A Versatile Synthesis of α-Amino Acid Derivatives via the Ugi Four-­Component Condensation with a Novel Convertible Isonitrile

Abstract

The Ugi four-component condensation (4CC) reaction with the carbonate-type isonitrile 9 proceeded smoothly, and subsequent base treatment of the Ugi products 10 provided the N-acyloxazolidinones 11 in high yield. The N-acyloxazolidinones derivatives can be reacted with several hetero-nucleophiles, namely, reaction of 11 with thiolates gave thiol ester derivatives 16 efficiently.

Key words

Ugi four-component condensation (4CC) - α-amino acid derivatives - convertible isonitrile - N-acyloxazolidinones - thiol esters

References

• For reviews of multicomponent reactions with isonitriles, see:
• 1a Hulme C. Gore V. Curr. Med. Chem.  2003,  10:  51 
• 1b Dömling A. Ugi I. Angew. Chem. Int. Ed.  2000,  39:  3168 
• 1c Gokel G. Lüdke G. Ugi I. In Isonitrile Chemistry   Ugi I. Academic; New York: 1971.  p.145-199  
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12

Synthesis of Isonitrile 9: To a stirred solution of 4,4-dimethyloxazoline (0.997 g, 10.1 mmol) in THF (10 mL) under Ar atmosphere at -78 °C, was added dropwise n-BuLi (1.1 M solution in hexane, 9.60 mL, 10.6 mmol) in a duration of 5 min and stirred at same temperature for 1 h followed by dropwise addition of phenyl chloroformate (1.40 mL, 10.7 mmol). After stirring for 5 min, the reaction mixture was warmed to ambient temperature and diluted with Et₂O, and water was added to the mixture. The organic layer was separated and washed with brine, dried over Na₂SO₄ and filtered. The solvent was removed under reduced pressure, the resulting residue was purified by silica gel chromatography (EtOAc-hexane = 1:9-1:4) to afford 1.10 g of 9 (49.7%). IR (film): 2991, 2136, 1767, 1592, 1496, 1457, 1401, 1378, 1258, 1074, 1024, 970, 879, 835, 775, 735 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 7.45-7.19 (m, 5 H), 4.21 (s, 2 H), 1.53 (s, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 156.1, 153.1, 150.8, 129.4, 126.1, 120.8, 72.9, 56.1, 25.6. HRMS (FAB): m/z calcd for C₁₂H₁₃NO₃: 219.0895; found: 219.0900.

14

General Procedure for the Ugi 4CC with Isonitrile 9, Synthesis of 10a: To a stirred solution of isonitrile 9 (223 mg, 1.02 mmol), isobutylaldehyde (0.310 mL, 3.40 mmol), and benzoic acid (128 mg, 1.05 mmol) in MeOH (3.5 mL, 0.19 M) was added dropwise benzylamine (74 µL, 0.68 mmol) within 2 min at ambient temperature. After stirring for 60 min, the solvent was removed under reduced pressure. The resultant crude mixture was purified by silica gel chromatography (EtOAc-hexane = 1:9-1:2) to afford 310 mg of 10a (91%) as a pale yellow foam. IR (film): 3301, 3063, 2968, 1764, 1680, 1620, 1545, 1496, 1457, 1367, 1263, 1211, 1066, 963, 918, 774, 734 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 7.60-7.05 (m, 15 H), 4.67 (d, J = 16 Hz, 1 H), 4.45 (m, 2 H), 4.29 (d, J = 11 Hz, 2 H), 2.80 (m, 1 H), 1.36 (m, 6 H), 1.02 (m, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 174.0, 170.2, 153.6, 151.1, 136.6, 136.5, 129.8, 129.5, 128.5, 128.4, 127.8, 127.7, 126.7, 126.0, 121.0, 76.7, 72.1, 54.0, 52.8, 27.2, 24.3, 23.8, 19.8, 19.5. HRMS (FAB): m/z calcd for C₃₀H₃₄N₂O₅: 502.2468; found: 502.2485.

