Introduction of chiral 2-(aminoalkyl) substituents into 5-amino-1,3-oxazol-4-ylphosphonic acid derivatives and their use in phosphonodipeptide synthesis

Abstract

Starting from phthalimidoalkanoylamines 1 (amino-protected derivatives of L-alanine, L-valine, and L-leucine), we have suggested a straightforward synthetic route to 5-amino-1,3-oxazol-4-ylphosphonic acid derivatives 5 containing a chiral aminoalkyl substituent on the 2-position of the oxazole ring. Compounds 5 have been further used to obtain phosphonodipeptides 8, 9, and 10 with the original optical purity retained.

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Introduction

The past several decades have witnessed the vigorous development of 5-amino-1,3-oxazole chemistry since the compounds of this family are well recognized for their potent and diverse bioregulation activity. For instance, substituted 5-amino-1,3-oxazole-4-carboxylic acids exhibit a stimulating effect on the GABA_B receptor,¹ inhibit enzymes (such as kinase cRaf1),^2,3 and also have antibacterial properties.⁴ Of particular interest are 1,3-oxazoles bearing optically active 2-substituents as they can be regarded as masked peptides; these derivatives are potent α₄β₁ integrin antagonists⁵ and also target arthritis-associated class II MHC A^q and DR4 proteins.

Substitution of the carboxyl group by a phosphoryl group in molecules of this kind appears promising, as the latter residue has significantly different properties: it is more bulky, tetrahedral-shaped in contrast to the planar carboxyl moiety, and more acidic. As a result of these features, phosphorylated analogues of carboxylic acids have been applied in the research of enzyme–substrate interactions^6–10 and new enzyme inhibitors can be found among such compounds.

As shown previously,¹¹ introduction of a phosphoryl group at position 4 of the 1,3-oxazole ring makes the heterocycle drastically more susceptible to acid cleavage leading to the formation of phosphonoglycine derivatives. It is notable that phosphorylated glycines have received much recent attention in bioorganic, biological, and medicinal chemistry, as evidenced by the plethora of publications including reviews^12,13 on the synthesis and biological activity of aminophosphonic acids and mimetics of phosphonopeptides. However, peptidomimetics derived from phosphonoglycine appear to be the least studied. They have been employed in the synthesis of peptide natural products, e.g. azinomycins A and B,¹⁴ (±)-tunichrome An-1,¹⁵ tunichromes Mm-1 and Mm-2,¹⁶ hexaacetylcelenamide A,^17,18 clionamide,¹⁹ etc. Also, some compounds of this class are effective glutathione transferase inhibitors.^20,21

Among several available methods to synthesize such mimetics of phosphonopeptides, there is the reaction of α-metallated diethyl isocyanomethylphosphonates with allyl α-isocyanatocarboxylates at −70 °C.²² Another approach is based on the reaction between glyoxylic acid derivatives and N-benzylurethane to give N-benzyloxycarbonyl-2-ethoxyglycine derivatives. On their successive treatment with phosphorus trichloride and triethyl phosphite, the corresponding N-benzyloxycarbonyl substituted phosphonoglycines are formed which, after reductive debenzylation with hydrogen on palladium, are used in peptide synthesis.²³ As an example, the cores of the natural macrocyclic peptides mucronine B and chlamydocin were constructed by this strategy.^24,25 A recently reported route to phosphonoglycine peptidomimetics involves the formation of 2-aminoalkyl-5-amino-1,3-oxazol-4-ylphosphonic acids followed by acid cleavage of the oxazole ring.^26,27 The method is advantageous because of easily accessible and cheap starting materials as well as for high yields and facile isolation of products in each reaction step. However, it has not hitherto been attempted in the stereoselective synthesis of acylated phosphonoglycine derivatives starting from substrates with chiral 2-(aminoalkyl) substituents on the oxazole ring.

To find a pathway to 4-phosphorylated 5-amino-2-aminoalkyl-1,3-oxazoles with a chiral center in the aminoalkyl moiety, we have invoked the synthesis of 5-amino-1,3-oxazol-4-ylphosphonates developed in the seventies of the last century.^28,29 It is based on the mild and smooth reaction of easily accessible dialkyl 1-acylamino-2,2,2-trichloroethylphosphonates with primary or secondary aliphatic amines. The thus obtained 2-unsubstituted 4-phosphorylated 5-amino-1,3-oxazoles were found to afford, in hydrochloric acid medium, phosphonoglycine amides or their N-formyl derivatives.³⁰ As recently reported,²⁷ this synthetic approach enables the introduction of phthalimidoalkyl groups (contained in amino-protected glycine, β-alanine, γ-amino butyric, δ-amino valeric, and ε-amino capronic acids) at position 2 of the oxazole ring; subsequent phthaloyl deprotection provides 2-(aminoalkyl) substituted 4-phosphorylated oxazoles which have proved to be valuable building blocks in peptide synthesis.

The present work is aimed to extend the scope of the method to the amides of natural α-amino acids, L-alanine, L-valine, and L-leucine (containing the protective phthaloyl group), which have for the first time been reacted as described above to produce the corresponding phosphorylated oxazoles with chiral 2-(aminoalkyl) substituents and, finally, phosphonodipeptides.

Results and discussions

Heating phthalimido derivatives of α-amino acid amides 1a–c with chloral in the presence of a catalytic amount of concentrated sulfuric acid leads to high yields of condensation products, corresponding chloralamides 2a–c obtained in the form of colourless crystals (see Scheme 1). HPLC-MS/GC-MS demonstrate that due to the formation of a new asymmetric center, compounds 2a–c are isolated as a 1 : 1 diastereomeric mixture. When treated with a small excess of thionyl chloride, they yield N-(1,2,2,2-tetrachloroethyl)amides 3a–c appearing as thick viscous oils. Products 3a–c react with triethyl phosphite by the Arbuzov reaction to give a 1 : 1 diastereomeric mixture of diethyl 1-acylamino-2,2,2-trichloroethylphosphonates 4a–c (as determined by HPLC-MS and ³¹P NMR spectroscopy).

Scheme 1

We have studied the reaction of compounds 4a–c with morpholine in order to establish whether they can be cyclized to 1,3-oxazoles^26,27 containing optically active aminoalkyl 2-substituents. Heating diethyl 1-acylamino-2,2,2-trichloroethylphosphonates 4a–c with an equivalent quantity of morpholine in the presence of excess triethylamine affords the corresponding esters of 2-aminoalkyl-5-morpholino-1,3-oxazol-4-ylphosphonic acids 5a–c (see Scheme 2).

Scheme 2

On treatment of compounds 4a–c with excess morpholine at 20–25 °C, oxazole cyclization is accompanied by the cleavage of the phthalimido moiety resulting in phthalic diamides 6a–c. The amino group in compounds 5a–c and 6a–c is readily deprotected with hydrazine hydrate in ethanol to furnish phosphorylated 2-aminoalkyloxazoles 7a–c in 80–86% yields (see Scheme 2).

As already mentioned, 4-phosphorylated 1,3-oxazoles (e.g. 5-amino derivatives^26,27) are highly unstable in acidic medium to undergo ring opening. We have found that oxazoles 5a–c, when heated to 70 °C in 70% aqueous acetic acid, are cleaved to dipeptides 8a–c in 90–93% yields, with the phthaloyl protection of amino groups retained. Contrastingly, the action of acetic acid on oxazoles 6a–c leads to a mixture of unidentified products. To avoid this pathway, the oxazole ring cleavage in these compounds was performed under milder conditions, using p-toluenesulfonic acid in aqueous tetrahydrofuran at 20–25 °C. The resulting mimetics of phosphonopeptides 9a–c were isolated in 90–92% yields. Noteworthy, acidic cleavage of compounds 7 leads to a nonseparated mixture containing, among with 10, products of interaction of a primary amino group with AcOH or TsOH, which was detected by LCMS.

Deprotection of products 8a–c and 9a–c with hydrazine hydrate in ethanol at 20–25 °C provides phosphonodipeptides 10a–c (see Scheme 3).

Scheme 3

Both the ¹H and ¹³C NMR spectra of compounds 8a–c, 9a–c, and 10a–c contain double sets of signals thus suggesting the presence of diastereomeric pairs. Likewise, two ³¹P resonances of equal intensity emerge. These data indicate that acid-assisted oxazole ring cleavage to phosphonoglycine derivatives proceeds non-stereoselectively. To investigate the effect of the 1 → 2 → 3 → 4 → 5 → 7 and 5 → 8 → 10 conversions on the asymmetric center of the L-alanine, L-valine, and L-leucine residues, compounds 5a–c, 7a, 8a, and 10a were chromatographed on a HPLC chiral column. As found, oxazoles 5a–c and 7a are represented by one enantiomer, whereas phosphonodipeptides 8a and 10a are both formed as a diastereomeric pair. This is clear evidence that the asymmetric center of L-alanine, L-valine, and L-leucine remains intact when the amino acid residues are introduced into phosphonoglycine amides. For comparison, the approach developed was used with the same starting amino acids derivatives (phthalimido amides 1) in racemic form to give, finally, two diastereomeric pairs in each case.

