Simple and Expedient Access to Novel Fluorinated Thiazolo- and Oxazolo[3,2-a]pyrimidin-7-one Derivatives and Their Functionalization via Palladium-Catalyzed Reactions

An efficient, versatile, and one-pot method for the preparation of novel fluorinated thiazolo- and oxazolo[3,2-a]pyrimidin-7-ones is described from 2-aminothiazoles or 2-amino-oxazoles and fluorinated alkynoates. This transformation, performed under transition-metal-free conditions, offers new fluorinated cyclized products with good to excellent yields. Moreover, the functionalization of these N-fused scaffolds via the Suzuki-Miyaura and Sonogashira cross-coupling reactions led to the synthesis of highly diverse thiazolo- and oxazolo[3,2-a]pyrimidin-7-ones.

In recent years, fluorinated heterocycles have acquired a crucial role in the pharmaceutical and agrochemical industries due to their wide range of biological properties [18,19]. In addition, fluorinated compounds are present in about 20% of drugs [20][21][22]. In general, the introduction of fluorine atoms or fluoroalkyl (perfluoroalkyl) groups into heterocyclic compounds increases lipophilicity, stability, solubility, reactivity, and biological properties [23][24][25][26][27]. Accordingly, the combination of two pharmacophoric entities such as the thiazolo-or oxazolo[3,2-a]pyrimidin-7-one scaffold and perfluoroalkyl groups could be an effective method to enhance the biological activity of these heterocycles.
Despite the numerous approaches described in the literature for the synthesis of non-fluorinated thiazolo-or oxazolo[3,2-a]pyrimidin-7-one derivatives, the incorporation of fluorinated groups into these scaffolds has never been reported to date. Thus, the development of a simple and efficient method using readily available starting materials to access new fluorinated thiazolo-and oxazolo[3,2-a]pyrimidin-7-one derivatives is highly desired.
Despite the numerous approaches described in the literature for the synthesis of nonfluorinated thiazolo-or oxazolo[3,2-a]pyrimidin-7-one derivatives, the incorporation of fluorinated groups into these scaffolds has never been reported to date. Thus, the development of a simple and efficient method using readily available starting materials to access new fluorinated thiazolo-and oxazolo[3,2-a]pyrimidin-7-one derivatives is highly desired.

Results and Discussion
In order to find the optimal reaction conditions for the synthesis of the target compounds, 2-aminothiazole, 1a, and ethyl 4,4,4-trifluorobut-2-ynoate, 2a, were selected as model substrates for the development of this condensation/heterocyclisation reaction by varying different conditions (solvents, temperature, and catalyst). The reaction conditions investigated leading to thiazolo[3,2-a]pyrimidin-7-one 3a are summarized in Table 1. report here a simple and regioselective synthesis of a novel series of fluorinated thiazolo and oxazolo[3,2-a]pyrimidin-7-one derivatives, by [3+3] cyclocondensation of 2-aminothiazoles or 2-amino-oxazoles with fluorinated alkynoates (Figure 2c). This simple synthetic strategy, which relies on the use of easily prepared fluorinated alkynoates [48] and of commercially available 2-aminothiazoles and oxazoles, represents a highly efficient one-step route and regioselective access to fluorinated thiazolo-and oxazolo[3,2-a]pyrimidin-7-ones.

