Diastereoselective ZnCl2-Mediated Joullié–Ugi Three-Component Reaction for the Preparation of Phosphorylated N-Acylaziridines from 2H-Azirines

We disclose a direct approach to the diastereoselective synthesis of phosphorus substituted N-acylaziridines based on a one-pot ZnCl2-catalyzed Joullié–Ugi three-component reaction of phosphorylated 2H-azirines, carboxylic acids and isocyanides. Hence, this robust protocol offers rapid access to an array of N-acylaziridines in moderate-to-good yields and up to 98:2 dr for substrates over a wide scope. The relevance of this synthetic methodology was achieved via a gram-scale reaction and the further derivatization of the nitrogen-containing three-membered heterocycle. The diastereo- and regioselective ring expansion of the obtained N-acylaziridines to oxazole derivatives was accomplished in the presence of BF3·OEt2 as an efficient Lewid acid catalyst.


Introduction
Multicomponent reactions (MCRs) are highly convergent processes that can be used to create new potent bioactive molecules.On the basis of green chemistry [1], MCRs benefit from efficiency, energy, time and atom economies, use environmentally favorable conditions and decrease the amount of byproducts and/or waste [2][3][4][5][6][7][8].In addition, MCRs provide one of the most attractive prospects concerning complexity and diversity for the preparation of chemical libraries compared with other traditional synthetic organic methods.A large number of MCRs have been developed and are currently in a promising place in the chemist's toolbox of sustainable synthetic methodologies.
Among them, the more versatile isocyanide-based MCRs form the backbone of today's MCRs, such as the Ugi four-component reaction (U-4CR), which comprises the condensation of primary amines, carbonyl compounds, carboxylic acids and isocyanides to furnish dipeptide-like structures [9][10][11][12][13].As the U-4CR mechanism occurred via the in situ formation of an imine intermediate, the employment of a cyclic imine instead of the amine and the carbonyl components in the Ugi protocol is a simple concept that supplies the robust Joullié-Ugi three-component reaction (JU-3CR).The Ugi variant was first reported by Joullié et al. [14,15] in 1982, and it is an appealing synthetic methodology not only because nitrogen-containing heterocycles can be directly attained in a single step, but also because of its greater stereochemical control [16].The ring strain and the impossibility of imine E/Z isomerization contribute to a greater diastereoselectivity.Likewise, Joullié-Ugi adducts are privileged structures of interest in medicinal chemistry due to the synergistic effect of peptidic moieties linked to the nitrogen heterocycles leading to products with unique pharmaceutical activities [17][18][19][20].
Due to a better understanding of the benefits of covalent binding mechanisms and to the FDA support of effective and innocuous covalent drugs, there is great interest in covalent binding therapeutics [21][22][23].Targeted covalent inhibitors are designed by incorporating an electrophile into a ligand that would bind the target protein.The integrated electrophile, acting as "warheads" [24], including ketone, α,β-unsaturated carbonyl, nitrile, ester, epoxide or aziridine, binds irreversibly to endogenous nucleophilic functionalities, including lysine, tyrosine, serine, cysteine and threonine, among others, on the target protein, introducing a covalent interaction.In this regard, aziridines, well known for their robust alkylating properties, possess the potential to function as potent covalent drugs by virtue of their ability to serve as DNA cross-linking agents.This is achieved through the nucleophilic ring opening of the three-membered nitrogen-containing heterocycle [25].Aziridines are an important class of synthetic targets because they often exhibit a broad range of biological activities; for example, aziridine-containing mitomycin C (I) [26], azinomycin A (II) [27] and imexon (III) [28] show antitumor activity (Figure 1).Other natural aziridines, also known as aziridine alkaloids, display antibacterial and antimicrobial activity against selected microorganisms.For instance, aziridine-2-phosphonates IV have been claimed to show antibacterial properties [29] (Figure 1).We have recently reported the synthesis of phosphorus-substituted N-acylaziridines V [30] and VI [31], which exhibited a very good cytotoxic effect inhibiting the growth of human tumor cell line A549 (adenocarcinomic human alveolar basal epithelial cells).Recently, oxazoles have been emerged as the critical pharmacophore for various biological and medicinal applications.They serve as the key structural motif in numerous naturally occurring compounds and exhibit a broad spectrum of pharmacological properties, such as anti-cancer, anti-tubercular, anti-bacterial, anti-fungal, anti-parasitic and anti-viral properties, among others [32].Hence, they can be utilized as primary building blocks in the pharmaceutical sector for the synthesis of several drugs.Likewise, from a biological perspective, organophosphorus compounds are very interesting due to their ability to modify the reactivity of heterocycles and regulate essential biological functions [33].Thus, the anticancer agents 1,3-oxazol-4-ylphosphonium perchlorates VII [34], or the oxazole-phosphine oxide derivative VIII [31], and the oxazol-4yl-phosphonate derivative IX displaying interesting anti-human cytomegalovirus (HCMV) properties [35], are only some examples of phosphorus-substituted oxazole derivatives with high potential for medical applications.
Molecules 2024, 29, x FOR PEER REVIEW 2 of 25 Due to a better understanding of the benefits of covalent binding mechanisms and to the FDA support of effective and innocuous covalent drugs, there is great interest in covalent binding therapeutics [21][22][23].Targeted covalent inhibitors are designed by incorporating an electrophile into a ligand that would bind the target protein.The integrated electrophile, acting as "warheads" [24], including ketone, α,β-unsaturated carbonyl, nitrile, ester, epoxide or aziridine, binds irreversibly to endogenous nucleophilic functionalities, including lysine, tyrosine, serine, cysteine and threonine, among others, on the target protein, introducing a covalent interaction.In this regard, aziridines, well known for their robust alkylating properties, possess the potential to function as potent covalent drugs by virtue of their ability to serve as DNA cross-linking agents.This is achieved through the nucleophilic ring opening of the three-membered nitrogen-containing heterocycle [25].Aziridines are an important class of synthetic targets because they often exhibit a broad range of biological activities; for example, aziridine-containing mitomycin C (I) [26], azinomycin A (II) [27] and imexon (III) [28] show antitumor activity (Figure 1).Other natural aziridines, also known as aziridine alkaloids, display antibacterial and antimicrobial activity against selected microorganisms.For instance, aziridine-2-phosphonates IV have been claimed to show antibacterial properties [29] (Figure 1).We have recently reported the synthesis of phosphorus-substituted N-acylaziridines V [30] and VI [31], which exhibited a very good cytotoxic effect inhibiting the growth of human tumor cell line A549 (adenocarcinomic human alveolar basal epithelial cells).Recently, oxazoles have been emerged as the critical pharmacophore for various biological and medicinal applications.They serve as the key structural motif in numerous naturally occurring compounds and exhibit a broad spectrum of pharmacological properties, such as anti-cancer, anti-tubercular, anti-bacterial, anti-fungal, anti-parasitic and anti-viral properties, among others [32].Hence, they can be utilized as primary building blocks in the pharmaceutical sector for the synthesis of several drugs.Likewise, from a biological perspective, organophosphorus compounds are very interesting due to their ability to modify the reactivity of heterocycles and regulate essential biological functions [33].Thus, the anticancer agents 1,3-oxazol-4-ylphosphonium perchlorates VII [34], or the oxazole-phosphine oxide derivative VIII [31], and the oxazol-4-yl-phosphonate derivative IX displaying interesting antihuman cytomegalovirus (HCMV) properties [35], are only some examples of phosphorussubstituted oxazole derivatives with high potential for medical applications.Many of the reported JU-3CR examples in the literature primarily employ 5, 6 or 7membered cyclic imines for the preparation of pyrrolidine [36,37], thiazolidine [38], indolines [39,40] (Scheme 1, Equation ( 1)), piperidine [36,41] or oxazepine [42] peptidomimetics (Scheme 1, Equation ( 2)).However, only two examples have been reported in relation to the use of 2H-azirines as cyclic imines in the three-component Joullié-Ugi reaction for the synthesis of N-acylaziridines.Kanizsai et al. [43] first described the Lewis Many of the reported JU-3CR examples in the literature primarily employ 5, 6 or 7-membered cyclic imines for the preparation of pyrrolidine [36,37], thiazolidine [38], indolines [39,40] (Scheme 1, Equation ( 1)), piperidine [36,41] or oxazepine [42] peptidomimetics (Scheme 1, Equation ( 2)).However, only two examples have been reported in relation to the use of 2H-azirines as cyclic imines in the three-component Joullié-Ugi reaction for the synthesis of N-acylaziridines.Kanizsai et al. [43] first described the Lewis acid-promoted version of the Joullié-Ugi reaction using 2H-azirine-2-carboxylates for the diastereoselective synthesis of N-acylaziridines-2-carboxamide derivatives (Scheme 1, Equation ( 3)).

