accessed by a Hatzsch- Our interest in developing general methods for the synthesis of pharmaceutically relevant heterocyclic compounds [29][30][31] induced us to launch a program aiming to establish widely applicable procedures for the direct synthesis of the desired thiazoles. In drug discovery, the chemical modifications of thiazole ring moieties could be a useful tool in the discovery of new ways to make variations on existing drugs. But this approach is limited for organic chemists because there are only so many changes that can be made to a complex heterocyclic compound. The cyclization of polyfunctionalized acyclic precursors is much more advantageous for medical and biotechnological applications [32]. Taking into account a potential industrial development of the methodology, it was essential to avoid harsh reaction conditions, issues of regioselectivity that may result in the formation of multiple products, the need for costly catalysts, elaborate reaction protocols, and complex purification procedures.

Results
Among the numerous methods for thiazole synthesis [33][34][35], the venerable Hantzsch reaction [36] and its variants [37,38], i.e., the cyclocondensation of α-halocarbonyl compounds with thioamides or thioureas (Scheme 1, Equation (1)), remains especially popular. This transformation reliably produces 1,3-thiazoles having alkyl, aryl, or heterocyclic substituents in good to excellent yields. Furthermore, the reaction requires no metallic catalysts, expensive reagents, or stringent measures to exclude moisture and air: a significant advantage in terms of environmental impact and total cost of the synthetic procedure. It appeared that the target compounds 4-5 could be accessed by a Hatzsch-like reaction between an α-chloroglycinate, 8, and a thioamide, 9, or thiourea, 11 (Scheme 2). Compounds 8 are readily available starting with a Ben-Ishai addition of a primary amide, 6, to, e.g., ethyl glyoxylate, followed by reaction of the resultant 7 with SOCl 2 [39][40][41][42]. They are perfectly isolable and fairly stable on storage at −20 • C with exclusion of moisture (two weeks at least) [43][44][45][46][47], even though the halogen atom is quite labile. Also, they are normally obtained is a state of good to excellent purity; therefore, it is generally expedient to use them directly. A caveat is that they are sensitive to the action of bases, which cause rapid formation of polymeric products [41]. A noteworthy illustration of this was provided in connection with their use in a useful oxazole synthesis: displacement of the chlorine with a poorly basic aluminum acetylide results in the efficient formation of polysubstituted oxazoles, but the action of basic alkali metal acetylides rapidly converts them into intractable mixtures of products [43][44][45][46][47]. On such grounds, it seemed plausible that poorly basic, but highly S-nucleophilic, thioamides/thioureas should combine with chlorogycinates 8 as desired. like reaction between an α-chloroglycinate, 8, and a thioamide, 9, or thiourea, 11 (Scheme 2). Compounds 8 are readily available starting with a Ben-Ishai addition of a primary amide, 6, to, e.g., ethyl glyoxylate, followed by reaction of the resultant 7 with SOCl2 [39][40][41][42]. They are perfectly isolable and fairly stable on storage at −20 °C with exclusion of moisture (two weeks at least) [43][44][45][46][47], even though the halogen atom is quite labile. Also, they are normally obtained is a state of good to excellent purity; therefore, it is generally expedient to use them directly. A caveat is that they are sensitive to the action of bases, which cause rapid formation of polymeric products [41]. A noteworthy illustration of this was provided in connection with their use in a useful oxazole synthesis: displacement of the chlorine with a poorly basic aluminum acetylide results in the efficient formation of polysubstituted oxazoles, but the action of basic alkali metal acetylides rapidly converts them into intractable mixtures of products [43][44][45][46][47]. On such grounds, it seemed plausible that poorly basic, but highly S-nucleophilic, thioamides/thioureas should combine with chlorogycinates 8 as desired. The exploration of the new methodology started with a study of the reaction of Nbenzoylchloroglycine ethyl ester, 8a, with thiobenzamide, 9a, (Scheme 3). When a solution of the reactants in THF was stirred at room temperature overnight, a precipitate appeared. This material consisted (NMR, MS) of a mixture of tautomers 10aa and 4aa of the expected product [48]. Unfortunately, the yield of product never exceeded 40%, regardless of solvent used (THF, DMF, and MeCN). Also, conduct of the reaction at higher temperatures (refluxing conditions) resulted in formation of complex mixtures. An HPLC-MS analysis of the reaction mixtures showed the presence of a dimer of tentative structure 13, the formation of which is attributable to water contamination of the solvents. The formation of presumed 13 was accelerated substantially when hydroxyglycinate 7a, R 1 = Ph, was exposed to the CeCl3 . 