Chemistry of naphthazarin derivatives 9. Direct observation of prototropic tautomerism of (poly)hydroxynaphthazarins by IR spectroscopy

Article abstract of DOI:10.1023/A:1022425121552

A series of substituted (poly)hydroxylated naphthazarins (5,8-dihydroxy-1,4-naphthoquhiones) were synthesized. In general, (poly)hydroxynaphthazarins exist in organic aprotic solvents as mixtures of tautomeric 1,4-naphthoquinonoid forms (IR data). The ratio of tautomers was determined for the first time. The effects of the nature of substituents and the solvent polarity on the tautomeric equilibrium were qualitatively estimated.

Full text of DOI:10.1023/A:1022425121552

198
Russian Chemical Bulletin, International Edition, Vol. 52, No. 1, pp. 198—207, January, 2003
Chemistry of naphthazarin derivatives
9.* Direct observation of prototropic tautomerism of
(poly)hydroxynaphthazarins by IR spectroscopy**
V. P. Glazunov,a A. Ya. Yakubovskaya,a N. D. Pokhilo,a N. V. Bochinskaya,b and V. Ph. Anufrieva
ꢀ
aPacific Institute of Bioorganic Chemistry, FarꢀEastern Branch of the Russian Academy of Sciences,
159 prosp. 100ꢀletiya Vladivostoka, 690022 Vladivostok, Russian Federation.
Fax: +7 (423 2) 31 4050. Eꢀmail: anufriev@piboc.dvo.ru
bInstitute of Chemistry and Applied Ecology, FarꢀEastern State University,
27 ul. Oktyabr´skaya, 690600 Vladivostok, Russian Federation.
Fax: +7 (423 2) 25 7609
A series of substituted (poly)hydroxylated naphthazarins (5,8ꢀdihydroxyꢀ1,4ꢀnaphthoꢀ
quinones) were synthesized. In general, (poly)hydroxynaphthazarins exist in organic aprotic
solvents as mixtures of tautomeric 1,4ꢀnaphthoquinonoid forms (IR data). The ratio of tauꢀ
tomers was determined for the first time. The effects of the nature of substituents and the
solvent polarity on the tautomeric equilibrium were qualitatively estimated.
Key words: naphthazarin, 5,8ꢀdihydroxyꢀ1,4ꢀnaphthoquinone, naphthopurpurin, hydrꢀ
oxydroseron, 3(6)ꢀethylꢀ2ꢀhydroxynaphthazarin, cristazarin, aureoquinone, (poly)hydroxyꢀ
naphthazarins; prototropic tautomerism; IR spectroscopy in solutions.
Scheme 1
Prototropic tautomerism of naphthazarin (5,8ꢀdiꢀ
hydroxyꢀ1,4ꢀnaphthoquinone, 1) and its derivatives has
been known for a rather long time.2 However, the high
rate of tautomeric exchange makes this phenomenon difꢀ
ficult to study. According to the quantumꢀchemical
data,3,4 rapid (20—40 MHz) synchronous double tunnelꢀ
ing of the OH protons occurs in naphthazarin tautomers
1(A)—(D) (Scheme 1). The calculations showed that
1,4ꢀnaphthoquinonoid forms 1(A) and 1(B) are thermoꢀ
dynamically more stable by 25 kcal mol–1 than alternative
1,5ꢀnaphthoquinonoid forms 1(C) and 1(D) and by
29 kcal mol–1 than centrosymmetric structure 1(E)***.
1H, 13C, and 17O NMR studies in solutions revealed a
rapid proton exchange in a naphthazarin molecule beꢀ
tween the hydroxy and carbonyl groups, which averages
the corresponding signals for the H, C, and O atoms in
the quinonoid and benzenoid parts of the molecule on the
NMR time scale.6—8 Signals for the C atoms in posiꢀ
tions 1 and 4, 5 and 8, 2 and 3, 6 and 7, and 9 and 10 are
split only in the 13C NMR spectrum of crystalline naphthꢀ
azarin cooled to –160 °C.7
estimation of the content of their tautomeric forms
(Scheme 2). This estimation is only approximate since
the original tautomers differ in reactivities and the tautoꢀ
meric equilibrium can be affected by methylating agents.
This approach cannot be applied to the study of
hydroxynaphthazarins. For instance, the quantumꢀchemiꢀ
cal calculations12 show that the βꢀOH group in naphthoꢀ
purpurin (2a) completely shifts the tautomeric equilibꢀ
Earlier,9—11 Oꢀmethylation of substituted naphthꢀ
1
azarins followed by quantitative H NMR analysis of a
ratio of the resulting products was proposed for indirect
* For Part 8, see Ref. 1.
** In memory of Prof. O. B. Maximov (1911—2001).
*** When the present paper was prepared for publication, some
doubt was cast on these calculations in a recent communication.5
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 186—195, January, 2003.
1066ꢀ5285/03/5201ꢀ198 $25.00 © 2003 Plenum Publishing Corporation

Tautomerism of hydroxynaphthazarins
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
199
Scheme 2
or comparable with the halfꢀwidths (∆ν1/2) of these
bands, which complicates their interpretation. In addiꢀ
tion, deuteration experiments showed that the C=O and
αꢀOH stretching vibrations are mixed up in naphthazarin
itself.