Results
As outlined in Table 1, we started our investigation with the optimization of the threecomponent reaction conditions of 2H-azirine phosphine oxide 1a, benzoic acid (2a) and cyclohexyl isocyanide (3a) in THF at 60 • C. The Joullié-Ugi reaction without a catalyst led to the obtention of only a 10% yield of N-acylaziridine phosphine oxide 4a (entry 1).It is well known that the activation of 2H-azirines by Lewis acids may significantly enhance their reactivity [47,48].Thus, we next explored the Lewis or Brønsted acid-mediated JU-3CR.Only a 14% yield of 4a could be achieved when PTSA was used as the Brønsted acid catalyst in this process (entry 2).Trifluoromethanesulfonic acid (TfOH, entry 3) showed moderate catalytic activity since the reaction proceeded smoothly in THF at 60 • C to give the product 4a in a 35% yield.Increasing the amount of TfOH from 10 mol% to 25 mol% did not improve the yield of compound 4a.The JU-3CR was carried out in the presence of different Lewis acids.For instance, Ti(O i Pr) 4 , Sc(OTf) 3 , InCl 3 , BF 3 •OEt 2 , MgBr 2 and ZnCl 2 •H 2 O were not suitable for the current reaction since compound 4a could not be detected, and only the starting 2H-azirine 1a or decomposition products were recovered instead (entries 4-9).In general, the most active catalyst for the JU-3CR of 1a with carboxylic acid 2a and isocyanide 3a was found to be ZnCl 2 (Table 1, entry 10), which is consistent with literature reports of other JU-3CRs involving 2H-azirine-2-carboxylates [43] Therefore, the use of ZnCl 2 (25 mol%, entry 10) resulted in the formation of N-acylaziridine 4a in a 60% chemical yield, together with a small amount of the pyrazine that proceed from the thermal treatment of the corresponding 2H-azirine phosphine oxide 1a [49].This process yielded product 4a in a diastereomeric ratio of 96:4.