7H2O-NaI system [49] in an attempt to effect conversion into the corresponding iodide. Fortunately, the use of freshly dried THF suppressed the formation of the dimeric product and greatly improved the yield of thiazoles. Furthermore, it transpired that it was best to allow the reaction to proceed at r.t. for only 2 h. In all cases, the workup procedure involved the removal of volatiles under vacuum, the resuspension of the solid residue in ether, and the recovery of the solid product by filtration. The thiazoles thus obtained were of excellent quality and required no further purification. Some of them existed in solution as mixtures of keto (10) and enol Compounds 8 are readily available starting with a Ben-Ishai addition of a primary amide, 6, to, e.g., ethyl glyoxylate, followed by reaction of the resultant 7 with SOCl2 [39][40][41][42]. They are perfectly isolable and fairly stable on storage at −20 °C with exclusion of moisture (two weeks at least) [43][44][45][46][47], even though the halogen atom is quite labile. Also, they are normally obtained is a state of good to excellent purity; therefore, it is generally expedient to use them directly. A caveat is that they are sensitive to the action of bases, which cause rapid formation of polymeric products [41]. A noteworthy illustration of this was provided in connection with their use in a useful oxazole synthesis: displacement of the chlorine with a poorly basic aluminum acetylide results in the efficient formation of polysubstituted oxazoles, but the action of basic alkali metal acetylides rapidly converts them into intractable mixtures of products [43][44][45][46][47]. On such grounds, it seemed plausible that poorly basic, but highly S-nucleophilic, thioamides/thioureas should combine with chlorogycinates 8 as desired.

Scheme 2. Hypothetical Hantzsch-type Route to the Target Thiazoles 4-5.
The exploration of the new methodology started with a study of the reaction of Nbenzoylchloroglycine ethyl ester, 8a, with thiobenzamide, 9a, (Scheme 3). When a solution of the reactants in THF was stirred at room temperature overnight, a precipitate appeared. This material consisted (NMR, MS) of a mixture of tautomers 10aa and 4aa of the expected product [48]. Unfortunately, the yield of product never exceeded 40%, regardless of solvent used (THF, DMF, and MeCN). Also, conduct of the reaction at higher temperatures (refluxing conditions) resulted in formation of complex mixtures. An HPLC-MS analysis of the reaction mixtures showed the presence of a dimer of tentative structure 13, the formation of which is attributable to water contamination of the solvents. The formation of presumed 13 was accelerated substantially when hydroxyglycinate 7a, R 1 = Ph, was exposed to the CeCl3 . 7H2O-NaI system [49] in an attempt to effect conversion into the corresponding iodide. Fortunately, the use of freshly dried THF suppressed the formation of the dimeric product and greatly improved the yield of thiazoles. Furthermore, it transpired that it was best to allow the reaction to proceed at r.t. for only 2 h. In all cases, the workup procedure involved the removal of volatiles under vacuum, the resuspension of the solid residue in ether, and the recovery of the solid product by filtration. The thiazoles thus obtained were of excellent quality and required no further purification. Some of them existed in solution as mixtures of keto (10) and enol The exploration of the new methodology started with a study of the reaction of N-benzoylchloroglycine ethyl ester, 8a, with thiobenzamide, 9a, (Scheme 3). When a solution of the reactants in THF was stirred at room temperature overnight, a precipitate appeared. This material consisted (NMR, MS) of a mixture of tautomers 10aa and 4aa of the expected product [48]. Unfortunately, the yield of product never exceeded 40%, regardless of solvent used (THF, DMF, and MeCN). Also, conduct of the reaction at higher temperatures (refluxing conditions) resulted