mixture was refluxed. The insoluble ketal progressively dissolved
in acetone whilst the temperature was rising. At reflux, one lot of
benzyl bromide (0.90 mL, 2.3 equiv., 7.25 mmol) was added, and
the solution was refluxed. The reaction mixture turned orange
and after 2 hours it was allowed to cool to room temperature.
The solution was concentrated under reduced pressure and the
residue was diluted by Et2O (30 mL). The organic layer was
washed with brine (30 mL) and the aqueous layer was extracted
twice with Et2O (2 × 30 mL). The combined organic layers
were dried over anhydrous MgSO4, filtered and concentrated
under reduced pressure. The crude material was purified by flash
chromatography (EtOAc–hexane 1 : 4 then 1 : 1) to afford the
dibenzyl derivative as a pale yellow solid (0.75 g, yield: 60%)
(Found: C, 69.95; H, 6.28. C23H24O6 requires C, 69.68; H, 6.10%).
Rf 0.23 (EtOAc–hexane 1 : 4), 0.67 (EtOAc–hexane 1 : 1). dH
(250 MHz, CDCl3) 1.39 (3 H, s, CH3), 1.44 (3 H, s, CH3), 4.03
4.8 Hz), 4.35 (1 H, dd, -O-CHH-CH(OH)-CH-, 2J = 11.5 Hz,
3J = 7.0 Hz), 4.61 (1 H, broad s, -CH(OH)-), 4.69 (1 H, d, -O-
CH2-CH(OH)-CH-, 3J = 2.0 Hz), 5.14 (2 H, s, -O-CH2-C6H5),
CHH-C6H5, 2J = 11.5 Hz), 7.20–7.50 (10 H, m, ArH). ES-MS
m/z 570 (M + H)+, calcd for C31H39NO9 569.64 g mol−1.
2
5.15 (1 H, d, -O-CHH-C6H5, J = 11.5 Hz), 5.24 (1 H, d, -O-
The N-Boc derivative (1.13 g, 1.0 equiv., 2.0 mmol) was
dissolved in CH2Cl2 (25 mL). The solution was cooled to 0 ◦C
and trifluoroacetic acid (1.5 mL, 10.0 equiv., 20.0 mmol) was
added dropwise. The resulting reaction mixture was stirred for
2 hours at room temperature. The solvent and excess of TFA were
removed under reduced pressure. The residue was triturated in
Et2O and the title compound 6 was quantitatively isolated as a
white solid (1.15 g). dH (250 MHz, CDCl3) 1.24–1.78 (6 H, m,
H2N-CH2-(CH2)3-CH2-C(O)-O-), 2.39 (2 H, t, -CH2-C(O)-O-,
3J = 6.5 Hz), 2.98–3.18 (2 H, m, H2N-CH2-), 4.20 (2 H, broad s,
-O-CHH-CH(OH)-CH-, 2J = 11.5 Hz, 3J = 4.5 Hz), 4.39 (1 H,
2
3
(1 H, dd, -O-CHH-CH(O)-, J = 8.5 Hz, J = 6.8 Hz), 4.11
2
3
2
3
(1 H, dd, -O-CHH-CH(O)-, J = 8.5 Hz, J = 6.5 Hz), 4.28
dd, -O-CHH-CH(OH)-CH-, J = 11.5 Hz, J = 6.5 Hz), 4.71
(1 H, d, -O-CH2-CH(OH)-CH-, 3J = 1.8 Hz), 5.03 (2 H, s, -CH2-
(1 H, dt, -O-CH2-CH(O)-CH-, 3J = 6.8 Hz, 3J = 3.3 Hz), 4.56
3
1
(1 H, d, -O-CH2-CH(O)-CH-, J = 3.3 Hz), 4.99 (1 H, d, -O-
C6H5), 5.16 (1 H, d, -O-CHH-C6H5, J = 11.5 Hz), 5.22 (1 H,
CHH-C6H5, 2J = 11.3 Hz), 5.02 (1 H, d, -O-CHH-C6H5, 2J =
d, -O-CHH-C6H5, J = 11.5 Hz), 7.18–7.40 (10 H, m, ArH).
