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V. Dichiarante et al. / Journal of Photochemistry and Photobiology A: Chemistry 265 (2013) 41–48
Scheme 5. Synthesis of 1-ethyl-8-fluoro-4-oxo-7-(piperidin-1-yl)-1,4-dihydroquinoline-3-carboxylic acid (3) and 1-ethyl-6,8-difluoro-4-oxo-7-(piperidin-1-yl)-1,4-
dihydroquinoline-3-carboxylic acid (4).
16b: 1H NMR (CD3OD) ␦ 8.65 (s, 1H), 8.00 (t, 1H, J = 8 Hz), 4.35
(q, 2H, J = 7 Hz), 1.35 (t, 3H, J = 7 Hz). IR (nujol) 1717.
3: Colourless solid, m.p. 225–226 ◦C. 1H NMR (CDCl3) ␦ 8.60 (s,
1H), 8.20 (d, 1H, J = 9 Hz), 7.20 (t, 1H, J = 9 Hz), 4.60–4.45 (m, 2H),
3.35–3.20 (m, 4H), 1.90–1.65 (m, 6H), 1.60 (t, 3H, J = 7 Hz). 13C NMR
(CDCl3) ␦ 177.2, 167.0, 150.2 (CH), 145.9, 143.1 (q, J = 245 Hz), 129.8,
122.9 (CH), 120.9, 117.9 (CH), 108.0, 54.6 (d, CH2, J = 16 Hz), 51.7
(2 × CH2), 25.9 (2 × CH2), 24 (CH2), 16.2 (d, CH3, J = 4 Hz). IR (nujol)
1722. Anal. calcd for C17H19FN2O3, C 64.14, H 6.02, N 8.80, found C
63.5, H 6.9, N 8.9.
4.1.3. Step c [22]
Derivative 16 (6.12 mmol), K2CO3 (2.1 g, 15.3 mmol), ethyl
iodide (2.47 ml, 30.6 mmol) and DMF (8 ml) were placed in a flask
and stirred at 120 ◦C for 2 h. The mixture was filtered hot, to
remove potassium salts, and the salts were washed with ethanol.
The organic phases were cooled down to r.t. and the resulting pre-
cipitate (17) was collected by filtration and used in the next step
without further purification.
4: [24] Colourless solid, m.p. >250 ◦C. 1H NMR (CDCl3) ␦ 8.60 (s,
1H), 7.90 (dd, 1H, J = 2 and 12 Hz), 4.60–4.40 (m, 2H), 3.40 (bs, 4H),
1.85–1.65 (m, 6H), 1.60 (dt, 3H, J = 1 and 7 Hz). 13C NMR (CDCl3)
␦ 176.2, 166.7, 155.2 (dd, J = 7 and 250 Hz), 149.7 (CH), 145.6 (d,
J = 245 Hz), 135.4, 127.0 (d, J = 40 Hz), 120.5, 108.1 (dd, J = 3 and
23 Hz), 107.9 (CH), 54.4 (d, CH2, J = 16 Hz), 52.2 (2 × CH2), 26.4
(2 × CH2), 24.0 (CH2), 16.2 (d, CH3, J = 4 Hz). IR (nujol) 1720. Anal.
calcd for C17H18F2N2O3, C 60.71, H 5.39, N 8.33, found C 61.3, H 4.9,
N 8.2.
17a: m.p. 158–160 ◦C. 1H NMR (CDCl3) ␦ 8.20 (s, 1H), 8.10 (m,
1H), 7.10 (m,1H), 4.30 (m, 4H), 1.40 (m, 6H). IR (nujol) 1678. ESI-MS
m/z (%) 282.1 ([M+1]+, 100). Anal. calcd for C14H13F2NO3, C 59.79,
H 4.66, N 4.98, found C 59.7, H 4.7, N 5.1.
17b: 1H NMR (CDCl3) ␦ 8.40 (s, 1H), 8.20 (t, 1H, J = 8 Hz),
4.50–4.30 (m, 4H), 1.55 (t, 3H, J = 7 Hz), 1.40 (t, 3H, J = 7 Hz).
4.2. Reaction quantum yields
4.1.4. Step d [23]
Compound 17 (7.4 mmol) was dissolved in acetic acid (40 ml)
and heated at 100 ◦C.
Reaction quantum yields were measured on 2-ml degassed
samples, in 1-cm optical path spectrophotometric cuvettes, irradi-
ating with a 150 W high-pressure mercury arc and an interference
filter (317 nm). Consumption of the starting compound was
assessed by HPLC (Discovery® HS C18, 250 × 4.6 mm, 5 m, pH
2.5 water/acetonitrile (from 50:50 to 45:55) as eluant, flow
1–1.2 ml/min, ꢀ = 285 nm). The light flux was measured by ferriox-
alate actinometry.
3 N HCl (20 ml) was added dropwise over 2 h and the final solu-
tion was stirred at 100 ◦C for 2 more hours, then cooled down slowly
to r.t. Water was added and the resulting precipitate was collected
by filtration, washed with cold water, 2-propanol and diethyl ether
to give 5 as a colourless solid.
18a: m.p. 217–219 ◦C. 1H NMR (CDCl3) ␦ 8.70 (s, 1H), 8.40 (m,
1H), 7.50 (m,1H), 4.60 (q, 2H, J = 6.6 Hz), 1.6 (t, 3H, J = 6.6 Hz). IR
(nujol) 1653. ESI-MS m/z (%) 254.1 ([M+1]+, 100). Anal. calcd for
C12H9F2NO3, C 56.92, H 3.58, N 5.53, found C 56.9, H 3.6, N 5.4.
18b: 1H NMR (CDCl3) ␦ 8.75 (s, 1H), 8.25 (dt, 1H, J = 8 and 2 Hz),
4.65–4.50 (m, 2H), 1.75–1.55 (m, 3H). 13C NMR (CDCl3) ␦ 176.0,
165.7, 150.6 (CH), 149.0 (dd, J = 260 and 15 Hz)), 144.4 (dd, J = 260
and 15 Hz), 143.6 (t, J = 260 Hz), 126.6, 123.2, 109.4 (d, CH, J = 19 Hz),
108.9, 54.3 (d, CH2, J = 15 Hz), 16.1 (d, CH3, J = 4.5 Hz). IR (nujol)
1726.
4.3. Fluorescence quantum yields
Fluorescence quantum yields were measured by means of a
Perkin Elmer LS55 spectrofluorimeter by using quinine sulphate
as the standard.
4.4. Titration
pKa values were measured by titration with a dilute NaOH solu-
tion by using a Model 121 Microcomputer pHmeter.
4.1.5. Step e [23]
Compound 18 (4 mmol) was suspended in acetonitrile (12 ml). A
solution of piperidine (0.42 ml, 4.2 mmol) and DBU (0.6 ml, 4 mmol)
in acetonitrile (6 ml) was added, under stirring, to the starting sus-
pension. The resulting mixture was refluxed for 2.5 h, then stirred
at r.t. overnight.
The precipitate was collected by filtration and washed with ace-
tonitrile (12 ml), 80% aqueous acetonitrile (4.5 ml), ethanol (12 ml)
and diethyl ether (45 ml), affording products 3 or respectively 4.
4.5. Preparative irradiations
4.5.1. Irradiation of compounds 3,4 in H2O/CH3CN (4:6)
A solution of either 3 or 4 (2 × 0.1 mmol, 3.5 × 10−4 M) in
H2O/CH3CN (4:6) was placed in an immersion-well apparatus
(2 × 300 ml), stirred and flushed with argon for 20 min, then