in formation of complex mixtures. An HPLC-MS analysis of the reaction mixtures showed the presence of a dimer of tentative structure 13, the formation of which is attributable to water contamination of the solvents. The formation of presumed 13 was accelerated substantially when hydroxyglycinate 7a, R 1 = Ph, was exposed to the CeCl 3 .7H 2 O-NaI system [49] in an attempt to effect conversion into the corresponding iodide. Fortunately, the use of freshly dried THF suppressed the formation of the dimeric product and greatly improved the yield of thiazoles. Furthermore, it transpired that it was best to allow the reaction to proceed at r.t. for only 2 h. In all cases, the workup procedure involved the removal of volatiles under vacuum, the resuspension of the solid residue in ether, and the recovery of the solid product by filtration. The thiazoles thus obtained were of excellent quality and required no further purification. Some of them existed in solution as mixtures of keto (10) and enol (4) tautomers (NMR). The keto form exhibited a diagnostic 3 J coupling between the C-5 and the NH protons (≈7.4 Hz), consistent with literature values in related systems [50]. The enol form may be the dominant/exclusive tautomer present in the solid state, as suggested by the broad OH signal observed in the FT-IR spectrum (see Supplementary Materials). Representative examples of the new transformation are shown in Table 1. It is apparent that the reaction tolerates both electron-donating and electron-withdrawing substituents on either reactant (entries 5, 8 and 10).
It is worthy of note that chloroglycinates derived from conjugated amides are good substrates for the present reaction (entry 4), even though they are quite poor for the oxazole-forming one [43][44][45][46][47]. It should also be stressed that the procedure is readily amenable to high-throughput chemical synthesis and that its scope was found to be considerably broader than the 12 examples of Table 1 suggest.
Thus, various points of diversification can be introduced to generate more complex molecules with interesting biological activities. It is worthy of note that chloroglycinates derived from conjugated amides are good substrates for the present reaction (entry 4), even though they are quite poor for the oxazole-forming one [43][44][45][46][47]. It should also be stressed that the procedure is readily amenable to high-throughput chemical synthesis and that its scope was found to be considerably broader than the 12 examples of Table 1 suggest. Thus, various points of diversification can be introduced to generate more complex molecules with interesting biological activities. On a side note, substituted 2-thiazolinones / 2-hydroxythiazoles are subject to acid-catalyzed ring mutation reactions [51][52][53]. Concerns about the possible sensitivity of 5-thiazolinones / 5hydroxythiazoles 10/4 to analogous isomerization processes were rapidly allayed by the observation that all such compounds remained unchanged upon storage for several weeks at low temperature.   Table 1. It is apparent that the reaction tolerates both electron-donating and electron-withdrawing substituents on either reactant (entries 5, 8 and 10). It is worthy of note that chloroglycinates derived from conjugated amides are good substrates for the present reaction (entry 4), even though they are quite poor for the oxazole-forming one [43][44][45][46][47]. It should also be stressed that the procedure is readily amenable to high-throughput chemical synthesis and that its scope was found to be considerably broader than the 12 examples of Table 1 suggest. Thus, various points of diversification can be introduced to generate more complex molecules with interesting biological activities. On a side note, substituted 2-thiazolinones / 2-hydroxythiazoles are subject to acid-catalyzed ring mutation reactions [51][52][53]. Concerns about the possible sensitivity of 5-thiazolinones / 5hydroxythiazoles 10/4 to analogous isomerization processes were rapidly allayed by the observation that all such compounds remained unchanged upon storage for several weeks at low temperature. On a side note, substituted 2-thiazolinones/2-hydroxythiazoles are subject to acid-catalyzed ring mutation reactions [51][52][53]. Concerns about the possible sensitivity of 5-thiazolinones/5-hydroxythiazoles 10/4 to analogous isomerization processes were rapidly allayed by the observation that all such compounds remained unchanged upon storage for several weeks at low temperature.