Moreover, the frequencies of the βꢀOH stretching viꢀ
brations (ν(OH)) in the IR spectra of hydroxynaphthꢀ
azarins depend on their ability to form intramoꢀ
lecular hydrogen bonds (IMHB) and their strengths.13
Thus, hydroxynaphthazarins are a convenient system
for experimental investigations of prototropic tautoꢀ
merism.14
Recently developed methods for the synthesis of difꢀ
ferent (poly)hydroxynaphthazarin derivatives have opened
practical prospects for the study of their prototropic tauꢀ
tomerism.1,13—18 To a considerable extent, the study
has become possible due to Fourier transform infrared
(FTꢀIR) spectrophotometers put into laboratory practice.
Such instruments are more sensitive than common gratꢀ
ing spectrophotometers and enable reliably recording and
collecting quantitative spectral data for compounds sparꢀ
ingly soluble in low polar solvents; hydroxynaphthazarins
are among such compounds. Here we present the results
of the IR study of prototropic tautomerism of hydroxyꢀ
naphthazarins in aprotic organic solvents.
R1 = H, R2 = COMe;
R1 = R2 = Cl, Br;
R
1 = H, R2 = STol, SOTol, SO2Tol
rium towards tautomer A (Scheme 3). However, the βꢀOH
group is more reactive than the αꢀOH groups and thus is
primarily methylated to give methoxy derivative 3. Clearly,
the equilibrium conditions for forms 3(A) and 3(B) will
differ from those for the starting hydroxynaphthazarin 2a.
Scheme 3
Results and Discussion
As noted above, the ν(OH) vibration frequencies of
hydroxynaphthazarins depend on the ability of βꢀOH
group to form IMHB and on their strengths. The IR specꢀ
trum of 3,6ꢀdiethylꢀ2,7ꢀdihydroxynaphthazarin (4c) in
CDCl3 contains two distinct narrow absorption bands at
3427 and 3532 cm–1 with ∆ν1/2 = 46 and 37 cm–1 (Fig. 1,
spectrum 1), which were assigned to the stretching vibraꢀ
tions of the βꢀOH groups in the quinonoid (at the C(2)
atom) and benzenoid parts (at the C(7) atom) of the
molecule, respectively. The stretching vibrations of the
αꢀOH groups appear as a broad diffuse band in the
3400—2200 cm–1 range.13
IR spectroscopy is a significantly "faster" method comꢀ
pared to NMR spectroscopy so that IR spectral paramꢀ
eters are not usually timeꢀaveraged. However, IR specꢀ
troscopy is not applied widely to the study of tautomerism
of naphthazarins since the absorption bands of the C—H
and C=O stretching vibrations are not very informative.
Indeed, in the IR spectra of hydroxynaphthazarins, lowꢀ
intensity absorption bands ν(C—H) at 3100—3000 cm–1
overlap with a broad band of the αꢀOH groups and a shift
of the ν(C=O) bands for the tautomers is smaller than
4: R = H (a), Me (b), Et (c)
5: R = H (a), Et (b)
Dilution experiments and a study of the solvent polarꢀ
ity effect on the position of the ν(OH) band revealed a
rather strong IMHB (∼4.3 kcal mol–1) between the βꢀOH

200
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
Glazunov et al.
Absorbance/arbitrary units
3532
0.125
3427
3412
0.100
3409
0.075
1
0.050
2
3
0.025
3517
3600
3400
3200
3000
2800
2600
2400
ν/cm–1
Fig. 1. IR spectra of (1) 3,6ꢀdiethylꢀ2,7ꢀdihydroxynaphthazarin (4c), (2) naphthopurpurin (2a), and (3) 6ꢀethylꢀ2ꢀhydroxynaphthazarin
(2d) in CDCl3 (highꢀfrequency range).
group at C(2) of the quinonoid moiety and the carbonyl
group.13 At the same time, the βꢀOH group at C(7) of the
benzenoid ring is characterized by higher ν(OH) values
and bound to the O atom of the αꢀOH group at the C(8)
atom by a weaker IMHB.
It is easy to show that the composition of a tautomeric
mixture of monohydroxynaphthazarin derivatives can be
determined from the equations
[A] = 100/(1 + Ra/1.59)
(1)
In this study, symmetrical dihydroxynaphthazarins
such as mompain (4a),19 aureoquinone (4b)20 (prepared
by Cꢀmethylation of mompain with acetyl peroxide; see
Experimental), 3,6ꢀdiethylꢀ2,7ꢀdihydroxynaphthazarin
(4c), and 2,6ꢀdihydroxynaphthazarins 5a,b, each conꢀ
taining βꢀOH groups in both the quinonoid and benꢀ
zenoid parts of the molecule, were used as basic comꢀ
pounds to develop a method for quantitative determinaꢀ
tion of the tautomeric ratio of (poly)hydroxynaphthazarins
in solutions from the ratio between their absorbances in
the maxima or from the ratio between the benzenoid and
quinonoid ν(OH) band areas. In the IR spectra of these
compounds, the difference between the ν(OH) in the
maxima (∆ν = 103—113 cm–1) is larger than, or equal to,
the sum of their halfꢀwidths (∆ν1/2 = 84—103 cm–1); for
this reason, overlap of the bands may be ignored. Insofar
as these absorption bands overlap the lowerꢀfrequency
wing of a broad lowꢀintensity absorption band of the αꢀOH
stretching vibration, the spectral characteristics of ν(OH)
were measured with reference to a local base line linearly
extrapolated to the 3650—3300 cm–1 range (see Fig. 1,
spectrum 1). The ratio between the absorbances in the
maxima and the ratio between the areas of the benzenoid
and quinonoid ν(OH) bands, determined in such a way,
were found to be 1.59 0.05 and 1.15 0.01 for basic comꢀ
pounds 4a—c and 5a,b, respectively.
or
[A] = 100/(1 + Rs/1.15),
[B] = 100 – [A],
(2)
(3)
where [A] is the percentage of a tautomer of type A (e.g.,
form 2a(A), see Scheme 3); [B] is the percentage of a
tautomer of type B (e.g., form 2a(B), see Scheme 3); Ra is
the ratio between the absorbances in the maxima for the
benzenoid and quinonoid ν(OH) bands in the IR specꢀ
trum of a compound under study; and Rs is the ratio
between the areas of these bands.
