After 36 h the reaction mixture was filtered through Celite and
the solvent removed in vacuo to yield the carboxylic acid as a
slightly yellow oil. This was dissolved in a 50:50 mix of CH2Cl2–
diethyl ether (10 ml), and methylated using freshly prepared
diazomethane.31 Residual diazomethane was removed by
adding a drop of acetic acid, and removal of solvent in vacuo
yielded crude 24b as a yellow oil. Purification by flash chrom-
atography (15% CH3CN–85% EtOAc, Rf 0.10) yielded 24b as a
slightly yellow oil (0.960 g, 27%); δH(300 MHz; CDCl3) 7.00
(1H, d, J 7.7, NH), 6.72 (1H, m, NH), 4.83 (1H, m, αCH), 4.52
(1H, m, αCH), 4.18 (1H, m, αCH), 3.79 (3H, s, OCH3), 3.77
(3H, s, OCH3), 3.75 (3H, s, OCH3), 3.17 (1H, br s, OH), 2.97
(1H, dd, J 15.6, J 4.4, Asp βCH), 2.79 (1H, dd, J 15.6, 4.4,
Asp βCH), 2.05 (3H, s, CH3), 1.84 (2H, m, CH2), 1.68 (2H, m,
CH2), 1.47 (2H, m, CH2); δC(75.45 MHz; CDCl3), 175.2 (CO2),
172.6 (CO2), 171.5 (CO2), 170.4 (CONH), 170.2 (CONH), 70.0
(εCH), 52.7 (OCH3), 52.5 (OCH3), 52.0 (OCH3), 52.0 (αCH),
49.0 (αCH), 37.3 (CH2CO), 33.4 (CH2), 31.6 (CH2), 23.0
(CH3CO), 20.8 (γCH2); νmax(KBr)/cmϪ1 3362, 3066.4, 2955.8,
2866.9, 1736.2, 1660.1; m/z (EI) 391.2 (MHϩ, 28%); m/z (CI)
391 (MHϩ) [Calc. for C16H27N2O9 (MHϩ), 391.1717. Found:
m/z, 391.1724].
(CO), 173.0 (CO2), 171.9 (CO2), 171.0 (CONH), 152.0, 136.4
(Ph), 129.4 (Ph), 128.8 (Ph), 127.2 (Ph), 54.5 (OCH3), 53.1
(OCH3), 52.6 (αCH), 52.0 (αCH), 38.6 (δCH2), 38.2 (CH2Ph),
31.4 (CH2), 23.3 (CH2), 18.5 (γCH2); m/z (EI) 406 (Mϩ, 5%);
(CI) 407 (MHϩ, 90%) (Calc. for C20H26N2O7: C, 59.10; H, 6.45;
N, 6.89. Found: C, 58.73; H, 6.68; N, 6.64%).
Ac-Asp(OBn)-(ꢀ-keto)AP(OCH3)-OCH3 22a
To
a
stirred solution of Ac-Asp(OBn)-(ε--hydroxy)-
AP(OCH3)-OCH3 21a (99.1 mg, 212.7 µmol) in dry CH2Cl2
(3 ml) was added Dess–Martin periodinane (90.2 mg, 212.7
µmol). After 3 h the solution was added to saturated aq.
NaHCO3 (5 ml) containing Na2S2O3 (0.5 g). The mixture was
stirred vigorously for 5 min and then extracted into CH2Cl2
(3 × 20 ml). The dried (MgSO4) organic extracts were evapor-
ated in vacuo, and the residue was purified by flash chromato-
graphy (10% CH3CN in EtOAc, Rf 0.33) to afford compound
22a as an oil (31 mg, 67%); [α]D24 Ϫ1.36 (c 2.95, CH2Cl2);
νmax(KBr)/cmϪ1 3307, 3064, 2954, 1731, 1658, 1537, 1437,
1377, 1262, 1173, 1047; δH(300.40 MHz; CDCl3) 7.38 (5H, m,
Ph), 7.13 (1H, d, J 8.10, NH), 6.82 (1H, d, J 7.9, NH), 5.18 (1H,
d, J 12.3, Bn CH), 5.12 (1H, d, J 12.3, Bn CH), 4.87 (1H, m,
αCH), 4.53 (1H, m, αCH), 3.87 (3H, s, OCH3), 3.73 (3H, s,
OCH3), 3.02 (1H, dd, J 4.3, 17.1, Asp βCH), 2.87 (2H, t, J 7.2,
δCH2), 2.69 (1H, dd, J 6.8, 17.0, Asp βCH), 2.04 (3H, s, CH3),
1.88 (2H, m, CH2), 1.65 (4H, m, 2 × CH2); δC(75.45 MHz;
CDCl3) 193.3 (CO), 172.0 (CO2), 171.9 (CO2), 170.4 (CONH),
170.3 (CONH), 161.2 (CO2), 135.3 (Ph), 128.6 (Ph), 128.4 (Ph),
128.2 (Ph), 67.0 (OCH2), 54.0 (OCH3), 53.0 (OCH3), 52.0
(αCH), 49.0 (αCH), 38.4 (CH2CO), 35.7 (CH2CO), 31.0
(βCH2), 23.1 (γCH2), 18.4 (CH3); m/z (EI) 464 (MHϩ, 78%).
