C.-C. Zeng et al. / Tetrahedron 65 (2009) 4505–4512
4511
4. Experimental
4.1. General
(KBr): n
3428, 2925, 2854, 1646, 1603, 1279 cmꢀ1; ESI-MS: m/z 255
(Mþþ1), 277 (MþþNa), 531 (2MþþNa).
4.2.3. 4-(5-Methyl-1,3,4-thiadiazol-2-ylthio)benzene-1,2-diol (3c)
Cyclic voltammograms were measured by a 273A Potentiostat/
Galvanostat equipped with an electrochemical analysis software,
using a conventional three-electrode cell. The working electrode
was a glassy carbon disk (ca.¼3 mm), Pt disk (ca.¼2 mm) or Au disk
(ca.¼2 mm) electrodes. The auxiliary and reference electrodes in
these studies were Pt net and Ag/AgCl (in 3 M KCl), respectively. All
electrodes for CV experiments were from CH Instruments, Inc., USA.
Mixed solution of acetate buffer (pH¼6.0) and acetonitrile
(v:v¼2:1) was used as a supporting electrolyte system. The con-
centrations of substrates were 2 mmol Lꢀ1, while those of the
supporting electrolyte was 0.2 mol Lꢀ1. Controlled-potential elec-
trolyses were conducted in a 250 mL beaker-type cell (undivided)
equipped with a platinum plate (2.0 cm2) cathode and an assembly
of 7-carbon rod (ca. 8 mm diameter and 60 mm length) anode,
which were connected to the 273A Potentiostat/Galvanostat.
Catechols 1a–e and 5-methyl-2-mercaptothiadiazole 2 were re-
agent grade from ‘Alfa Aesar China (Tianjin) Co., Ltd’. Other chem-
icals and solvents were from Beijing Chemicals Co. These chemicals
were used without further purification. Doubly distilled de-ionized
water was used for preparation of acetate buffer. All experiments
were performed at room temperature and ambient pressure.
Melting points (mp) were determined on an XT4A Electrothermal
apparatus equipped with a microscope and were uncorrected. In-
frared spectra (IR) were recorded as thin films on KBr plates on
Yield 31%; mp 210–211 ꢂC; 1H NMR (400 MHz, acetone-d6):
d 2.62
(s, 3H, CH3), 6.95 (d, 1H, J¼8.0 Hz, Ar-H), 7.08 (dd, 1H, J¼8.4 Hz,
J¼1.6 Hz, Ar-H), 7.17 (d, 1H, J¼1.6 Hz, Ar-H), 8.51 (br s, 2H, OH); 13C
NMR (100 MHz, acetone-d6):
d 15.4, 117.5, 120.9, 122.4, 128.1, 147.2,
148.8,166.2,170.8; IR (KBr): n
3412,1596,1511,1286 cmꢀ1; ESI-MS: m/
z 238.6 (Mþꢀ1), 262.7 (MþþNa), 478.7 (2Mþꢀ1), 502.7 (2MþþNa).
4.2.4. 4,5-Bis(5-methyl-1,3,4-thiadiazol-2-ylthio)benzene-
1,2-diol (4c)
Yield 33%; mp 219–221 ꢂC; 1H NMR (400 MHz, DMSO-d6):
d
2.63
(s, 6H, CH3), 7.19 (s, 2H, Ar-H), 10.17 (s, 2H, OH); 13C NMR (100 MHz,
DMSO-d6):
d 15.8, 123.2, 124.4, 149.4, 167.1, 167.3; IR (KBr): n 3435,
2925, 1637, 1385 cmꢀ1; ESI-MS: m/z 368.7 (Mþꢀ1).
4.2.5. 3-Methoxy-5-(5-methyl-1,3,4-thiadiazol-2-ylthio)benzene-
1,2-diol (3d)
Yield 31%; mp 179–180 ꢂC; 1H NMR (400 MHz, acetone-d6):
d
2.58 (s, 3H, CH3), 3.77 (s, 1H, OCH3), 6.76 (d, 1H, J¼2.4 Hz, Ar-H),
6.82 (s, 1H, J¼2.4 Hz, Ar-H), 8.97 (s, 1H, Ar-OH), 9.45 (s, 1H, Ar-OH);
13C NMR (100 MHz, acetone-d6):
15.6, 19.9, 117.7, 118.8, 123.1,
d
133.6, 144.9, 149.0, 165.9, 170.6; IR (KBr):
n
3415, 1650, 1294 cmꢀ1
;
ESI-MS: m/z 269 (Mþꢀ1).
4.2.6. 3-Methoxy-4,5-bis(5-methyl-1,3,4-thiadiazol-2-
ylthio)benzene-1,2-diol (4d)
a Bruker IR spectrophotometerand are expressed in n
(cmꢀ1).1H NMR
and 13C NMR spectra were obtained using an AV 400M Bruker
spectrometer in solvent (DMSO-d6 or acetone-d6) with TMS as in-
ternal reference. The MS data (ESI) were recorded on a Bruker esquire
6000 mass spectrometer.
Yield 16%; mp 195–196 ꢂC; 1H NMR (400 MHz, DMSO-d6):
d
2.59
(s, 3H, CH3), 2.65 (s, 3H, CH3), 3.75 (s, 3H, OCH3), 7.00 (s, 1H, Ar-H);
13C NMR (100 MHz, DMSO-d6): 15.7, 15.8, 61.3, 117.4, 118.0, 126.0,
142.0, 150.9, 151.2, 166.0, 166.5, 167.6, 168.4; IR (KBr): n 3434, 1639,
1389, 1310 cmꢀ1; ESI-MS: m/z 399 (Mþꢀ1), 799 (2Mþꢀ1).