Conclusions

Starting from L-alanine, L-valine, and L-leucine derivatives with phthaloyl-protected amino groups, we have presented a facile preparative route to 5-amino-1,3-oxazol-4-ylphosphonic acid derivatives 5 and 7 containing a chiral aminoalkyl substituent on the 2-position of the oxazole ring. It has been demonstrated that oxazoles 5 and 6 can be employed in peptide chain construction. As found, the chiral center of aminoalkyl moieties remains intact in the conversions studied, so that optical purity is retained for oxazoles 5 and 7 as well as for phosphonodipeptides 8 and 10.

Experimental section

The NMR spectra were obtained on a Bruker Avance DRX-500 instrument [¹H (500 MHz), ³¹P (202 MHz), ¹³C (125 MHz)] in a solution of DMSO-d₆, relative to internal TMS or external 85% phosphoric acid. The IR spectra were recorded on a Vertex 70 spectrometer with an ATR adaptor from KBr pellets or dichloromethane solution. The melting points were determined on a Fisher-Johns instrument. Elemental analysis was carried out in the analytical laboratory of IBOPC NASU. The LC/MS spectra were recorded on an Agilent 1100 Series high-performance liquid chromatograph equipped with a diode matrix with an Agilent LC\MSD SL mass selective detector allowing fast switching between positive and negative ionization modes. The LC/MS parameters were set as follows: column, Zorbax SBC18 1.8 μm, 4.6 × 15 mm (PN 821975-932); solvents A, acetonitrile–water mixture (95 : 5), 0.1% trifluoroacetic acid, and B, 0.1% aqueous trifluoroacetic acid; eluent flow rate, 3 mL min⁻¹; injection volume, 1 μL; UV detection, 215, 254, 265 nm; ionization method, atmospheric-pressure chemical ionization (APCI); scanning range, m/z 80–1000. Optical purity was measured on the Agilent 1100 system with a diode array detector on a CHIRALPAK® IA column (5 μm, 4.6 × 250 mm); mobile phase, hexane : 2-propanol. The reaction progress was TLC-monitored on Silica gel 60 F₂₅₄ (Merck). Optical rotations were measured with an Anton Paar MCP 300 polarimeter.

Phthalimido amides

Commercially unavailable compounds 1a–c (ref. 31) were obtained by known methods.

General procedure for the preparation of 2a–c. Condensation of amides 1a–e with chloral

A mixture of one of compounds 1a–c (0.20 mol), anhydrous chloral (44.22 g, 0.30 mol), and conc. sulfuric acid (1 mL) was kept at a temperature of 95–100 °C for 8 h and cooled by pouring into water (500 mL). The crystalline precipitate was filtered, washed on the filter with water to pH 7, and recrystallized from ethanol.

2-(1,3-Dioxo-2,3-dihydro-1H-isoindol-2-yl)-N-(2,2,2-trichloro-1-hydroxyethyl)propanamide (2a). Yield 60.35 g, 82% as a colourless solid; mp 149–151 °C; [found: C, 42.65; H, 3.13; Cl, 29.11; N, 7.69. C₁₃H₁₁Cl₃N₂O₄ requires C, 42.71; H, 3.03; Cl, 29.09; N, 7.66%]; [α]²⁰_D −9.5 (c 0.7, CH₂Cl₂); ν_max (KBr) 3323, 1775, 1709, 1513 cm⁻¹; δ_H (DMSO-d₆) 8.98–8.92 (1H, m, NH), 7.88–7.85 (4H, m, aromatic), 7.76–7.75 (1H, m, OH), 5.76–5.74 (1H, m, CH), 4.90–4.83 (1H, m, CH), 1.67–1.65 (3H, m, CH₃); δ_C (DMSO-d₆) 169.27, 169.21 (C=O), 167.30, 167.14 (C=O), 134.05, 133.95, 131.38, 131.28, 122.55, 122.50 (aromatic), 101.95, 101.51 (CCl₃), 80.45, 80.28 (NCOH), 47.90, 47.88 (NCH), 14.10, 13.98 (CH₃).

2-(1,3-Dioxo-2,3-dihydro-1H-isoindol-2-yl)-3-methyl-N-(2,2,2-trichloro-1-hydroxyethyl)butanamide (2b). Yield 65.83 g, 84% as a colourless solid; mp 154–156 °C; [found: C, 45.63; H, 3.97; Cl, 27.21; N, 7.01. C₁₅H₁₅Cl₃N₂O₄ requires C, 45.79; H, 3.84; Cl, 27.02; N, 7.10%]; [α]²⁰_D +7.1 (c 1.3, CH₂Cl₂); ν_max (KBr) 3280, 1773, 1711, 1550 cm⁻¹; δ_H (DMSO-d₆) 8.81–8.79 (1H, m, NH), 7.93–7.87 (4H, m, aromatic), 7.72–7.71 (1H, m, OH), 5.74–5.71 (1H, m, CH), 4.51–4.48 (1H, m, CH), 2.91–2.84 (1H, m, CH), 1.06 (3H, d, J 6.2 Hz, CH₃), 0.82 (3H, d, J 6.2 Hz, CH₃); δ_C (DMSO-d₆) 168.22 (C=O), 167.51, 167.46 (C=O), 134.32, 134.09, 130.93, 130.62, 122.82, 122.65 (aromatic), 101.27, 94.34 (CCl₃), 79.91 (NCOH), 59.53, 57.50 (NCH), 26.10, 25.92 (CH), 19.86, 18.76, 18.30, 18.28 (CH₃).

2-(1,3-Dioxo-2,3-dihydro-1H-isoindol-2-yl)-4-methyl-N-(2,2,2-trichloro-1-hydroxyethyl)pentanamide (2c). Yield 67.46 g, 82% as a colourless solid; mp 135–137 °C; [found: C, 47.07; H, 4.21; Cl, 26.14; N, 6.73. C₁₆H₁₇Cl₃N₂O₄ requires C, 47.14; H, 4.20; Cl, 26.09; N, 6.87%]; [α]²⁰_D +4.7 (c 1.3, CH₂Cl₂); ν_max (KBr) 3351, 1774, 1715, 1526 cm⁻¹; δ_H (DMSO-d₆) 9.10–8.97 (1H, m, NH), 7.91–7.85 (4H, m, aromatic), 7.84–7.82 (1H, m, OH), 5.76–5.73 (1H, m, CH), 4.95–4.82 (1H, m, CH), 2.41–2.33 (1H, m, CH_aH_b), 1.85–1.79 (1H, m, CH_aH_b), 1.45–1.39 (1H, m, CH), 0.87–0.85 (6H, m, 2CH₃); δ_C (DMSO-d₆) 168.05 (C=O), 166.65, 166.51 (C=O), 133.22, 133.15, 129.93, 129.91, 121.68, 121.65, (aromatic), 100.82, 100.47 (CCl₃), 79.39, 79.26 (NCOH), 50.37, 50.31 (NCH), 34.40, 34.33 (CH), 22.85, 22.83 (CH₂), 21.11, 21.05 (CH₃), 18.43, 18.31 (CH₃).

General procedure for the preparation of 3a–c. Tetrachloroethylamides

A mixture of one of compounds 2a–c (0.16 mol), thionyl chloride (14 mL, 0.19 mol), and dry toluene (100 mL) was refluxed with stirring until no more gas was evolved and for another 30 min. The solvent was evaporated under reduced pressure to dryness to give the analytically pure product.

2-(1,3-Dioxo-2,3-dihydro-1H-isoindol-2-yl)-N-(1,2,2,2-tetrachloroethyl)propanamide (3a). Yield 58.49 g, 96% as a yellowish oil; [found: C, 40.80; H, 2.55; Cl, 37.01; N, 7.39. C₁₃H₁₀Cl₄N₂O₃ requires C, 40.66; H, 2.62; Cl, 36.93; N, 7.29%]; [α]²⁰_D −22.1 (c 1.3, CH₂Cl₂); ν_max (ATR) 3317, 1779, 1702, 1508 cm⁻¹; δ_H (CDCl₃) 7.88 (2H, m, aromatic), 7.77 (2H, m, aromatic), 7.49 (1/2H, d, J 9.3 Hz, NH), 7.34 (1/2H, d, J 9.3 Hz, NH), 6.51–6.48 (1H, m, CHN), 5.05–5.01 (1H, m, CH), 1.77–1.74 (3H, m, CH₃); δ_C (CDCl₃) 167.80 (C=O), 168.71, 168.58 (C=O), 134.70, 134.62, 131.57, 131.54, 123.86, 123.82 (aromatic), 99.30 (CCl₃), 73.92, 73.76 (NCHCl), 50.02, 49.90 (NCH), 15.27, 15.10 (CH₃).

