Pezzella et al.
(2.9 mL) in a cuvette thermostated by circulation of ice-cold water
and treated under stirring with 110 µL of a 2.5 mM solution of
o-chloranil in ethyl acetate. Absorption spectra were immediately
recorded by a diode array spectrophotometer over 1 min period at
15 s time intervals. In other experiments, the phosphate solution
containing 1a was taken to pH 7 with 2 M HCl. An aliquot of the
solution (50 µL) was diluted with cold 50 mM phosphate buffer
pH 7.0 (2.9 mL) and treated under the conditions described above
with 110 µL of a solution of o-chloranil in methanol. When
necessary the oxidation mixtures at 15 s were analyzed by HPLC
(eluant as for LC/MS analysis) to estimate residual chloranil.
Experiments using sodium periodate were run under the conditions
described above in phosphate buffer pH 6.0 with addition of the
oxidant in water.
In other experiments, the solution of 1a in ethyl acetate obtained
as described above but starting from 20 mg of 1b was oxidized
with an equimolar solution of o-choranil in ethyl acetate. After 20
s, the organic layer was washed with a 1% Na2S2O4 solution (5
mL) and then directly treated with an equal volume of acetic
anhydride containing 5% pyridine overnight at rt. After removal
of the volatile components the residue was analyzed by TLC and
was found to consist mainly of 1b which was purified by preparative
TLC (8 mg, Rf 0.68 (CHCl3/MeOH, 9:1)).
of the structure and chemical properties of 5,6-indolequinones
and underscore the investigative potential of the combined
chemical, pulse radiolytic and quantum mechanical approach
reported in the present study.
Experimental Section
5,6-Dihydroxyindole32 and 5,6-diacetoxyindole33 were synthe-
sized as previously reported. For general experimental methods and
other materials, see the Supporting Information.
Preparation of 5,6-Diacetoxy-3-iodoindole (1b). A general
procedure reported in the literature34 was adopted with modifica-
tions. Briefly, Oxone (11.9 g, 19.2 mmol) was added to a stirred
solution of NH4I (1.4 g, 9.6 mmol), I2 (2.4 g, 9.6 mmol), and 5,6-
diacetoxyindole (1.5 g, 6.4 mmol) in acetonitrile (30 mL), and the
reaction mixture was taken under stirring at room temperature. After
1 h, at complete substrate consumption (TLC evidence, Rf 0.62
eluant CHCl3/CH3OH 9:1), the reaction mixture was diluted with
ethyl acetate (60 mL) and washed with a 0.1% Na2S2O3/1% NaCl
solution (2 × 50 mL) and then with water (2 × 50 mL). The
combined organic layers were dried over sodium sulfate, and the
solvent was removed under reduced pressure. The resulting dark
oil was purified by column chromatography (eluant CHCl3) to give
1b (1.27 g; 55% yield, Rf 0.68 (CHCl3/MeOH, 9:1, > 98% purity)
as a yellow solid. Crystallization from ethanol gave colorless
needles, decomposing over 110 °C.
Isolation of 3-Iodo-1H-pyrrolo[2,3-b]phenazine (2). To a
solution of 1a in ethyl acetate prepared as described above from
50 mg of 1b taken in an ice bath was added a cooled solution of
o-chloranil (69 mg, 0.28 mmol) in ethyl acetate (6.7 mL). After 30
s, the reaction mixture was added to 16 mL of glacial acetic acid
at rt, followed by a solution of o-phenylenediamine (76 mg, 0.70
mmol) in ethyl acetate (6.7 mL). After 2 min, the reaction mixture
was washed with water (40 mL), dried over sodium sulfate and
taken to dryness to give a brown residue. This was dissolved in
ethyl acetate and analyzed by LC/ESI+MS. Preparative TLC
fractionation (eluant CHCl3/CH3OH 9:1 v/v) gave a band at Rf 0.47
(5 mg) that was further purified by semipreparative HPLC (eluant
0.1% TFA/acetonitrile 1:1 v/v) to give 2 (tR 46 min, 1 mg, 80%
1b: UV (MeOH) λmax 284, 292 (sh) nm; IR (CCl4) νmax 3479,
3395, 1770, 1468, 1370, 1326, 1232, 1116, 1009, 920, 871 cm-1
;
1H NMR (acetone-d6) δ 2.28 (3H, s), 2.29 (3H, s), 7.17 (1H, s,
H-4), 7.36 (1H, s, H-7), 7.56 (1H, d, J ) 3.0 Hz, H-2), 10.88 (1H,
bs, NH); 13C NMR (acetone-d6) 20.0 (2 × CH3), 54.2 (C-3), 106.2
(C-7), 113.6 (C-4), 127.4 (C-9), 131.2 (C-2), 133.0 (C-8), 137.5
1
(C-5), 139.4 (C-6), 168.3 (CO), 168.6 (CO); H and 13C NMR
spectra are provided as Supporting Information; ESI+/MS m/z 360
[M + H]+, 382 [M + Na]+, 719 [2M + H]+, 741 [2M + Na]+;
HR ESI+/MS found 359.9730 ([M + H]+), calcd for C12H10NO4I
m/z 359.9733.