Fmoc-Ala-Ala-(ꢀ-DL-hydroxy)AP(OCH3)-OCH3 27b
Fmoc-Ala-Ala-OH 17b (1.19 g, 3.12 mmol), EDCI (660 mg,
3.43 mmol) and HOBt (452 mg, 3.43 mmol) were dried under
high vacuum for 90 min before being dissolved in anhydrous
THF (20 ml). Pyridine (280 µl, 3.43 mmol) was added to the
solution, which was stirred at RT for 15 min during which time
a white suspension formed. The amino alcohol hydrochloride
11 (400 mg, 1.46 mmol) was added as a solution in anhydrous
CH2Cl2 (10 ml) and the reaction mixture stirred at RT for a
further 16 h. Solvent was removed in vacuo and water (150 ml)
was added. The mixture was extracted with EtOAc (3 × 50 ml)
and CH2Cl2 (2 × 50 ml). The combined organic extracts were
dried (MgSO4), and evaporated in vacuo. The crude product
was purified by flash chromatography (10% CH3CN–90%
EtOAc, Rf 0.38) which yielded 27b as a fluffy colourless solid
(350 mg, 39%); νmax(KBr)/cmϪ1 3065, 2953, 1735, 1650, 1531;
δH(270 MHz; CDCl3, two diastereomers) 7.4 (8H, m, Ph),
6.2 (1H, d, J 6.2, NH), 5.9 (2H, m, OCH2), 4.55 (2H, m, 2 ×
CHCH3), 4.13 (2H, m, αCH ϩ εCH), 3.68 (3H, s, OCH3), 3.66
(3H, s, OCH3), 1.70 (2H, m, CH2), 1.38 (6H, m, 2 × CHCH3),
1.45 (2H, m, CH2), 1.36 (2H, m, CH2); δC(75.45 MHz; CDCl3)
175.1 (CO2CH3), 172.5 (CO2CH3), 172.4 (CONH), 172.3
(CONH), 156.2 (OCONH), 143.7 (Ph), 141.1 (Ph), 127.6 (Ph),
126.9 (Ph), 125.0 (Ph), 119.8 (Ph), 70.1 (OCH2), 66.9 (Fmoc
CH), 52.2 (OCH3), 52.0 (OCH3), 50.4 (εCH), 48.8 (αCH), 46.9
(2 × Ala αCH), 33.4 (δCH2), 31.4 (βCH2), 20.6 (γCH2), 18.8
(Ala CH3), 17.9 (Ala CH3); m/z (ESϩ) 622 [(M ϩ K)ϩ, 15%],
606 [(M ϩ Na)ϩ,70], 584 [(MH)ϩ, 51].
Ac-Asp(OCH3)-(ꢀ-keto)AP(OCH3)-OCH3 25a
A solution of Ac-Asp(OCH3)-(ε-DL-hydroxy)AP(OCH3)-OCH3
24a (316.3 mg, 811 µmol) in anhydrous CH2Cl2 (10 ml) was
stirred at RT under dry N2. Dess–Martin periodinane (413 mg,
1.2 equiv. ) was added and the reaction mixture stirred for 1 h.
A further 50 mg of periodinane was added and stirring was
continued for 20 min. After this time the mixture was poured
into a vigorously stirred solution of Na2S2O3 in 1 M aq.