The scatter of the values [A] calculated by Eqs. (1)
or (2) did not exceed 1.5—5%. For this reason, the perꢀ
centage of tautomers of types A and B were measured by
a simpler and more convenient method, namely, usꢀ
ing Eq. (1).
Monohydroxynaphthazarins
We examined compounds 2a—j, including natural
hydroxynaphthazarins 2a—d,i and monomethyl ethers of
dihydroxynaphthazarins (2e—h,j) (hereafter, the [A] : [B]
ratio in CDCl3 is given in parentheses).
Most of the compounds studied were prepared acꢀ
cording to the known procedures (see Experimental).

Tautomerism of hydroxynaphthazarins
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
201
Scheme 4
Reagents and conditions: i. Me2SO4, NaOH. ii. AlCl3, NaCl,
190 °C, 5 min. iii. Fe, AcOH. iv. MnO2, H2SO4, ∼20 °C, 2 h.
Scheme 5
R = H (a), Me (b), Et (c)
6ꢀEthylꢀ2ꢀhydroxynaphthazarin (2d) was synthesized
from ethylhydroquinone (6) in several steps (Scheme 4).21
Cycloacylation of ether 7 (prepared by methylation of
hydroquinone 6) with dichloromaleic anhydride gave
dichloronaphthazarin 8, which was transformed into comꢀ
pound 9 by dehalogenation. Oxidation of naphthazarin
9 with MnO2 in conc. H2SO4 afforded a mixture of
7ꢀethylꢀ (10) and 6ꢀethylꢀ2ꢀhydroxynaphthazarins (2d),
the latter being isolated by chromatography.
a
H
H
H
b
Me
H
c
Et
H
d
H
Et
H
e
OMe
H
f
H
OMe
H
R1
R2
R3
H
H
H
The highꢀfrequency range of the IR spectrum of a
solution of naphthopurpurin (2a) in CDCl3 contains
one narrow absorption band at 3412 cm–1 with a halfꢀ
width of 50 cm–1, which corresponds to ν(OH) (see
Fig. 1, spectrum 2). This indicates that the equilibrium
g
H
H
h
Et
OMe
i
Et
H
j
H
Et
k
Et
Et
R1
R2
R3 OMe
H
OMe
OMe
OMe
2a(A)
2a(B) in the above solution is completely
The introduction of electronꢀdonating alkyl substituꢀ
ents at C(3) of naphthopurpurins (2b,c) does not shift the
tautomeric equilibrium towards Bꢀtype tautomers. At the
same time, the introduction of the ethyl substituent at
position 6 of naphthopurpurin (compound 2d) results in
certain shift of the equilibrium towards tautomer B. Inꢀ
deed, the IR spectrum of compound 2d contains, along
with the ν(OH) absorption band at 3409 cm–1 (form A),
shifted to tautomer 2a(A), i.e., no tautomerism of comꢀ
pound 2a occurs under these conditions (Scheme 5). Thus,
the IMHB between the βꢀOH group at the C(2) atom and
the C(1) carbonyl group stabilizes compound 2a in the
tautomeric form A. This is in good agreement with the
results of the quantumꢀchemical calculations12 (see
above).

202
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
Glazunov et al.
Absorbance/arbitrary units
of compound 2g, the latter was assumed to exist in chloꢀ
roform as a ∼75 : 25 mixture of two tautomers 2g(A) and
2g(B), which correlates with our IR data (70 : 30). Introꢀ
duction of the electronꢀdonating ethyl substituent at poꢀ
sition 3 of hydroxynaphthazarins 2f,g (compounds 2h,i),
shifts the tautomeric equilibrium, though differently, to
form A. Interestingly, ethyl derivatives of monomethyl
ether of mompain (2i,j) exist in chloroform as mixtures of
tautomers A and B in the ∼85 : 15 ratios irrespective of the
position of the ethyl radical relative to the βꢀOH group
(see Fig. 2, spectrum 3). At the same time, the presence of
two ethyl substituents in compound 2k reduces the perꢀ
centage of form B to 4.5% (see Fig. 2, spectrum 2).
0.125
0.100
0.075
0.050
0.025
4
3
2
The halfꢀwidths of the ν(OH) absorption bands
strongly depend on the solvent polarity. For instance, for
the benzenoid βꢀOH group they change from 35—40 cm–1
in CDCl3 to 15—17 cm–1 in hexane, and those for the
1
3600
3500
3400
ν/cm –1
Fig. 2. Fragments of the IR spectra of (1) 6ꢀethylꢀ2ꢀhydrꢀ
oxynaphthazarin (2d), (2) 3,6ꢀdiethylꢀ2ꢀhydroxyꢀ7ꢀmethoxyꢀ
naphthazarin (2k), (3) 3ꢀethylꢀ2ꢀhydroxyꢀ7ꢀmethoxynaphthꢀ
azarin (2i), and (4) 2ꢀhydroxyꢀ7ꢀmethoxynaphthazarin (2g) in
CDCl3 (ν(OH) frequency range).
quinonoid βꢀOH group, from 50—60 to 17—20 cm–1
,
respectively, which is clearly illustrated with compound
2k (Fig. 3).