Results and Discussion
In order to find the optimal reaction conditions for the synthesis of the target compounds, 2-aminothiazole, 1a, and ethyl 4,4,4-trifluorobut-2-ynoate, 2a, were selected as model substrates for the development of this condensation/heterocyclisation reaction by varying different conditions (solvents, temperature, and catalyst). The reaction conditions investigated leading to thiazolo[3,2-a]pyrimidin-7-one 3a are summarized in First, the reaction was carried out using a variety of solvents without any catalyst. The use of a non-polar solvent, such as toluene, gave only traces of the desired product 3a ( Table 1, entry 1). The use of aprotic polar solvents such as DCE, THF, 1,4-dioxane, MeCN, DMF, and DMSO resulted in very low to moderate yields of 3a (Table 1, entries 2-7). Switching to a protic polar solvent such as H2O did not provide any improvement in the conversion of the starting material, and the expected product 3a was isolated with a yield not exceeding 28% ( First, the reaction was carried out using a variety of solvents without any catalyst. The use of a non-polar solvent, such as toluene, gave only traces of the desired product 3a (Table 1, entry 1). The use of aprotic polar solvents such as DCE, THF, 1,4-dioxane, MeCN, DMF, and DMSO resulted in very low to moderate yields of 3a (Table 1, entries 2-7). Switching to a protic polar solvent such as H 2 O did not provide any improvement in the conversion of the starting material, and the expected product 3a was isolated with a yield not exceeding 28% ( Table 1, entry 8), presumably due to the low solubility of the starting materials in water. In contrast, a significant improvement in the yield of target product 3a occurred, reaching 72%, when the reaction was performed with ethanol as the reaction solvent (Table 1, entry 9). To our delight, when the cyclocondensation reaction was run in MeOH as a solvent, the expected product was obtained with a good yield reaching 88% ( but did not improve the reaction yield and provided the cyclized product 3a with yields not exceeding 70% (Table 1, entries [13][14][15][16][17][18][19]. The molecular structure of the new compound 3a was unambiguously confirmed by X-ray crystal analysis, as depicted in Figure 3 [49]. was run in MeOH as a solvent, the expected product was obtained with a good yield reaching 88% (Table 1, entry 10). Increasing the reaction temperature to 100 °C led to a decrease in the yield of compound 3a (Table 1, entry 11). At room temperature and under the same conditions, the reaction proceeded with a very low yield and incomplete conversion of 1a after 72 h (Table 1, entry 12). Further optimizations were undertaken using various catalysts, including AgSO3CF3, Ag2CO3, AgOAc, Cu(OAc)2, Pd(Oac)2, ZnCl2, and CuBr, but did not improve the reaction yield and provided the cyclized product 3a with yields not exceeding 70% (Table 1, entries [13][14][15][16][17][18][19]. The molecular structure of the new compound 3a was unambiguously confirmed by X-ray crystal analysis, as depicted in Figure 3 [49]. The results are summarized in Scheme 1. As shown in Scheme 1, activated alkynes substituted with different fluorinated groups (CF3, C2F5, CF2Ar) reacted efficiently with 2-aminothiazole 1a to generate novel fluorinated thiazolo[3,2-a]pyrimidin-7-ones 3a-d with yields ranging from 65 to 88%. It is noteworthy that the presence of an electron-withdrawing atom such as bromine at the 4-position of 2-aminoathiazole did not prevent the success of this condensation/lactamization reaction, providing the brominated cyclized products 3e-g with yields of 63%, 67%, and 62%, respectively. These brominated compounds could serve as key intermediates to access novel functionalized thiazolo[3,2-a]pyrimidin-7-ones. With the optimized conditions in hand [1a (1 equiv), 2a (1.3 equiv), MeOH, 70 • C, 12 h], the substrate scope and limitation of the [3+3] cyclocondensation reaction were investigated using various 2-amino-thiazole or oxazole derivatives and fluorinated alkynes. The results are summarized in Scheme 1. As shown in Scheme 1, activated alkynes substituted with different fluorinated groups (CF 3 , C 2 F 5 , CF 2 Ar) reacted efficiently with 2-aminothiazole 1a to generate novel fluorinated thiazolo[3,2-a]pyrimidin-7-ones 3a-d with yields ranging from 65 to 88%. It is noteworthy that the presence of an electron-withdrawing atom such as bromine at the 4-position of 2-aminoathiazole did not prevent the success of this condensation/lactamization reaction, providing the brominated cyclized products 3e-g with yields of 63%, 67%, and 62%, respectively. These brominated compounds could serve as key intermediates to access novel functionalized thiazolo[3,2-a]pyrimidin-7-ones. The synthetic scope of this reaction was also successfully extended to benzo[d]thiazol-2-amine derivatives to access new fluorinated tricyclic compounds 3h-i in good yields (61 and 60%, respectively). It should be noted that introducing a substituent such as a bromine atom on the aromatic ring of the benzo[d]thiazol-2-amine had a negligible effect on the reactions, offering the possibility of diversifying the range of ben-Scheme 1. Substrate scope studies. The synthetic scope of this reaction was also successfully extended to benzo[d]thiazol-2-amine derivatives to access new fluorinated tricyclic compounds 3h-i in good yields (61 and 60%, respectively). It should be noted that introducing a substituent such as a bromine atom on the aromatic ring of the benzo[d]thiazol-2-amine had a negligible effect on the reactions, offering the possibility of diversifying the range of benzo[d]thiazolo[3,2a]pyrimidin-7-ones.