Results
As outlined in Table 1, we started our investigation with the optimization of the threecomponent reaction conditions of 2H-azirine phosphine oxide 1a, benzoic acid (2a) and cyclohexyl isocyanide (3a) in THF at 60 °C.The Joullié-Ugi reaction without a catalyst led to the obtention of only a 10% yield of N-acylaziridine phosphine oxide 4a (entry 1).It is well known that the activation of 2H-azirines by Lewis acids may significantly enhance their reactivity [47,48].Thus, we next explored the Lewis or Brønsted acid-mediated JU-3CR.Only a 14% yield of 4a could be achieved when PTSA was used as the Brønsted acid catalyst in this process (entry 2).Trifluoromethanesulfonic acid (TfOH, entry 3) showed moderate catalytic activity since the reaction proceeded smoothly in THF at 60 °C to give the product 4a in a 35% yield.Increasing the amount of TfOH from 10 mol% to 25 mol% did not improve the yield of compound 4a.The JU-3CR was carried out in the presence of different Lewis acids.For instance, Ti(O i Pr)4, Sc(OTf)3, InCl3, BF3•OEt2, MgBr2 and ZnCl2•H2O were not suitable for the current reaction since compound 4a could not be detected, and only the starting 2H-azirine 1a or decomposition products were recovered instead (entries 4-9).In general, the most active catalyst for the JU-3CR of 1a with carboxylic acid 2a and isocyanide 3a was found to be ZnCl2 (Table 1, entry 10), which is consistent with literature reports of other JU-3CRs involving 2H-azirine-2-carboxylates [43] Therefore, the use of ZnCl2 (25 mol%, entry 10) resulted in the formation of N-acylaziridine 4a in a 60% chemical yield, together with a small amount of the pyrazine that proceed from the thermal treatment of the corresponding 2H-azirine phosphine oxide 1a [49].This process yielded product 4a in a diastereomeric ratio of 96:4.The effect of the solvent on the JU-3CR was also tested.The ZnCl 2 catalyst in this process was incompatible with some solvents such as MeOH or MeCN (entries 11 and 12), and in both cases, no reaction product was observed.The degree of consumption of the starting 2H-azirine 1a was found to depend on reaction temperature.In the reaction conducted at -10 • C (entry 13), a significant amount of unreacted 2H-azirine 1a was recovered and the expected product 4a was obtained in a low yield.The JU-3CR was found to work better when the reaction was carried out at room temperature, and the almost complete conversion of the 2H-azirine 1a was then observed (entry 14).Finally, different amounts of ZnCl 2 were examined, which showed that decreasing the amount of ZnCl 2 (10 mol%, entry 15) led to the desired product 4a in a low yield together with α-ketamide derived from the nucleophilic addition of the carboxylic acid to 2H-azirine 1a [50].Increasing the amount of ZnCl 2 up to 30 mol% (entry 16) did not affect the yield of compound 4a.
Given that the results of this preliminary investigation seemed to define ZnCl 2 as the best catalyst in the JU-3CR in THF and room temperature as the best reaction condition, we adopted these conditions for further studies.Then, a range of N-acylaziridine phosphine oxides 4 with diverse substitution patterns (Figure 2) on the aziridine ring were prepared.A considerable selection of aromatic carboxylic acid partners 2 were well tolerated in the JU-3CR with 1a and 3 as coupling partners (Scheme 2).Both electron-donating (OMe) and electron-withdrawing groups (F, NO 2 ) at the para-phenyl position of aromatic carboxylic acid 2 yielded desired products 4c, 4d, 4e, 4j and 4k in 20-74% yields and very good diastereoselectivities.Among them, 4-fluor derivatives 4d and 4j were achieved with the best yields (74%).
The effect of the solvent on the JU-3CR was also tested.The ZnCl2 catalyst in this process was incompatible with some solvents such as MeOH or MeCN (entries 11 and 12), and in both cases, no reaction product was observed.The degree of consumption of the starting 2H-azirine 1a was found to depend on reaction temperature.In the reaction conducted at -10 °C (entry 13), a significant amount of unreacted 2H-azirine 1a was recovered and the expected product 4a was obtained in a low yield.The JU-3CR was found to work better when the reaction was carried out at room temperature, and the almost complete conversion of the 2H-azirine 1a was then observed (entry 14).Finally, different amounts of ZnCl2 were examined, which showed that decreasing the amount of ZnCl2 (10 mol%, entry 15) led to the desired product 4a in a low yield together with α-ketamide derived from the nucleophilic addition of the carboxylic acid to 2H-azirine 1a [50].Increasing the amount of ZnCl2 up to 30 mol% (entry 16) did not affect the yield of compound 4a.
Given that the results of this preliminary investigation seemed to define ZnCl2 as the best catalyst in the JU-3CR in THF and room temperature as the best reaction condition, we adopted these conditions for further studies.Then, a range of N-acylaziridine phosphine oxides 4 with diverse substitution patterns (Figure 2) on the aziridine ring were prepared.A considerable selection of aromatic carboxylic acid partners 2 were well tolerated in the JU-3CR with 1a and 3 as coupling partners (Scheme 2).Both electron-donating (OMe) and electron-withdrawing groups (F, NO2) at the para-phenyl position of aromatic carboxylic acid 2 yielded desired products 4c, 4d, 4e, 4j and 4k in 20-74% yields and very good diastereoselectivities.Among them, 4-fluor derivatives 4d and 4j were achieved with the best yields (74%).The electron-donating group (Me) at the meta-phenyl position of aromatic carboxylic acid also furnished N-acylaziridine 4b in a 63% yield (Scheme 2).Other aromatic carboxylic acids such as 2-naphthoic acid 2g or 4-benzoylbenzoic acid 2i were selected as suitable candidates for this transformation, providing products 4 in moderate-to-good yields.Even heteroaromatic carboxylic acids were well tolerated in the Joullié-Ugi three-component reaction.For instance, nicotinic acid 2f, 2-furoic acid 2h and or quinoline-6-carboxylic acid 2j gave desired products 4h, 4m, 4t and 4u in 43-85% yields.Phenylacetic acid 2m yielded N-acylaziridines 4f and 4q in a 68 and 71% yield, respectively.However, worse yields of N-acylaziridines 4o, 4p and 4r, derived, respectively, from acetic acid 2k, trifluoroacetic acid 2l and 4-biphenylacetic acid 2n were attained (Scheme 2).Moreover, propargylic acids, such as propiolic acid 2p, can also be subjected to the JU-3CR to give compound 4g in a low yield but with high diastereoselectivity (96:4 trans:cis dr).This confirms the strength of the carboxylic acid scope in the JU-3CR.
In order to assess the applicability of the JU-3CR, we next investigated the scope of this process with regard to the isocyanide partner.Besides the cyclohexyl isocyanide (3a), other aliphatic isocyanides such as tert-butyl isocyanide (3b) or cyclopropyl isocyanide (3c) have been tested in the JU-3CR.All the isocyanides studied are well tolerated, giving the N-azylaziridines 4 in a moderate-to-good yield (see Scheme 2).The isocyanide partner does not affect the outcome of this protocol.For instance, compare the chemical yields of 4a, 4i and 4s (60-80% yields), 4d and 4j (74% yield) or 4m and 4u, which proceeded smoothly in 85 and 75% yields (Scheme 2).
A careful examination of the spectroscopic data of the crude reaction mixture of N-acylaziridine 4a, showed two well-resolved doublets in the 1 H NMR spectrum corresponding to the H3 methine proton of the aziridine ring, which is consistent with the presence of two diastereoisomers.The major diastereoisomer appears at δ H ~3.81 ppm with the coupling constant 2 J PH = 24.0Hz, and the minor one at δ H ~3.35 ppm with a lower coupling constant of 2 J PH = 21.7 Hz in a ratio of 96:4.Substrates 4 were extensively characterized on the basis of their 1 H, 13 C, 31 P, 19 F NMR and 2D NMR spectra and HRMS (see the Supporting Information).The most characteristic signals for N-acylaziridine 4a (major diastereoisomer) in the 1 H NMR spectrum are the two well-resolved doublets at δ H ~5.76 ppm ( 3 J HH = 8.1 Hz) and δ H ~3.81 ppm with a coupling constant of 2 J PH = 24.0Hz, corresponding to the NH of amide group and the H3 methine proton of the aziridine ring, respectively.A singlet at δ H ~1.96 ppm was attributed to the methyl group.In the 13 C NMR spectrum, the formation of compound 4a is evident from the presence of two carbonyl groups at δ C ~176.2 ( 3 J PC = 3.3 Hz) and 165.6 ppm, while the quaternary carbon C2 appears as a doublet at δ C ~49.6 ppm ( 2 J PC = 3.0 Hz) and the methine carbon C3 shows a chemical shift at δ C ~42.0 ppm with a large coupling constant ( 1 J PC = 100.4Hz).