To elucidate how the equilibrium 2(A)
2(B) is
affected by the nature of substituents, we studied not only
natural hydroxynaphthazarins 2a—d,i and some monoꢀ
methyl ethers of dihydroxynaphthazarins (2e—h,j), but
also their analogs 2l—r.
a very weak band at 3517 cm–1 corresponding to tauꢀ
tomer B (see Fig. 1, spectrum 3 and Fig. 2, spectrum 1).
The ratio of tautomeric forms A and B, estimated for hydrꢀ
oxynaphthazarin 2d from the ratios between the absorꢀ
bances (Ra) in the maxima of these bands, was found to be
98.5 : 1.5 for tautomers 2d(A) and 2d(B) in chloroform.
When a hydroxynaphthazarin molecule contains an
electronꢀwithdrawing group (e.g., a Cl atom in comꢀ
pound 2l) in the ring bearing the βꢀOH group, the tautoꢀ
meric equilibrium is considerably shifted to the tautomer
of type B as compared with compounds 2a—c. At the
same time, replacement of the electronꢀdonating methyl
substituents in compounds 2q,r by electronꢀwithdrawing
ones (e.g., Cl atoms in compounds 2m,n) in the other ring
produces no significant shift of the equilibrium. Moreꢀ
over, within the homologous series of compounds 2m—p,
the tautomeric ratio in chloroform noticeably changes
when passing from 2n to 2o,p.
The tautomeric equilibrium 2(A)
2(B) is signifiꢀ
cantly affected by methoxy groups. Thus 2ꢀhydroxyꢀ3ꢀ
methoxynaphthazarin (2e) exists in chloroform as a mixꢀ
ture of 2e(A) and 2e(B) in the 89.0 : 11.0 ratio. Methoxy
groups in position 6 or 7 (compounds 2f,g) have nearly
equal effects, shifting the equilibrium even more strongly
to tautomer B (∼30%) (see Fig. 2, spectrum 4).
Earlier,6 when interpreting chemical shifts of the proꢀ
tons at the C(3) and C(6) atoms in the 1H NMR spectrum
Absorbance/arbitrary units
0.125
0.100
0.075
0.050
0.025
3
2
1
3600
3500
3400
ν/cm–1
Fig. 3. Fragments of the IR spectra of 3,6ꢀdiethylꢀ2ꢀhydroxyꢀ7ꢀmethoxynaphthazarin (2k) in (1) hexane, (2) CCl4, and (3) CDCl3
(ν(OH) frequency range).

Tautomerism of hydroxynaphthazarins
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
203
The IR spectra of spinazarins 11a—c in CDCl3
contain, along with the ν(OH) absorption band at
3431—3433 cm–1 of form A,
a weak band at
∼3520—3525 cm–1; this indicates the presence of a secꢀ
ond tautomer B in solution contrary to the expected single
form A. In our opinion, tautomeric form A of compounds
11a—c can be destabilized by the formation of other types
of IMHB which involve the βꢀOH groups in positions 2
and 3 (see Scheme 6). This is evidenced by broadened
benzenoid ν(OH) bands in the IR spectra; their halfꢀ
widths (∆ν1/2 = 48—52 cm–1) are comparable with ∆ν1/2
of the quinonoid bands. In this case, the tautomeric
[A] : [B] ratio of compounds 11a—c in CDCl3 is deterꢀ
mined with a lower accuracy and provides no information
on the effect of the Me group on the equilibrium for
compounds 11a,b in chloroform*. At the same time, this
approach enables one to estimate, though less accurately,
a difference in tautomeric compositions for compounds
11a,b and naphthazarin 11c containing two Me groups.
Thus, the effect of the nature of substituents on the
tautomeric equilibrium of hydroxynaphthazarins is hard
to analyze since each substituent in a conjugated system
interacts with both the βꢀOH and C=O groups, exertꢀ
ing opposite effects on the strength of the IMHB beꢀ
tween them.
Dihydroxynaphthazarins
2,3ꢀDihydroxynaphthazarins. As already noted, the
main factor stabilizing monohydroxynaphthazarins in the
tautomeric form A (i.e., 2ꢀhydroxyꢀ1,4ꢀnaphthoquinone
form, see Scheme 5) is an IMHB between the βꢀOH
group at the C(2) atom and the C(1) carbonyl group.
However, analysis of the tautomeric equilibrium for
2,3ꢀdihydroxynaphthazarins 11a—c gave rather unexꢀ
pected results (Scheme 6).
Scheme 6
Compounds 11a,b were obtained according to the
known procedures, and dimethylspinazarin (11c) was synꢀ
thesized by cycloacylation of 1,2,3,4ꢀtetramethoxybenꢀ
zene with dimethylmaleic anhydride in an AlCl3—NaCl
melt (see Experimental).
2,6(7)ꢀDihydroxynaphthazarins. As noted above, the
IR spectra of isomeric 2,7ꢀ and 2,6ꢀdihydroxynaphthꢀ
azarins containing the same substituents in positions 3
and 6(7) contain two narrow absorption bands at
3540—3390 cm–1 corresponding to the βꢀOH stretching
vibrations in the quinonoid and benzenoid moieties. In
the IR spectra of nonꢀsymmetrically substituted 2,6ꢀ and
2,7ꢀdihydroxynaphthazarins, four ν(OH) absorption bands
should be expected for two tautomers.
