Synthesis of Dityrosine, Trityrosine, and Pulcherosine
stirred at 80 °C for 21 h. The resulting mixture was extracted
with EtOAc (2 × 30 mL) and the combined organic extracts
were washed with saturated sodium chloride solution (2 × 30
mL), dried (MgSO4), and concentrated in vacuo to give a dark
brown oil. The oil was chromatographed on silica, eluting with
EtOAc/hexane (1:2-1:1 gradient) to give iodo-dityrosine de-
rivative 16 (7.4 mg, 21%) as a pale yellow oil: [R]D +15.6 (c
0.54, CHCl3); IR (neat) υmax (cm-1) 3373, 2089, 1636, 1454,
1342, 1215, 1057, 694; 1H NMR (400 MHz, CDCl3) δ 7.54 (1H,
d, J ) 1.2 Hz), 7.28-7.12 (30H, m), 6.99-6.94 (3H, m), 6.78
(1H, dd, J ) 1.2, 5.8 Hz), 5.28 (1H, br d, J ) 7.9 Hz), 5.23
(1H, br d, J ) 9.7 Hz), 5.14-5.00 (8H, m), 4.90-4.89 (4H, m),
4.64 (1H, m), 4.37 (1H, m), 3.03-2.96 (4H, m); 13C NMR (100
MHz, CDCl3) δ 171.4 (s), 171.2 (s), 155.6 (s), 155.3 (s), 155.0
(s), 139.1 (d), 137.4 (s), 137.0 (s), 136.5 (s), 136.2 (s), 134.9 (s),
133.2(8) (d), 133.1(6) (s), 132.6 (d), 132.3 (d), 130.0 (d), 129.2
(d), 128.6(6) (d), 128.5(8) (s), 128.5(4) (d), 128.4(8) (s), 128.4(1)
(d), 128.3 (d), 128.1(4) (s), 128.1(0) (d), 127.9 (d), 127.6 (d), 127.4
(d), 126.9 (d), 126.6 (d), 113.0 (d), 92.9 (s), 74.9 (t), 70.3 (t),
67.3(7) (t), 67.2(6) (t), 67.1 (t), 67.0 (t), 54.8(7) (d), 54.8(0) (d),
37.3 (t), 37.0 (t) (2 coincident C-C and 6 C-H); MS (ESI, +ve)
m/z 1137 (M + Na+, 57%), 1011 (M - I + Na+, 100%).
6.69 (5H, m), 6.43 (1H, s), 5.24-5.19 (2H, br m), 5.04-4.81
(10H, m), 4.59-4.57 (2H, m), 3.01-2.85 (4H, m); 13C NMR (100
MHz, CDCl3) δ 171.3 (s), 171.2 (s), 155.6 (s), 154.8 (s), 154.3
(s), 151.8 (s), 139.2 (d), 136.2 (s), 135.0 (s), 132.9 (d), 132.7 (d),
130.5 (d), 129.5 (d), 129.4 (s), 128.6 (d), 128.5 (d), 128.4 (d),
128.3 (d), 128.1 (d), 128.0 (d), 127.1 (d), 127.0 (d), 126.9 (s),
125.6 (s), 114.1 (d), 86.1 (s), 71.1 (t), 67.3 (t), 67.2 (t), 67.0 (t),
66.4 (t), 55.8 (d), 55.0 (d), 37.7 (t), 36.9 (t) (4 coincident C-C
and 6 C-H); MS (ESI, +ve) m/z 1048 (M + Na+, 44%); HRMS
(ESI, +ve) m/z calcd for (M + Na)+ 1047.2325, found 1047.2314.
Trityrosine 3. To a solution of protected trityrosine deriva-
tive 23 (90.3 mg, 0.06 mmol) in H2O/MeOH/THF (1:5:5, 10 mL)
was added 10% palladium on charcoal (7.0 mg). The reaction
mixture was stirred under hydrogen at room temperature for
7 d. The resulting mixture was filtered through Celite, washed
with NH3(aq)/MeOH/CHCl3 (2:18:80, 3 × 20 mL), and concen-
trated in vacuo to give the deprotected trityrosine in quantita-
tive yield. For analytical purposes, the crude product was
chromatographed on reverse-phase HPLC using H2O/CH3CN/
TFA (95:5:0.05) as eluent to give pure trityrosine 3 as an off-
white solid: mp 152.5-154 °C; [R]D -15.7 (c 0.30, 1 M HCl);
1H NMR (400 MHz, D2O) δ 7.17 (2H, dd, J ) 2.2, 8.3 Hz), 7.13
(2H, d, J ) 2.2 Hz), 7.12 (2H, s), 6.95 (2H, d, J ) 8.3 Hz),
4.09-4.03 (3H, m, RH’s), 3.24-3.08 (6H, m, âH’s);33 13C NMR
(100 MHz, D2O) δ 173.1 (s), 173.0 (s), 152.6 (s), 149.9 (s), 132.5
(d), 132.0 (d), 130.7 (d), 127.1 (s), 126.8 (s), 126.5 (s), 125.3 (s),
117.8 (d), 55.4(0) (d), 55.3(9) (d), 35.1(2) (t), 35.0(6) (t); MS (ESI,
+ve) m/z 540 (M + H+, 90%).