Since it was not possible to assign the stereochemistry of N-acylaziridines 4 via 1 H and 13 C NMR, their structure has been unambiguously determined via X-ray diffraction analysis [51][52][53], establishing the trans-relationship between the amide group at the C2 position and the phosphorus moiety at the C3 position of the major diastereoisomer 4i.The CIF data are presented in the Supporting Information, and the ORTEP drawing of 4i is shown in Figure 3.   Encouraged by the abovementioned obtained results of the Joullié-Ugi three-component reaction between phosphorylated 2H-azirines 1, carboxylic acids 2 and isocyanides 3, we further investigated the substrate scope using N-Fmoc-protected amino acids as carboxylic acid partners for the preparation of phosphorylated aziridine peptidomimetics.To our delight, it was found that the reaction proceeded smoothly when 2H-azirine phosphonate 1b reacted with tert-butyl isocyanide (3b) and Fmoc-Leu (2r, R 1 = i Bu) in the standard conditions to yield derivative 6a in a 65% isolated yield (Scheme 4).The 1 H NMR spectrum of the crude reaction mixture confirmed the presence of two well-resolved doublets at δH = 3.03 and 2.90 ppm with a coupling constant of 2 JPH = 18.6 and 18.4 Hz, respectively, for the H3 methine proton of the aziridine ring of both trans-diastereoisomers, in a ratio of 1:1: Conversely, another doublet appeared at δH = 2.71 ppm with a lower coupling constant of 2 JPH = 14.7 Hz corresponding to H3 of the cis-diastereoisomer, while the fourth doublet corresponding to the other cis-diastereoisomer appeared overlapped in the range of δH = 2.83-2.75ppm.The diastereomeric ratio between trans-and cis-diastereoisomers is approximately 92:8.After purification via flash-chromatography, it was possible to identity both trans-diastereoisomers of 6a.The Joullié-Ugi reaction between 2H-azirine 1b, tert-butyl isocyanide (3b) and Fmoc-Ala (2s, R 1 = Me) using 25 mol% of ZnCl2 in THF and at room temperature led to the formation of derivative 6b in a lower yield (Scheme 4).Via Encouraged by the abovementioned obtained results of the Joullié-Ugi three-component reaction between phosphorylated 2H-azirines 1, carboxylic acids 2 and isocyanides 3, we further investigated the substrate scope using N-Fmoc-protected amino acids as carboxylic acid partners for the preparation of phosphorylated aziridine peptidomimetics.To our delight, it was found that the reaction proceeded smoothly when 2H-azirine phosphonate 1b reacted with tert-butyl isocyanide (3b) and Fmoc-Leu (2r, R 1 = i Bu) in the standard conditions to yield derivative 6a in a 65% isolated yield (Scheme 4).The 1 H NMR spectrum of the crude reaction mixture confirmed the presence of two well-resolved doublets at δ H = 3.03 and 2.90 ppm with a coupling constant of 2 J PH = 18.6 and 18.4 Hz, respectively, for the H3 methine proton of the aziridine ring of both trans-diastereoisomers, in a ratio of 1:1: Conversely, another doublet appeared at δ H = 2.71 ppm with a lower coupling constant of 2 J PH = 14.7 Hz corresponding to H3 of the cis-diastereoisomer, while the fourth doublet corresponding to the other cis-diastereoisomer appeared overlapped in the range of δ H = 2.83-2.75ppm.The diastereomeric ratio between transand cis-diastereoisomers is approximately 92:8.After purification via flash-chromatography, it was possible to identity both trans-diastereoisomers of 6a.The Joullié-Ugi reaction between 2H-azirine 1b, tert-butyl isocyanide (3b) and Fmoc-Ala (2s, R 1 = Me) using 25 mol% of ZnCl 2 in THF and at room temperature led to the formation of derivative 6b in a lower yield (Scheme 4).Via the 1 H NMR of the crude compound, it was possible to determine the 1:1 ratio between both trans-diastereoisomers.Nevertheless, in this case, determining the diastereomeric ratio (trans/cis) was infeasible.
The gram-scale synthesis of phosphorylated N-acylaziridines was accomplished, as shown in Scheme 3. The use of 4.0 mmol of 2H-azirine phosphonate 1b, under the JU standard conditions, gave N-acylaziridine phosphonate 5a in a 70% yield (1.11 g) after recrystallization.Scheme 5 outlines a plausible mechanism for the JU-3CR.This process carried out in a polar solvent, suggesting the formation of polar intermediates, is compatible with a stepwise mechanism.The addition of Lewis acids (in our case ZnCl2) increases the electrophilicity of the iminic C-N double bond in 2H-azirine 1.Thus, the electrophilic imine and nucleophilic carboxylic acid 2 add to the carbon atom of isocyanide 3. The amino group of the adduct thus formed promotes the irreversible Mumm rearrangement in the presence of a zinc catalyst and the intramolecular acylation of the amine nitrogen atom, which after subsequent hydroxylimine → amide tautomerization leads to phosphorylated N-acylaziridines 4, 5 or 6.We performed further derivatization in order to illustrate the utility of the Joullié-Ugi adducts.Thus, we explored the isomerization reaction of N-acylaziridines 4 and 5 to oxazole derivatives.For this purpose, and taking into account that the regio-and stereochemical outcomes of these rearrangements strongly depend on the reaction conditions, as well as the substitution pattern of the N-acylaziridine, we started exploring thermal conditions for the ring opening of compounds 4 and 5. Thus, phosphorus-substituted N-acylaziridine 4a was heated in refluxing CHCl 3 .Under these conditions, the corresponding oxazole derivative was not observed, and the unreacted starting substrate was recovered instead.Next, the rearrangement of 4a was also tested under nucleophilic conditions [31,[54][55][56][57]. When 4a reacted with 0.2 equivalents of NaI in THF at 60 • C, as in the previous case, no satisfactory results were attained, observing only decomposition products.Likewise, the isomerization of the N-acylaziridine to oxazole derivative under mild acidic conditions was examined.We performed further derivatization in order to illustrate the utility of the Joullié-Ugi adducts.Thus, we explored the isomerization reaction of N-acylaziridines 4 and 5 to oxazole derivatives.For this purpose, and taking into account that the regio-and stereochemical outcomes of these rearrangements strongly depend on the reaction conditions, as well as the substitution pattern of the N-acylaziridine, we started exploring thermal conditions for the ring opening of compounds 4 and 5. Thus, phosphorus-substituted Nacylaziridine 4a was heated in refluxing CHCl3.Under these conditions, the corresponding oxazole derivative was not observed, and the unreacted starting substrate was recovered instead.Next, the rearrangement of 4a was also tested under nucleophilic conditions [31,[54][55][56][57]. When 4a reacted with 0.2 equivalents of NaI in THF at 60 °C, as in the previous case, no satisfactory results were attained, observing only decomposition products.Likewise, the isomerization of the N-acylaziridine to oxazole derivative under mild acidic conditions was examined.Compound 4a was treated with both Brønsted acids, including ptoluenesulfonic acid (PTSA), and Lewis acids, including BF3•OEt2.Only the use of BF3•OEt2 gave satisfactory results.Hence, when N-acylaziridine 4a reacted in the presence of 1.2 equivalents of BF3•OEt2 in MeCN at 90 °C and under microwave irradiation for 10 min, the formation of 4-diphenylphosphoryl-4,5-dihydrooxazole-5-carboxamide 7a was achieved in a very good yield and in a regio-and diastereoselective fashion (Scheme 6).Spectroscopic data confirmed the isomerization of N-acylaziridine 4a into oxazole derivative 7a.While the 1 H NMR spectrum of 4a shows a signal for the methyl group at δ H = 1.96 ppm and the methine hydrogen resonates at δ H = 3.81 ppm as a well-resolved doublet ( 2 J PH = 24 Hz, see above), in dihydrooxazole-5-carboxamide 7a, these signals appear at δ H = 1.63 and 5.32 ppm as a singlet and a well-resolved doublet with a much lower coupling constant ( 2 J PH = 6.0 Hz), respectively.Similarly, other N-acylaziridines derived from phosphine oxide 4b, 4i and 4l reacted with BF 3 •OEt 2 under the same reaction conditions, providing 60, 92 and 91% yields of oxazole derivatives 7b-d (Scheme 6).This synthetic methodology was extended to the use of N-acylaziridines 5 derived from phosphonate.Thus, the ring expansion of 5a and 5b easily occurred via the slight excess of BF 3 •OEt 2 (3 equivalents) in CHCl 3 at 71 • C and under microwave irradiation for 15 min, to obtain regio-and diasteroselective 7e and 7f in high yields (Scheme 6).
Since it was not possible to assign the stereochemistry of oxazole derivatives 7 via 1 H and 13 C NMR, their structure has been unambiguously determined via X-ray diffraction analysis [51][52][53], establishing not only the regioselectivity of the isomerization process, but also the anti-relationship between the amide group at the C5 position and the phosphorus moiety at the C4 position of 7a (Figure 4).
from phosphine oxide 4b, 4i and 4l reacted with BF3•OEt2 under the same reaction conditions, providing 60, 92 and 91% yields of oxazole derivatives 7b-d (Scheme 6).This synthetic methodology was extended to the use of N-acylaziridines 5 derived from phosphonate.Thus, the ring expansion of 5a and 5b easily occurred via the slight excess of BF3•OEt2 (3 equivalents) in CHCl3 at 71 °C and under microwave irradiation for 15 min, to obtain regio-and diasteroselective 7e and 7f in high yields (Scheme 6).
Since it was not possible to assign the stereochemistry of oxazole derivatives 7 via 1 H and 13 C NMR, their structure has been unambiguously determined via X-ray diffraction analysis [51][52][53], establishing not only the regioselectivity of the isomerization process, but also the anti-relationship between the amide group at the C5 position and the phosphorus moiety at the C4 position of 7a (Figure 4).A reasonable mechanism that would explain the formation of 7 is exemplified in Scheme 7. First, BF3•OEt2 would activate the carbonyl group of N-acylaziridine 4 or 5, thus assisting the ring-opening reaction, through the N-C2 bond of N-acylaziridine, with the concomitant generation of the most stable carbocation.This intermediate enables the ring expansion of N-acylaziridine 4 or 5 to oxazole derivative 7, as the only regio-and diastereoisomer.