Previous13,14 IR studies of nonꢀsymmetrically substiꢀ
tuted 2,6(7)ꢀdihydroxynaphthazarins in CDCl3 showed
that changes in frequencies of the absorption bands of the
* Compounds 11a—c are negligibly soluble in hexane, which
makes it impossible to use the aforementioned effect of narrowꢀ
ing ν(OH) absorption bands in their IR spectra.
R1 = R2 = H (а); R1 = Me, R2 = H (b); R1 = R2 = Me (c)

204
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
Glazunov et al.
benzenoid and quinonoid βꢀOH groups do not exceed
20 cm–1, depending on the donorꢀacceptor effect of subꢀ
stituents at the C(3) and C(6(7)) atoms, while ∆ν1/2 is
32—42 and 44—53 cm–1, respectively. According to the
Rayleigh criterion for band resolution, these bands canꢀ
not be resolved as separate spectral peaks.
As noted above, the ν(OH) band halfꢀwidths depend
on the solvent polarity. This property allowed us to detect
two tautomers of types A and B for 3ꢀchloroꢀ6ꢀethylꢀ2,7ꢀ
dihydroxynaphthazarin (4d) and 3ꢀchloroꢀ7ꢀethylꢀ2,6ꢀdiꢀ
hydroxynaphthazarin (5c) in hexane (Scheme 7).
∼3400 cm–1. In this case, the problem of band deꢀ
convolution for the estimation of a tautomeric composiꢀ
tion cannot be solved unambiguously. At the same time,
this absorption band in hexane appears as a distinct douꢀ
blet at 3400 and 3376 cm–1 (see Fig. 4, spectrum 2). The
first component of the doublet belongs to tautomer 5c(B)
(which follows from comparison of the ν(OH) band
frequencies for 3,7ꢀdiethylꢀ2,6ꢀdihydroxynaphthazarin
(5b)), while the second component corresponds to tauꢀ
tomer 5c(A).13 The βꢀOH stretching vibration in the benꢀ
zenoid ring appears as a singlet at 3522 cm–1, probably
because of a smaller difference between their frequencies
in different tautomers. Resolution of the observed douꢀ
blet for the quinonoid ν(OH) band into components (see
Fig. 4, curves 3, 3´) allows one to estimate the ratio of
tautomers A and B simply by determining the ratio of the
areas of the resulting components. For compounds 4d
and 5c, the ratio was found to be 64 : 36. As expected, the
dominant tautomers in solution are those of type A with a
stronger IMHB and the Cl atom in the quinonoid ring.
Hence, we propose a simple and fairly precise method
for quantitative determination of the tautomeric ratio of
hydroxynaphthazarins in solutions, which is based on meaꢀ
suring the IR spectroscopic parameters of the βꢀOH
groups. No simple correlations were found between the
character of substituents and the tautomeric equilibrium
state for such a strongly conjugated system as hydroxyꢀ
naphthazarins.
Scheme 7
Experimental
In the IR spectrum of compound 5c in CDCl3 (Fig. 4,
spectrum 1), the higherꢀfrequency wing of the quinonoid
ν(OH) band contains only an illꢀresolved shoulder at
Melting points were determined on a Boetius hot stage and
are given uncorrected. IR spectra were recorded on a Bruker
Vectorꢀ22 FTIR spectrophotometer (resolution 2.0 cm–1) in
Absorbance/arbitrary units
3508
3522
0.015
1
0.010
3376
3400
2
0.005
3´
3
3550
3500
3450
3400
3350
3300 ν/cm–1
Fig. 4. Fragments of the IR spectra of 3ꢀchloroꢀ6ꢀethylꢀ2,6ꢀdihydroxynaphthazarin (5c) in (1) CDCl3, (2) hexane, and (3, 3´) in
hexane with band contour splitting (ν(βꢀOH) frequency range).

Tautomerism of hydroxynaphthazarins
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
205
CDCl3 in matched cells with CaF2 windows supplied with polyꢀ
ethylene spacers; the layer thickness was 0.40—1.50 mm. The
spectra were "smoothed" before measuring ν(OH) values. For
ν(OH) with ∆ν1/2 = 35—60 cm–1, this procedure did not distort
the measured spectroscopic parameters. Frequency measureꢀ
ments were performed with an accuracy of 0.5 cm–1. The
concentrations of solutions of the compounds studied were
(2—5)×10–3 mol L–1. The 1H NMR spectra were recorded on a
Bruker ACꢀ250 spectrometer (250.13 MHz) in CDCl3, acetoneꢀ
d6, and DMSOꢀd6 with Me4Si as the internal standard. Mass
spectra (EI) were obtained with an LKBꢀ9000S instrument (diꢀ
rect inlet probe, ionizing energy 70 eV). The course of the reacꢀ
tion was monitored and the purity of the products was checked
by TLC on Merck 60Fꢀ254 plates in hexane—acetone (2 : 1).
Individual compounds were isolated from mixtures of the reacꢀ
tion products by preparative TLC on plates (20×20 cm) with
nonꢀfixed silica gel layer (5—40 µm, H+ form)18 or by column
chromatography on L 40/100 silica gel (H+ form). The yields of
the compounds obtained were not optimized.