1
purity): UV λmax (CH3OH) 244, 280, 395, 483 (b) nm; H NMR
Preparation of 5,6-Dihydroxy-3-iodo-indole (1a). A solution
of 1b (5 mg, 0.014 mmol) in methanol (1.0 mL) was slowly added
under vigorous stirring to a 0.025 M trisodium phosphate solution
(2.5 mL) that had previously been purged with argon. The reaction
mixture was stirred under an argon atmosphere and after 10 min
acidified to pH 4 with 2 M HCl, and the mixture was extracted
with ethyl acetate (2 × 2.5 mL). Note that the solutions of 1a in
ethyl acetate are highly unstable and cannot be taken to dryness
even in the cold. For NMR analysis of 1a the reaction was carried
out on 5 (or 50) mg of 1b in 0.025 M trisodium phosphate solution
(1 (or 50) mL) and the ethyl acetate extract (1 (or 3 × 50) mL)
was added to acetone-d6 (5 mL) or DMSO-d6 (600 µL, in this case
the ethyl acetate was removed by fluxing the solution with
anhydrous argon).
(acetone-d6) δ 7.86 (2H, m, H-7, H-8), 8.08 (1H, br s, H-2), 8.21-
8.25 (2H, m, H-6, H-9), 8.28 (1H, s, H-4), 8.35 (1H, s, H-7); 13C
NMR (acetone-d6) δ 55 (C-3) , 107 (C-11), 118 (C-4), 128 (C-6/
C-9), 129 (C-7/C-8), 136-139 (C-3a/C4a), 139 (C-2), 139-141
(C-10a/C-11a); (LC-ESI+/MS tR 27.5 min, m/z 346 ([M + H]+);
HR ESI+/MS found 345.9872 ([M + H]+), calcd for C14H9N3I
m/z 345.9841.
Pulse Radiolysis. The pulse radiolysis experiments were per-
formed with the 12 MeV linear accelerator at the Daresbury
Laboratory, using the Free Radical Research Facility.35 This
accelerator provides pulse lengths of between 0.2 and 2 µs with
doses up to 30 Gy using quartz capillary cells of optical path 2.5
•-
cm. Absorbed doses were determined from the transient (CNS)2
formation in air-saturated potassium thiocyanate solutions (10 mM)
1
1a: UV λmax (pH 7.0) 300 nm; H NMR (acetone-d6) δ 6.71
using a G of 0.30 µM/Gy and ꢀ (500 nm) ) 7100 M-1 cm-1 36
.
(1H, s, H-4), 6.87 (1H, s, H-7), 7.28 (1H, d, J ) 3.0 Hz, H-2); 1H
NMR (DMSO-d6) δ 6.59 (1H, s, H-4), 6.80 (1H, s, H-7), 7.17 (1H,
bs, H-2), 8.66, 8.77 (bs, OH), 10.87 (1H, bs, NH); 13C NMR
(DMSO-d6) δ 55.8 (C-3), 97.6 (C-7), 104.2 (C-4), 122.4 (C-9),
The estimates of the molar absorption coefficients and rate constants
are considered to be correct to (15%. In a typical experiment, a
solution of 1b in methanol (15 mM) was treated under a N2O
atmosphere with a solution of 0.025 M trisodium phosphate
containing 0.5 M KBr up to a 0.15 mM concentration, and after
10 min the pH of the solution was taken to 7.0 by addition of NaH2-
1
127.1 (C-2), 130.0 (C-8), 141.8 (C-5), 143.9 (C-6); H and 13C
NMR spectra are provided as Supporting Information; ESI+/MS
m/z 276 [M + H]+, 298 [M + Na]+ , 573 [2M + Na]+.
Chemical Oxidation of 1a. All of the following experiments
were performed on ice-bath cooled solutions and solvents. For the
spectrophotometric experiments, an aliquot (100 µL) of the solution
of 1a obtained as described above was diluted with ethyl acetate
(35) (a) Butler, J.; Hodgson, B. W.; Hoey, B. M.; Land, E. J.; Lea, J. S.;
Lindley, E. J.; Rushton, F. A. P.; Swallow, A. J. Radiat. Phys. Chem. 1989,
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1602 J. Org. Chem., Vol. 72, No. 5, 2007