4.4. General Procedure of Deprotection of tert-Butyl Esters to Carboxylic Acids 8a–c

To a stirred solution of tert-butyl ester 7a,f,g (1 mmol) in dry CH₂Cl₂ (1 mL), TFA (1 mL) was added and the reaction mixture was left stirring for 3 hrs. After removal of the solvent, the residue was diluted in diethyl ether and precipitation by petroleum ether (40–60 °C) and filtration afforded the desired product.

Yield 96%; White solid; mp: 101–103 °C; [α]_D = +13 (c = 1 in CHCl₃); ¹H NMR (400 MHz, CDCl₃): δ = 8.88 (br s, 1H, COOH), 7.42 (d, J = 7.6 Hz, 1H, NH), 4.58 (qu, J = 7.3 Hz, 1H, CHCH₃), 2.90 (t, J = 7.3 Hz, 2H, CH₂CO), 1.65–1.56 (m, 2H, CH₂CH₂CO), 1.52 (d, J = 7.2 Hz, 3H, CHCH₃), 1.36–1.20 (m, 22H, 11 × CH₂), 0.87 (t, J = 6.6 Hz, 3H, CH₂CH₃); ¹³C NMR (100 MHz, CDCl₃): δ = 198.4, 176.8, 160.0, 48.1, 36.8, 32.0, 29.77, 29.75, 29.7, 29.54, 29.45, 29.4, 29.1, 23.2, 22.8, 17.8, 14.2; HRMS (ESI) [M-H]^- m/z: 340.2492; (calculated for [C₁₉H₃₄NO₄]⁻ 340.2493).

Yield 84%; White solid; mp: 157–158 °C; [α]_D = +8 (c = 1 in DMF); ¹H NMR (400 MHz, DMSO-d₆): δ = 12.52 (br s, 1H, COOH), 8.35 (d, J = 7.9 Hz, 1H, NHCOCO), 8.25 (d, J = 7.3 Hz, 1H, NHCHCOOH), 4.32 (qu, J = 7.1 Hz, 1H, CHNHCOCO), 4.20 (qu, J = 7.3 Hz, 1H, CHCOOH), 2.78 (t, J = 7.3 Hz, 2H, CH₂CO), 1.55–1.43 (m, 2H, CH₂CH₂CO), 1.30–1.20 (m, 28H, 11 × CH₂, 2 × CH₃CH), 0.86 (t, J = 6.8 Hz, 3H, CH₃CH₂); ¹³C NMR (100 MHz, DMSO-d₆) δ = 198.8, 173.8, 171.1, 160.6, 47.9, 47.5, 36.5, 31.3, 29.03, 29.02, 29.00, 28.99, 28.96, 28.84, 28.78, 28.7, 28.4, 22.7, 22.1, 18.0, 17.1, 13.9; HRMS (ESI) [M-H]⁻ m/z: 411.2864; (calculated for [C₂₂H₃₉N₂O₅]⁻ 411.2864).

Yield 59%; White solid; mp: 53–54 °C; [α]_D = +6 (c = 1 in CHCl₃); ¹H NMR (400 MHz, CDCl₃): δ = 10.59 (s, 1H, COOH), 7.39 (d, J = 9.1 Hz, 1H, NH), 4.57–4.44 (m, 1H, CHNH), 2.89 (t, J = 7.5 Hz, 2H, CH₂CO), 2.36–2.23 (m, 1H, CH(CH₃)₂), 1.65–1.52 (m, 2H, CH₂CH₂CO), 1.36–1.16 (m, 14H, 7 × CH₂), 1.03–0.79 (m, 9H, 3 × CH₃); ¹³C NMR (100 MHz, CDCl₃): δ = 198.5, 176.1, 160.3, 57.2, 36.9, 32.0, 31.2, 29.6, 29.5, 29.43, 29.40, 29.1, 23.3, 22.8, 19.1, 17.6, 14.2; HRMS (ESI) [M-H]⁻ m/z: 312.2176; (calculated for [C₁₇H₃₀NO₄]⁻ 312.2180).