N-Cbz-3-Iodo-L-tyrosine Benzyl Ester 30. To a solution
of silver sulfate (0.77 g, 2.47 mmol) and iodine (0.63 g, 2.47
mmol) in MeOH (60 mL) was added N-Cbz-L-tyrosine benzyl
ester 20 (1.00 g, 2.47 mmol). The mixture was stirred under
nitrogen in the dark at room temperature for 5 h. The yellow
precipitate was removed by filtration, a 1:1 mixture of satu-
rated sodium hydrogen carbonate solution and saturated
sodium chloride solution (100 mL) were added to the filtrate,
and the mixture was extracted with EtOAc (2 × 100 mL). The
combined organic extracts were washed with 50% saturated
sodium chloride solution (100 mL) and saturated sodium
thiosulfate solution (100 mL), dried (MgSO4), and concentrated
in vacuo to give a dark brown oil. The oil was chromatographed
on silica, eluting with ether/DCM/hexane (1:3:4) to give the
diiodo-tyrosine derivative 11 (0.21 g, 13%) and the iodo-
tyrosine derivative 30 (0.57 g, 43%) as a yellow solid: mp
131.5-133.0 °C; [R]D +11.9 (c 2.06, CHCl3); IR (Nujol) υmax
(cm-1) 3354, 1765, 1725, 1498, 1223, 1027, 697; 1H NMR (400
MHz, CDCl3) δ 7.39-7.27 (11H, m), 6.85 (1H, dd, J ) 2.0, 8.3
Hz), 6.76 (1H, d, J ) 8.3 Hz), 5.76 (1H, br s, OH), 5.35 (1H, br
d, J ) 8.0 Hz), 5.19-5.07 (4H, m), 4.65 (1H, m, RH), 3.04-
2.94 (2H, m, âH’s);34 13C NMR (100 MHz, CDCl3) δ 171.2 (s),
155.6 (s), 154.1 (s), 138.9 (d), 136.0 (s), 134.8 (s), 131.0 (d),
129.5 (s), 128.6 (d), 128.6 (d), 128.5 (d), 128.5 (d), 128.2 (d),
128.0 (d), 115.0 (d), 85.4 (q), 67.4 (t), 67.1 (t), 54.9 (d), 36.8 (t);
MS (EI) m/z 380 (M+• - CbzNH, 59%), 335 (13), 334 (21), 253
(53), 233 (M+• - CbzNHCHCO2Bn, 29), 207 (19), 91 (100).
N-Cbz-4-O-p-Methoxybenzyl-3-iodo-L-tyrosine Benzyl
Ester 31. To a solution of potassium carbonate (2.65 g, 19.2
mmol) and tetrabutylammonium iodide (0.18 g) in dry acetone
(100 mL) was added iodotyrosine derivative 30 (2.55 g, 4.80
mmol). The mixture was stirred under nitrogen at room
temperature for 30 min. p-Methoxybenzyl chloride (2.61 mL,
19.2 mmol) was added and the mixture was heated at reflux
in the dark for 24 h. The resulting mixture was cooled and
concentrated in vacuo. To the residue was added water (100
mL) and the mixture was extracted with DCM (3 × 200 mL).
The combined organic extracts were dried (Na2SO4) and
concentrated in vacuo to give a brown/maroon oil. The oil was
Potassium N-Cbz-4-O-Benzyl-L-tyrosyl-3-trifluorobo-
rate Benzyl Ester 21. To a solution of tyrosine boronate
derivative 8 (50.0 mg, 0.08 mmol) in MeOH (1 mL) was added
potassium hydrogen fluoride (4.5 M, 0.13 mL). The mixture
was stirred at room temperature for 2 h. The resulting mixture
was filtered and the solid washed with DCM and hot acetone.