2ꢀEthylꢀ5,8ꢀdihydroxyꢀ1,4ꢀnaphthoquinone (9). A mixture of
naphthoquinone 8 (1 g, 3.6 mmol) and powdered Fe (2 g,
36 mgꢀat.) in 100 mL of AcOH was heated for 30 min. After
cooling, the reaction mixture was filtered, and the solvent was
removed under reduced pressure. The residue was alkalified with
5% NaOH to pH 8—9, and the resulting mixture was stirred
without access for air until the solution turned dark blue
(15—40 min). After the reaction was completed, the mixture
was acidified with 10% HCl to pH 4—5. The product was exꢀ
tracted with ether. Compound 9 was isolated by column chroꢀ
matography in hexane—acetone (50 : 1). The yield of 9 was
785 mg (73%), m.p. 125—127 °C (cf. Ref. 25: m.p. 123—124 °C).
IR (CDCl3), ν/cm–1: 1656 vw, 1611 s (C=O); 1571 m (C=C).
1H NMR (CDCl3), δ: 1.24 (t, 3 H, Me, J = 7.3 Hz); 2.66 (dq,
2 H, CH2, J1 = 7.3 Hz, J2 = 1.5 Hz); 6.86 (t, 1 H, H(3), J =
1.5 Hz); 7.21 (s, 2 H, H(6), H(7)); 12.47, 12.61 (both s, 1 H each,
αꢀOH). MS, m/z (Irel (%)): 219 [M + 1]+ (13); 218 [M]+ (100);
203 [M – Me]+ (14); 200 [M – H2O]+ (13); 190 [M – CO]+
(15); 189 (10); 175 (19); 172 (8).
2,5,8ꢀTrihydroxyꢀ1,4ꢀnaphthoquinone (2a) was prepared acꢀ
cording to the known procedure.22 IR (CDCl3), ν/cm–1: 3412 m
(βꢀOH); 1663 w, 1657 m, 1610 s (C=O); 1575 m (C=C).
2,5,8ꢀTrihydroxyꢀ3ꢀmethylꢀ1,4ꢀnaphthoquinone (2b) was
prepared according to the known procedure.16 IR (CDCl3),
ν/cm–1: 3418 m (βꢀOH); 1662 w, 1638 m, 1602 s (C=O);
1571 m (C=C).
Oxidation of ethylnaphthazarin 9. Powdered MnO2 (570 mg,
0.8 mmol) was added in small portions at 35 °C over 1 h to a
vigorously stirred solution of ethylnaphthazarin 9 (300 mg,
1.4 mmol) in 15 mL of conc. H2SO4. The course of the reaction
was monitored by TLC. After the reaction was completed, the
mixture was poured into ice, and the products were extracted
with AcOEt (300 mL). The extract was washed with brine
(30 mL), dried with anhydrous Na2SO4, and concentrated
under reduced pressure. Column chromatography on SiO2
(1500×15 mm) in CCl4 gave compounds 2d 19 (46 mg, 14%) and
10 6 (98 mg, 30%).
3ꢀEthylꢀ2,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone (2c) was preꢀ
pared according to the known procedure.23 IR (CDCl3), ν/cm–1
:
3416 m (βꢀOH); 1655 w, 1634 m, 1603 s (C=O); 1572 m (C=C).
6ꢀEthylꢀ2,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone (2d). 1H NMR
(CDCl3), δ: 1.27 (t, 3 H, Me, J = 7.8 Hz); 2.77 (q, 2 H, CH2,
J = 7.8 Hz); 6.35, 7.07 (both s, 1 H each, H(3), H(7)); 7.42 (s,
1 H, βꢀOH); 11.60, 13.31 (both s, 1 H each, αꢀOH). IR (CDCl3),
ν/cm–1: 3517 vw, 3409 m (βꢀOH); 1662 w, 1631 m, 1607 s
(C=O); 1575 m (C=C).
7ꢀEthylꢀ2,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone (10). 1H NMR
(CDCl3), δ: 1.27 (t, 3 H, Me, J = 7.8 Hz); 2.74 (q, 2 H, CH2,
J = 7.8 Hz); 6.34, 7.18 (both s, 1 H each, H(3), H(6)); 7.42 (s,
1 H, βꢀOH); 11.99, 12.82 (both s, 1 H each, αꢀOH).
Synthesis of 6ꢀethylꢀ2,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone (2d)
Dimethyl ether of ethylhydroquinone (7) was obtained as deꢀ
scribed earlier24 by methylation of ethylhydroquinone (6)21 with
dimethyl sulfate in aqueous NaOH. The yield of compound 7
was 44%, b.p. 98—100 °C (7 Torr). IR (CDCl3), ν/cm–1: 1590,
1501 (C=C). 1H NMR (CDCl3), δ: 1.19 (t, 3 H, Me, J =
7.8 Hz); 2.64 (q, 2 H, CH2, J = 7.8 Hz); 3.77, 3.76 (both s,
3 H each, OMe); 6.67 (dd, 1 H, H(5), J1 = 8.8 Hz, J2 = 2.9 Hz);
6.75 (d, 1 H, H(3), J = 2.9 Hz); 6.76 (d, 1 H, H(6), J = 8.8 Hz).
MS, m/z (Irel (%)): 166 [M]+ (40); 153 (10); 152 (100); 151 (17);
137 (41).
2,3ꢀDichloroꢀ6ꢀethylꢀ5,8ꢀdihydroxyꢀ1,4ꢀnaphthoquinone (8).
A mixture of dichloromaleic anhydride (23.5 g, 0.14 mol) and
ether 7 (10 g, 0.06 mol) were added at 140 °C to a stirred melt of
anhydrous AlCl3 (195 g, 1.46 mol) and NaCl (37 g, 0.63 mol).