2-(1,3-Dioxo-2,3-dihydro-1H-isoindol-2-yl)-3-methyl-N-(1,2,2,2-tetrachloroethyl)butanamide (3b). Yield 64.63 g, 98% as a yellowish oil; [found: C, 43.62; H, 3.50; Cl, 34.48; N, 6.65. C₁₅H₁₄Cl₄N₂O₃ requires C, 43.72; H, 3.42; Cl, 34.41; N, 6.80%]; [α]²⁰_D +9.4 (c 1.3, CH₂Cl₂); ν_max (ATR) 3281, 1779, 1706, 1512 cm⁻¹; δ_H (CDCl₃) 8.74 (1/2H, d, J 9.5 Hz, NH), 8.50 (1/2H, d, J 9.5 Hz, NH), 7.88 (2H, m, aromatic), 7.78 (2H, m, aromatic), 6.53–6.47 (1H, m, CH), 4.57–4.52 (1H, m, CH), 2.88–2.74 (1H, m, CH), 1.11–1.09 (3H, m, CH₃), 0.91–0.83 (3H, m, CH₃); δ_C (CDCl₃) 168.92, 168.78 (C=O), 168.22 (C=O), 135.26, 135.18, 131.79, 131.57, 123.81, 123.74, 102.90 (aromatic), 102.90, 102.52 (CCl₃), 81.43, 81.19 (NCHCl), 60.89, 60.20 (NCH), 27.61, 27.50 (CH), 20.55, 20.39, 20.17, 19.92 (CH₃).

2-(1,3-Dioxo-2,3-dihydro-1H-isoindol-2-yl)-4-methyl-N-(1,2,2,2-tetrachloroethyl)pentanamide (3c). Yield 64.54 g, 95% as a yellowish oil; [found: C, 45.12; H, 3.87; Cl, 33.36; N, 6.49. C₁₆H₁₆Cl₄N₂O₃ requires C, 45.10; H, 3.78; Cl, 33.28; N, 6.57%]; [α]²⁰_D +24.25 (c 0.7, CH₂Cl₂); ν_max (ATR) 3315, 1776, 1704, 1512; δ_H (CDCl₃) 7.92–7.72 (5H, m, aromatic, NH), 6.50–6.47 (1H, m, CHN), 5.03–4.99 (1H, m, CH), 2.37–2.20 (1H, m, CH_aH_b), 1.92–1.83 (1H, m, CH_aH_b), 1.50–1.43 (1H, m, CH), 0.94–0.90 (6H, m, 2CH₃); δ_C (CDCl₃) 169.92, 166, 70 (C=O), 166.51, 166.44 (C=O), 132.81, 132.76, 126.35, 122.96, 121.90, 121.83 (aromatic), 97.17, 97.00 (CCl₃), 71.70, 71.44 (NCHCl), 51.60, 51.50 (NCH), 34.99, 34.90 (CH₂), 22.66, 22.55 (CH), 20.24, 20.15, 17.98, 17.66 (CH₃).

General procedure for the preparation of 4a–c. Arbuzov rearrangement

A mixture of one of compounds 3a–c (0.14 mol), triethyl phosphite (30 mL, 0.17 mol), and dry dioxane (150 mL) was refluxed for 3 h and evaporated in vacuo to dryness. To isolate 4a and 4c, the residue was recrystallized from benzene; in the case of 4b, the solvent was evaporated under reduced pressure to dryness to give the analytically pure product.

Diethyl {2,2,2-trichloro-1-[2-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)propanamido]ethyl}phosphonate (4a). Yield 58.44 g, 84% as a colourless solid; mp 142–144 °C; [found: C, 42.13; H, 4.26; Cl, 21.81; N, 5.70; P, 6.43. C₁₇H₂₀Cl₃N₂O₆P requires C, 42.04; H, 4.15; Cl, 21.90; N, 5.77; P, 6.38%]; [α]²⁰_D −15.8 (c 1.3, CH₂Cl₂); ν_max (KBr) 3248, 1778, 1716, 1540, 1260, 1023 cm⁻¹; δ_H (DMSO-d₆) 9.43 (1/2H, d, J 6.7 Hz, NH), 9.34 (1/2H, d, J 6.7 Hz, NH), 7.89 (4H, m, aromatic), 5.28–5.18 (1H, m, CHP), 5.04–4.98 (1H, m, CH), 4.15–4.05 (4H, m, 2OCH₂CH₃), 1.77–1.72 (3H, m, CH₃), 1.30–1.20 (6H, m, 2OCH₂CH₃); δ_C (DMSO-d₆) 170.71 (d, J 4.5 Hz, C=O), 170.57 (d, J 4.5 Hz, C=O), 168.05, 167.95 (C=O), 135.06, 135.00, 132.10, 132.01, 123.59, 123.57 (aromatic), 97.62, (d, J 14.5 Hz, CCl₃), 97.17 (d, J 14.5 Hz, CCl₃), 64.07–63.86 (d, J 6.5 Hz, OCH₂CH₃), 62.10 (d, J 159.0 Hz, CP), 61.73 (d, J 159.0 Hz, CP), 49.73, 49.45 (CH), 16.74 (d, J 4.8 Hz, OCH₂CH₃), 16.56 (d, J 4.8 Hz, OCH₂CH₃), 15.85, 15.78 (CH₃); δ_P (DMSO-d₆) 14.6, 14.3; LCMS: found m/z 486.8 MH⁺. C₁₇H₂₀Cl₃N₂O₆P requires 485.7.

Diethyl {2,2,2-trichloro-1-[2-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)-3-methylbutanamido]ethyl}phosphonate (4b). Yield 58.84 g, 80% as a yellow oil; [found: C, 44.55; H, 4.86; Cl, 20.82; N, 5.53; P, 6.19. C₁₉H₂₄Cl₃N₂O₆P requires C, 44.42; H, 4.71; Cl, 20.70; N, 5.45; P, 6.03%]; [α]²⁰_D +5.5 (c 1.2, CH₂Cl₂); ν_max (ATR) 2974, 1779, 1712, 1519, 1259, 1014, 972 cm⁻¹; δ_H (DMSO-d₆) 9.32 (1/2H, d, J 9.3 Hz, NH), 9.22 (1/2H, d, J 9.3 Hz, NH), 7.92–7.88 (4H, m, aromatic), 5.29–5.21 (1H, m, CHP), 4.68–4.61 (1H, m, CH), 4.10–4.00 (4H, m, 2OCH₂CH₃), 2.97–2.90 (1H, m, CH), 1.22–1.15 (6H, m, 2OCH₂CH₃), 1.11–1.05 (3H, m, CH₃), 0.88–0.79 (3H, m, CH₃); δ_C (DMSO-d₆) 168.61 (d, J 4.5 Hz, C=O), 167.41 (d, J 4.5 Hz, C=O), 134.41, 134.36, 130.55, 130.53, 122.85, 122.80 (aromatic), 96.21 (d, J 14.5 Hz, CCl₃), 96.06 (d, J 14.5 Hz, CCl₃), 62.30 (d, J 6.5 Hz, OCH₂CH₃), 62.07 (d, J 6.5 Hz, OCH₂CH₃), 60.31 (d, J 158.8 Hz, CP), 60.26 (d, J 158.6 Hz, CP), 59.31, 59.22 (CH), 25.92, 25.84 (CH), 18.55, 18.50 (CH₃), 18.49, 18.36 (CH₃), 14.97 (d, J 6.0 Hz, OCH₂CH₃), 14.86 (d, J 6.0 Hz, OCH₂CH₃); δ_P (DMSO-d₆) 14.4–14.2; LCMS: found m/z 515.0 MH⁺. C₁₉H₂₄Cl₃N₂O₆P requires 513.7.

