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269
2.1.6. Pyridoxal-50-phosphate-6-azo-(4-chlorophenyl)-
2,4-disulfonate (8)
Yield = 48%. 1H NMR (DMSO-d6) d 2.32 (3H, s, –CH3),
5.62 (2H, d, J = 5.1 Hz, –CH2O–), 7.49 (2H, d, J = 8.4 Hz,
phenyl), 7.84 (2H, d, J = 8.4 Hz, phenyl), 10.40 (1H, s,
CHO).
2.1.13.1. 6-(Benzyl)pyridoxal-4-dimethylacetal (32). 3-
Trimethylcarbonyl-6-(benzyl)pyridoxal monomethylace-
tal (30) (0.77 g, 2.16 mmol), prepared as described [17],
was treated with a solution of 10% sulphuric acid (2 ml) in
acetone (4 ml) and heated at 48 8C for 48 h. Workup with
ether/saturated sodium bicarbonate followed by silica col-
umn chromatography eluting with CHCl3:MeOH (150:1)
yielded 31 (0.52 g, 79%). 1H NMR (CDCl3) d 1.38 (9H, s,
t-Bu), 2.44 (3H, s, –CH3), 3.64 (d, OH, J = 7.81 Hz), 4.07
(2H, s, –CH2–), 4.74 (2H, dd, J = 12.7 Hz, J = 58.6 Hz,
CH2O–), 5.93 (1H, s, –CH–), 7.16–7.28 (5H, m, phenyl).
MS (Cl, NH3) 342 (M + H).
2.1.7. Pyridoxal-50-phosphate-6-azo-(3-fluorophenyl)-
2,4-disulfonate (9)
Yield = 87%. 1H NMR (D2O) d 2.47 (3H, s, –CH3), 5.68
(2H, d, J = 4.5 Hz, –CH2O–), 7.24 (1H, pseudo t,
J = 7.8 Hz, J = 8.4 Hz phenyl), 7.48 (1H, s, phenyl),
7.51 (1H, d, J = 8.4 Hz, phenyl), 7.66 (1H, d,
J = 7.8 Hz, phenyl), 10.42 (1H, s, CHO).
Compound 31 (150 mg, 0.439 mmol) was dissolved in
1 ml of 10% methanolic potassium hydroxide solution and
the mixture was allowed to stand for 10 min. The mixture
was cooled in an ice bath, and 1 equivalent of acetic acid
was added for neutralisation. The product was purified by
preparative thin layer chromatography with CHCl3:MeOH
(20:1) and crystallized from hexane:ethyl acetate (10:1) to
afford 90 mg of 31 as a white solid (yield 68%). 1H NMR
(CDCl3) d 2.50 (3H, s, –CH3), 3.44 (6H, s, 2x-OCH3), 4.25
(2H, s, –CH2–), 4.59 (2H, s, –CH2–), 5.93 (1H, s, –CH–),
7.16–7.29 (5H, m, phenyl), 8.73 (1H, s, –OH). MS (CI,
NH3) 304 (M + H).
2.1.8. Pyridoxal-50-phosphate-6-azo-(3-nitrophenyl)-
2,4-disulfonate (10)
Yield = 30%. 1H NMR (D2O) d 2.45 (3H, s, –CH3), 5.69
(2H, d, J = 4.4 Hz, –CH2O–), 7.72 (1H, t, J = 8.0 Hz,
phenyl), 8.20 (1H, d, J = 8.0 Hz, phenyl), 8.25 (1H, d,
J = 8.0 Hz, phenyl), 8.48 (1H, s, phenyl), 10.41 (1H, s,
CHO).
2.1.9. Pyridoxal-50-phosphate-6-azo-(4-chloro-3-
nitrophenyl)-2,4-disulfonate (13)
Yield = 25%. 1H NMR (D2O) d 2.39 (3H, s, –CH3), 5.65
(2H, d, J = 5.4 Hz, –CH2O–), 7.74 (1H, d, J = 9.3 Hz,
phenyl), 8.10 (1H, d, J = 9.3 Hz, phenyl), 8.39 (1H, s,
phenyl), 10.39 (1H, s, CHO). HRMS (FAB-) calcd.:
429.0003; found: 428.9980.