2
11.3 Hz), 5.07 (1 H, d, -O-CHH-C6H5, 2J = 11.3 Hz), 5.12 (1 H,
MS-ES m/z 470 (M + H)+, calcd for C26H31NO7 469.53 g mol−1
2
d, -O-CHH-C6H5, J = 11.3 Hz), 7.11–7.34 (10 H, m, ArH).
and for C26H31NO7.CF3COOH 583.55 g mol−1.
MS-ES m/z 397 (M + H)+, calcd for C23H24O6 396.43 g mol−1.
The tetra protected ascorbic acid derivative (0.72 g,
1.82 mmol) was dissolved in a mixture of 2 mL of THF and 2 mL
of MeOH. An aqueous solution of HCl 2 M was added and the
reaction mixture was stirred overnight at room temperature. The
solution was concentrated under reduced pressure. The residue
was diluted in EtOAc (10 mL) and washed with H2O (2 × 5 mL).
The organic layer was dried over anhydrous MgSO4, filtered
and concentrated under reduced pressure to afford the desired
compound 5 as a yellow oil which slowly solidified (0.64 g,
quantitative) (Found: C, 67.63; H, 5.71. C20H20O6 requires C,
67.41; H, 5.66%). Rf 0.20 (EtOAc–hexane 1 : 1). dH (250 MHz,
CDCl3) 3.00 (2 H, broad s, 2 OH), 3.56–3.72 (2 H, m, -CH2-OH),
3.80–3.84 (1 H, m, -CH-CH(OH)-CH2OH), 4.56 (1 H, d, -CH-
6-{N-[N-(3,5-Difluorophenylacetyl)-L-alanyl]-L-phenylglycyl}-
aminocaproyl 2,3-di-O-benzyl-L-ascorbic acid, 7
The title compound was synthesized according to a similar
procedure that described previously for the synthesis of DAPT 1
by using BOP as coupling agent. Compound 7 was isolated as a
white solid (800 mg, yield: 73%) after flash chromatography
(EtOAc) (Found: C, 65.23; H, 5.88; N, 4.95. C45H47F2N3O10
requires C, 65.29; H, 5.72; N, 5.08%). Rf 0.47 (EtOAc). dH
(250 MHz, CDCl3) 1.11–1.63 (9 H, m, -NH-CH2-(CH2)3-CH2-
C(O)-O- + CH3 b Ala, 3J = 7.5 Hz), 2.18–2.33 (2 H, m, -CH2-
C(O)-O-), 2.93–3.30 (3 H, m, -NH-CH2- + OH), 3.51 (2 H,
broad s, -CH2-C(O)-NH), 4.06–4.36 (4 H, m, -O-CH2-CH(OH)-
3
CH- + CH a Ala), 4.71 (1 H, d, -O-CH2-CH(OH)-CH-, J =
3
CH(OH)-CH2OH, J = 2.0 Hz), 5.06 (1 H, d, -O-CHH-C6H5,
3.1 Hz), 5.05 (2 H, broad s, -O-CH2-C6H5), 5.11 (1 H, d, -O-
CHH-C6H5, 2J = 10.9 Hz), 5.23 (1 H, d, -O-CHH-C6H5, 2J =
10.9 Hz), 5.74 (1 H, d, CH a Phg, 3J = 7.8 Hz), 6.42–6.81 (3 H,
m, ArH), 7.18–7.44 (15 H, m, ArH) 7.81 (1 H, broad s, NH),
7.99 (1 H, broad s, NH), 8.50 (1 H, broad s, NH). MS-ES m/z
828 (M + H)+, calcd for C45H47F2N3O10 827.87 g mol−1.
2J = 11.3 Hz), 5.11 (1 H, d, -O-CHH-C6H5, 2J = 11.3 Hz), 5.14
(1 H, d, -O-CHH-C6H5, 2J = 11.8 Hz), 5.21 (1 H, d, -O-CHH-
2
C6H5, J = 11.8 Hz), 7.07–7.25 (10 H, m, ArH). MS-ES m/z
357 (M + H)+, calcd for C20H20O6 356.37 g mol−1.