To further extend the synthetic scope of this reaction, we sought to examine the reactivity of other 1,3-bis nucleophile reagents, such as 2-amino oxazole derivatives. Interestingly, 2-amino-oxazole derivatives were also efficiently cyclocondensed with fluorinated alkynoates (substituted with a CF 3 , C 2 F 5 , or CF 2 -Ar group) to give the expected products 3jl with a yield ranging from 60 to 68%. Continuing the evaluation of the cyclocondensation process, treatment of 2-aminobenzo-oxazole derivatives with ethyl 4,4,4-trifluorobutynoate under the same reaction conditions provided the corresponding oxazolo[3,2-a]pyrimidin-7-ones 3m and 3n with yields of 61% and 56%, respectively. Again, the presence of a chlorine atom on the aromatic ring had no significant influence on the efficiency of the reaction. It is noteworthy that 1 H, 19 F, and 13 C NMR spectrum analysis of the crude mixture of all examples confirmed that no trace of the second regioisomer is observed, showing full regioselectivity of the [3+3] cyclocondensation process. Thus, this synthetic process appears efficient and versatile for the regioselective synthesis of new thiazolo-and oxazolo[3,2-a]pyrimidin-7-ones 3a-n bearing fluoroalkyl groups.
Functionalization via the Palladium-catalyzed cross-coupling reaction of the C-Br bond at position 2 of compounds 3e-f will allow the preparation of a large library of novel thiazolo[3,2-a]pyrimidin-7-ones with a high structural diversity [50][51][52]. First, compounds 3e-f were subjected to Suzuki-Miyaura cross-coupling under known standard reaction conditions [53-55] using 1.2 equiv of boronic acid, 10 mol% of PdCl 2 (PPh 3 ) 2 , and 2 equiv of Na 2 CO 3 in a 1,4-dioxane/water (4/1) mixture at 80 • C for 1 h. Under these conditions, these coupling reactions allowed complete conversion of the starting materials and efficient access to the new 2-arylated 5-fluorinated thiazolo[3,2-a]pyrimidin-7-ones 4a-l (Scheme 2). As shown in Scheme 2, different boronic acids bearing electron-donating or electron-withdrawing groups on the aromatic ring provided the arylated products 4a-l in good to excellent yields. Notably, phenylboronic acid was successfully coupled with compound 3e leading to the arylation product 4a with a yield of 91%. Arylboronic acids bearing electron-donating groups, such as a methoxy group in the ortho, meta, or pa-As shown in Scheme 2, different boronic acids bearing electron-donating or electronwithdrawing groups on the aromatic ring provided the arylated products 4a-l in good to excellent yields. Notably, phenylboronic acid was successfully coupled with compound 3e leading to the arylation product 4a with a yield of 91%. Arylboronic acids bearing electron-donating groups, such as a methoxy group in the ortho, meta, or para-position, were readily coupled with 3e to provide the corresponding products 4b (82%), 4c (92%), and 4d (95%), respectively. It should be noted that a slight drop in yield was observed in the case of compound 4b, presumably due to the steric hindrance generated by the methoxy group on the ortho-position. As expected, the coupling reaction of compound 3f carrying a pentafluoroethyl group with p-methoxyphenylboronic acid led to the arylated product 4e with a moderate yield of 55%. This result seems to indicate that the nature of the fluoroalkyl group plays a significant role in the efficiency of this coupling reaction. Moreover, the coupling reactions of compound 3e with 1,4-dimethoxyboronic acid and 2,3-ethylenedioxyboronic acid were easily converted to the desired products 4f and 4g with yields of 98% and 60%, respectively. Interestingly, it is worth noting that the reaction was tolerant of the free amino group at the meta position of the phenylboronic acid, providing the corresponding compound 4h in a yield of 52%. Likewise, phenylboronic acid substituted at the para or meta position with an electron-withdrawing group such as CF 3 yielded the expected products 4i (75%) and 4j (57%), respectively. Gratifyingly, the coupling reaction was extended to a heteroarylboronic acid such as 3-thiophenyl, providing the cross-coupling product 4l in excellent yield (95%).
