1
, δ, ppm, Coupling Constant,
TABLE 2. H NMR Spectra of 4-7
J (Hz)
Com-
pound
CH2, s, 2H
OH, NH, br. s,
2-H, s, 1H
HAr
(6-H, s, 1H)
2H (OH, s, 1H) (NH2, s, 2H)
5.47
5.43
5.45
5.39
5.55
5.28
5.51
5.62
7.21
7.27
7.22
7.27
7.47
6.20
6.25
5.70
7.97
7.89
7.92
7.97
8.30
(6.53)
(6.53)
(6.55)
(6.55)
(6.50)
8.37
8.41
8.44
8.40
(6.86)
(6.86)
(6.99)
7.69-8.10, m, 5H
4а
7.64 d and 8.06 d, 4H 3J = 7.8
7.66 d and 8.03 d, 4H 3J = 8.0
7.72 d and 8.06 d, 4H 3J = 8.0, 4Н
8.42 d and 8.55 d, 4H 3J = 8.0
7.45-7.98 m
4b
4c
4d
4e
5a
7.31 d and 7.94 d, 4Н 3J = 7.8
8.10 d and 8.45 d, 4H 3J = 8.0
7.40-8.12 m, 5Н
5b
5e·HCl
5f·HCl
5g·HCl
6a
5.54
5.57
3.30
3.25
7.80-8.42 m, 5Н
(9.43)
(9.40)
(9.42)
(9.47)
(9.20)
(9.22)
(9.29)
(12.75)
(12.70)
(12.85)
(12.75)
(11.35)
(11.40)
(11.30)
7.65-8.32 m, 5Н
7.65 d and 8.33 d, 4Н 3J = 8.0
7.70 d and 8.38 d, 4H 3J = 7.5
7.69-8.38 m, 5Н
6b
6d
6f
7.50-8.12 m, 5Н
7a
7.32 d and 8.05 d, 4Н 3J = 8.0
7.69 d and 8.04 d, 4Н 3J = 7.5
7b
7d
The reactions of α-bromoacetophenones with
o-phenylenediamine and its derivatives proceed under
milder conditions involving brief heating of the starting compounds in ethanol at reflux in an inert atmosphere
to give dihydroquinoxalines in satisfactory yield [4, 5]. These products are sensitive to atmospheric oxygen and
readily oxidize to 2-arylquinoxalines. The stability of 1,2-dihydroquinoxalines is enhanced upon substitution at
the NH or CH2 group [6]. On the other hand, dihydropteridines 4a-e, 5a,b,d-g are more stable than
1,2-dihydroquinoxalines. The stabilization of these compounds may be related to the formation of
intermolecular associates since heating 4a-e, 5a,b,d,g in acetone, DMSO, or DMF facilitates their oxidation to
pteridines 6 or 7, respectively. The presence of the electron-withdrawing pyrimidine ring in 4a-4e, 5a,b,d,g is
also a stabilizing factor.
Since an intramolecular hydrogen bond is impossible for dihydropteridines 4, quantum-chemical ab
initio (6-31G** [7]) calculations were carried out for model compounds A and B (Table 5). The results showed
that there is no O···HN hydrogen bond since that the O···H distance is greater than the sum of the corresponding
van der Waals radii. On the other hand, the shortening of the N···HO distance for form B is sufficient for
hydrogen bond formation. The increase of the dipole moment for molecule B (up to 1.380) also supports this
conclusion [8]. However, the only slight difference between the energies for A and B (4 kcal/mole in favor of
form A) makes hydrogen bonding improbable.
show two rather resolved π–π* bands at
The electronic absorption spectra for 4a-e, 5a,b,d-g
220-380 nm. The long-wavelength band undergoes a bathochromic shift upon introduction of substituent R1 in
(Δλ = 35 nm), which is
the para position of the phenyl group and the greatest shift is seen for nitro derivative 4e
characteristic for azomethine chromophores. Aromatization is regularly accompanied by a hypsochromic shift of
λ
max of the long-wavelength band (Table 1). The addition of one or two drops of concentrated sulfuric acid to the
solution measured for dihydro derivatives 5a,b,d,f,g leads to a slight hypsochromic shift of the long-wavelength
band and bathochromic shift of the short-wavelength band while both these bands undergo a hypsochromic shift
for their aromatic analogs 7a,b,d (Table 3). Comparison of the experimental data and results of CNDO/S
calculations [9] for 5a and 7a suggested that protonation occurs at the primary amino group (Table 4).
The electronic spectra remain unchanged upon the addition of several drops of base such as ammonia or
aqueous NaOH. However, an increase in the basicity of the medium has a marked effect on pterines 7a,b,d and
a new strong band appears in the short-wavelength region, which is independent of the base concentration
(Table 3). This finding is probably the result of formation of enolate forms of 7a,b,d.
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