The melt was heated to 190 °C and stirred for an additional
3 min. Then the reaction mixture was cooled and hydrolyzed
with 5% HCl. The product was separated, washed with water
(100 mL), dried, and purified by column chromatography in
hexane—acetone (50 : 1). The yield of compound 8 was 15 g
(88%), m.p. 123—125 °C. IR (CDCl3), ν/cm–1: 1617 s (C=O);
1575 m.sh, 1562 s (C=C). 1H NMR (CDCl3), δ: 1.27 (t, 3 H,
Me, J = 7.3 Hz); 2.73 (dq, 2 H, CH2, J1 = 7.3 Hz, J2 = 1.4 Hz);
7.06 (t, 1 H, H(7), J = 1.4 Hz); 12.56, 12.90 (both s, 1 H each,
αꢀOH). MS, m/z (Irel (%)): 286, 288, 290 [M]+ (100); 285, 287,
289 [M – 1]+ (21); 271, 273, 275 [M – Me]+ (7); 268, 270,
272 [M – H2O]+ (8); 258, 260, 262 [M – CO]+ (12); 257,
259, 261 (5).
2,5,8ꢀTrihydroxyꢀ3ꢀmethoxyꢀ1,4ꢀnaphthoquinone (2e) was
obtained according to the known procedure.26 IR (CDCl3),
ν/cm–1: 3492 w, 3403 m (βꢀOH); 1652 m, 1643 m, 1604 s
(C=O); 1573 m (C=C).
2,5,8ꢀTrihydroxyꢀ6ꢀmethoxyꢀ1,4ꢀnaphthoquinone (2f) was
obtained according to the known procedure.1 IR (CDCl3),
ν/cm–1: 3512 m, 3392 m (βꢀOH); 1628 m, 1602 s (C=O);
1582 sh.s (C=C).
2,5,8ꢀTrihydroxyꢀ7ꢀmethoxyꢀ1,4ꢀnaphthoquinone (2g) was
obtained according to the known procedure.1 IR (CDCl3),
ν/cm–1: 3525 m, 3422 m (βꢀOH); 1661 w, 1629 m, 1606 s
(C=O); 1585 sh.m (C=C).
3ꢀEthylꢀ2,5,8ꢀtrihydroxyꢀ6ꢀmethoxyꢀ1,4ꢀnaphthoquinone
(2h) was obtained according to the known procedure.1 IR
(CDCl3), ν/cm–1: 3575 w, 3397 m (βꢀOH); 1629 m, 1596 s
(C=O); 1580 sh.m (C=C).
3ꢀEthylꢀ2,5,8ꢀtrihydroxyꢀ7ꢀmethoxyꢀ1,4ꢀnaphthoquinone
(2i) was obtained according to the known procedure.1 IR
(CDCl3), ν/cm–1: 3526 w, 3424 m (βꢀOH); 1654 w, 1629 m,
1599 s (C=O); 1577 sh.m (C=C).

206
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
Glazunov et al.
6ꢀEthylꢀ2,5,8ꢀtrihydroxyꢀ7ꢀmethoxyꢀ1,4ꢀnaphthoquinone
IR (CDCl3), ν/cm–1: 3525 m, 3418 m (βꢀOH); 1653 vw, 1630 m,
1604 s (C=O); 1586 sh.m (C=C).
(2j) was obtained according to the known procedure.14 IR
(CDCl3), ν/cm–1: 3523 w, 3414 m (βꢀOH); 1662 m, 1620 s,
1614 s (C=O); 1588 s, 1575 sh.s (C=C).
2,5,6,8ꢀTetrahydroxyꢀ1,4ꢀnaphthoquinone (5a) was obtained
according to the known procedure.18 IR (CDCl3), ν/cm–1
3509 m, 3392 m (βꢀOH); 1602 s (C=O, C=C).
:
3,6ꢀDiethylꢀ2,5,8ꢀtrihydroxyꢀ7ꢀmethoxyꢀ1,4ꢀnaphthoꢀ
quinone (2k) was obtained according to the known procedure.1
IR (CDCl3), ν/cm–1: 3524 w, 3417 m (βꢀOH); 1654 w, 1623 m,
1595 s (C=O); ∼1575 sh.m (C=C).
3ꢀChloroꢀ2,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone (2l) was synꢀ
thesized as described earlier.27 IR (CDCl3), ν/cm–1: 3502 w,
3395 m (βꢀOH); 1652 sh.w, 1637 m, 1611 s (C=O); 1577 m,
1519 m (C=C).
3,7ꢀDiethylꢀ2,5,6,8ꢀtetrahydroxyꢀ1,4ꢀnaphthoquinone (5b)
was obtained according to the known procedure.13 IR (CDCl3),
ν/cm–1: 3512 m, 3394 m (βꢀOH); 1653 w, 1630 sh.m, 1599 s
(C=O); 1580 sh.m (C=C).
3ꢀChloroꢀ7ꢀethylꢀ2,5,6,8ꢀtetrahydroxyꢀ1,4ꢀnaphthoquinone
(5c) was obtained according to the known procedure.13 IR
(CDCl3), ν/cm–1: 3508 m, 3377 m (βꢀOH); 1631 sh.m, 1606 s
(C=O); 1597 s, 1587 sh.s (C=C).
6,7ꢀDichloroꢀ2,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone (2m) was
synthesized as described earlier.28 IR (CDCl3), ν/cm–1: 3517 w,
3419 m (βꢀOH); 1661 w, 1630 s, 1613 s (C=O); 1558 w (C=C).
