Na-[N-(tert-Butyloxycarbonyl)-2-aminoethyl]phenylalanine
tert-butyl ester (7b). Aldehyde 6 (0.5 g, 3.14 mmol) and H–Phe–
OtBu ꢂ HCl (5b; 0.68 g, 2.62 mmol) were dissolved in 3% AcOH
in MeOH (6.5 ml) containing molecular sieves (3 A). To this
NaBH3CN (0.22 g, 3.53 mmol) was slowly added. After stirring
for 2 h, EtOAc (50 ml) and 5% aq. NaHCO3 (20 ml) were
added. The aqueous layer was extracted with EtOAc (3 ꢁ 25
ml) and the combined organic layers were washed with 5% aq.
NaHCO3 (20 ml), brine (20 ml) and dried over MgSO4. After
evaporation, the residue was purified by column chromatogra-
phy (3% TEA in hexane–EtOAc, 5 : 1 - 1 : 1 v/v), affording 7b
in 66% yield (0.63 g, 1.73 mmol). 1H NMR (CDCl3): d ¼ 7.27
(m, 5H, Phe arom.), 4.97 (br s, 1H, NH), 3.35 (t, J ¼ 7.1 Hz,
1H, aPhe), 3.15–3.08 (m, 2H, BocNCH2CH2N), 2.87 (d, J ¼
7.1 Hz, 2H, bPhe), 2.74 (m, 1H, BocNCH2CH2N), 2.56 (m, 1H,
BocNCH2CH2N), 1.45 (s, 9H, Boc), 1.36 (s, 9H, tBu); ESI-MS:
m/z 365 [M þ H]1.
graphy (1% AcOH in EtOAc–hexane, 1 : 1 v/v). The product-
containing fraction was washed with 5% aq. Na2CO3, H2O,
brine and dried over MgSO4. Product 4a was obtained as a
white powder in 58% (from 8a) yield (0.815 g, 1.41 mmol). 1H
NMR (d4 MeOD, 55 1C): d ¼ 7.78 (m, 4H, Fmoc), 7.60 (m, 4H,
Fmoc), 7.39–7.29 (m, 8H, Fmoc), 4.48–4.17 (m, 7H, CH
Fmoc þ CH2 Fmoc þ aAla), 3.33–2.90 (br m, 4H, NC2H4N),
1.37 (br m, 3H, bAla); 13C NMR (d4 MeOD, 55 1C): d ¼ 175.4
(C , CO H), 158.6, 157.7 (C , 2 ꢁ C O), 145.2 (C arom., 4 ꢁ
Q
q
2
q
q
Fmoc), 142.6 (Cq arom., 4 ꢁ Fmoc), 128.8, 128.1, 126.0, 120.9
(CH arom., 16 ꢁ Fmoc), 68.6 (CH2, Fmoc), 67.5 (CH2, Fmoc),
57.6 (CH, aAla), 56.6 (CH, 2 ꢁ Fmoc), 47.1 (CH2, NC2H4N),
40.8 (CH2, NC2H4N), 15.6 (CH3, bAla); ESI-MS: m/z 575
[M ꢀ H]ꢀ.