Additionally, phenylacetylene substituted at the 4-position with an electron-donating group, such as methoxy, generated the coupled product 5b in a yield of 65%. Similarly, coupling with 4-chlorophenylacetylene provided the desired product 5c in 58% of yield, showing that an electron-withdrawing group at position 4 of the aromatic ring is also tolerated. We also briefly investigated the effect of the position of the fluorine atom on the aromatic ring on the efficiency of the coupling reaction. Overall, the electron-withdrawing atom fluor at the para or ortho position was tolerated to give rise to the alkynylated products 5d (85%) and 5e (68%), respectively, although a lower yield was observed when the fluorine atom was positioned in the ortho position, which is probably due to steric hindrance. Finally, when aliphatic alkynes, such as 1-hexyne and ethynylcyclohexane were subjected to the standard reaction conditions, the expected products 5g and 5h were obtained in yields of 75% and 55%, respectively.
As shown in Schemes 1-3, a wide variety of fluorinated thiazolo-and oxazolo[3,2a]pyrimidin-7-ones were efficiently prepared with a large substrate scope and with good to excellent yields (34 examples). All products are new and have been fully characterized by 1 H, 19 F, and 13 C NMR spectroscopy and HRMS.
Some synthesized derivatives were subjected to in vitro evaluation on human hMAO-A and hMAO-B by using p-tyramine as a nonspecific substrate. Screening of the compounds was done at 100 µM and 10 µM, and compounds with residual activities (RAs) below 50% at 100 µM were subjected to IC 50 determination using a serial dilution of the inhibitors. The results expressed in IC 50 showed that compound 3g selectively inhibited human monoamine oxidase A (hMAO-A) with an IC 50 value of 54.08 µM ( Table 2).

General Information
All reactions were performed under an inert atmosphere of argon in glassware equipped with a magnetic stir bar. Solvents for reactions were ob Thermo Fisher Scientific in extra dry quality and stored under argon over ac sieves. All reagents were purchased from Fluorochem and used as received ditional purification. Reactions were monitored by thin-layer chromatogr analysis using silica gel 60 F254 plates. All products were visualized by expo light (longwave at 365 nm or shortwave at 254 nm). Column chromatograph formed using silica gel 60 (230-400.13 mesh, 0.040-0.063 mm). Eluents were the standard methods before each use. All new compounds were characteriz spectroscopy ( 1 H, 19 F, and 13 C), high-resolution mass spectroscopy (HRMS), a point (if solids). NMR spectra were recorded at 300 MHz for 1 H, 282 MHz fo MHz for 13 C with a Bruker ® 300 MHz NMR spectrometer. Proton and carbo resonance spectra ( 1 H NMR and 13 C NMR) were recorded using tetramethyls as an external standard and CDCl3 (7.28 ppm for 1 H NMR and 77.04 ppm for 1 DMSO-d6 (2.50 ppm for 1 H NMR and 40.0 ppm for 13 C NMR) as internal sta spectra were unreferenced. Data for NMR are reported as follows: chemical sh multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, sep = septet, m = m br = broad resonance) and coupling constants J are reported in Hertz (Hz). A spectra were processed in MestReNova. HRMS experiments were performed tandem quadrupole/time-of-flight (Q-TOF) instrument, equipped with a pn 20/67 IC 50 = 54.08 ± 10.45 µM n.a. c a IC50 values are means ± standard error of the mean (SEM) of three independent experiments, each performed in duplicate, b RA, residual activity; c n.a., not active (RA at 100 µM ≥ 50%).