NaHCO3 (20 ml). After 10 min the mixture was extracted into
CH2Cl2 (4 × 25 ml), and the organic extracts were combined,
dried (MgSO4) and evaporated. The residue was purified by
flash chromatography (15% CH3CN in EtOAc, Rf 0.65), yielding
compound 25a as a colourless solid (132.2 mg, 42%); mp 93–
102 ЊC; [α]D24 Ϫ4.2 (c 2.1, CH2Cl2); δH(300 MHz; CDCl3) 7.17
(1H, d, J 8.1, AP NH), 6.91 (1H, d, J 8.1, Asp NH), 4.86 (1H,
ddd, J 4.2, 6.8, 11.0, Asp αCH), 4.54 (1H, m, AP αCH), 3.87
(3H, s, OCH3), 3.74 (3H, s, OCH3), 3.72 (3H, s, OCH3), 2.98
(1H, dd, J 4.0, 17.0, Asp βCH), 2.89 (2H, t, J 6.8, CH2CO), 2.65
(1H, dd, J 6.8, 17.1, Asp βCH), 2.25 (1H, m, AP βCH), 2.07
(3H, s, CH3), 1.90 (1H, m, AP βCH), 1.68 (2H, m, γCH2);
δC(75.45 MHz; CDCl3) 193.4 (CO), 172.6 (CO2), 171.9 (CO2),
170.5 (CO2), 170.4 (CONH), 161.2 (CONH), 53.0 (OCH3), 52.6
(OCH3), 52.2 (OCH3), 52.0 (αCH), 49.2 (αCH), 38.5 (CH2),
35.5 (CH2), 31.1 (CH2), 23.2 (CH3CO), 18.5 (CH2); νmax (KBr)/
cmϪ1 3387, 2960, 1731, 1656, 1536, 1440, 1374, 1280; m/z (EI)
389 (MHϩ, 0.2%), 388 (Mϩ, 0.2); m/z (CI) 389 (MHϩ, 50%), 357
(MHϩ Ϫ CH3OH, 22) (Calc. for C16H24N2O9: M, 388.1482.
Found: Mϩ, 388.1494) (Calc. for C16H24N2O9: C, 49.48; H,
6.23; N, 7.21. Found: C, 48.96; H, 6.11; N, 7.03%).
Ac-Phe-(ꢀ-keto)AP(OCH3)-OCH3 19
To a stirred solution of α-hydroxy ester 18 (70 mg, 170 µmol)
in dry CH2Cl2 (2 ml) was added Dess–Martin periodinane
(72 mg, 170 µmol). After 90 min the solution was added to
saturated aq. NaHCO3 (5 ml) containing Na2S2O3 (0.5 g). The
mixture was stirred vigorously for 5 min and then extracted into
CH2Cl2 (3 × 15 ml). The dried (MgSO4) extracts were evapor-
ated in vacuo, and the residue was purified by flash chromato-
graphy (10% CH3CN in EtOAc, Rf 0.28) to afford 19 as an oil
(60 mg, 87%): [α]D24 ϩ1.48 (c 6.10, CH2Cl2); νmax(KBr)/cmϪ1
3583.1, 3282.5, 3061.7, 2362.2, 1732.3, 1648.7, 1542.5, 1437.6,
1373.4, 1261.0, 1042.6, 746.1; δH(300.40 MHz; CDCl3) 7.25–
7.13 (5H, m, Ph), 6.40 (1H, d, J 7.7, NH), 6.10 (1H, d, J 7.9,
NH), 4.70 (1H, m, αCH), 4.48 (1H, m, αCH), 3.79 (3H, s,
OCH3), 3.64 (3H, s, OCH3), 3.06 (2H, d, J 7.0, Phe βCH2), 2.85
(2H, t, J 7.1, δCH2), 1.92 (3H, s), 1.84 (1H, m, βCH), 1.66 (1H,
m, βCH), 1.60 (2H, m, γCH2); δC(75.45 MHz; CDCl3), 194.0
Ac-Asp([ꢀ-keto]AP[OCH3]-OCH3)-OCH3 25b
To a stirred solution of Ac-Asp([ε--hydroxy]AP[OCH3]-
OCH3)-OCH3 24b (720 mg, 1.9 mmol) in dry CH2Cl2 (20 ml)
was added Dess–Martin periodinane (1.22 g, 2.9 mmol, 1.6
equiv.). After one hour the reaction mixture was poured onto
aq. sodium thiosulfate (10 g) in water (100 ml) and stirred
J. Chem. Soc., Perkin Trans. 1, 2000, 2023–2036
2033