The filtrate was concentrated in vacuo to give the potassium
trifluoroborate derivative 21 (48.0 mg, 100%) as white crystals;
[R]D +10.4 (c 0.95, CHCl3); IR (Neat) υmax (cm-1) 3416, 2359,
1
1717, 1645, 1452, 1211, 1018, 737, 696; H NMR (400 MHz,
CD3CN) δ 7.52-7.50 (3H, m), 7.38-7.28 (13H, m), 6.86 (1H,
dd, J ) 2.5, 8.3 Hz), 6.68 (1H, d, J ) 8.3 Hz), 6.05 (1H, br d,
J ) 8.1 Hz), 5.15-4.98 (6H, m), 4.45 (1H, m), 3.05 (1H, dd, J
) 5.5, 13.8 Hz), 2.86 (1H, dd, J ) 8.5, 13.8 Hz); 13C NMR (100
MHz, CD3CN) δ 173.1 (s), 161.5 (s), 157.1 (s), 139.7 (s), 138.0
(s), 136.9 (s), 135.4 (d), 129.5(2) (d), 129.4(7) (d), 129.4(1) (d),
129.2 (d), 129.0 (d), 128.9 (d), 128.7 (d), 112.8 (d), 75.4 (d), 70.4
(t), 67.6 (t), 67.2 (t), 57.0 (d), 41.0 (d), 37.8 (t), 25.2 (d) (1
coincident C-C) (C-B not observed); MS (ESI, -ve) m/z 563
([M - K]-, 100%), 1163 ([M2 - K]-, 20%); HRMS (ESI, -ve)
m/z calcd for (M - K)- 562.2010, found 562.1970.
Protected Trityrosine 23. To a solution of diiodo-tyrosine
derivative 11 (50.1 mg, 0.08 mmol) and tyrosyl-3-trifluorobo-
rate derivative 21 (91.7 mg, 0.15 mmol) in H2O:THF (1:10, 3
mL) was added potassium carbonate (31.6 mg, 0.23 mmol) and
Pd(dppf)Cl2‚CH2Cl2 (5.50 mg). The reaction mixture was
stirred under nitrogen at reflux for 26 h. The resulting mixture
was filtered through Celite, washed with DCM (3 × 20 mL),
and concentrated in vacuo to give a brown oil. The oil was
chromatographed on silica eluting with EtOAc/hexane (1:2-
1:1 gradient) to give protected trityrosine derivative 23 (70.8
mg, 67%) as an off-white solid: mp 55.5-58 °C; [R]D +5.78 (c
1.98, CHCl3); IR (Nujol) υmax (cm-1) 3346, 3032, 1720, 1499,
1456, 1340, 1213, 1057, 1026, 696; 1H NMR (400 MHz, CDCl3)
δ 7.35-7.16 (40H, m), 6.97-6.93 (6H, m), 6.82-6.79 (2H, m),
6.23 (1H, br s), 5.29-5.25 (3H, br m), 5.14-4.89 (16H, m),
4.68-4.65 (3H, m), 3.07-2.97 (6H, m); 13C NMR (100 MHz,
CDCl3) δ 171.4 (s), 162.9 (s), 155.6 (s), 154.7 (s), 150.2 (s), 136.7
(s), 136.2 (s), 135.9 (s), 135.1 (s), 133.5 (d), 133.1 (d), 132.1 (d),
131.8 (d), 129.5 (d), 128.9 (d), 128.7 (d), 128.5 (d), 128.4(2) (s),
128.3(8) (d), 128.2 (d), 128.1 (d), 128.0(3) (d), 127.9(6) (d), 127.8
(d), 127.7 (d), 126.9 (d), 126.6 (s), 120.8 (d), 114.0 (d), 111.4
(d), 71.0 (t), 67.3 (t), 67.2 (t), 67.0 (t), 66.9 (t), 55.7 (d), 55.0
(d), 37.9 (t), 37.4 (t) (4 coincident C-C); MS (ESI, +ve) m/z
1415 (M + Na+, 100%); HRMS (ESI, +ve) m/z calcd for (M +
Na)+, 1414.525, found 1414.526. Further elution gave the
iododityrosine derivative 22 (17.2 mg, 22%) as an off-white
solid: mp 59-61 °C; [R]D +7.04 (c 1.95, CHCl3); IR (Nujol) υmax
(cm-1) 3346, 3032, 1720, 1499, 1456, 1340, 1213, 1059, 1026,
696; 1H NMR (400 MHz, CDCl3) δ 7.46-7.10 (25H, m), 6.97-
(33) Nomura, K.; Suzuki, N.; Matsumoto, S. Biochemistry 1990, 29,
4525.
(34) Burke, T. R. Jr; Yao, Z.-J.; Zhao, H.; Milne, G. W. A.; Wu, L.;
Zhang, Z.-Y.; Voigt, J. H. Tetrahedron 1998, 54, 9981.
J. Org. Chem, Vol. 70, No. 18, 2005 7361