General Information
All reactions were performed under an inert atmosphere of argon in oven-dried glassware equipped with a magnetic stir bar. Solvents for reactions were obtained from Thermo Fisher Scientific in extra dry quality and stored under argon over activated 3 Å sieves. All reagents were purchased from Fluorochem and used as received without additional purification. Reactions were monitored by thin-layer chromatography (TLC) analysis using silica gel 60 F254 plates. All products were visualized by exposure to UV light (longwave at 365 nm or shortwave at 254 nm). Column chromatography was performed using silica gel 60 (230-400.13 mesh, 0.040-0.063 mm). Eluents were distilled by the standard methods before each use. All new compounds were characterized by NMR spectroscopy ( 1 H, 19 F, and 13 C), high-resolution mass spectroscopy (HRMS), and melting point (if solids). NMR spectra were recorded at 300 MHz for 1 H, 282 MHz for 19 F, and 75 MHz for 13 C with a Bruker ® 300 MHz NMR spectrometer. Proton and carbon magnetic resonance spectra ( 1 H NMR and 13 C NMR) were recorded using tetramethylsilane (TMS) as an external standard and CDCl 3 (7.28 ppm for 1 H NMR and 77.04 ppm for 13 C NMR) or DMSO-d6 (2.50 ppm for 1 H NMR and 40.0 ppm for 13 C NMR) as internal standards. 19 F spectra were unreferenced. Data for NMR are reported as follows: chemical shift (δ ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, sep = septet, m = multiplet and br = broad resonance) and coupling constants J are reported in Hertz (Hz). All the NMR spectra were processed in MestReNova. HRMS experiments were performed on a hybrid tandem quadrupole/time-of-flight (Q-TOF) instrument, equipped with a pneumatically assisted electrospray (Z-spray) ion source (Micromass, Manchester, UK) operated in the positive mode.

Conclusions
In conclusion, we have developed a simple and convenient method for straightforward access to an important range of fluorinated thiazolo[3,2-a]pyrimidin-7-ones and oxazolo[3,2a]pyrimidin-7-ones. We first established a concise one-pot strategy for the synthesis of 5-fluoroalkylated thiazolo -and oxazolo[3,2-a]pyrimidin-7-ones by condensation of 2-amino thiazole or 2-amino oxazole derivatives with fluorinated ethyl propiolates. The synthesized 2-bromo-5-trifluoromethyl thiazolo[3,2-a]pyrimidin-7-ones were used as building blocks for the synthesis of a series of new 2-arylated and 2-alkynylated thiazolo[3,2-a]pyrimidin-7ones containing a fluoroalkyl group. A preliminary biological evaluation carried out on a few synthesized compounds on human hMAO-A and hMAO-B showed that compound 3g exhibits a selective micromolar inhibition of hMAO-A, which is a promising target in the symptomatic treatment and potentially disease-modifying treatment of neurodegenerative disorders.
Further exploration of this strategy and further evaluation of the biological potential of the synthesized compounds are currently under investigation in our laboratory.