A Straightforward and High-Yielding Synthesis of 1,2,4-Oxadiazoles from Chiral N-Protected α-Amino Acids and Amidoximes in Acetone-Water: An Eco-Friendly Approach

A straightforward and high-yielding methodology for the synthesis of a high structural diversity of 1,2,4-oxadiazoles from different chiral N-protected α-amino acids and amidoximes under microwave (MW) irradiation is described herein. This greener approach gives the desired products using acetone/water as solvent in very short reaction times.

Despite the diverse number of processes allowing the preparation of the desired 1,2,4-oxadiazoles, there are some drawbacks such as the use of high temperature, long reaction times, and the use of hazardous solvents such as DMF, diglyme, and 1,4-dioxane [19-21, 23, 24, 27].
In recent years, the use of water as a solvent has gained increasing attention in the field of organic synthesis.Water has many advantages over conventional organic solvents, mainly due to its low cost and its nontoxic, nonpolluting, and nonflammable characteristics [29,30].On the contrary, the use of microwave mediating organic transformations has arisen as a very useful and effective tool for synthetic protocols due to the strict reaction control, high reaction rates, and energy savings [31,32].e use of water as the solvent in microwave transformations has been described.Several reports have shown that, at higher temperature and pressure, water behaves as a pseudoorganic solvent, as the dielectric constant decreases substantially and an ionic product increases the solvating power towards organic molecules to be similar to that of acetone or ethanol [33][34][35].
Searching for an alternative protocol for the synthesis of 1,2,4-oxadiazoles, we decided to perform the preparation of these compounds under microwave irradiation, which from a "green" point of view associated with solvent-free conditions represents an environmentally benign alternative in organic synthesis [34,36,37].
As part of our growing interest in using α-amino acids as chiral building blocks in organic synthesis [21,[38][39][40][41][42][43][44][45] and in connection with the increasing importance of the synthesis of small libraries of compounds with programmed variations of substituents, we describe herein an assay, an inexpensive synthetic route for the preparation of a set of chiral Nprotected α-amino acids-derived 1,2,4-oxadiazoles under microwave irradiation, as depicted in Scheme 1.

Experimental
2.1.General Information.All commercially available reagents and solvents were used without further purification unless otherwise stated.e experiments were performed in MW Discover CEM using mode of operation with simultaneous cooling. 1 H NMR and 13 C NMR spectra were recorded at 200 ( 1 H at 200.13 MHz and 13 C at 50.32 MHz) or 400 MHz ( 1 H at 400.13 MHz and 13 C at 100.62 MHz), using a Bruker Avance III HD-400 MHz or a Bruker Avance DPX-200 MHz spectrometers.Spectra were recorded in CDCl 3 solutions.Chemical shifts (δ) are reported in parts per million, referenced to TMS.Samples were diluted in 1 : 1 (v/ v) acetonitrile: water mixture, containing 0.1% of formic acid.Analyses were performed by infusion mode in an ACQUITY UPLC system from Waters Corp. (Milford, MA, USA) equipped with sampler manager and quadrupole timeof-flight (Q-Tof ) MS detector.
e Q-Tof Xevo G2 mass spectrometer was equipped with an electrospray ionization source (ESI).Detections were performed in positive ion mode (ESI + ) and resolution mode.Optimized MS conditions were capillary voltage 2.50 kV, cone voltage 15 V, extractor cone 3.30 V, desolvation gas 300 L/h, cone gas 10 L/ h, desolvation temperature 300 °C, and source temperature 150 °C.e acquisition mass range was monitored from 50 to 1000 Da.System control and data acquisition were performed using MassLynx V 4.1 software.Column chromatography was performed using Merck Silica Gel (230-400 mesh).
in-layer chromatography (TLC) was performed using Merck Silica Gel GF254 (0.25 mm thickness).For visualization, TLC plates were placed under ultraviolet light (254 nm) and then soaked in acidic vanillin, followed by heating.e product yields included in all tables refer to isolated yields.

General Procedure for Preparation of Arylamidoximes
a-e.To a stirred solution of appropriate nitrile (a-e) (50 mmol) in EtOH (100 mL) was added hydroxylamine hydrochloride (2.2 equiv) and then Et 3 N (2.3 equiv).is solution was stirred under reflux for 18 h and then diluted with water.e solvent (EtOH) was removed under reduced pressure, and the aqueous layer extracted 2 times DCM.e organic layer was dried over MgSO 4 and concentrated under vacuum.e crude product was purified by recrystallization using chloroform-heptane or column chromatography over silica gel using a mixture of hexane/EtOAc as the eluent to afford pure aryl amidoximes.

General Procedure for the Synthesis of Chiral N-Protected
Amino Acid-Derived 1-5.
e L-amino acid (10.0 g) was dissolved in H 2 O (300 mL), and Na 2 CO 3 (2.0 equiv) and NaHCO 3 (1.0 equiv) were added at room temperature, with stirring, to give a clear solution.Acetone (4.0 vol.) was added, and the slightly turbid solution was cooled in an ice water bath to 15-20 °C.en, ethyl chloroformate (1.5 equiv) was added slowly, with stirring, and the reaction mixture allowed to warm, to r.t.After stirring for an additional 3 h, the organic layer was concentrated under vacuum.To the aqueous phase was slowly added aqueous HCl, to give a pH of 2.
e resulting mixture was extracted into EtOAc (150 mL), and this was washed with H 2 O (100 mL) and then concentrated in vacuum to give the N-protected amino acid, an oil.