General Experimental Information
Solvents for extraction and chromatography were reagent-grade.All solvents used in reactions were freshly distilled and dried over 4Å molecular sieves before use.Unless otherwise mentioned, all other solvents and chemicals were purchased from commercial vendors and recrystallized or distilled as necessary, or used without further purification.All A reasonable mechanism that would explain the formation of 7 is exemplified in Scheme 7. First, BF 3 •OEt 2 would activate the carbonyl group of N-acylaziridine 4 or 5, thus assisting the ring-opening reaction, through the N-C2 bond of N-acylaziridine, with the concomitant generation of the most stable carbocation.This intermediate enables the ring expansion of N-acylaziridine 4 or 5 to oxazole derivative 7, as the only regioand diastereoisomer.
from phosphine oxide 4b, 4i and 4l reacted with BF3•OEt2 under the same reaction conditions, providing 60, 92 and 91% yields of oxazole derivatives 7b-d (Scheme 6).This synthetic methodology was extended to the use of N-acylaziridines 5 derived from phosphonate.Thus, the ring expansion of 5a and 5b easily occurred via the slight excess of BF3•OEt2 (3 equivalents) in CHCl3 at 71 °C and under microwave irradiation for 15 min, to obtain regio-and diasteroselective 7e and 7f in high yields (Scheme 6).
Since it was not possible to assign the stereochemistry of oxazole derivatives 7 via 1 H and 13 C NMR, their structure has been unambiguously determined via X-ray diffraction analysis [51][52][53], establishing not only the regioselectivity of the isomerization process, but also the anti-relationship between the amide group at the C5 position and the phosphorus moiety at the C4 position of 7a (Figure 4).A reasonable mechanism that would explain the formation of 7 is exemplified in Scheme 7. First, BF3•OEt2 would activate the carbonyl group of N-acylaziridine 4 or 5, thus assisting the ring-opening reaction, through the N-C2 bond of N-acylaziridine, with the concomitant generation of the most stable carbocation.This intermediate enables the ring expansion of N-acylaziridine 4 or 5 to oxazole derivative 7, as the only regio-and diastereoisomer.