The use of a thiourea in lieu of a thioamide in the reaction just described successfully led to the formation of compounds 5 in moderate to good yield ( Table 2). No improvement in yields was observed when the reaction was carried out in the presence of 1,8-bis-(dimethylamino)naphthalene (proton sponge) [54]. The rate of product formation was also unaffected, providing additional evidence that the target thiazoles do not form by an initial dehydrohalogenation of 8 to an acylimine and subsequent nucleophilic addition thereto. Instead, they are likely to arise upon cyclization of intermediates 14 (Scheme 4, reaction pathway a), formed in turn by displacement of chlorine from 8 by the nucleophilic sulfur center of the thioamide. Interestingly, all attempts to detect 14 or other possible intermediates by ESI-MS techniques [55,56] met with failure (only reactants and products apparent in the spectra), indicating that the cyclization of 14 to 12/5 must be very fast. We note in passing that substance 14 could theoretically produce heterocycle 15 by a cyclization reaction involving the amide group (pathway b). However, no products of the type 15 were ever observed in our reactions, undoubtedly because of the weaker electrophilic reactivity of the amide relative to the ester and the lack of aromatic character in 15.  The use of a thiourea in lieu of a thioamide in the reaction just described successfully led to the formation of compounds 5 in moderate to good yield ( Table 2). No improvement in yields was observed when the reaction was carried out in the presence of 1,8-bis-(dimethylamino)naphthalene (proton sponge) [54]. The rate of product formation was also unaffected, providing additional evidence that the target thiazoles do not form by an initial dehydrohalogenation of 8 to an acylimine and subsequent nucleophilic addition thereto. Instead, they are likely to arise upon cyclization of intermediates 14 (Scheme 4, reaction pathway a), formed in turn by displacement of chlorine from 8 by the nucleophilic sulfur center of the thioamide. Interestingly, all attempts to detect 14 or other possible intermediates by ESI-MS techniques [55,56] met with failure (only reactants and products apparent in the spectra), indicating that the cyclization of 14 to 12/5 must be very fast. We note in passing that substance 14 could theoretically produce heterocycle 15 by a cyclization reaction involving the amide group (pathway b). However, no products of the type 15 were ever observed in our reactions, undoubtedly because of the weaker electrophilic reactivity of the amide relative to the ester and the lack of aromatic character in 15.  Et (8c) 4-CH3CO-C6H4 (11d) 12cd d + 5cd 80 a Typical procedure: a thiourea (1.0 mmol) was added to a stirred solution of α-chloroglycinate (1.0 mmol) in dry THF under nitrogen. After 2 h, the solvent was removed under reduced pressure, and the residue was re-suspended in Et2O and stirred for 1 h. The solid was collected by filtration to obtain a thiazole that required no further purification. b Equilibrium mixture of keto (12) and enol (5) form. c As a mixture of tautomers. d Predominant tautomer in DMSO-d6.
The use of a thiourea in lieu of a thioamide in the reaction just described successfully led to the formation of compounds 5 in moderate to good yield ( Table 2). No improvement in yields was observed when the reaction was carried out in the presence of 1,8-bis-(dimethylamino)naphthalene (proton sponge) [54]. The rate of product formation was also unaffected, providing additional evidence that the target thiazoles do not form by an initial dehydrohalogenation of 8 to an acylimine and subsequent nucleophilic addition thereto. Instead, they are likely to arise upon cyclization of intermediates 14 (Scheme 4, reaction pathway a), formed in turn by displacement of chlorine from 8 by the nucleophilic sulfur center of the thioamide. Interestingly, all attempts to detect 14 or other possible intermediates by ESI-MS techniques [55,56] met with failure (only reactants and products apparent in the spectra), indicating that the cyclization of 14 to 12/5 must be very fast. We note in passing that substance 14 could theoretically produce heterocycle 15 by a cyclization reaction involving the amide group (pathway b). However, no products of the type 15 were ever observed in our reactions, undoubtedly because of the weaker electrophilic reactivity of the amide relative to the ester and the lack of aromatic character in 15.  In conclusion, a Hantzsch construction of thiazoles 4-5 and 18 through the reaction of α-chloroglycinate esters and congeners with thioamides or thioureas has been established. The target compounds are obtained under mild conditions from readily available, inexpensive building blocks through an environmentally benign process that requires no stringent control of reaction parameters/atmosphere and no catalysts. The medicinal chemistry of the products is being actively researched and pertinent results will be reported in due course.

