Synthetic Procedures
Oxo-analogues of Santacruzamate A (4a–c), general procedure: To a magnetically stirred solution of the corresponding N-ethoxycarbonyl amino acid (1 mmol) in CH2Cl2 (5 mL), N-methylmorpholine (1 mmol, 0.11 mL) was added. The solution was then put in an ice bath and ethyl chloroformate (1 mmol, 0.1 mL) was added. The mixture was left to stir for 5 min and after that a solution of enamino amide 2 (1 mmol) and DMAP (0.2 mmol) in CH2Cl2 (10 mL) was added in one go. The ice bath was then removed, and the reaction mixture was left to stir for one more hour at r.t. The reaction mixture was then transferred to a separatory funnel with additional 30 mL of CH2Cl2 and washed with aqueous (10:1) HCl. The aqueous layer was extracted with 30 more mL of CH2Cl2, the combined organic layers were dried with anhydrous sodium sulfate, the drying agent was removed by filtration, and the solvent was evaporated under reduced pressure. The intermediates 3 solidified upon trituration with small volume of diethyl ether. The ethereal washings were filtered off and the crude compounds 3 were dissolved in TFA (1 mL TFA per 100 mg of 3). The TFA solutions were stirred for 5 min at r.t. and then 3 mol/L aqueous solution of NaOAc (10 mL for each mL of TFA) was added, followed by CH2Cl2 (30–50 mL). The mixture was left to stir intensely for 2 h. The layers were then separated, and the aqueous layer was extracted two more times with CH2Cl2. The organic layers were combined, washed with saturated aqueous NaHCO3 (20 mL), and then dried over Na2SO4. The solvent was removed on a rotary evaporator. Compounds 4a and 4c crystallized and were rinsed with small volumes of diethyl ether or ether-petroleum. Compound 4b was isolated as clear oil. Chromatography of the ethereal washings through a short plug of silica gel can afford small additional amounts of 4a,c.
(2-Oxo-3-phenethylcarbamoyl-propyl)-carbamic acid ethyl ester (4a): white solid, mp 137 –139 °C; Rf = 0.55 (Et2O:CH3OH 20:1); 1H-NMR (600 MHz, CDCl3, δ ppm, J Hz): 7.34–7.20 (m, 5H), 6.71 (br s, 1H), 5.40 (br s, 1H), 4.15 (q, J = 7.0, 2H) overlapped with 4.12 (d, J = 5.3, 2H), 3.55 (dt, J = 6.4, J = 7.0, 2H), 3.39 (s, 2H), 2.84 (t, J = 7.0, 2H), 1.27 (t, J = 7.0, 3H); 13C-NMR (150 MHz, CDCl3, δ ppm): 201.6, 164.8, 156.5, 138.6, 128.8, 128.7, 126.6, 61.4, 51.1, 47.0, 40.9, 35.5, 14.6; ESI-MS (m/z): 315.1320 [M + Na]+ (calcd for C15H20N2NaO4+ 315.1315); 291.1353 [M − H]− (calcd for C15H19N2O4− 291.1350).
Methyl-(2-oxo-3-phenethylcarbamoyl-propyl)-carbamic acid ethyl ester (4b): clear oil; Rf = 0.35 (Et2O:CH3OH 20:1); 1H-NMR (600 MHz, CDCl3, δ ppm, J Hz), only signals for the major rotamer are listed: 7.34–7.21 (m, 5H), 6.87 (br s, 1H), 4.17–4.09 (m, 4H) 3.55 (m, 2H), 3.38 (s, 2H), 2.94 (s, 3H), 2.85 (t, J = 7.0, 2H), 1.30 (t, J = 7.0, 3H); 13C-NMR (150 MHz, CDCl3, δ ppm): 202.0, 165.1, 157.0, 138.7, 128.8, 128.6, 126.6, 62.0, 58.9, 46.7, 41.0, 35.6, 35.5, 14.6; ESI-MS (m/z): 329.1475 [M + Na]+ (calcd for C16H22N2NaO4+ 329.1472).