2.1.13.2. Dimethyl[4-formyl-3-hydroxy-2-methyl-6-ben-
zyl-pyrid-5-yl]methylphosphonate (33). Compound 32
(80 mg, 0.263 mmol) was treated with 0.5 ml of thionyl
chloride and the mixture was stirred for 5 min. The excess
thionyl chloride was removed by a N2 stream and reduced
pressure. The residue was dissolved in anhydrous CH2Cl2,
neutralized with triethylamine and filtered through a bed of
silica gel eluting with CHCl3/MeOH (100/1). After evapora-
tion, 85 mg of the chloromethyl compound was obtained
(100%). 1H NMR (CDCl3) d 2.50 (3H, s, –CH3), 3.44 (6H, s,
2x-OCH3), 4.25 (2H, s, –CH2–), 4.55 (2H, s, –CH2–), 5.86
(1H, s, –CH–), 7.17–7.29 (5H, m, phenyl), 8.81 (1H, s, –
OH). HRMS (EI) calcd.: 321.1132; found 321.1129.
2.1.10. Pyridoxal-50-phosphate-6-azo-(2,4-
difluorophenyl)-2,4-disulfonate (14)
Yield = 26%. 1H NMR (D2O) d 2.49 (3H, s, –CH3), 5.73
(2H, d, J = 3.3 Hz, –CH2O–), 7.07 (1H, d, J = 6.6 Hz,
phenyl), 7.17 (1H, pseudo t, J = 9.0 Hz, J = 9.6 Hz,
phenyl), 7.93 (1H, d, J = 6.6 Hz, phenyl), 10.46 (1H, s,
CHO).
The chloromethyl compound was dissolved in 1 ml of
trimethylphosphite, and heatedat 65 8C for 2 days and 70 8C
for 1 day. After evaporation of the excess trimethylpho-
sphite, the residue was purified by preparative thin layer
chromatography with CHCl3:MeOH (20:1) to afford 30 mg
of 33 as a light yellow oil with recovery of 20 mg of the
starting compound (combined yield 38%). 1H NMR
(CDCl3) d 2.48 (3H, d, J = 2.6 Hz, –CH3), 3.16 (2H, d,
J = 21.6 Hz, –CH2P–), 3.43 (6H, s, 2x-OCH3), 3.68 (6H, d,
J = 11.0 Hz, –P(O)(OCH3)2), 4.28(2H, s, –CH2–), 6.16(1H,
2.1.11. Pyridoxal-50-phosphate-6-azo-(2,3,4-
trifluorophenyl)-2,4-disulfonate (15)
Yield = 31%. 1H NMR (D2O) d 2.50 (3H, s, –CH3), 5.22
(2H, d, J = 6.9 Hz, –CH2O–), 6.48 (1H, m, phenyl), 7.50
(1H, m, phenyl), 10.44 (1H, s, CHO).
2.1.12. Pyridoxal-50-phosphate-6-azo-(4-fluoro-3-
nitrophenyl)-2,4-disulfonate (16)
Yield = 86%. 1H NMR (D2O) d 2.52 (3H, s, –CH3), 5.22
(2H, d, J = 6.78 Hz, –CH2O–), 6.51 (1H, d, J = 7.2 Hz,
phenyl), 7.52 (1H, s, phenyl), 7.78 (1H, d, J = 7.2 Hz,
phenyl), 10.46 (1H, s, CHO).
s, –CH–), 7.09–7.28 (5H, m, phenyl), 9.01 (1H, s, –OH). 31
NMR (CDCl3) 28.76 (m). MS (CI, NH3) 396 (M + H).
P
2.1.13.3. [4-Formyl-3-hydroxy-2-methyl-6-benzyl-pyrid-
5-yl]methylphosphonic acid monotriethylamine salt
(23). A solution of 50 mg (0.126 mmol) of 33 in 3 ml of
anhydrous CH2Cl2 was treated with 70 ml (0.5 mmol) of
bromotrimethylsilane at 25 8C. The mixture was stirred
2.1.13. Synthesis of the 6-benzyl analogue of
PPADS (23)
The general scheme of synthesis is described in
Section 3.