650
D. Franck et al. / Bioorg. Med. Chem. 21 (2013) 643–652
4.8. Methyl O-[trans-3-(benzyloxy)cyclobutyl]-N-(tert-
butoxycarbonyl)- -tyrosine (13)
TLC showed full conversion, the pH of the mixture was adjusted
to pH 5 with 1 M HCl and concentrated in vacuo. The resulting
oil was dissolved in ethyl acetate, washed with brine and evapo-
rated to dryness, was then re-dissolved in ethyl acetate, dried with
sodium sulfate, filtered and concentrated in vacuo to give 16
(80 mg, 97%) as a white solid. TLC (ethyl acetate/hexane 1:1, spot
at start). 1H NMR (400 MHz, CDCl3): d ppm 1.41 (s, 9H, 3 CH3),
2.33–2.51 (m, 2H, CH), 2.75 (d, 1H, CH), 2.90 (dd, 2H, CH), 3.12
(d, 2H, CH), 4.16–4.27 (m, 1H, CH), 4.52 (d, J = 5.56 Hz, 1H, CH),
4.76 (quin, 0.5H, CH–F), 4.90 (quin, 0.6H, CH–F), 4.98 (d,
J = 7.58 Hz, 1H, NH), 6.05 (br s, COOH), 6.74 (d, J = 8.34 Hz, 2H,
CHarom), 7.09 (d, J = 8.34 Hz, 2H, CHarom). 19F NMR (376 MHz,
CDCl3): d ppm ꢂ169.2. ESI-MS: m/e 354 [M+H]+.
L
To a solution of Boc-Tyr-OMe L-12 (1.02 g; 3.35 mmol) and cis-
3-(benzyloxy)cyclobutanol 5 (1.33 g; 7.37 mmol) in dry DMF
(25 mL) was added diethyl diazocarboxylate (1.20 mL; 7.37 mmol).
The yellow solution was stirred under nitrogen and triphenylphos-
phine (1.98 g; 7.37 mmol) was added. The mixture was stirred at
room temperature for 23 h and concentrated in vacuo. The crude
oil was dissolved in chloroform (50 mL) and washed with water
(3 ꢀ 30 mL) to remove dimethylformamide. The organic layer
was dried with sodium sulfate, filtered and concentrated in vacuo
to give a brown oil. The crude product was purified by silica chro-
matography, eluting with a gradient of ethyl acetate/hexane to give
the L-13 (1.35 g, 88%) as a clear colorless oil. TLC (ethyl acetate/
hexane 1:2, Rf ꢁ0.46). 1H NMR (400 MHz, CDCl3): d ppm 1.43
(s, 9H, CH3), 2.38–2.56 (m, 2H, CH), 2.94–3.11 (m, 2H, CH), 3.72
(s, 3H, CH3), 4.30–4.39 (m, 1H, CH), 4.46 (s, 2H, CH2), 4.50–4.60
(m, 1H, CH), 4.78–4.88 (m, 1H, CH), 4.90–5.00 (m, 1H, NH), 6.71
(d, 2H, CHarom), 7.02 (d, 2H, CHarom), 7.29–7.42 (m, 5H, CHarom).
CI-MS: m/e 356 [MꢂBoc]+.
4.12. O-(cis-3-Fluorocyclobutyl)-L-tyrosine hydrochloride salt
(L-17)
A solution of N-(tert-butoxycarbonyl)-O-(cis-3-fluorocyclobu-
tyl)- -tyrosine 16 (79 mg; 0.22 mmol) in DMF (0.6 mL) was treated
L
with 4 M HCl in dioxane (0.5 mL; 2.24 mmol) for 1 h at room tem-
perature. The mixture was concentrated in vacuo, re-suspended in
ethanol/water (1:1) and extracted with a small amount of dichlo-
romethane. To reduce the organic solvent, the aqueous phase
was concentrated and gave after freeze–drying the product 17
(25 mg, 44%) as a white solid. TLC (DCM/MeOH 9:1, spot at start).
1H NMR (300 MHz, CD3OD): d ppm 2.19–2.36 (m, 2H, CH), 2.95–
3.12 (m, 2H, CH), 3.24–4.16 (m, 1H, CH), 4.27–4.37 (m, 1H, CH),
4.84 (quin, 1H, CH–F), 4.92 (quin, 1H, CH–F), 6.85 (d, 2H, CHarom),
7.20 (d, 2H, CHarom), COOH and NH2 not visible. 13C NMR
(151 MHz, CD3OD): d ppm 36.7 (CH2), 40.3 (CH2), 40.5 (CH2),
55.8 (CH), 63.6 (CH), 63.8 (CH), 81.0 (CH), 82.4 (CH), 116.6 (CH),
128.1 (CH), 131.7 (CH), 158.3 (C), 171.8 (C). ESI-MS: m/e 254
[M+H]+. HPLC (chiral): ratio D/L 3%: 97%, rt = 3.84 min.30
4.9. Methyl N-(tert-butoxycarbonyl)-O-(trans-3-
hydroxycyclobutyl)-L-tyrosine (14)
A solution of methyl O-[trans-3-(benzyloxy)cyclobutyl]-N-(tert-
butoxycarbonyl)- -tyrosine 13 (1.35 g; 2.96 mmol) in methanol
L
(20 mL) was stirred with 10% palladium on charcoal (500 mg,
50% wet). The mixture was stirred under a positive pressure of
hydrogen (balloon) at room temperature for 2 h. The mixture
was filtered and the solvent concentrated in vacuo. The crude oil
was dissolved in dichloromethane, filtered through CeliteÒ,
washed with dichloromethane and concentrated in vacuo to give
14 (1.02 g, 94%) as a clear colorless oil which was used without fur-
ther purification. TLC (ethyl acetate, Rf ꢁ0.54). 1H NMR (400 MHz,
CDCl3): d ppm 1.42 (s, 9H, CH3), 1.80 (br s, 1H, OH), 2.34–2.59 (m,
2H, CH), 3.02 (m, 2H, CH), 3.72 (s, 3H, CH3), 4.47–4.59 (m, 1H, CH),
4.60–4.70 (m, 1H, CH), 4.79–4.90 (m, 1H, CH), 4.96 (d, J = 7.91 Hz,
1H, NH), 6.71 (d, 2H, CHarom), 7.02 (d, 2H, CHarom). ESI-MS: m/e
366 [M+H]+.