Diethyl {2,2,2-trichloro-1-[2-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)-4-methylpentanamido]ethyl}phosphonate (4c). Yield 61.60 g, 82% as a colourless solid; mp 133–134 °C; [found: C, 45.48; H, 4.84; Cl, 20.03; N, 5.41; P, 5.98. C₂₀H₂₆Cl₃N₂O₆P requires C, 45.52; H, 4.97; Cl, 20.15; N, 5.31; P, 5.87%]; [α]²⁰_D +6.0 (c 1.1, CH₂Cl₂); ν_max (KBr) 3212, 1776, 1717, 1530, 1246, 1023, 980 cm⁻¹; δ_H (DMSO-d₆) 9.63 (1/2H, d, J 9.3 Hz, NH), 9.43 (1/2H, d, J 9.3 Hz, NH), 7.89 (m, 4H, aromatic), 5.27–5.17 (1H, m, CHP), 5.06–5.03 (1/2H, m, CH), 5.02–4.98 (1/2H, m, CH), 4.15–4.03 (4H, m, 2OCH₂CH₃), 2.45–2.36 (1H, m, CH_aH_b), 1.95–1.86 (1H, m, CH_aH_b), 1.51–1.44 (1H, m, CH), 1.31–1.19 (6H, m, 2OCH₂CH₃), 0.89 (6H, m, 2CH₃); δ_C (DMSO-d₆)‡,§ 168.76 (d, J 4.5 Hz, C=O), 166.63 (C=O), 133.33, 129.75, 121.72 (aromatic), 95.32 (d, J 14.5 Hz, CCl₃) 61.72 (d, J 6.5 Hz, OCH₂CH₃), 61.20 (d, J 6.5 Hz, OCH₂CH₃), 59.28 (d, J 159.0 Hz, CP), 50.65 (CH), 34.21 (CH₂), 21.96, 21.11 (CH), 18.23 (CH₃), 14.03 (d, J 6.5 Hz, OCH₂CH₃), 13.91 (d, J 4.5 Hz, OCH₂CH₃); δ_P (DMSO-d₆) 14.6, 14.3; LCMS: found m/z 528.8 MH⁺. C₂₀H₂₆Cl₃N₂O₆P requires 527.8.

General procedure for the preparation of 5a–c. Oxazole cyclization

A mixture of one of compounds 4a–e (30 mmol), morpholine (31.5 mmol), and triethylamine (25.05 mL, 180 mmol) in dry dioxane (125 mL) was refluxed for 24 h under TLC control, cooled, and evaporated in vacuo to dryness. If necessary, the reaction product can be purified by extraction with boiling petroleum ether (bp 80–110 °C), followed by evaporation of the combined extracts in vacuo.

Diethyl {2-[1-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)ethyl]-5-(morpholin-4-yl)-1,3-oxazol-4-yl}phosphonate (5a). Yield 9.00 g, 65% as a colourless solid; mp 174–175 °C; [found: C, 54.56; H, 5.54; N, 9.16; P, 6.79. C₂₁H₂₆N₃O₇P requires C, 54.43; H, 5.65; N, 9.07; P, 6.68%]; [α]²⁰_D +2.8 (c 0.6, CH₃OH); ν_max (KBr) 1777, 1715, 1612, 1573, 1263, 1025, 969 cm⁻¹; δ_H (DMSO-d₆) 7.93–7.85 (4H, m, aromatic), 5.45 (1H, q, J 6.8 Hz, CH), 4.02–3.91 (4H, m, 2OCH₂CH₃), 3.64 (4H, m, 2CH₂), 3.42 (4H, m, 2CH₂), 1.74 (3H, d, J 6.8 Hz, CH₃), 1.22–1.15 (6H, d, J 7.0 Hz, 2OCH₂CH₃); δ_C (DMSO-d₆)‡ 166.70 (C=O), 160.89 (d, J 38.0 Hz, O–C=C–P), 151.07 (d, J 21.4 Hz, O–C=N), 134.34, 130.80, 122.83 (aromatic), 100.34 (d, J 251.3 Hz, CP), 64.55 (OCH₂, morpholine), 60.94–60.88 (m, OCH₂CH₃), 46.88 (NCH₂, morpholine), 41.71 (CH), 15.76 (CH₃), 14.85–14.77 (m, OCH₂CH₃); δ_P (DMSO-d₆) 12.4; LCMS: found m/z 464.5 MH⁺. C₂₁H₂₆N₃O₇P requires 463.4. Chiral HPLC: 1 peak; eluent, hexane/IPA (80 : 20, v/v); flow rate, 0.5 mL min⁻¹.

Diethyl {2-[1-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)-2-methylpropyl]-5-(morpholin-4-yl)-1,3-oxazol-4-yl}phosphonate (5b). Yield 10.22 g, 64% as a yellow oil; [found: C, 56.32; H, 6.24; N, 8.53; P, 6.25. C₂₃H₃₀N₃O₇P requires C, 56.21; H, 6.15; N, 8.55; P, 6.30%]; [α]²⁰_D +7.6 (c 0.7, CH₂Cl₂); ν_max (ATR) 1769, 1717, 1611, 1573, 1266, 1017, 961 cm⁻¹; δ_H (DMSO-d₆) 7.94–7.88 (4H, m, aromatic), 4.93 (1H, d, J 10.0 Hz, CH), 4.03–3.92 (4H, m, 2OCH₂CH₃), 3.66–3.63 (4H, m, 2CH₂), 3.46–3.42 (4H, m, 2CH₂), 2.93–2.86 (1H, m, CH), 1.23–1.16 (6H, m, 2OCH₂CH₃), 1.08 (3H, d, J 7.0 Hz, CH₃), 0.89 (3H, d, J 7.0 Hz, CH₃); δ_C (DMSO-d₆)‡,¶ 165.91 (C=O), 159.70 (d, J 38.0 Hz, O–C=C–P), 148.71 (d, J 21.4 Hz, O–C=N), 134.45, 129.40, 121.90 (aromatic), 98.71 (d, J 251.3 Hz, CP), 63.63, 63.49 (OCH₂, morpholine), 60.00–59.79 (m, OCH₂CH₃), 46.00, 45.79 (NCH₂, morpholine), 36.63 (CH), 25.91 (CH), 17.94 (CH₃), 16.83 (CH₃), 13.95–13.78 (m, OCH₂CH₃); δ_P (DMSO-d₆) 12.3; LCMS: found m/z 492.5 MH⁺. C₂₃H₃₀N₃O₇P requires 491.5. Chiral HPLC: 1 peak; eluent, hexane/IPA (90 : 10, v/v); flow rate, 0.6 mL min⁻¹.

Diethyl {2-[1-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)-3-methylbutyl]-5-(morpholin-4-yl)-1,3-oxazol-4-yl}phosphonate (5c). Yield 9.281 g, 61% as a yellow viscous oil; [found: C, 57.15; H, 6.29; N, 8.29; P, 6.09. C₂₄H₃₂N₃O₇P requires C, 57.02; H, 6.38; N, 8.31; P, 6.13%]; [α]²⁰_D −12.2 (c 1.0, CH₂Cl₂); ν_max (ATR) 1771, 1714, 1611, 1572, 1265, 1019, 960 cm⁻¹; δ_H (DMSO-d₆) 7.92–7.84 (4H, m, aromatic), 5.38–5.34 (1H, m, CH), 3.99–3.87 (4H, m, 2OCH₂CH₃), 3.64–3.61 (4H, m, 2CH₂), 3.42–3.39 (4H, m, 2CH₂), 2.32–2.23 (1H, m, CH_aH_b), 2.07–1.99 (1H, m, CH_aH_b), 1.59–1.49 (1H, m, CH), 1.19–1.12 (6H, m, 2OCH₂CH₃), 0.92–0.88 (6H, m, 2CH₃); δ_C (DMSO-d₆) 165.93 (C=O), 159.80 (d, J 36.9 Hz, O–C=C–P), 149.57 (d, J 21.4 Hz, O–C=N), 133.48, 125.51, 121.96 (aromatic), 99.8 (d, J 250.3 Hz, CP), 63.51 (OCH₂ morpholine), 59.94–59.88 (m, OCH₂CH₃), 45.81 (NCH₂, morpholine), 35.26 (CH), 22.26 (CH₂), 20.70 (CH), 19.09 (CH₃), 13.83–13.74 (m, OCH₂CH₃); δ_P (DMSO-d₆) 12.2; LCMS: found m/z 506.6 MH⁺. C₂₄H₃₂N₃O₇P requires 505.5. Chiral HPLC: 1 peak; eluent, hexane/IPA (80 : 20, v/v); flow rate, 0.5 mL min⁻¹.

General procedure for the preparation of 6a–c. Oxazole cyclization

A mixture of one of compounds 6a–c (30 mmol) and morpholine (11.78 mL, 135 mmol) in dry THF (125 mL) was stirred at 20–25 °C for 48 h under TLC control. The precipitate was filtered and washed on the filter with dry dioxane, and the combined filtrates were evaporated in vacuo to dryness. If necessary, the reaction product can be purified by extraction with boiling water, followed by evaporation of the combined extracts in vacuo.

