Photoinduced Aromatization of Unsymmetrically Substituted 1,4 ...

136 J. Chin. Chem. Soc., Vol. 54, No. 1, 2007 Memarian et al.
Table 5. Irradiation of various dihydropyridines in CHCl3 under oxygen atmosphere
Time of total oxidation (h)
Unsymmetrical 1,43,5-Diacetyl-1,41,4-Dihydropyridinesdihydropyridinesdihydropyridines
3,5-diester
1a 0.66 4a 0.66 5a 0.75
1b 5.5 4b 6 5b 6.5
1d 6 4d 6 5d 9
1e 9 4e 9.75 5e 19
1f 12.5 4f 11 5f 21
1h 6 4h 8.5 5h 9
1k 1.5 4k 2.5 5k 3.5
spectra were obtained on a Sisonn, TRIO 1000, EI-mode at
70 eV. The elemental analyses were measured on a Euro EA-
CHNS analyzer. UV spectra were measured on a Shimadzu
UV-160 spectrometer. Preparative layer chromatography
(PLC) was carried out on 20 × 20 cm 2 plates, coated with a
1 mm layer of Merck silica gel PF254, prepared by applying
the silica as slurry and drying in air. All irradiations were
performed using a 400 W high-pressure lamp from NARVA
with cooling of samples in Duran glass ( 280 nm) by
running cold water. Argon (99%) or oxygen (99%) was
bubbled through the solutions during irradiation.
General procedure for the irradiation of dihydropyridines
(1a-1l)
A 15.10 -3 M solution of 1a-1l in chloroform was irradiated
under Ar or O2 atmosphere until total disappearance
of dihydropyridines was observed (TLC; the corresponding
irradiation times are given in Table 1). When the reaction
was complete, the product was purified by chromatography.
Ethyl 5-acetyl-2,6-dimethylpyridine-3-carboxylate (2)
PLC (petroleum ether/ethyl acetate, 3:1). m.p. 35-36
C. UV (MeOH) max [nm] (log ): 275 (3.77), 242 (4.05),
218 (4.20); IR max: 1724 (CO2C2H5), 1691 (COCH3)cm -1 ;
1 H NMR : 1.45 (t, 3H,J= 7.13 Hz, CO2CH2CH3), 2.65 (s,
3H, 6-CH3), 2.80 (s, 3H, 2-CH3), 2.88 (s, 3H, COCH3), 4.44
(q, 2H, J = 7.12 Hz, CO2CH2CH3), 8.53 ppm (s, 1H, 4-H);
MS m/z (rel. int. %): 221 [M + ] (36), 206 [M + -CH3] (100),
178 [M + - COCH3] (21), 176 [M + -CH3CH2O] (20), 160
[M + -CH3CH2OH-CH3] (5), 150 [M + -COCH3 -C2H4]
(16), 106 [M + -CO2CH2CH3 - COCH2] (25), 43 [CH3CO + ]
(47); Anal. Calcd. for C12H15NO3 (221.26): C, 65.14; H,
6.83; N, 6.33; O, 21.69. Found: C, 65.16; H, 6.88; N, 6.31.
Ethyl 5-acetyl-2,6-dimethyl-4-(2-nitrosophenyl)pyridine-3-carboxylate
(3a)
Recrystallized from ethyl acetate. m.p. 94-97 C. UV
(MeOH) max [nm] (log ): 310 (3.90), 279 (4.10), 219 (sh,
4.39); IR max: 1723 (CO2C2H5), 1705 (COCH3), 1551 (NO)
cm -1 ; 1 H NMR : 0.81 (t, 3H, 3 J = 7.13 Hz, CO2CH2CH3),
2.04 (s, 3H, 6-CH3), 2.63 (s, 3H, 2-CH3), 2.70 (s, 3H,
COCH3), 3.89 (qq, 2H, J = 7.07 Hz, CO2CH2CH3), 6.52 (d,
1H, J = 8.04 Hz, 6-H), 7.50 (mc, 1H, 4-H), 7.58 (d, 1H,
3-H, J = 7.58 Hz), 7.78 ppm (mc, 1H, 5-H); MS m/z (rel.
int. %): 327 [M + +1] (3), 284 [M + - COCH2] (100), 267 [M +
-NO-CH2CH3] (20), 253 [M + -CO2CH2CH3] (27), 239
[M + -CH3CHO - COCH3] (33), 237 [M + -CH3CH2OH -
COCH3] (14), 211 [M + - COCH2 -CO2CH2CH3] (7), 193
[M + -H2O - COCH2 -CO2CH2CH3] (15), 192 [M + -
C6H4NO-C2H4] (11), 43 [CH3CO + ] (90); Anal. Calcd. for
C18H18N2O4 (326.36): C, 66.25; H, 5.56; N, 8.58; O, 19.61.
Found: C, 66.14; H, 5.54; N, 8.50.
Ethyl 5-acetyl-2,6-dimethyl-4-(3-nitrophenyl)pyridine-
3-carboxylate (3b)
PLC (petroleum ether/ethyl acetate, 2:1). m.p. 64-65
C. UV (MeOH) max [nm] (log ): 259 (sh, 3.91); 211
(4.27); IR max: 1724 (CO2C2H5), 1710 (COCH3), 1533 and
1353 (NO2) cm -1 ; 1 H NMR : 1.05 (t, 3H, J = 7.13 Hz,
CO2CH2CH3), 2.07 (s, 3H, 6-CH3), 2.59 (s, 3H, 2-CH3),
2.67 (s, 3H, COCH3), 4.11 (q, 2H, J = 7.11 Hz, CO2CH2CH3),
7.65 (mc, 2H, 5-H and 6-H), 8.21 (s, 1H, 2-H), 8.33 ppm
(dd, 1H, J = 7.85 Hz and 1.53 Hz, 4-H); MS m/z (rel. int.
%): 342 [M + ] (28), 327 [M + -CH3] (99), 325 [M + - OH]
(49), 299 [M + - COCH3] (23), 281 [M + -CH3 -NO2] (29),
43 [CH3CO + ] (100); Anal. Calcd. for C18H18N2O5 (342.35):
C, 63.15; H, 5.30; N, 8.18; O, 23.37. Found: C, 63.15; H,
5.33; N, 8.10.