General Experimental Information
Solvents for extraction and chromatography were reagent-grade.All solvents used in reactions were freshly distilled and dried over 4Å molecular sieves before use.Unless otherwise mentioned, all other solvents and chemicals were purchased from commercial vendors and recrystallized or distilled as necessary, or used without further purification.All

General Experimental Information
Solvents for extraction and chromatography were reagent-grade.All solvents used in reactions were freshly distilled and dried over 4 Å molecular sieves before use.Unless otherwise mentioned, all other solvents and chemicals were purchased from commercial vendors and recrystallized or distilled as necessary, or used without further purification.All reactions were performed under an atmosphere of dry nitrogen.The reaction progress was monitored via 31 P NMR or analytical thin-layer chromatography (TLC) performed on precoated Merck silica gel 60 F 254 TLC aluminum plates, and spot-visualized with UV light or permanganate stain.Melting points were uncorrected. 1H (400 MHz), 13 C (100 MHz), 19 F (376 MHz) and 31 P NMR (160 MHz) spectra were recorded using a Bruker Avance 400 (400 MHz) spectrometer in CDCl 3 at room temperature.Chemical shifts (δ) are reported in parts per million (ppm) with the internal chloroform signal at 7.26 ppm as a standard for 1 H, the internal chloroform signal at 77.2 ppm as a standard for 13 C, the external fluorotrichloromethane (CFCl 3 ) signal at 0.0 ppm for 19 F or the external H 3 PO 4 (50%) signal at 0.0 ppm as a standard for 31 P NMR spectra.All coupling constants (J) values are reported in Hz. 19 F and 13 C NMR spectra were recorded in a broadband decoupled mode from hydrogen nuclei.Distortionless enhanced polarization transfer (DEPT) supported peak assignments for 13 C NMR. Data for 1H NMR spectra are reported for the following: chemical shift, multiplicity, coupling constant and integration.Multiplicity abbreviations are reported as s = singlet; d = doublet; t = triplet; q = quartet; m = multiplet; dd = double doublet; bs = broad singlet.IR spectra were measured using a Nicolet iS10 Termo Scientific spectrometer using an attenuated total reflectance technique (ATR).Absorbance frequencies are given at maximum of intensity in cm -1 .High-resolution mass spectra (HRMS) were obtained via the positive-ion electrospray ionization (ESI) method with a time of flight Q-TOF method.Data are reported in the form m/z (intensity relative to base = 100).2H-Azirine phosphine oxide 1a [58] and phosphonate 1b [59] were prepared according to procedures in the literature and characterized using NMR spectra.