(4-Benzylcarbamoyl-3-oxo-butyl)-carbamic acid ethyl ester (4c): white solid, mp 100–102 °C; Rf = 0.50 (Et2O:CH3OH 20:1); 1H-NMR (600 MHz, DMSO-d6, δ ppm, J Hz): 8.33 (br t, J = 5.9, 1H), 7.15–7.04 (m, 5H), 6.86 (br t, J = 5.9, 1H), 4.10 (d, J = 5.9, 2H) 3.77 (q, J = 7.0, 2H) 3.21 (s, 2H), 2.98(dt, J = 5.9, J = 7.0, 2H), 2.50 (t, J = 7.0, 2H), 0.95 (t, J = 7.0, 3H); 13C-NMR (150 MHz, DMSO-d6, δ ppm): 204.3, 166.5, 156.6, 139.6, 128.8, 127.7, 127.3, 60.0, 50.9, 42.9, 42.7, 35.7, 15.1; ESI-MS (m/z): 315.1317 [M + Na]+ (calcd for C15H20N2NaO4+ 315.1315);
Hydroxy-analogues of Santacruzamate A (5): To the corresponding keto amide 4 (100 mg) in methanol (10 mL) was added NaBH4 in small portions (5–7 mg every 10 min) until TLC indicated the absence of the starting material. The mixture was then diluted with water (50 mL) and extracted with CH2Cl2 (3 × 20 mL). The organic layers were combined, dried over Na2SO4, and the solvent was removed on a rotary evaporator to afford practically clean hydroxy amides 5.
(2-Hydroxy-3-phenethylcarbamoyl-propyl)-carbamic acid ethyl ester (5a): white solid, mp 104–105 °C; Rf = 0.50 (Et2O:CH3OH 20:1); 1H-NMR (600 MHz, CDCl3, δ ppm, J Hz): 7.25–7.11 (m, 5H), 6.20 (br s, 1H), 5.24 (br s, 1H), 4.02 (q, J = 7.0, 2H), 3.96 (m, 1H), 3.45 (dt, J = 5.9, J = 7.0, 2H), 3.22 (dt, 2J = 14.1, 3J = 4.7, 1H), 3.09 (dt, 2J = 14.1, 3J = 5.9, 1H), 2.75 (t, J = 7.0, 2H), 2.24 (m, 2H), 1.16 (t, J = 7.0, 3H); 13C-NMR (150 MHz, CDCl3, δ ppm): 172.04, 157.6, 138.6, 128.74, 128.70, 126.6, 68.2, 61.1, 45.9, 40.6, 39.6, 35.5, 14.6; ESI-MS (m/z): 317.1475 [M + Na]+ (calcd for C15H22N2NaO4+ 317.1472).
(2-Hydroxy-3-phenethylcarbamoyl-propyl)-methyl-carbamic acid ethyl ester (5b): white solid, mp 79–81 °C; Rf = 0.33 (Et2O:CH3OH 20:1); 1H-NMR (600 MHz, CDCl3, δ ppm, J Hz), only signals for the major rotamer are listed: 7.33–7.21 (m, 5H), 6.47 (br s, 1H), 4.15 (br s, 1H) overlapped with 4.13 (q, J = 7.0, 2H), 3.55 (m, 2H), 3.33 (m, 2H), 2.99 (s, 3H), 2.84 (t, J = 7.0, 2H), 2.33 (m, 2H), 1.28 (t, J = 7.0, 3H); 13C-NMR (150 MHz, CDCl3, δ ppm): 171.9, 158.0, 138.7, 128.8, 128.6, 126.6, 68.2, 61.8, 54.6, 40.6, 40.2, 36.2, 35.6, 14.7; ESI-MS (m/z): 331.1630 [M + Na]+ (calcd for C16H24N2NaO4+ 331.1628).