4.13. O-(cis-3-Fluorocyclobutyl)-D-tyrosine hydrochloride salt
(D-17)
O-(cis-3-Fluorocyclobutyl)-D-tyrosine hydrochloride salt D-17
was synthesized according to the procedure described for its L-iso-
mer L-17.
4.10. Methyl N-(tert-butoxycarbonyl)-O-(cis-3-
4.13.1. Methyl O-[trans-3-(benzyloxy)cyclobutyl]-N-(tert-
fluorocyclobutyl)-
L
-tyrosine (15)
butoxycarbonyl)-D-tyrosine (D-13)
Clear oil, 1.29 g (3.81 mmol) of D-12 gave D-13 (1.16 g, 72%).
TLC (ethyl acetate/hexane 1:2, Rf ꢁ0.39). 1H NMR (300 MHz,
CDCl3): d ppm 1.43 (s, 9H, CH3), 2.37–2.58 (m, 4H, CH), 2.92–3.11
(m, 3H, CH3), 3.72 (s, 3H, CH3), 4.30–4.39 (m, 1H, CH), 46 (s, 2H,
CH2), 4.54 (m, J = 7.16 Hz, 1H, CH), 4.79–4.88 (m, 1H, CH), 4.96
(d, J = 7.91 Hz, 1H, NH), 6.71 (d, J = 8.48 Hz, 2H, CHarom), 7.02 (d,
J = 8.48 Hz, 2H, CHarom), 7.28–7.41 (m, 5H, CHarom). ESI-MS: m/e
456 [M+H]+.
To an ice-cooled solution of methyl N-(tert-butoxycarbonyl)-O-
(trans-3-hydroxycyclobutyl)- -tyrosine 14 (658 mg; 1.80 mmol) in
dry dichloromethane (25 mL) was added DAST (358 L;
L
l
2.70 mmol). The mixture was stirred at 0 °C for 3 h and was al-
lowed to reach room temperature overnight. The crude product
was concentrated in vacuo and purified by column chromatogra-
phy with a gradient of ethyl acetate/hexane to give 15 (257 mg,
38%) as white crystals. TLC (ethyl acetate/hexane 1:2, Rf ꢁ0.62).
1H NMR (300 MHz, CDCl3): d ppm 1.42 (s, 9H, CH3), 2.36–2.53
(m, 2H, CH), 2.95–3.08 (m, 2H, CH), 3.72 (s, CH3), 4.17–4.27 (m,
1H, CH), 4.50–4.60 (m, 1H, CH), 4.81 (quin, 0.5H, CH-F), 4.94 (quin,
0.5H, CH-F), 4.96 (d, J = 7.91 Hz, 1H, NH), 6.73 (d, J = 8.59 Hz, 2H,
CHarom), 7.03 (d, J = 8.59 Hz, 2H, CHarom). 19F NMR (376 MHz,
CDCl3): d ppm ꢂ169.3. ESI-MS: m/e 368 [M+H]+.
4.13.2. Methyl N-(tert-butoxycarbonyl)-O-(trans-3-
hydroxycyclobutyl)-D-tyrosine (D-14)
Clear oil, 1.52 g (2.96 mmol) of D-13 gave D-14 (1.35 g, 97%).
TLC (DCM/MeOH 9:1, Rf ꢁ0.59). 1H NMR (300 MHz, CDCl3): d
ppm 1.42 (s, 9H, CH3), 1.63 (br s, 1H, OH), 2.34–2.57 (m, 4H, CH),
2.93–3.11 (m, 2H, CH2), 3.72 (s, 3H, CH3), 4.48–4.59 (m, 1H, CH),
4.59–4.70 (m, 1H, CH), 4.80–4.89 (m, 1H, CH), 4.97 (d, J = 8.10 Hz,
1H, NH), 6.71 (d, J = 8.67 Hz, 2H, CHarom) 7.02 (d, J = 8.48 Hz, 2H,
CHarom). ESI-MS: m/e 366 [M+H]+.
4.11. N-(tert-Butoxycarbonyl)-O-(cis-3-fluorocyclobutyl)-L-
tyrosine (16)
To a solution of methyl N-(tert-butoxycarbonyl)-O-(cis-3-flu-
orocyclobutyl)- -tyrosine 15 (85 mg; 23 mol) in methanol
(3.5 mL) was added 1 M lithium hydroxide (650 L; 65 mol).
The clear mixture was stirred for 6 h at room temperature. After
4.13.3. Methyl N-(tert-butoxycarbonyl)-O-(cis-3-
fluorocyclobutyl)-D-tyrosine (D-15)
L
l
l
l
White solid, 147 mg (4.16 mmol) of D-14 gave of D-15 (1.52 mg,
8%). TLC (ethyl acetate/hexane 1:2, Rf ꢁ0.62). 1H NMR (300 MHz,