Photoinduced Aromatization of Unsymmetrically Substituted 1,4 ...

Photoinduced Aromatization of 1,4-Dihydropyridines J. Chin. Chem. Soc., Vol. 54, No. 1, 2007 133
mechanism for this conversion, in which chloroform is involved
in the reaction (Scheme II). According to the proposed
mechanism, excited 1,4-dihydropyridine (PyH2*),
especially 1b-1l, donates an electron to chloroform under
formation of PyH2· + and CHCl3· . Elimination of HCl from
both intermediates leads to the formation of a radical pair,
namely hydropyridyl (PyH·) and dichloromethyl (·CHCl2)
radicals. Hydrogen abstraction by ·CHCl2 radical completes
the reaction by formation of the pyridine compound (Py)
and dichloromethane. Other studies also have proposed
this mechanism for the photo-oxidation of symmetrical
1,4-dihydropyridines in CCl4 15 and CBrCl3 solutions. 17,20
The interesting point in our study is the expulsion of
the 4-substituent in the cases of 1e, 1k and 1l with a 4-hydroxy-3-methoxyphenyl,
5-methyl-2-furyl and 2-furyl
substituent, respectively, in the 4-position-. The loss of the
C-4 substituent upon irradiation of Hantzsch esters has
been reported earlier only in the case of carboxy groups, 31
some heterocyclic groups, 25 and secondary alkyl and benzyl
groups. 25 The expulsion of the 4-substituent has also
been reported upon irradiation of diketo-dihydropyridines
with 5-methyl-2-furyl and 2-furyl substituents in the 4 position.
26 Thermal oxidation of Hantzsch esters with expulsion
of benzylic and secondary alkyl substituents by various
oxidants has also been cited. 26 Most of the product derived
from 1d in our work is converted to an unidentified
compound, which stays on the start line when separation is
attempted by PLC and which we considered to be the pyridinium
salt. However, shaking of this zone with NaOHsolution
and extraction of the aqueous phase with CHCl3
did not give any isolable product.
IR (Table 2), 1 H NMR (Table 3) and UV data (Table
4) gave useful information on the structural assignment of
Scheme II
Table 2. Comparison of the IR spectra I (/cm -1 ) 1a-1l with
those of 3a-3j
1 NH CO2C2H5 COCH3 3 CO2C2H5 COCH3 a 3333 1675 1655 a II
1723 1705
b 3289 1700 1645 b 1724 1710
c 3333 1675 1662 c 1724 1688
d 3264 1675 1643 d 1740 1726
e 3291 1677 1640 e 1724 1702
f 3300 1663 1640 f 1724 1700
g 3341 1700 1654 g 1724 1697
h 3290 1692 1648 h 1724 1694
i 3033 1691 1662 i 1724 1705
j 3162 1643 1632 j 1737 1722
k 3307 1691 1646 2 1724 1691
l 3231 1676 1660 2 1724 1691
I
The spectra have been taken as KBr disc, except for 3d which
has been taken in CHCl3-solution. II The absorption at 1551 cm -1
indicated the presence of a NO-group.
the photoproducts 2 and 3a-3j.
A comparison of the IR spectra showed the disappearance
of the NH band and also the shift of both CO vibrations
to higher frequency due to the aromatization of the
ring, which changes the role of the ring from enamine-like
in 1 to acceptor-like in 3. Comparison of the 1 H NMR spectra
indicated the loss of the signals for NH and also for the
substituent in position 4 because of the expulsion of this
substituent upon photo-oxidation of 1e, 1k and 1l with formation
of the identical product 2. In this case, 4-H appears
in the aromatic region at 8.53 ppm. In the 1 H NMR spectra
of photoproducts containing the 4-substituent, the loss of
NH and 4-H resonances was observed. Owing to aromatization
of the ring and diminished conjugation of the C-C
double bonds with both CO groups, the methyl protons in