4.2. General procedure for the synthesis of compounds 3–5
4.2.7. 3-Methyl-4-(5-methyl-1,3,4-thiadiazol-2-ylthio)benzene-
1,2-diol (5e)
To a 250 mL of undivided beaker-type cell were added a mixture
of 100 mL of 0.1 mol/L acetate buffer solution (pH¼6.0) and 50 mL
of acetonitrile, which were pre-electrolyzed (to remove impurities
present in the electrolytic system) at 0.50 V (vs SCE). Subsequently,
2 mmoL of catechols and 2 mmoL of 5-methyl-2-mercaptothia-
diazole 2 were added to the cell and continued to electrolysis. After
electrolysis, the solvent was acidified to pH¼1 with 1 mol/L aque-
ous HCl and then extracted by ethyl acetate (3ꢁ20 mL) and washed
with water (20 mL). The separated organic layer was dried over
MgSO4, filtered, and evaporated. The crude product was purified by
column chromatography on silica gel and eluted with a mixture of
petroleum ether and acetone (v:v¼1:1).
Yield 20%; mp 184–185 ꢂC; 1H NMR (400 MHz, acetone-d6):
d
2.37 (s, 3H, CH3), 2.61 (s, 3H, CH3), 6.85 (d, 1H, J¼8.4 Hz, Ar-H), 7.12
(d, 1H, J¼8.4 Hz, Ar-H); 13C NMR (100 MHz, acetone-d6):
d
14.1, 15.4,
3411,
114.4, 121.4, 128.8, 129.9, 145.7, 148.3, 165.9, 171.1; IR (KBr):
n
1646, 1290 cmꢀ1; ESI-MS: m/z 253 (Mþꢀ1), 507 (2Mþꢀ1).
4.2.8. 3-Methyl-4,5-bis(5-methyl-1,3,4-thiadiazol-2-
ylthio)benzene-1,2-diol (4e)
Yield 13%; mp 203–204 ꢂC; 1H NMR (400 MHz, acetone-d6):
2.47 (s, 3H, CH3), 2.64 (s, 3H, CH3), 2.69 (s, 3H, CH3), 7.21 (s, 1H, Ar-
d
H); 13C NMR (100 MHz, acetone-d6):
d 14.3, 14.6, 14.7, 118.2, 124.2,
128.0, 131.9, 146.3, 147.6, 165.2, 166.2, 166.6, 167.5; IR (KBr): n 3429,
1640, 1390, 1295 cmꢀ1; ESI-MS: m/z 383 (Mþꢀ1).
4.2.1. 5-tert-Butyl-4-(5-methyl-1,3,4-thiadiazol-2-ylthio)benzene-
1,2-diol (3a)
4.3. Yield determination by HPLC method
Yield 92%; mp 172–174 ꢂC; 1H NMR (400 MHz, DMSO-d6):
d 1.22
(s, 9H, C(CH3)3), 2.58 (s, 3H, CH3), 6.95 (d, 1H, J¼2.4 Hz, Ar-H), 6.99
Similar to the preparative-scale electrolysis, 0.11 mmol of cate-
chols 1 and 0.1 mmol of 2 were added to the beaker-type cell and
were electrolyzed under different conditions (amount of passed
charge, anodic material, pH, applied potential, concentration of 1
and 2). After reaction, the electrolytic solution was diluted and
subjected to HPLC analysis. HPLC analysis conditions were: Di-
amond (250ꢁ4.6 mm) C18 column (Dikma Technologies), an eluent
with 2:3 CH3OHþH2O at 1.0 mL/min, a Waters 2487 UV detector
with detection at 270 nm. Typically, un-reacted starting material 1
and the corresponding products were checked by an HPLC com-
parison with the authentic samples, which were isolated and
characterized by 1H NMR, 13C NMR, MS, and IR. The yields of the
products and the amount of un-reacted starting 2 were determined
(d, 1H, J¼2.4 Hz, Ar-H), 9.17 (br s, 1H, OH), 9.61 (br s, 1H, OH); 13C
NMR (100 MHz, DMSO-d6):
d 15.6, 31.6, 34.3, 115.9, 116.0, 122.3,
143.1, 144.9, 146.3, 165.9, 169.0; IR (KBr):
n 3482, 2967, 1647, 1479,
1400, 1288 cmꢀ1; ESI-MS: m/z 297 (Mþþ1), 319 (MþþNa), 615
(2MþþNa).
4.2.2. 5-Methyl-4-(5-methyl-1,3,4-thiadiazol-2-ylthio)benzene-
1,2-diol (3b)
Yield 65%; mp 191–194 ꢂC; 1H NMR (400 MHz, DMSO-d6):
d 2.24
(s, 3H, CH3), 2.57 (s, 3H, CH3), 6.79 (s, 1H, Ar-H), 7.02 (s, 1H, Ar-H),
9.29 (br s, 1H, OH), 9.54 (br s, 1H, OH); 13C NMR (100 MHz, DMSO-
d6): d 15.6, 19.9, 117.7, 118.8, 123.1, 133.6, 144.9, 149.0, 165.9, 170.6; IR