15

General Procedure for the Synthesis of N -Acyloxazol-idinones, Synthesis of 11a: To a stirred solution of 10a (220 mg, 0.44 mmol) in anhyd THF (2.0 mL, 0.22 M) at 0 °C was added freshly activated MS 4 Å (powder, 280 mg), and stirred for 10 min, followed by dropwise addition of 1.0 M t-BuOK in t-BuOH (0.45 mL, 0.45 mmol) in a duration of
2 min. After stirring for 5 min, 10% aq citric acid was added and the mixture was extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO₄ and filtered. The solvent was removed under reduced pressure, the resultant crude mixture was purified by silica gel chromatography (EtOAc-hexane = 1:9-1:2) to afford 11a (149 mg, 83%) as a pale yellow foam. IR (film): 2968, 2360, 1779, 1701, 1646, 1496, 1374, 1305, 1231, 1173, 1089, 1031, 761, 734 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 7.36-7.19 (m, 10 H), 5.63 (m, 1 H), 4.88 (m, 2 H), 3.94 (m, 2 H), 2.43 (m, 1 H), 1.49 (s, 3 H), 1.33 (s, 3 H), 0.93 (d, J = 15 Hz, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 175.8, 173.9, 172.3, 152.9, 138.7, 136.7, 129.6, 128.6, 128.3, 128.2, 127.4, 126.8, 126.3, 75.1, 61.0, 29.5, 24.3, 23.8, 19.4, 18.6. HRMS (FAB): m/z calcd for C₂₄H₂₈N₂O₄: 408.2049; found: 408.2033.

16

In the absence of MS 4 Å, the reaction resulted in low yields, accompanied by the N-acylamino alcohols 12 and/or carboxylic acid 13 (Figure [2] ).

23

General Procedure for the Synthesis of Thiolesters, Synthesis of 16a: To a solution of n-dodecanethiol (0.26 mL, 1.08 mmol) in anhyd THF (2.5 mL) at 0 °C was added dropwise n-BuLi (1.0 M solution in hexane, 0.40 mL, 0.40 mmol) for 2 min. After stirring for 5 min, the resulting white suspension was added to a solution of N-acyloxazolidinone 11a (93 mg, 0.23 mmol) in THF (1.5 mL) at 0 °C. After stirring for 10 min, 10% aq citric acid was added and the mixture was extracted with EtOAc. The combined organic layer was washed with brine, dried over MgSO₄ and filtered. The solvent was removed under reduced pressure, the resultant crude mixture was purified by silica gel chromatography (EtOAc-hexane = 1:9-1:2) to afford 16a (94 mg, 83%) as a white solid. IR(film): 2915, 2854, 1684, 1653, 1457, 1300, 1129, 731 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 7.58-6.98 (m, 10 H), 4.88 (d, J = 14 Hz, 1 H), 4.65 (d, J = 15 Hz, 1 H), 4.10 (d, J = 7.3 Hz, 1 H), 2.77 (t,
J = 15 Hz, 2 H), 2.43 (m, 1 H), 1.30-1.00 (m, 20 H), 0.88 (t, J = 14 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 196.8, 173.5, 138.1, 136.2, 129.7, 128.5, 128.3, 128.1, 127.8, 127.5, 126.8, 73.5, 62.7, 50.3, 45.8, 31.9, 29.6, 29.5, 29.4, 29.1, 29.0, 22.7, 19.5, 19.1, 14.1. HRMS (FAB): m/z calcd for C₂₄H₂₈N₂O₄: 495.3171; found: 495.3169.

24

In order to avoid the thiolate addition to the carbonyl group on the oxazolidinone ring, the reaction should be carried out at a temperature lower than 0 °C. Reactions at higher temperature often provided the undesired N-acylamino alcohols 12. This tendency was notably observed in compounds possessing a bulky substituent at the Z-position.