Experimental Procedure and Characterization Data for Phosphorylated Aziridine Peptidomimetics 6
In a flame-dried flask, corresponding amino acid 2r or 2s (1.3 mmol, 1.3 eq.), tert-butyl isocyanide 3b (1.3 mmol, 1.3 eq.) and a 1M diethyl ether solution of ZnCl 2 (0.25 mL, 0.25 mmol, 0.25 eq.) were added to 1 mL of dry THF.Then, 2H-azirine phosphonate 1b (1 mmol, 1 eq.) was added at room temperature.The reaction mixture was stirred until TLC showed the disappearance of 2H-azirine 1b (6 h).The solvent was removed under ity and satisfactory yields.This protocol was applicable to a wide range of substrates, including 2H-azirines derived from phosphine oxide and phosphonate, various aromatic and heteroaromatic, aliphatic, acrylic and propargylic acids, and isocyanide with different alkyl substitutions.Even N-Fmoc protected amino acids as carboxylic acid partners are well tolerated and phosphorylated aziridine peptidomimetics are achieved in a simple procedure.This strategy for the preparation of phosphorylated N-acylaziridines represents a valued method owing to the high degree of diastereoselectivity observed, the high atom economy and the reaction stages.The synthetic potential of this JU-3CR was established with the preparative-scale reaction and useful transformations of the JU-3CR adducts.The regio-and stereospecific ring expansion of N-acylaziridines to oxazole derivatives was accomplished in the presence of BF 3 •OEt 2 as an efficient Lewis acid catalyst.