Diethyl [5-(morpholin-4-yl)-2-[1-({2-[(morpholin-4-yl)carbonyl]phenyl}formamido)ethyl]-1,3-oxazol-4-yl]phosphonate (6a). Yield 10.42 g, 63% as a yellow viscous oil; [found: C, 54.69; H, 6.50; N, 10.20; P, 5.52. C₂₅H₃₅N₄O₈P requires C, 54.54; H, 6.41; N, 10.18; P, 5.63%]; [α]²⁰_D +15.3 (c 1.1, CH₂Cl₂); ν_max (ATR) 3474, 1717, 1614, 1573, 1276, 1016, 960 cm⁻¹; δ_H (DMSO–d₆) 8.95 (1H, d, J 8.0 Hz, NH), 7.68 (1H, d, J 7.3 Hz, aromatic), 7.55–7.52 (1H, m, aromatic), 7.49 (1H, t, J 7.3 Hz, aromatic), 7.30 (1H, d, J 7.3 Hz, aromatic), 5.14–5.08 (1H, m, CH), 4.03–3.99 (4H, m, 2OCH₂CH₃), 3.68–3.63 (8H, m, 4CH₂), 3.54–3.46 (8H, m, 4CH₂), 1.48 (3H, d, J 7.0 Hz, CH₃), 1.26–1.21 (6H, m, 2OCH₂CH₃); δ_C (DMSO-d₆) 167.58 (C=O), 165.01 (C=O), 159.60 (d, J 36.9 Hz, O–C=C–P), 152.89 (d, J 22.1 Hz, O–C=N), 135.08, 133.32, 131.89, 129.16, 126.94, 126.24, 125.12, 121.80 (aromatic), 104.79 (d, J 259.3 Hz, CP), 63.65 (OCH₂, morpholine), 59.85 (d, J 5.0 Hz, OCH₂CH₃), 45.99 (NCH₂, morpholine), 40.51 (CH), 15.89 (CH₃), 13.93 (d, J 6.5 Hz, OCH₂CH₃); δ_P (DMSO-d₆) 12.7; LCMS: found m/z 549.5 M − H⁻. C₂₅H₃₅N₄O₈ requires 550.4.

Diethyl {2-[2-methyl-1-({2-[(morpholin-4-yl)carbonyl]phenyl}formamido)propyl]-5-(morpholin-4-yl)-1,3-oxazol-4-yl}phosphonate (6b). Yield 10.93 g, 63% as a yellow viscous oil; [found: C, 56.19; H, 6.63; N, 9.58; P, 5.27. C₂₇H₃₉N₄O₈P requires C, 56.05; H, 6.79; N, 9.68; P, 5.35%]; [α]²⁰_D −26.7 (c 1.0, CH₂Cl₂); ν_max (ATR) 1719, 1613, 1572, 1262, 1016, 960 cm⁻¹; δ_H (DMSO-d₆) 8.84 (1H, d, J 8.3 Hz. NH), 7.67 (1H, d, J 6.0 Hz, aromatic), 7.54 (1H, t, J 6.0 Hz, aromatic), 7.48–7.52 (1H, m, aromatic), 7.31 (1H, d, J 6.0 Hz, aromatic), 4.81–4.78 (1H, m, CH), 4.04–3.95 (4H, m, 2OCH₂CH₃), 3.74–3.59 (8H, m, 4CH₂), 3.56–3.49 (8H, m, 4CH₂), 2.26–2.23 (1H, m, CH), 1.24–1.20 (6H, m, 2OCH₂CH₃), 1.00 (3H, d, J 5.2 Hz, CH₃), 0.91 (3H, d, J 5.2 Hz, CH₃); δ_C (DMSO-d₆) 169.21 (C=O), 167.36 (C=O), 161.03 (d, J 36.4 Hz, O–C=C–P), 153.58 (d, J 20.9 Hz, O–C=N), 136.70, 134.11, 131.01, 128.94, 128.48, 123.07 (aromatic), 101.84 (d, J 249.8 Hz, CP), 65.98 (OCH₂, morpholine), 62.27–62.19 (m, OCH₂CH₃), 48.46 (NCH₂, morpholine), 42.13 (CH), 31.3 (CH), 19.30 (CH₃), 19.28 (CH₃), 16.54 (d, J 6.5 Hz, OCH₂CH₃); δ_P (DMSO-d₆) 12.6; LCMS: found m/z 579.6 MH⁺. C₂₇H₃₉N₄O₈P requires 578.6.

Diethyl {2-[3-methyl-1-({2-[(morpholin-4-yl)carbonyl]phenyl}formamido)butyl]-5-(morpholin-4-yl)-1,3-oxazol-4-yl}phosphonate (6c). Yield 12.15 g, 68% as a yellow viscous oil; [found: C, 56.69; H, 6.84; N, 9.39; P, 5.31. C₂₈H₄₁N₄O₈P requires C, 56.75; H, 6.97; N, 9.45; P, 5.23%]; [α]²⁰_D −21.2 (c 0.9, CH₂Cl₂); ν_max (ATR) 3459, 1718, 1616, 1574, 1535, 1270, 1019, 963 cm⁻¹; δ_H (DMSO-d₆) 8.88 (1H, d, J 8.0 Hz, NH), 7.64 (1H, d, J 6.6 Hz, aromatic), 7.55–7.49 (2H, m, aromatic), 7.31 (1H, d, J 7.0 Hz, aromatic), 5.10–5.04 (1H, m, CH), 4.03–3.95 (4H, m, 2OCH₂CH₃), 3.71–3.47 (16H, m, 8CH₂), 2.43 (1H, m, CH_aH_b), 1.82–1.64 (2H, m, CH_aH_b, CH), 1.27–1.21 (6H, m, 2OCH₂CH₃); 0.97–0.89 (6H, m, 2CH₃); δ_C (DMSO-d₆) 169.65 (C=O), 167.49 (C=O), 159.53 (d, J 33.4 Hz, O–C=C–P), 152.64 (d, J 22.4 Hz, O–C=N), 134.98, 133.47, 129.52, 129.10, 126.95, 126.18, 125.09, 121.94 (aromatic), 99.17 (d, J 248.8 Hz, CP), 64.28 (OCH₂, morpholine), 59.86–59.78 (m, OCH₂CH₃), 45.97 (NCH₂, morpholine), 39.41 (CH), 26.66 (CH₂), 22.16 (CH), 20.61 (CH₃), 19.49 (CH₃), 13.92 (d, J 7.0 Hz, OCH₂CH₃); δ_P (DMSO-d₆) 12.7; LCMS: found m/z 563.6 MH⁺. C₂₈H₄₁N₄O₈P requires 592.6.

General procedure for the preparation of 7a–c. Removal of the phthaloyl protection by hydrazinolysis

A mixture of one of compounds 5a–c or 6a–c (4 mmol), hydrazine hydrate (0.4 mL, 8 mmol), and ethanol (50 mL) was kept at a temperature of 45–50 °C for 3 h. The precipitate was filtered and washed on the filter with ethanol, and the combined filtrates were evaporated in vacuo to dryness. The residue was dissolved in dry dichloromethane (50 mL), followed by filtering the precipitate and washing it on the filter with dry dichloromethane. The combined filtrates were evaporated under reduced pressure to dryness to give the analytically pure product.

Diethyl [2-(1-aminoethyl)-5-(morpholin-4-yl)-1,3-oxazol-4-yl]phosphonate (7a). Yield 1.14 g, 86% as a yellow viscous oil; [found: C, 46.75; H, 7.36; N, 12.55; P, 9.39. C₁₃H₂₄N₃O₅P requires C, 46.84; H, 7.26; N, 12.61; P, 9.29%]; [α]²⁰_D +3.6 (c 7.6, CH₂Cl₂); ν_max (ATR) 2978, 1606, 1570, 1265, 1018, 959 cm⁻¹; δ_H (DMSO-d₆) 4.04–3.95 (4H, m, 2OCH₂CH₃), 3.91–3.87 (1H, q, J 6.6 Hz, CH), 3.69–3.67 (4H, m, 2CH₂), 3.48–3.46 (4H, m, 2CH₂), 1.31–1.29 (3H, d, J 6.6 Hz, CH₃), 1.25–1.21 (6H, m, 2OCH₂CH₃). δ_C (DMSO-d₆) 159.52 (d, J 37.4 Hz, O–C=C–P), 157.03 (d, J 20.4 Hz, O–C=N), 98.81 (d, J 51.8 Hz, CHP), 63.65 (OCH₂, morpholine), 59.74 (d, J 5.5 Hz, OCH₂CH₃), 46.05 (NCH₂-morpholine), 42.62 (CH), 19.16 (CH₃), 13.94 (d, J 6.0 Hz, OCH₂CH₃); δ_P (DMSO-d₆) 12.3; LCMS: found m/z 334.3 MH⁺. C₁₃H₂₄N₃O₅P requires 333.3. Chiral HPLC: 1 peak; eluent, hexane/IPA (80 : 20, v/v); flow rate, 0.5 mL min⁻¹.