Na-[N-(9-Fluorenylmethoxycarbonyl)-2-aminoethyl]-Na-(9-
fluorenylmethoxycarbonyl)phenylalanine (4b) from 8b. To crude
oil 9b in dioxane (7 ml) was added a solution of DiPEA (284
ml, 1.6 mmol) and Fmoc–OSu (202 mg, 0.60 mmol) in dioxane
(2 ml). After stirring overnight, EtOAc (7 ml) was added and
the solution was washed with 1 M HCl (7 ml), H2O (7 ml) and
brine (7 ml). After evaporation, the product was purified by
column chromatography (EtOAc–hexane, 1 : 1 v/v), furnishing
Na-[N-(tert-Butyloxycarbonyl)-2-aminoethyl]-Na-(9-fluorenyl-
methoxycarbonyl)alanine tert-butyl ester (8a). Crude 7a was
dissolved in DCM (30 ml), then DiPEA (2.19 ml, 12.6 mmol)
and Fmoc–OSu (2.44 g, 7.22 mmol) were added. After stirring
overnight the solvent was evaporated and the residue purified
by column chromatography (hexane–EtOAc, 3 : 2 v/v), afford-
1
4b in 46% (from 8b) yield (0.16 g, 0.25 mmol). H NMR (d4
1
ing 8a in 39% (from 5a) yield (1.24 g, 2.43 mmol). H NMR
MeOD, 55 1C): d ¼ 7.72 (m, 4H, Fmoc arom.), 7.56–7.46 (br
m, 4H, Fmoc arom.), 7.33 (m, 4H, Fmoc arom.), 7.24 (m, 4H,
Fmoc arom.), 7.21–7.01 (m, 5H, Phe), 4.42 (br m, 2H, CH
Fmoc), 4.28 (m, 2H, CH2 Fmoc), 4.21 (m, 1H, aPhe), 4.14 (m,
2H, CH2 Fmoc), 3.20 (br m, 2H, bPhe), 3.12–2.46 (m, 4H,
NC2H4N); 13C NMR (d4 MeOD, 55 1C): d ¼ 175.8 (Cq,
(CDCl3): d ¼ 7.75 (m, 2H, Fmoc arom.), 7.56 (m, 2H, Fmoc
arom.), 7.38–7.30 (m, 4H, Fmoc arom.), 4.92 (br s, 1H, NH),
4.54–4.07 (br m, 4H, CH2 Fmoc þ CH Fmoc þ aAla), 3.48–
2.89 (br m, 4H, NC2H4N), 1.41 (br s, 21H, tBu þ Boc þ bAla);
ESI-MS: m/z 511 [M þ H]1, 455 [M ꢀ tBu]1.
CO H), 158.3 (C , C O), 157.3 (C , C O), 145.2, 145.2,
Q
Q
2
q
q
Na-[N-(tert-Butyloxycarbonyl)-2-aminoethyl]-Na-(9-fluorenyl-
methoxycarbonyl)phenylalanine tert-butyl ester (8b). Compound
7b (0.63 g, 1.73 mmol) was dissolved in DCM (9 ml), then
DiPEA (0.59 ml, 3.48 mmol) and Fmoc–OSu (0.67 g, 2.0 mmol)
were added at 0 1C. After stirring overnight the solvent was
evaporated and the residue purified by column chromatogra-
phy (hexane–EtOAc, 4 : 1 v/v). Product 8b was obtained in 31%
yield (0.32 g, 0.54 mmol). 1H NMR (CDCl3): d ¼ 7.78 (m, 2H,
Fmoc arom.), 7.69–7.55 (m, 2H, Fmoc arom.), 7.43 (m, 2H,
Fmoc arom.), 7.34 (m, 2H, Fmoc arom.), 7.27–7.10 (m, 5H,
Phe arom.), 5.12 (br s, 1H, NH), 4.98 (br m, 1H, aPhe), 4.66 (m,
1H, CH Fmoc), 4.45 (m, 1H, CH2 Fmoc), 4.27 (m, 1H, CH2
Fmoc), 3.28 (br d, J ¼ 6.1 Hz, 2H, bPhe), 3.08 (m, 1H,
NC2H4N), 2.87 (m, 1H, NC2H4N), 2.66 (m, 1H, NC2H4N),
2.46 (m, 1H, NC2H4N), 1.50 (s, 9H, Boc), 1.42 (s, 9H, tBu);
ESI-MS: m/z 588 [M þ H]1.
145.2, 145.1, 142.9, 142.6 (Cq arom., Fmoc), 139.6 (Cq arom.,
Phe), 129.9-123.7 (CH arom., 5 ꢁ Phe þ 12 ꢁ Fmoc), 121.0
(CH arom., 4 ꢁ Fmoc), 68.5 (CH2, Fmoc), 67.6 (CH2, Fmoc),
64.7 (CH, aPhe), 57.1 (CH, 2 ꢁ Fmoc), 40.2 (CH2, NC2H4N),
38.7 (CH2, NC2H4N), 35.9 (CH2, bPhe); ESI-MS: m/z 651
[M ꢀ H]ꢀ.