6,7ꢀDichloroꢀ2,5,8ꢀtrihydroxyꢀ3ꢀmethylꢀ1,4ꢀnaphthoquinone
2,3,5,8ꢀTetrahydroxyꢀ1,4ꢀnaphthoquinone (11a) was obꢀ
tained according to the known procedure.22 IR (CDCl3), ν/cm–1
:
∼3520 vw, 3433 m (βꢀOH); 1692 m, 1663 w, 1602 s (C=O);
1571 m (C=C).
(2n) was synthesized as described earlier.15 IR (CDCl3), ν/cm–1
:
3510 w, 3426 m (βꢀOH); 1660 w, 1632 m, 1607 s (C=O);
1558 w (C=C).
2,3,5,8ꢀTetrahydroxyꢀ6ꢀmethylꢀ1,4ꢀnaphthoquinone (11b)
was obtained according to the known procedure.32 IR (CDCl3),
ν/cm–1: ∼3525 vw, 3432 m (βꢀOH); 1694 m, 1641 w, 1599 s
(C=O); 1576 m (C=C).
6,7ꢀDichloroꢀ3ꢀethylꢀ2,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone
(2o) was synthesized as described earlier.15 IR (CDCl3), ν/cm–1
:
3512 w, 3423 m (βꢀOH); 1654 w, 1630 s, 1606 s (C=O);
1554 w (C=C).
2,3,5,8ꢀTetrahydroxyꢀ6,7ꢀdimethylꢀ1,4ꢀnaphthoquinone
(11c). A mixture of 1,2,3,4ꢀtetramethoxybenzene (7.5 g,
0.04 mol) and dimethylmaleic anhydride (10.3 g, 0.08 mol) was
added at 140 °C to a stirred melt of anhydrous AlCl3 (81.0 g,
0.60 mol) and NaCl (16.2 g, 0.28 mol). The melt was heated to
180 °C and stirred for an additional 8 min. The reaction mixture
was cooled and hydrolyzed with 5% HCl (1 L). Crude product
11c precipitated within 12 h was separated, washed with hot
water (200 mL), dried, and purified by column chromatography
in hexane—acetone with a gradient from 10 : 1 to 4 : 1. The yield
of compound 11c was 6.2 g (62%), Rf 0.37, m.p. 283—285 °C
(from dioxane). IR (CDCl3), ν/cm–1: 3525 w, 3431 m (βꢀOH);
1693 m, 1642 w, 1598 s (C=O), 1576 m (C=C). 1H NMR
(acetoneꢀd6), δ: 2.26 (s, 6 H, 2 Me); 8.96 (s, 2 H, 2 βꢀOH);
12.71 (s, 2 H, 2 αꢀOH). MS, m/z (Irel (%)): 251 [M + 1]+ (16);
250 [M]+ (100); 249 (34); 222 (31); 221 (16).
6,7ꢀDichloroꢀ2,5,8ꢀtrihydroxyꢀ3ꢀpropylꢀ1,4ꢀnaphthoquinone
(2p) was synthesized as described earlier.15 IR (CDCl3), ν/cm–1
:
3514 w, 3422 m (βꢀOH); 1651 w, 1629 m, 1607 s (C=O);
1553 w (C=C).
2,5,8ꢀTrihydroxyꢀ6,7ꢀdimethylꢀ1,4ꢀnaphthoquinone (2q)
was synthesized as described earlier.29 IR (CDCl3), ν/cm–1
:
3520 w, 3406 m (βꢀOH); 1660 w, 1621 m, 1603 s (C=O);
1578 sh.m (C=C).
2,5,8ꢀTrihydroxyꢀ3,6,7ꢀtrimethylꢀ1,4ꢀnaphthoquinone (2r)
was synthesized as described earlier.30 IR (CDCl3), ν/cm–1
:
3524 w, 3414 m (βꢀOH); 1662 w, 1624 m, 1583 s (C=O);
1575 sh.m (C=C).
2,5,7,8ꢀTetrahydroxyꢀ1,4ꢀnaphthoquinone (mompain, 4a)
was synthesized as described earlier.18 IR (CDCl3), ν/cm–1
:
3530 m, 3428 m (βꢀOH); 1602 s (C=O, C=C). IR (dioxane),
ν/cm–1: 3198 (βꢀOH); 1651 w, 1630 m, 1607 s (C=O);
∼1585 sh.m (C=C).
This work was financially supported in part by the
Russian Foundation for Basic Research (Project 96ꢀ15ꢀ
97316), the Civilian Research and Development Founꢀ
dation of the United States (Grant RECꢀ003), and the
Ministry of Education of the Russian Federation.
2,5,7,8ꢀTetrahydroxyꢀ3,6ꢀdimethylꢀ1,4ꢀnaphthoquinone
(aureoquinone, 4b) (see Ref. 20). Acetyl peroxide31 was added in
small portions to a boiling solution of mompain 4a 18 (110 mg,
0.5 mmol) in 30 mL of ButOH. The reaction was terminated
when the conversion was ∼60% (TLC). The solvent was reꢀ
moved; preparative TLC of the residue gave 2,5,7,8ꢀtetraꢀ
hydroxyꢀ3ꢀmethylꢀ1,4ꢀnaphthoquinone1 (Rf 0.32) and product
References
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:
3532 m, 3428 m (βꢀOH); 1662 w, 1633 m, 1609 s, 1589 s (C=O);
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Received May 17, 2002;
in revised form October 8, 2002