Diethyl [2-(1-amino-2-methylpropyl)-5-(morpholin-4-yl)-1,3-oxazol-4-yl]phosphonate (7b). Yield 1.198 g, 83% as a yellow viscous oil; found: C, 49.75; H, 7.96; N, 11.61; P, 8.69. C₁₅H₂₈N₃O₅P requires C, 49.86; H, 7.81; N, 11.63; P, 8.57%; [α]²⁰_D −5.3 (c 1.2, CH₂Cl₂); ν_max (ATR) 1611, 1572, 1264, 1017, 962 cm⁻¹; δ_H (DMSO-d₆) 4.10–4.04 (1H, m, CH), 4.02–3.94 (4H, m, 2OCH₂CH₃), 3.69–3.46 (10H, m, 4CH₂, NH₂), 1.23–1.19 (6H, m, 2OCH₂CH₃), 0.89 (3H, d, J 6.3 Hz, CH₃), 0.81 (3H, d, J 6.32 Hz, CH₃); δ_C (DMSO-d₆) 159.46 (d, J 37.4 Hz, O–C=C–P), 155.73 (d, J 20.9 Hz, O–C=N), 98.63 (d, J 250.8 Hz, CHP), 63.63 (OCH₂, morpholine), 59.80 (d, J 5.0 Hz, OCH₂CH₃), 52.96 (NCH₂, morpholine), 46.11 (NCH), 30.80 (CH), 16.65 (CH₃), 16.06 (CH₃), 13.90 (d, J 6.0 Hz, OCH₂CH₃); δ_P (DMSO-d₆) 12.2; LCMS: found m/z 362.5 MH⁺. C₁₅H₂₈N₃O₅P requires 361.4.

Diethyl [2-(1-amino-3-methylbutyl)-5-(morpholin-4-yl)-1,3-oxazol-4-yl]phosphonate (7c). Yield 1.200 g, 80% as a yellow viscous oil; found: C, 51.35; H, 8.06; N, 11.21; P, 8.19. C₁₆H₃₀N₃O₅P requires C, 51.19; H, 8.05; N, 11.19; P, 8.25%]; [α]²⁰_D −17.2 (c 1.3, CH₂Cl₂); ν_max (ATR) 1607, 1570, 1265, 1019, 959 cm⁻¹; δ_H (DMSO-d₆) 4.03–3.94 (4H, m, 2OCH₂CH₃), 3.78–3.74 (1H, t, J 7.4 Hz, CH), 3.68–3.66 (4H, m, 2CH₂), 3.48–3.45 (4H, m, 2CH₂), 1.65–1.47 (3H, m, CHCH₂), 1.24–1.20 (6H, m, 2OCH₂CH₃), 0.86 (3H, d, J 6.3 Hz, CH₃), 0.83 (3H, d, J 6.3 Hz, CH₃); δ_C (DMSO–d₆) 159.40 (d, J 37.4 Hz, O–C=C–P), 156.70 (d, J 20.9 Hz, O–C=N), 98.76 (d, J 250.8 Hz, CHP), 63.66 (OCH₂, morpholine), 59.74 (d, J 5.5 Hz, OCH₂CH₃), 46.09 (NCH₂, morpholine), 42.47 (CH), 22.44 (CH₂), 20.20 (CH), 19.96 (CH₃), 13.88 (d, J 6.0 Hz, OCH₂CH₃). δ_P (DMSO-d₆) 12.2; LCMS: found m/z 376.5 MH⁺. C₁₆H₃₀N₃O₅P requires 375.4.

General procedure for the preparation of peptidomimetics 8a–c. Oxazole ring opening

The compounds were obtained as previously described.²⁷ One of oxazoles 5a–c (6 mmol) was heated in a 5 : 1 (v/v) AcOH–H₂O mixture (20 mL) for 8 h at 75 °C, followed by evaporation of the solvents in vacuo to dryness. To remove AcOH completely, the residue was treated with a 5% Na₂CO₃ solution (20 mL) and extracted with dichloromethane (3 × 50 mL). The extract was dried over MgSO₄ and evaporated under reduced pressure to dryness. The resulting product was analyzed without further purification.

Diethyl {1-[2-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)propanamido]-2-(morpholin-4-yl)-2-oxoethyl}phosphonate (8a). Yield 2.773 g, 96% as a colourless solid; mp 114–115 °C; [found: C, 52.23; H, 5.79; N, 8.61; P, 6.51. C₂₁H₂₈N₃O₈P requires C, 52.39; H, 5.86; N, 8.73; P, 6.43%]; [α]²⁰_D −14.4 (c 0.6, CH₂Cl₂); ν_max (KBr) 3303, 1778, 1716, 1674, 1640, 1529, 1260, 1021, 971 cm⁻¹; δ_H (DMSO-d₆) 8.75 (1/2H, d, J 8.5 Hz, NH), 8.68 (1/2H, d, J 8.5 Hz, NH), 7.89–7.86 (4H, m, aromatic), 5.49–5.36 (1H, m, CHP), 4.99–4.87 (1H, m, CH), 4.12–3.98 (4H, m, 2OCH₂CH₃), 3.60–3.37 (8H, m, 4CH₂), 1.69–1.63 (3H, m, CH₃), 1.26–1.14 (6H, m, 2OCH₂CH₃); δ_C (DMSO-d₆)‡ 169.46 (d, J 5.0 Hz, C=O), 169.22 (d, J 5.0 Hz, C=O), 168.10, 167.98 (C=O), 164.44 (d, J 2.5 Hz, C=O), 164.37 (d, J 2.5 Hz, C=O), 135.01, 134.97, 132.11, 132.07, 123.54, 123.52 (aromatic), 66.42, 66.39 (OCH₂, morpholine), 63.55–63.46 (m, OCH₂CH₃), 63.21–63.11 (m, OCH₂CH₃), 49.43, 49.20 (NCH₂, morpholine), 48.46 (d, J 149.0 Hz, CHP), 48.25 (d, J 149.0 Hz, CHP), 46.81 (NCH₂, morpholine), 42.92, 42.84 (CH), 16.81–16.52 (m, OCH₂CH₃), 15.98, 15.74 (CH₃); δ_P (DMSO-d₆) 18.2; 17.9; LCMS: found m/z 482.4 MH⁺. C₂₁H₂₈N₃O₈P requires 481.4. Chiral HPLC: 1 peak; eluent, hexane/IPA (50 : 50, v/v); flow rate, 0.3 mL min⁻¹.

Diethyl {1-[2-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)-3-methylbutanamido]-2-(morpholin-4-yl)-2-oxoethyl}phosphonate (8b). Yield 2.923 g, 95% as a yellow viscous oil; [found: C, 54.10; H, 6.38; N, 8.37; P, 6.19. C₂₃H₃₂N₃O₈P: C, 54.22; H, 6.33; N, 8.25; P, 6.08%]; [α]²⁰_D −19.2 (c 0.8, CH₂Cl₂); ν_max (ATR) 3320, 1769, 1712, 1644, 1514, 1250, 1014, 965 cm⁻¹; δ_H (DMSO-d₆) 8.60 (1/2H, d, J 8.5 Hz, NH), 8.39 (1/2H, d, J 8.5 Hz, NH), 7.94–7.86 (4H, m, aromatic), 5.47–5.38 (1H, m, CHP), 4.54 (1/2H, d, J 10.5 Hz, CH), 4.49 (1/2H, d, J 10.5 Hz, CH), 4.04–3.95 (4H, m, 2OCH₂CH₃), 3.60–3.43 (8H, m, 4CH₂), 2.88–2.80 (1H, m, CH), 1.24–1.10 (6H, m, 2OCH₂CH₃), 1.04–1.00 (3H, m, CH₃), 0.82–0.76 (3H, m, CH₃); δ_C (DMSO-d₆)‡,|| 166.49, 166.45 (C=O), 166.36, 166.33, 166.28 (C=O), 166.45 (d, J 2.0 Hz, C=O), 162.40 (d, J 2.0 Hz, C=O), 133.36, 129.55, 129.51, 121.81, 121.77 (aromatic), 64.04, 64.00 (OCH₂, morpholine), 61.13 (d, J 6.5 Hz, OCH₂CH₃), 60.96 (d, J 6.0 Hz, OCH₂CH₃), 60.76 (d, J 6.5 Hz, OCH₂CH₃), 60.64 (d, J 6.0 Hz, OCH₂CH₃), 58.58, 58.50 (NCH₂, morpholine), 45.66 (d, J 147.6 Hz, CHP), 45.54 (d, J 147.6 Hz, CHP), 44.42, 44.29 (NCH₂, morpholine), 40.48, 40.34 (CH), 24.92, 24.75 (CH), 17.60, 17.52, 17.26 (CH₃), 14.04–13.79 (m, OCH₂CH₃); δ_P (DMSO-d₆) 18.2, 17.9; LCMS: found m/z 510.5 MH⁺. C₂₃H₃₂N₃O₈P requires 509.5.