Na-[N-(tert-Butyloxycarbonyl)-2-aminoethyl]phenylalanine
(11b). Phenylalanine 10b (205 mg, 1.24 mmol) was suspended
in MeOH (2 ml) and 6 (297 mg, 1.87 mmol) was added,
followed by NaBH3CN (86 mg, 1.37 mmol). The mixture
was stirred overnight, filtered and washed with MeOH, afford-
ing insoluble white solid 11b in 52% yield (200 mg, 0.65 mmol),
which was used without characterization.
Na-[N-(tert-Butyloxycarbonyl)-2-aminoethyl]-Na-(9-fluorenyl-
methoxycarbonyl)phenylalanine (12b). Bis(trimethylsilyl)aceta-
mide (281 ml, 1.14 mmol) and DiPEA (0.11 ml, 0.65 mmol)
were added to 11b (0.20 g, 0.65 mmol) suspended in DCM (1.3
ml). The mixture was stirred until it was clear, after which Fmoc
–OSu (0.23 g, 0.68 mmol) was added. After stirring overnight,
the solution was co-evaporated with toluene and the residue
purified by column chromatography [hexane–EtOAc (2: 1 v/v)
- AcOH–EtOAc (1: 99 v/v)], producing 12b in 74% yield (0.25
g, 0.48 mmol) as a colorless oil. 1H NMR (CDCl3): d ¼ 7.71 (m,
2H, Fmoc), 7.52 (m, 2H, Fmoc), 7.38 (m, 2H, Fmoc), 7.36–7.07
(m, 7H, Fmoc þ Phe), 4.70 (m, 1H, aPhe), 4.44 (m, 2H, CH
Fmoc), 4.24 (m, 1H, CH2 Fmoc), 4.16 (m, 1H, CH2 Fmoc), 3.31
(m, 2H, bPhe), 3.06 (br m, 2H, BocNCH2CH2N), 2.83 (br m,
2H, BocNCH2CH2N), 1.37 (s, 9H, Boc).
Na-[2-Aminoethyl]-Na-(9-fluorenylmethoxycarbonyl)alanine
(9a). Compound 8a (1.24 g, 2.43 mmol) was dissolved in DCM
(1.6 ml) and TFA (9 ml) was added. After stirring for 2 h, the
solvent was co-evaporated with toluene, resulting in a brown
oil of 9a.
Na-[2-Aminoethyl]-Na-(9-fluorenylmethoxycarbonyl)phenyl-
alanine (9b). Compound 8b (0.32 g, 0.54 mmol) was dissolved
in DCM (0.34 ml) at 0 1C and TFA (2 ml) was added. After
stirring for 1 h, the solvent was co-evaporated with toluene,
resulting in a brown oil of 9b.
Na-[N-(9-Fluorenylmethoxycarbonyl)-2-aminoethyl]-Na-(9-
fluorenylmethoxycarbonyl)alanine (4a). The crude oil 9a was
dissolved in dioxane (30 ml), then DiPEA (1.27 ml, 7.29 mmol)
and Fmoc–OSu (0.90 g, 2.67 mmol) were added and the
reaction mixture was stirred overnight. EtOAc (30 ml) was
added and the solution was washed with 1 M HCl (20 ml), H2O
(20 ml) and brine (20 ml), and dried over MgSO4. After
evaporation, the product was purified by column chromato-
Na-[N-(9-Fluorenylmethoxycarbonyl)-2-aminoethyl]-Na-(9-
fluorenylmethoxycarbonyl)phenylalanine (4b) from 12b. Com-
pound 12b (0.24 g, 0.45 mmol) was dissolved in TFA–DCM
(1 : 1 v/v, 6 ml) and stirred for 2 h. The solvent was co-
evaporated with toluene and the resulting colorless oil 9b was
dissolved in dioxane (5.7 ml). DiPEA (237 ml, 1.35 mmol) and
N e w J . C h e m . , 2 0 0 5 , 2 9 , 2 2 0 – 2 2 5
223