Figure 1 .
Figure 1.Aziridine-containing natural products and selected examples of phosphorus-substituted oxazole derivatives with high potential for medical applications.

Figure 1 .
Figure 1.Aziridine-containing natural products and selected examples of phosphorus-substituted oxazole derivatives with high potential for medical applications.

Scheme 2 .
Scheme 2. Substrate scope of the N-acylaziridine phosphine oxides 4 in the Joullié-Ugi three-component reaction.See the SI for experimental details.Reactions were carried out on a 0.5 mmol scale; 2H-azirine 1a (0.5 mmol), carboxylic acid 2 (1.3 eq), isocyanide 3 (1.3 eq), ZnCl2 (25 mol%) and solvent (0.5 mL).Diastereomeric ratio (trans/cis) determined from the crude reaction mixture via 1 H NMR. It was not possible to determine the diastereomeric ratio of compounds 4c and 4h as the crude reaction NMR spectra were not clean enough.

Scheme 2 .
Scheme 2. Substrate scope of the N-acylaziridine phosphine oxides 4 in the Joullié-Ugi threecomponent reaction.See the Supporting Information for experimental details.Reactions were carried out on a 0.5 mmol scale; 2H-azirine 1a (0.5 mmol), carboxylic acid 2 (1.3 eq.), isocyanide 3 (1.3 eq.), ZnCl 2 (25 mol%) and solvent (0.5 mL).Diastereomeric ratio (trans/cis) determined from the crude reaction mixture via 1 H NMR. It was not possible to determine the diastereomeric ratio of compounds 4c and 4h as the crude reaction NMR spectra were not clean enough.

Scheme 4 .Scheme 5 .
Scheme 4. Substrate scope of phosphorylated aziridine peptidomimetics 6 through the Joullié-Ugi three-component reaction using N-Fmoc amino acids.See the Supporting Information for experimental details.Reactions were carried out on a 1 mmol scale; 2H-azirine 1b (1 mmol), Fmoc-protected amino acid 2r or 2s (1.3 eq.), isocyanide 3b (1.3 eq.), ZnCl 2 (25 mol%) and solvent (1 mL). a Diastereomeric ratio (trans/trans) determined from the crude reaction mixture via 1 H NMR. The diastereomeric ratio between transand cis-diastereoisomers is approximately 92:8.b The diastereomeric ratio (trans/trans) determined from the crude reaction mixture via 1 H NMR. The diastereomeric ratio between trans and cis-diastereoisomers could not be determined.The gram-scale synthesis of phosphorylated N-acylaziridines was accomplished, as shown in Scheme 3. The use of 4.0 mmol of 2H-azirine phosphonate 1b, under the JU standard conditions, gave N-acylaziridine phosphonate 5a in a 70% yield (1.11 g) after recrystallization.Scheme 5 outlines a plausible mechanism for the JU-3CR.This process carried out in a polar solvent, suggesting the formation of polar intermediates, is compatible with a stepwise mechanism.The addition of Lewis acids (in our case ZnCl 2 ) increases the electrophilicity of the iminic C-N double bond in 2H-azirine 1.Thus, the electrophilic imine and nucleophilic carboxylic acid 2 add to the carbon atom of isocyanide 3. The amino group of the adduct thus formed promotes the irreversible Mumm rearrangement in the presence of a zinc catalyst and the intramolecular acylation of the amine nitrogen atom, which after subsequent hydroxylimine → amide tautomerization leads to phosphorylated N-acylaziridines 4, 5 or 6.

Scheme 7 .
Scheme 7. Rational mechanism for the stereospecific and regioselective ring expansion of N-acylaziridines 4 or 5 to oxazole derivatives 7.

Scheme 7 .
Scheme 7. Rational mechanism for the stereospecific and regioselective ring expansion of N-acylaziridines 4 or 5 to oxazole derivatives 7.

Scheme 7 .
Scheme 7. Rational mechanism for the stereospecific and regioselective ring expansion of Nacylaziridines 4 or 5 to oxazole derivatives 7.

Table 1 .
Reaction condition optimization a .