6-Aminocaproyl 2,3-di-O-benzyl-L-ascorbic acid trifluoroacetic
acid salt, 6
6-{N-[N-(3,5-Difluorophenylacetyl)-L-alanyl]-L-phenylglycyl}-
aminocaproyl L-ascorbic acid, 2
6-N-(tert-Butyloxycarbonyl)-aminocaproic acid 4 (0.80 g, 1.0
equiv., 3.46 mmol) was dissolved in freshly distilled CH2Cl2
(10 mL) with 1.2 equiv. (1.84 g, 4.15 mmol) of BOP reagent.
The reaction mixture was cooled to 0 ◦C and then 1.0 equiv. of
DIEA (600 lL, 3.46 mmol) was added dropwise. The reaction
mixture was stirred for ◦1 hour at room temperature and then
once again cooled to 0 C. A CH2Cl2 solution (30 mL) of 1.1
equiv. (1.36 g, 3.81 mmol) of alcohol 5 and 2.0 equiv. (3.12 mL,
17.92 mmol) of DIEA was added dropwise. The solution was
allowed to warm and stirred overnight at room temperature.
The solvent was removed under reduced pressure and the residue
was dissolved in EtOAc (100 mL). The organic layer was washed
successively by using 5% aqueous citric acid (3 × 50 mL), brine
(50 mL), 5% aqueous NaHCO3 (3 × 50 mL) and brine (50 mL),
was dried over anhydrous MgSO4, filtered and concentrated
under reduced pressure. The crude residue was purified by flash
chromatography (EtOAc–hexane 2 : 3 then 1 : 1) to give the
desired compound (1.13 g, yield: 57%) as a white solid (Found:
C, 65.42; H, 7.05; N, 2.35. C31H39NO9 requires C, 65.36; H,
6.90; N, 2.46%). Rf 0.50 (EtOAc–hexane 1 : 1). dH (250 MHz,
CDCl3) 1.26–1.71 (15 H, m, CH3 Boc + -NH-CH2-(CH2)3-CH2-
C(O)-O-), 2.36 (2 H, t, -CH2-C(O)-O-, 3J = 6.3 Hz), 3.04–3.18
(2 H, m, -NH-CH2-), 4.14 (1 H, broad s, -O-CH2-CH(OH)-CH-
The protected ascorbic acid derivative 7 (200 mg, 1.0 equiv.,
0.24 mmol) was dissolved in 3 mL of MeOH. 10% in weight
of Pearlman’s catalyst (Pd(OH)2 over activated charcoal) was
added to the previous solution and the resulting suspension
was stirred at room temperature under H2 atmosphere for 2
1
hours. The solution was filtered and concentrated under
2
reduced pressure to yield the deprotected ascorbic acid derivative
2 (140 mg, yield: 90%) as a white solid (Found: C, 57.71; H, 5.25;
N, 6.54. C31H35F2N3O10 requires C, 57.49; H, 5.45; N, 6.49%). dH
(250 MHz, CD3OD) 1.22–1.63 (9 H, m, -NH-CH2-(CH2)3-CH2-
C(O)-O- + CH3 b Ala, 3J = 7.2 Hz), 2.34–2.72 (2 H, m, -CH2-
C(O)-O-), 3.06–3.21 (2 H, m, -NH-CH2-), 3.58 (2 H, broad s,
CH2-C(O)-NH), 4.05–4.41 (4 H, m, -O-CH2-CH(OH)-CH- +
CH a Ala), 4.71–4.73 (1 H, m, -O-CH2-CH(OH)-CH-), 5.32
(1 H, s, CH a Phg), 6.78–6.94 (3 H, m, ArH), 7.29–7.35 (5 H,
m, ArH). ES-MS m/z 648 (M + H)+, calcd for C31H35F2N3O10
647.62 g mol−1.
6-{N-[N-(3,5-Difluorophenylacetyl)-L-alanyl]-L-phenylglycyl}-
aminocaproic acid, 8
The ester derivative 7 (200 mg, 1.0 equiv., 0.24 mmol) was
smoothly hydrolysed by LiOH (13 mg, 1.5 equiv., 0.36 mmol) in
2
3
), 4.23 (1 H, dd, -O-CHH-CH(OH)-CH-, J = 11.5 Hz, J =
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 4 5 0 – 2 4 5 7
2 4 5 5