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General procedure A for esterification of benzoyl protected amino acids.
Bz
N
H
O
56c
2-Benzoylaminopropionic acid prop-2-ynyl ester (56c). To a solution of propargyl alcohol
(9.7 mL, 0.17 mol) in CH2Cl2 (400 mL) was added DMAP (100 mg, 0.820 mmol) and DCC
(22.9 g, 0.111 mol) followed by N-Bz-alanine (21.4 g, 0.111 mol). A white precipitate forms
immediately. After stirring for 36 h at rt, hexanes (~100 mL) were added. A 150 mL fritted
funnel equipped with vacuum outlet was filled with silica gel and connected to a 1 L round-
bottomed flask. The reaction mixture was filtered over the plug of silica gel under vacuum
eluting with hexanes : EtOAc (1 : 1). The resulting colorless solution was concentrated under
vacuum and dissolved in EtOAc. Recrystallization (hexanes-EtOAc, 4 : 1, v/v) afforded 56c
(17.4 g, 68%).
mp range 94.5-97.5 °C; 1 H NMR (300 MHz, CDCl3): δ 7.78-7.76 (m, 2H), 7.75-7.35 (m, 3H),
6.98 (br d, J = 6.9 Hz, 1H), 4.79 (quin, J = 7.2 Hz, 1H), 4.76 (dd, J = 15.5, 2.5 Hz, 1H), 4.69 (dd,
J = 15.5, 2.5 Hz, 1H), 2.51 (t, J = 2.4 Hz, 1H), 1.50 (d, J = 7.2 Hz, 3H); 13 C NMR (75 MHz,
CDCl3): δ 172.3, 166.9, 133.6, 131.6, 128.4, 127.0, 76.9, 75.4, 52.7, 48.3, 18.0; IR (thin film): ν
3294, 2940, 2128, 1749, 1642, 1536, 1164 cm -1 ; MS m/z (%) 231 (43), 176 (20), 148 (63), 105
(100); EI-HRMS calcd for C13H13NO3 m/z [M] + 231.0895; found 231.0899.
Bz
N
H
56d
2-Benzoylamino-4-methylpentanoic acid prop-2-ynyl ester (56d). Prepared by following
general procedure A with: N-Bz-leucine (35.0 g, 0.15 mol), propargyl alcohol (26.0 mL, 0.45
143
O
O
O