Diethyl {1-[2-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)-4-methylpentanamido]-2-(morpholin-4-yl)-2-oxoethyl}phosphonate (8c). Yield 3.078 g, 98% as a yellow viscous oil; [found: C, 55.19; H, 7.09; N, 8.14; P, 5.81. C₂₄H₃₄N₃O₈P requires C, 55.06; H, 6.55; N, 8.03; P, 5.93%]; [α]²⁰_D −7.4 (c 0.6, CH₂Cl₂); ν_max (ATR) 3318, 1765, 1714, 1645, 1516, 1251, 1016, 964 cm⁻¹; δ_H (DMSO-d₆) 8.90 (1/2H, d, J 8.5 Hz, NH), 8.87 (1/2H, d, J 8.5 Hz, NH), 7.91–7.86 (4H, m, aromatic), 5.46–5.38 (1H, m, CHP), 4.99–4.90 (1H, m, CH), 4.12–3.98 (4H, m, 2OCH₂CH₃), 3.61–3.36 (8H, m, 4CH₂), 2.45–2.27 (1H, m, CH_aH_b), 1.90–1.79 (1H, m, CH_aH_b), 1.43–1.37 (1H, m, CH), 1.25–1.14 (6H, m, 2OCH₂CH₃), 0.86–0.83 (6H, m, 2CH₃); δ_C (DMSO-d₆)‡,** 167.54 (d, J 5.0 Hz, C=O), 167.24 (d, J 5.0 Hz, C=O), 166.67, 166.59 (C=O), 162.60–162.56 (m, C=O), 133.23, 129.82, 129.80, 121.67 (aromatic), 64.05, 64.03 (OCH₂, morpholine), 61.16–61.10 (d, J 6.0 Hz, OCH₂CH₃), 61.05–60.74 (d, J 6.0 Hz, OCH₂CH₃), 50.34, 50.20 (NCH₂, morpholine), 46.95 (d, J 147.6 Hz, CHP), 45.77 (d, J 147.6 Hz, CHP), 44.33 (NCH₂, morpholine) 40.41, 40.33 (CH), 34.55, 34.35 (CH₂), 22.95, 22.85 (CH), 18.35, 18.24 (CH₃), 14.15–13.84 (m, OCH₂CH₃); δ_P (DMSO-d₆) 18.0, 17.9; LCMS: found m/z 524.2 MH⁺. C₂₄H₃₄N₃O₈P requires 523.5.

General procedure for the preparation of peptidomimetics 9a–c. Oxazole ring opening

A mixture of one of compounds 6a–c (6 mmol) and p-toluenesulfonic acid monohydrate (6 mmol, 0.64 g) in THF (50 mL) was stirred at 20–25 °C for 48 h under TLC control, followed by evaporation of the solvents in vacuo to dryness. To remove p-toluenesulfonic acid completely, the residue was treated with a 5% Na₂CO₃ solution (20 mL) and extracted with dichloromethane (3 × 50 mL). The extract was dried over MgSO₄ and evaporated under reduced pressure to dryness. The resulting product was analyzed without further purification.

Diethyl [2-(morpholin-4-yl)-1-[2-({2-[(morpholin-4-yl)carbonyl]phenyl}formamido)propanamido]-2-oxoethyl]phosphonate (9a). Yield 3.070 g, 90% as a yellow viscous oil; [found: C, 52.69; H, 6.66; N, 9.73; P, 5.3: C₂₅H₃₇N₄O₉P requires C, 52.81; H, 6.56; N, 9.85; P, 5.45%]; [α]²⁰_D +20.5 (c 0.9, CH₂Cl₂); ν_max (ATR) 3290, 1716, 1635, 1526, 1251, 1013, 973 cm⁻¹; δ_H (DMSO-d₆) 8.76 (1/2H, d, J 8.7 Hz, NH), 8.69 (1/2H, d, J 8.7 Hz, NH), 8.54 (1/2H, d, J 7.4 Hz, NH), 8.37 (1/2H, d, J 7.4 Hz, NH), 7.76–7.72 (1H, m, aromatic), 7.56–7.46 (2H, m, aromatic), 7.30–7.27 (1H, m, aromatic), 5.49–5.38 (1H, m, CHP), 4.68–4.56 (1H, m, CH), 4.07–4.01 (4H, m, 2OCH₂CH₃), 3.68–3.31 (16H, m, 8CH₂), 1.31–1.29 (3H, d, J 6.3 Hz, CH₃), 1.25–1.15 (6H, m, 2OCH₂CH₃); δ_C (DMSO-d₆)‡,§ 170.67 (d, J 4.5 Hz, C=O), 170, 59 (d, J 4.5 Hz, C=O), 167.63, 167.60 (C=O), 166.35, 166.22 (C=O), 162.69, 162.66 (C=O), 134.98, 134.90, 133.06, 133.03, 130.14, 130.09, 129.21, 129.17, 126.93, 126.88, 126.43 (aromatic), 64.07, 64.04 (OCH₂, morpholine), 61.26 (d, J 6.5 Hz, OCH₂CH₃), 61.14 (d, J 6.5 Hz, OCH₂CH₃), 60.94 (d, J 6.5 Hz, OCH₂CH₃), 60.76 (d, J 6.5 Hz, OCH₂CH₃), 50.47 (d, J 139.1 Hz, CP), 50.21 (d, J 139.1 Hz, CP), 47.74, 46.45, 45.05, 44.33 (NCH₂, morpholine), 40.45, 40.32 (CH), 15.85, 15.70 (CH₃), 14.15–13.96 (OCH₂CH₃); δ_P (DMSO-d₆) 17.6, 17.5; LCMS: found m/z 541.5 MH C₂₅H₃₇N₄O₉P requires 540.5.

Diethyl {1-[3-methyl-2-({2-[(morpholin-4-yl)carbonyl]phenyl}formamido)butanamido]-2-(morpholin-4-yl)-2-oxoethyl}phosphonate (9b). Yield 3.293 g, 92% as a yellow viscous oil; [found: C, 54.23; H, 6.70; N, 9.71; P, 5.51. C₂₇H₄₁N₄O₉P requires C, 54.36; H, 6.56; N, 9.85; P, 5.45%]; [α]²⁰_D −19.2 (c 0.9, CH₂Cl₂); ν_max (ATR) 3288, 1718, 1637, 1527, 1252, 1014, 965 cm⁻¹; δ_H (DMSO-d₆) 8.72–8.67 (1/2H, m, NH), 8.41–8.38 (1/2H, m, NH), 8.25–8.22 (1/2H, m, NH), 7.74–7.70 (1H, m, aromatic), 7.59–7.47 (2H, m, aromatic), 7.33–7.29 (1H, m, aromatic), 5.52–5.41 (1H, CHP), 4.59–4.52 (1/2H, m, CH), 4.45–4.42 (1/2H, m, CH), 4.11–3.97 (4H, m, 2OCH₂CH₃), 3.67–3.46 (16H, m, 8CH₂), 2.10–2.07 (1H, m, CH), 1.27–1.20 (6H, m, 2OCH₂CH₃), 0.95–0.90 (6H, m, 2CH₃); δ_C (DMSO-d₆)‡,§,|| 169.37, 169.30 (C=O), 167.57, 167.51 (C=O), 165.40, 165.26 (C=O), 162.76 (d, J 3.0 Hz, C=O), 162.68 (d, J 3.0 Hz, C=O), 134.60, 133.57, 132.07, 131.84, 129.11, 127.03, 126.94, 126.71, 126.51, 125.14, 125.05 (aromatic), 64.09, 63.98, 63.85, 63.77 (OCH₂, morpholine), 61.19 (d, J 6.0 Hz, OCH₂CH₃), 61.07 (d, J 6.5 Hz, OCH₂CH₃), 60.87 (d, J 6.0 Hz, OCH₂CH₃), 60.56 (d, J 6.5 Hz, OCH₂CH₃), 52.00 (d, J 128.6 Hz, CP), 51.90 (d, J 128.6 Hz, CP), 45.16–44.41 (m, NCH₂, morpholine), 40.32, 39.63 (CH), 28.48, 28.35 (CH), 17.07, 16.93 (CH₃), 16.26, 16.17 (CH₃), 14.12–13.93 (m, OCH₂CH₃); δ_P (DMSO-d₆) 17.7, 17.5; LCMS: found m/z 597.6 MH⁺. C₂₇H₄₁N₄O₉P requires 596.6.

Diethyl {1-[4-methyl-2-({2-[(morpholin-4-yl)carbonyl]phenyl}formamido)pentanamido]-2-(morpholin-4-yl)-2-oxoethyl}phosphonate (9c). Yield 3.334 g, 91% as a yellow viscous oil; [found: C, 55.23; H, 7.19; N, 9.31; P, 8.61. C₂₈H₄₃N₃O₆P requires C, 55.07; H, 7.10; N, 9.18; P, 8.82%]; [α]²⁰_D −20.2 (c 1.1, CH₂Cl₂); ν_max (ATR) 3288, 1719, 1635, 1524, 1255, 1016, 964 cm⁻¹; δ_H (DMSO-d₆) 8.90 (1/2H, d, J 8.20 Hz, NH), 8.78 (1/2H, d, J 8.2 Hz, NH), 8.67 (1/2H, d, J 7.8 Hz, NH), 8.56 (1/2H, d, J 7.8 Hz, NH), 7.76–7.65 (1H, m, aromatic), 7.67–7.47 (2H, m, aromatic), 7.39–7.27 (1H, m, aromatic), 5.50–5.37 (1H, m, CHP), 5.00–4.90 (1/2H, m, CH), 4.71–4.55 (1/2H, m, CH), 4.11–4.00 (4H, m, 2OCH₂CH₃), 3.62–3.45 (16H, m, 8CH₂), 2.42–2.28 (1H, m, CH_aCH_b), 1.93–1.82 (1H, m, CH_aCH_b), 1.54–1.38 (1H, m, CH), 1.26–1.14 (6H, m, 2OCH₂CH₃), 0.91–0.65 (6H, m, 2CH₃); δ_C (DMSO-d₆) 169.45, 169.37, 167.63 (C=O), 165.60, 162.35 (C=O), 162.80 (d, J 4.0 Hz, C=O), 162.75 (d, J 4.0 Hz, C=O), 134.63, 133.60, 132.13, 131.90, 129.23, 127.10, 126.96, 126.78, 126.56, 125.24, 125.15 (aromatic), 64.05, 63.92, 63.81, 63.70 (OCH₂, morpholine), 61.15 (d, J 6.0 Hz, OCH₂CH₃), 60.83 (d, J 6.0 Hz, OCH₂CH₃), 51.97 (d, J 128.6 Hz, CP), 51.15 (d, J 128.6 Hz, CP), 45.20, 43.42 (NCH₂, morpholine), 40.31, 39.72 (CH), 33.48, 33.32 (CH₂), 22.03, 22.10 (CH), 18.42, 18.35 (CH₃), 14.10–13.83 (m, OCH₂CH₃); δ_P (DMSO-d₆) 17.1, 17.0; LCMS: found m/z 611.5 MH⁺. C₂₈H₄₃N₃O₆P requires 610.6.

General procedure for the preparation of peptidomimetics 10a–c. Removal of the phthaloyl protection by hydrazinolysis

The compounds were obtained starting from 8a–c or 9a–c by the same procedure as used for 7a–c.

Diethyl [1-(2-aminopropanamido)-2-(morpholin-4-yl)-2-oxoethyl]phosphonate (10a). Yield 1.110 g, 79% as a yellow viscous oil; [found: C, 44.31; H, 7.36; N, 11.91; P, 8.89. C₁₃H₂₆N₃O₆P requires C, 44.44; H, 7.46; N, 11.96; P, 8.82%]; [α]²⁰_D +3.6 (c 0.6, CH₃OH); ν_max (ATR) 3357, 1639, 1506, 1242, 1012, 974 cm⁻¹; δ_H (CDCl₃) 8.24 (1/2H, m, NH), 8.13 (1H, m, N₂), 5.48–5.41 (1H, m, CHP), 4.20–4.13 (4H, m, 2OCH₂CH₃), 3.85–3.46 (9H, m, 4CH₂, CH), 2.15 (2H, br. s, NH₂), 1.36–1.29 (9H, m, 2OCH₂CH₃, CH₃); δ_C (DMSO-d₆)‡,§,|| 174.76–174.67 (m, C=O), 163.76–163.66 (m, C=O), 65.24, 65.21, 65.08 (OCH₂, morpholine), 62.26–62.16 (m, OCH₂CH₃), 61.88 (d, J 6.0 Hz, OCH₂CH₃), 49.12, 48.73 (NCH₂, morpholine), 45.84 (d, J 146.8 Hz, CHP), 45.90 (d, J 146.8 Hz, CHP), 45.36, 45.11 (NCH₂, morpholine), 41.40, 40.24 (CH), 20.15, 19.88 (CH₃), 15.13–14.92 (m, OCH₂CH₃); δ_P (CDCl₃) 17.5, 17.3; LCMS: found m/z 352.4 MH⁺. C₁₃H₂₆N₃O₆P requires 351.4. Chiral HPLC: 2 peaks of equal intensity; eluent, EDA/IPA (70 : 30, v/v); flow rate, 0.4 mL min⁻¹.

Diethyl [1-(2-amino-3-methylbutanamido)-2-(morpholin-4-yl)-2-oxoethyl]phosphonate (10b). Yield 1.290 g, 85% as a viscous oil; found: C, 47.55; H, 8.06; N, 11.21; P, 8.09. C₁₅H₃₀N₃O₆P requires C, 47.49; H, 7.97; N, 11.08; P, 8.16%]; [α]²⁰_D −12.2 (c 1.0, CH₂Cl₂); ν_max (ATR) 3341, 1639, 1506, 1242, 1016, 963 cm⁻¹; δ_H (DMSO-d₆) 5.45–5.40 (1H, m, CHP), 4.08–4.02 (4H, m, 2OCH₂CH₃), 3.66–3.35 (9H, m, 4CH₂, CH), 2.07–2.00 (1/2H, m, CH), 1.99–1.92 (1/2H, m, CH), 1.24–1.19 (6H, m, 2OCH₂CH₃), 1.06–1.03 (1H, m, CH), 0.93–0.85 (3H, m, CH₃), 0.78–0.72 (3H, m, CH₃); δ_C (DMSO-d₆)§ 172.84 (d, J 5.0 Hz, C=O), 172.48 (d, J 5.0 Hz, C=O), 162.76 (d, J 2.0 Hz, C=O), 162.62 (d, J 2.0 Hz, C=O), 64.10, 64.06 (OCH₂, morpholine), 61.13 (d, J 6.5 Hz, OCH₂CH₃), 60.79 (d, J 6.0 Hz, OCH₂CH₃), 57.58, 57.17, 54.17, 54.11, 52.92 (NCH₂, morpholine), 44.74 (d, J 146.1 Hz, CHP), 45.54 (d, J 146.1 Hz, CHP), 44.40, 40.41 (CH), 29.20, 28.96 (CH), 17.18 (CH₃), 14.06–13.86 (m, OCH₂CH₃); δ_P (DMSO-d₆) 17.3, 17.1; LCMS: found m/z 380.5 MH⁺. C₁₅H₃₀N₃O₆P requires 379.4.

Diethyl [1-(2-amino-4-methylpentanamido)-2-(morpholin-4-yl)-2-oxoethyl]phosphonate (10c). Yield 1.432 g, 91% as a yellow viscous oil; [found: C, 48.71; H, 8.36; N, 10.50; P, 7.89. C₁₆H₃₂N₃O₆P requires C, 48.85; H, 8.20; N, 10.68; P, 7.87%]; [α]²⁰_D −9.8 (c 0.9, CH₂Cl₂); ν_max (ATR) 3345, 1639, 1507, 1242, 1015, 966 cm⁻¹; δ_H (DMSO-d₆) 5.43–5.37 (1H, m, CHP), 4.08–4.03 (4H, m, 2OCH₂CH₃), 3.73–3.67 (1H, m, CH), 3.64–3.55 (8H, m, 4CH₂), 2.54–2.52 (1H, m, CH_aCH_b), 1.78–1.69 (1H, m, CH_aCH_b), 1.47–1.40 (1H, m, CH), 1.24–1.19 (6H, m, 2OCH₂CH₃), 0.89–0.83 (6H, m, 2CH₃); δ_C (DMSO-d₆) 173.74 (d, J 5.5 Hz, C=O), 173.64 (d, J 5.5 Hz, C=O), 162.72 (d, J 2.5 Hz, C=O), 162.66 (d, J 2.5 Hz, C=O), 64.16, 64.06 (OCH₂, morpholine), 61.20 (d, J 6.5 Hz, OCH₂CH₃), 61.12 (d, J 6.0 Hz, OCH₂CH₃), 54.10, 50.90, 50.75, 50.44 (NCH₂, morpholine), 44.82 (d, J 145.4 Hz, CHP), 44.78 (d, J 145.4 Hz, CHP), 40.40, 40.37 (CH), 21.88 (CH₂), 21.20, 21.14 (CH), 19.47, 19.34 (CH₃), 14.14–13.93 (m, OCH₂CH₃); δ_P (DMSO-d₆) 17.4, 17.3; LCMS: found m/z 394.4 MH⁺. C₁₆H₃₂N₃O₆P requires 393.4.

Acknowledgements

The authors would like to thank Prof. I. Komarov and N. Komarova for their assistance in the HPLC analyses on a CHIRALPAK IA column.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13819h

‡ The diethoxyphosphoryl group signals appear as multiplets due to the nonequivalence of carbon atoms.

§ One of diastereomers.

¶ All carbon atoms in the morpholine residue are nonequivalent.

|| The carbonyl group signals appear as broad peaks.

** The carbon signals at 129.82–129.80 ppm appear as broad peaks.

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