General Procedure for the Synthesis of 1,2,4-Oxadiazoles
(1-5a-e) from Chiral N-Protected α-Amino Acids and Amidoximes under Microwave Irradiation.In a sealed tube in a microwave reactor, 0.8 mmol of L-N-protected amino acid (1-5) and DCC (0.96 mmol, 0.199 g) were dissolved in acetone (1.0 mL) and the mixture was magnetically stirred for approximately 40 minutes to form the reactive intermediate.en, 0.8 mmol of aryl amidoxime (a-e) was added, and the mixture was homogenized.

Results and Discussion
e synthetic approach to give the 1,2,4-oxadiazoles was performed, starting from the protected amino acids shown in Figure 1.
In order to determine the optimal reaction conditions for the synthesis of 1,2,4-oxadiazole derivatives, we carried out the reaction employing the protected amino acid methionine (5) and aryl amidoxime (b) as a model substrate.In this set of experiments, we studied the effect of the temperature, reaction time, and solvents in order to determine the optimal reaction conditions (Table 1).
As described in Table 1 (Entries 9-11), the use of less polar solvents such as toluene, THF, and 1,4-dioxane are clearly less suitable for microwave-assisted synthesis, because their dielectric constants are substantially lower [37,46].e use of a small amount of ionic liquid (BMIMBF 4 ) as a "doping agent" for microwave heating of the nonpolar solvents (THF, 1,4-dioxane, and toluene) allowed heating above boiling points in sealed vessels [37,[47][48][49], with the formation of the products in moderate to good yields (Table 1, Entries 2, 4, and 8).Since these less polar solvents poorly absorb MW irradiation due to their lower loss tangent, the use of ionic liquid as a doping agent becomes essential to achieve high temperature (160 °C).
e best result was obtained using DCC which furnished the desired product in the best yield.As observed in Table 1, the best solvent for this reaction was the mixture acetone/ water, affording the respective compound (5b) in 91% yield in 15 min (Table 1, Entry 13).Increasing the reaction temperature from 115 to 160 °C resulted in a decrease in yield to 50% (Entry 6).e influence of coupling agents was also studied in order to determine the most efficient promoter for this transformation.us, a series of coupling agents including CDI, DCC, DIC, and TBTU was used to afford the 1,2,4-oxadiazole.
e extensive reduction in the reaction time in the microwave-assisted conditions along with a very simple work-up, better yield as well as the use of acetone/water as solvent when compared with conventional heating [19], makes more benign protocol to the environment in the process synthesis of the compound (5b).
With the optimized reaction conditions determined, we extended the protocol to a broader range of protected amino acids (1-5) as shown in Figure 1 and to a set of aryl amidoximes (a-e), in the presence of DCC, using microwave irradiation (Table 2).e 1,2,4-oxadiazoles were obtained in moderate to good yields (62-92%).
An important point to note is the primary activation of the carboxyl in the amino acid by DCC and the subsequent aryl amidoxime addition in the reaction medium.is sequence of reagents is an important factor, especially because the amidoxime can also react with DCC, before the activation of the carboxyl in the amino acid residue, resulting in undesired byproducts.
e process of formation of the intermediate I was carried for approximately 40 minutes, at room temperature, with the use of acetone as the solvent, which is also considered a solvent with green features [50].e solvent was subsequently removed by evaporation under reduced pressure, without heating.After this, water was added to the formed intermediate I, and then, the reaction mixture was subjected to microwave irradiation (Table 2).
e electronic effect of the substituents at the aryl amidoximes was studied.Amidoximes containing electrondonating groups such as methyl and methoxy and electronwithdrawing groups such as chloro and nitro were prepared (Table 2, compounds 1-5c, d).

Journal of Chemistry
In this way, the presence of strong electron-withdrawing groups such as nitro (NO 2 ) yield slightly lower products compared with donating or soft electron-withdrawing groups (Table 2, compounds 1-5e and 1-5a-d, respectively). is result could be explained by the reduced nucleophilicity in aryl amidoximes when this group is directly attached.Electron-donating and soft withdrawing groups allowed the preparation of the respective 1,2,4oxadiazoles with similar e ciency.
In terms of amino acids, the nature of the side chain does not play a signi cant role in terms of conversion to the desired heterocycle, since the results obtained with lipophilic and sulfurated side chains were quite similar.An exception was observed for the derivatives of S-benzyl-Lcysteine (Table 2, compounds 4a-e) probably due to the formation of by-products.Despite the small di erence in the reactivity of the aryl amidoximes, all the 1,2,4-oxadiazoles were obtained in moderate to good yields for all  Mixture of 1.0 mL of solvent and 0.5 mL of BMIMBF 4 . 21.0 mL of acetone and 1.0 mL of water as solvent. 31.0 mL of solvent. 4Yields for isolated pure products.
Journal of Chemistry the amino acids studied, showing the versatility of the methodology.

Conclusions
In conclusion, we developed a straightforward, efficient, and greener procedure for the high structural diversity of 1,2,4-oxadiazoles under microwave irradiation using acetone/water as the solvent in very short reaction times.Moreover, the method using microwave irradiation and water as the solvent is more efficient and greener, allowing the synthesis of 1,2,4-oxadiazoles by carrying out all reactions with an easy work-up, in short reaction times and better yields, when compared to conventional heating [19].
Although a limited number of substituents have been presented, this method can be extended to other substrates, since amino acids are inexpensive and easily available starting materials.e reaction conditions proved to be compatible with the carbamate protective group.

Table 1 :
Optimization of the reaction conditions for the synthesis of 1,2,4-oxadiazoles 5b by microwave irradiation.