General
All reagents and solvents were purchased from commercial suppliers and used without further purification, except THF (freshly distilled over metallic sodium) and DCM (freshly distilled over CaCl 2 ). All reactions were performed under nitrogen atmosphere. All glassware was oven dried at 100 • C for at least 2 h prior to use. Merck pre-coated TLC plates (silica gel 60 GF254 0.25mm) furnished by Merck KGaA (Darmstadt, Germany) were used for thin-layer chromatography (TLC). Compounds were visualized under UV light, or in an iodine, chamber, or by staining with phosphomolybdic acid solution. Proton (400 MHz), 13 C (100 MHz), and 135DEPT spectra were recorded on a Varian Mercury 400 (Varian, Inc., Palo Alto, CA, USA). Chemical shifts are reported in ppm from TMS and are referenced to solvent signals (CDCl 3 : 7.26 ppm for the residual protio species in 1 H, 77.2 ppm in 13 C; DMSO-d 6 : 2.50 ppm in 1 H and 39.5 ppm in 13 C). Coupling constants, J, are reported in hertz (Hz). Splitting patterns are described as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet). IR spectra (cm −1 ) were recorded with a Perkin-Elmer FT-IR spectrometer Spectrum Two UATR (Perkin Elmer, Inc., Waltham, MA, USA). Low-resolution ESI/APCI mass spectra were recorded with an Agilent 1100 MSD ion-trap mass spectrometer (Agilent Technologies, Inc., Santa Clara, CA, USA) equipped with a standard ESI/APCI source. Nitrogen served both as the nebulizer gas and the dry gas. The analyte (10 mg) was dissolved in the appropriate mobile phase (1 mL) and introduced by direct infusion with a syringe pump. High-resolution mass spectra (HRMS) were obtained with a HPLC Ultimate 3000 (Thermofisher Scientific, MA, USA) coupled with a high-resolution Q Exactive Benchtop Quadrupole-Orbitrap (Thermofisher Scientific, MA, USA). The NMR spectra of compounds were provided in Supplementary  Materials (Figures S1-S66).

General Procedure for the Synthesis of α-Hydroxyglycinates (7)
An amide (1.0 mmol) was added to a solution of ethyl glyoxylate (technical, 50% solution in toluene, 1.2 eq) in toluene (1 mL) and the reaction was stirred overnight at 70 • C. The next morning a white precipitate had appeared. The solvent was removed under reduced pressure and the residue was suspended in Et 2 O. The precipitate of α-hydroxyglycinate ester was recovered by filtration and found to be pure enough for the next step. Yields were generally quantitative. The following compounds were thus prepared from appropriate amides:

General Procedure for the Synthesis of α-Chloroglycinates (8)
Thionyl chloride (10 eq) was added dropwise to a suspension of a hydroxyglycinate (7) (1 mmol) in dry DCM (1 mL) under nitrogen. The mixture was warmed to 40 • C and the progress of the reaction was periodically checked by 1 H-NMR. Full conversion typically required about 3 h. Excess thionyl chloride was removed under high vacuum and the residue of crude chloride, yellowish solid, was immediately used in subsequent coupling reactions without further purification to avoid degradation. Yields were essentially quantitative. Since the compounds are unstable in water solution it was not possible to perform an HPLC-MS analysis. The following compounds were thus prepared:  The suspension was stirred for 1 h, then the solid product was collected by filtration. This material was of excellent quality and required no further purification unless otherwise specified. The following thiazoles were thus obtained: N-(4-hydroxy-2-phenyl-1,3-thiazol-5-yl)benzamide (

General Procedure for the Synthesis of 5-Amido-2-Amino Thiazoles 5 and Their Keto Tautomers (12)
A thiourea (1 mmol) was added to a solution of a chloroglycinate 8 (1.0 mmol) in dry THF (2 mL) under nitrogen and the reaction was stirred at room temperature for 2 h, whereupon a precipitate appeared. The solvent was removed under reduced pressure and the residue was resuspended in Et 2 O. The suspension was stirred for 1 h, then the solid thiazole was collected by filtration. This material was of excellent quality and required no further purification unless otherwise specified. The following thiazoles were thus obtained: