Chemistry of naphthazarin derivatives 7. Determination of structures of substituted 2,6(7)-dihydroxynaphthazarins by UV and IR spectroscopy

Article abstract of DOI:10.1023/A:1009581319518

A set of substituted 2,6- and 2,7-dihydroxynaphthazarins were synthesized. The difference in the UV spectra of alkaline alcoholic solutions of 2,6- and 2,7-dihydroxynaphthazarins allows the reliable differentiation of the structures of these compounds. Th

Full text of DOI:10.1023/A:1009581319518

88
Russian Chemical Bulletin, International Edition, Vol. 50, No. 1, pp. 88—94, January, 2001
Chemistry of naphthazarin derivatives
7.* Determination of structures of substituted 2,6(7)-dihydroxynaphthazarins
by UV and IR spectroscopy
V. P. Glazunov,a A. Ya. Tchizhova,a M. I. Shuvalova,b and V. Ph. Anufrieva«
aPacific Institute of Bioorganic Chemistry, Far-Eastern Branch of the Russian Academy of Sciences,
159 prosp. 100-letiya Vladivostoka, 690022 Vladivostok, Russian Federation.
Fax: +7 (423 2) 31 4050. E-mail: piboc@stl.ru
bInstitute of Chemistry and Applied Ecology, Far Eastern State University,
27 ul. Octyabr'skaya, 690600 Vladivostok, Russian Federation.
Fax: +7 (423 2) 25 7609
A set of substituted 2,6- and 2,7-dihydroxynaphthazarins were synthesized. The difference
in the UV spectra of alkaline alcoholic solutions of 2,6- and 2,7-dihydroxynaphthazarins
allows the reliable differentiation of the structures of these compounds. The IR spectra of
2,6- and 2,7-dihydroxynaphthazarins have characteristic nonoverlapping intervals of stretching
vibration frequencies of the β-hydroxy groups. Based on these spectral regularities, the data on
cuculoquinone, which has been isolated previously from lichen Cetraria cucullata and has been
identified as 7,7´-bis(3-ethyl-2,5,6,8-tetrahydroxynaphthalene-1,4-dione), were revised and
the structure of 6,6´-bis(3-ethyl-2,5,7,8-tetrahydroxynaphthalene-1,4-dione) was assigned to
this compound.
Key words: 2,6-dihydroxynaphthazarin, 2,5,6,8-tetrahydroxy-1,4-naphthoquinone; 2,7-di-
hydroxynaphthazarin, 2,5,7,8-tetrahydroxy-1,4-naphthoquinone; 7,7´-bis(3-ethyl-1,4,5,8-
tetrahydroxynaphthalene-2,6-dione), 7,7´-bis(3-ethyl-2,5,6,8-tetrahydroxynaphthalene-1,4-
dione), 6,6´-bis(3-ethyl-2,5,7,8-tetrahydroxynaphthalene-1,4-dione); electronic absorption
spectra; IR absorption spectra; cuculoquinone, Cetraria cucullata.
Numerous examples that illustrate difficulties en-
carbonyl oxygen atom) as a result of which all pairs of
protons, including the protons of the α-hydroxy groups
at the C(5) and C(8) atoms, are magnetically equivalent
within the NMR time scale. To the contrary, the mol-
ecule of mompain (2) is unsymmetrical with respect to
the α-hydroxy groups, resulting in the magnetic non-
equivalence of their protons and, consequently, in the
different chemical shifts, whereas the protons of the
β-hydroxy groups (at the C(2) and C(7) atoms) and the
protons at the C(3) and C(6) atoms are magnetically
equivalent within the NMR time scale taking into ac-
count the tautomerism of the naphthazarin moiety. The
introduction of a substituent into the nuclei of 2,6(7)-di-
hydroxynaphthazarins at the C(3) atom or different sub-
stituents at the C(3) and C(7) (or C(6)) atoms gives rise
to unsymmetrical structures, which leads to the above-
mentioned difficulties associated with the determination
of the relative arrangement of the hydroxy groups in
molecules of these compounds.
countered in determining the relative arrangement of the
β-hydroxy groups in substituted 2,6(7)-dihydroxy-
napthazarins (2,5,6(7),8-tetrahydroxy-1,4-naphtho-
quinones) are surveyed in the monograph by R. H.
Thomson.2 Natural products are often available in small
amounts, which hinders the use of chemical methods for
the establishment of their structures, whereas the avail-
able physicochemical methods do not allow unambigu-
ous conclusions about the arrangement of the substitu-
ents in the quinoid moiety (at the C(2) and C(3) atoms)
with respect to the substituents at the C(6) and C(7)
atoms. We faced an analogous problem when studying
products of nucleophilic replacement of one of the
chlorine atoms in naphthazarin derivatives by the alkoxy
or amino groups.3,4
As for 2,6-dihydroxynaphthazarin (1), 2,7-dihydroxy-
naphthazarin (2), and their derivatives bearing identical
substituents at positions 3 and 7(6), the determination of
1
their structures by H NMR spectroscopy presents no
OH
O
OH
O
difficulties. Thus, the molecule of isomompain (1) is
totally symmetrical (taking into account the peculiar
tautomerism of naphthazarins involving intramolecular
proton transfer from the α-OH group to the adjacent
2
3
OH
HO
7
6
2
3
OH
7
1
4
1
4
8
5
8
5
6
HO
OH
O
OH
O
* For Part 6, see Ref. 1.
1
2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 85—91, January, 2001.
1066-5285/00/5001-0088 $25.00 © 2001 Plenum Publishing Corporation

2,6(7)-Dihydroxynaphthazarins
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 1, January, 2001
89
With the aim of searching for a simple and reliable
procedure, which could allow one to solve the above-
mentioned problem, we synthesized several pairs of sub-
stituted 2,6- and 2,7-dihydroxynaphthazarins. Thus, free-
radical ethylation of 2,6-dihydroxynaphthazarin (1) and
mompain (2)5 under the action of propionyl peroxide
under conditions of thermolysis afforded ethyliso-
mompain (3), ethylmompain (4), 3,7-diethyl-2,6-
dihydroxynaphthazarin (5), and 3,6-diethyl-2,7-dihydr-
oxynaphthazarin (6).
Previously, electronic absorption spectroscopy has
been used for studying the structures of naphthazarins.6,7
In the cited investigations, only absorption bands in the
visible region were considered as the major criterion.
However, detailed examination of the UV spectra of
2,6(7)-dihydroxy-substituted naphthazarins demonstrated
that this spectral region is equally informative. Thus, the
UV spectra of mompain (2) and its derivatives 4, 6, 12,
and 14 have an intense band in the region of
229—240 nm and a lower-intensity band in the region of
269—273 nm assigned to π→π* transitions. The UV
spectra of the corresponding 2,6-dihydroxynaphthazarins
1, 3, 5, 8, and 13 have a low-intensity long-wavelength
band as a shoulder (Table 1).
In the UV spectra of alkaline alcoholic solutions of
2,7-dihydroxynaphthazarins 2, 4, 6, 12, and 14, the
above-mentioned bands are shifted bathochromically.
The second band is shifted to 296—305 nm and its
absorption increases by a factor of 1.3—1.4. The spectra
of alkaline alcoholic solutions of 2,6-dihydroxy-
naphthazarins 1, 3, 5, 8, and 13 have no absorption
bands in this region. Therefore, the presence of an
intense absorption band at 296—305 nm in the UV
spectra of alkaline solutions of 2,7-dihydroxynaphth-
azarins can serve as a simple reliable criterion, which
allows one to differentiate these compounds from the
corresponding 2,6-dihydroxy derivatives.
In the visible region of the electronic spectra of
2,6-dihydroxy (1, 3, 5, 8, and 13) and 2,7-dihydroxy
derivatives (2, 4, 6, 12, and 14), a low-intensity band
with a pronounced fine structure is observed, which is
typical of the n→π* transition in the naphthazarin sys-
tem.6 It should be mentioned that the absorption in the
spectra of the 2,6-dihydroxy derivatives is observed at
460—529 nm, whereas the absorption of the 2,7-di-
hydroxy derivatives is observed in the longer-wavelength
region (480—556 nm). A comparison of the visible
regions of the spectra of 2,6-dihydroxy derivatives 1, 3,
OH
O
OH
O
R2
OH
R1
HO
R2
OH
R1
HO
OH
O
OH
O
3, 5
3, 4: R1 = Et, R2 = H; 5, 6: R1 = R2 = Et
4, 6
3-Chloro-2,6-dihydroxynaphthazarin (8) and 3-chlo-
ro-2,7-dihydroxynaphthazarin (12) were prepared from
2,3-dichloronaphthazarin (7) according to Scheme 1.
The structure of compound 8 was established by its
conversion into 2,6-dihydroxynaphthazarin.4 The nu-
cleophilic replacement of the chlorine atom in dichloro-
naphthazarin (10) by the methoxy group yielded pre-
dominantly product 11. After hydrolysis of the reaction
mixture, compound 8 (25%) was isolated along with the
major product 12 (43%). The corresponding chloroethyl
derivatives 13 and 14 were prepared by free-radical
alkylation of monochloronaphthazarins 8 and 12, re-
spectively.
Compounds 3, 5, 8, 13, 4, 6, 12, and 14, whose
structures were unambiguously established based on their
directed syntheses from 2,6-dihydroxynaphthazarin (1),
2,7-dihydroxynaphthazarin (2), and 2,3-dichloronaphth-
azarin (7), were studied by UV and IR spectroscopy.
Scheme 1
OH
OH
OH
OH
O
O
OH
O
O
OH
O
O
OH
O
O
O
O
MnO2,
H2SO4
H2SO4,
H3BO3
OH
OH
OH
Et
Cl
Cl
Cl
Cl
Cl
Cl
(EtCOO)2
HO
HO
OH
OH
OH
9
7
8
13
CH2N2
O
OH
O
OH
O
O
OH
OMe
MeO
OMe
HO
OH
HO
OH
Et
Cl
Cl
–
H+
(EtCOO)2
MeOH, F
Al2O3
Cl
Cl
Cl
O
OH
11
O
OH
OH
10
12
14

90
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 1, January, 2001
Glazunov et al.
Table 1. UV spectra of compounds 1—6, 8, and 12—14 in
methanolic and alkaline methanolic solutions
because of the low solubility of these compounds in
nonpolar or weakly polar aprotic solvents, although the
spectra recorded in such solvents enable one to analyze
the region of stretching vibrations of the β-hydroxy
groups (hereinafter ν(OH)) of hydroxynaphthazarins
Com-
pound
λ
max/nm
(lg ε)
–1
MeOH
MeOH+NaOH
(3350—3550 cm ). The latter region is practically im-
portant, but it is poorly informative in the case of spectra
recorded in KBr pellets.
1
2
3
4
229 (4.23), 315 (3.86),
461 sh (3.62), 488 (3.70), 255 (4.21), 357 (3.94),
526 sh (3.55)
206 (4.19), 223 (4.15),
Actually, the high-frequency region of the IR spec-
trum of mompain (2) in KBr pellets has a single absorp-
433 sh (3.82), 465 (3.87),
510 sh (3.66)
229 (3.79), 249 (3.92),
297 (3.74), 368 (3.19),
–1
tion band (3240 cm ),7 whereas the spectrum of 2 in
212 (4.26), 229 (4.43),
273 (4.18), 320 (3.98),
481 sh (3.74), 515 (3.79), 446 sh (3.17), 476 (3.13),
549 sh (3.64)
213 (4.28), 235 (4.30),
270 sh (3.90), 317 (3.96), 260 (4.28), 368 (4.04),
461 sh (3.67), 490 (3.73), 385 sh (4.01), 442 sh (3.93),
526 sh (3.59)
213 (4.28), 233 (4.36),
269 (3.94), 323 (3.87),
chloroform has two pronounced narrow ν(OH) absorp-
tion bands of the β-hydroxy groups at 3431 and
540 (3.13), 575 (3.07)
205 (4.29), 224 (4.24),
–1
–1
3524 cm with half-widths (∆ν1/2) of 46 and 37 cm ,
respectively. In addition, the IR spectrum of a chloroform
solution of mompain, like the spectrum of naphthazarin
(5,8-dihydroxy-1,4-naphthoquinone),9 has a low-inten-
463 (3.96)
230 (4.23), 249 (4.27),
296 (4.30), 375 (3.73),
sity diffuse absorption band at 3400—2200 cm–1 with the
maximum at ∼3000 cm
–1
belonging to the stretching
478 sh (3.69), 510 (3.74), 452 (3.65), 478 (3.67),
546 sh (3.57)
vibration of the α-hydroxy groups.
526 sh (3.56), 565 (3.65),
606 br.sh (3.56)
Apparently, the difference in the positions of the
ν(OH) absorption bands is associated with the difference
in the strength of intramolecular hydrogen bonds in
which the β-hydroxy groups of the quinoid (at the C(2)
atom) and aromatic (at the C(7) atom) moieties of
compound 2 can be involved. To elucidate this fact, we
studied the IR spectra of model compounds, viz.,
naphthopurpurin (15) and purpurin (16). Thus, the IR
spectrum of a chloroform solution of naphthopurpurin
containing the β-hydroxy group in the quinoid moi-
5
238 (4.28), 252 sh (4.40), 265 (4.26), 379 (3.90),
328 (3.85), 431 sh (3.63), 389 (3.89), 446 sh (3.86),
463 (3.68), 495 (3.77),
529 (3.65)
215 (4.24), 236 (4.36),
269 (3.96), 331 (3.90),
450 sh (3.63), 481 (3.68), 385 (3.68), 463 (3.71),
514 (3.21), 549 (3.53)
214 (4.27), 236 (4.35),
267 sh (3.86), 326 (4.00), 365 (4.13), 382 (4.09),
467 (3.84), 495 (3.91),
529 (3.77)
216 (4.26), 235 (4.32),
274 (3.87), 329 (3.81),
469 (3.90)
6
202 (4.21), 231 (4.45),
255 (4.28), 298 (4.24),
490 (3.75), 568 (3.60)
216 (4.21), 260 (4.39),
8
–1
ety10 has the ν(OH) absorption band at 3412 cm
437 sh (3.94), 463 (4.04),
521 (3.79)
223 (4.24), 250 (4.35),
299 (4.15), 366 (3.67),
–1
(∆ν1/2 51 cm ). The IR spectrum (recorded in the same
12
solvent) of purpurin containing the β-hydroxy group in
–1
the benzoid moiety has the absorption band at 3519 cm
455 sh (3.56), 488 (3.69), 457 (3.67), 483 (3.69),
546 (3.74), 559 (3.56)
–1
(∆ν1/2 40 cm ). Like the spectra of mompain (2) and
532 sh (3.64), 565 (3.67),
633 br.sh (3.50)
207 (4.21), 265 (4.32),
naphthazarin,9 the IR spectra of model compounds 15
and 16 have a broad diffuse absorption band of the
stretching vibration of the α-hydroxy groups in the
13
14
217 (4.22), 239 (4.32),
253 sh (4.20), 333 (3.92), 375 (3.99), 389 (3.98),
467 sh (3.74), 495 (3.82), 446 sh (3.92), 467 (3.96)
529 (3.70)
215 (4.24), 240 (4.30),
272 (4.06), 337 (3.79),
490 (3.79), 521 (3.78),
556 (3.67)
–1
region of 3400—2200 cm .
O ...H
OH
O
HO
H
230 (4.20), 251 (4.40),
305 (4.18), 385 (3.67),
460 (3.80), 485 (3.85),
556 (3.67), 602 br.sh (3.57)
O
ν 3412
O
ν 3519
OH
O
O
HO
5, 8, and 13 in neutral and alkaline methanolic solutions
revealed a hypsochromic shift accompanied by a sub-
stantial increase in the intensity of the bands (by a factor
of 1.3—1.5) and by leveling down of their fine structures.
To the contrary, 2,7-dihydroxy derivatives 2, 4, 6, 12,
and 14 are characterized by the bathochromic shift of
the absorption maximum.
The characteristic time of IR spectroscopy is smaller
than the period of interconversion of the tautomeric
forms of naphthazarin derivatives,8 which opens up new
opportunities for investigation of their structures. Previ-
ously, the IR spectra of naphthazarin derivatives were
virtually always measured in KBr pellets or Nujol mulls2,7
15
16
Variations in the substituents serving as electron
donors (Et) or acceptors (Cl) lead to a change in the
stretching vibration frequencies of the β-hydroxy groups
of the compounds under study. In the IR spectra of
2,7-dihydroxynaphthazarin derivatives 2, 4, 6, 12, and
14, the frequencies of the OH groups at the C(2) and
C(7) atoms are in the ranges of 3431—3413 and
–1
3533—3523 cm , respectively (Table 2).
In the IR spectra of chloroform solutions of 2,6-di-
hydroxynaphthazarins 1, 3, 5, 8, and 13, the stretching
vibration frequencies of both β-hydroxy groups are lower

2,6(7)-Dihydroxynaphthazarins
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 1, January, 2001
91
–1
Table 2. Stretching vibration frequencies of the OH groups in
the IR spectra of the compounds synthesized
purpurin (15) (3412 cm ) remained unchanged upon
dilution with chloroform or when the CHCl3 solvent was
changed for CCl4. The ν(OH) frequency in the spectrum
of purpurin (16) also remained unchanged upon dilution
–1
Com-
pound
ν/cm
–1
with chloroform, but it changed from 3519 to 3525 cm
2-ÎÍ
6-ÎÍ
Ñ=Î, Ñ=Ñ
when CHCl3 was changed for CCl4. This fact indicates
that the β-hydroxy group of the quinoid moiety in the
naphthopurpurin molecule is linked to the carbonyl
group at the C(1) atom through an intramolecular
hydrogen bond, whereas the β-hydroxy group in the
benzoid moiety of the purpurin molecule remains "free."
IR spectroscopy of solutions of 2,6(7)-dihydroxy-
naphthazarins in low-polarity aprotic solvents enables one
to reliably identify the quinoid and aromatic β-hydroxy
groups. Thus, 20-fold dilution of ethylmompain (4) with
(7-ÎÍ)
1
2
3392 m
3428 m
3391 m
3430 m
3394 m
3427 m
3373 m
3413 m
3377 m
3417 m
3509 m
(3530) m
3512 m
(3531) m
3512 m
(3533) m
3508 m
(3523) m
3508 m
1602 s
1603 s
1631 sh.m, 1600 s
1631 m, 1603 s
1599 s, 1580 sh.m
1630 sh.m, 1602 s, 1587 s
1610 s, 1594 sh. s
1632 m, 1609 ñ, 1595 m.sh
1631 sh.m, 1606 s, 1597 s
1630 s, 1604 s, 1586 sh.m
3
4
5
6
8
12
13
14
–2
–3
–1
(3526) m
chloroform (from 2•10 to 1•10 mol L ) did not
lead to changes either in the ν(OH) frequencies of the
Note. Stretching vibrations of the α-hydroxy groups in com-
–1
absorption bands (3430 and 3531 cm ) or in the ratio of
pounds 1—6, 8, and 12—14 are observed as a broad diffuse
–1
their peak intensities. The spectrum of ethylmompain in
band in the region of 3400—2200 cm
.
the nonpolar solvent (CCl4) demonstrates that the ν(OH)
–1
frequency of the first band (3430 cm ) remains un-
than those in the spectra of the corresponding 2,7-di-
hydroxynaphthazarins 2, 4, 6, 12, and 14 (see Table 2).
For the series of isomompain derivatives under study,
the ν(OH) frequencies of the OH groups at the C(2) and
C(6) atoms are observed in the ranges of 3392—3373
changed (i.e., the corresponding OH group is linked to
the carbonyl group at the C(1) atom through an in-
tramolecular hydrogen bond, like in the naphthopurpurin
molecule), whereas the frequency of the second band
–1
increases from 3531 to 3538 cm (i.e., the OH group
–1
and 3512—3508 cm , respectively.
corresponding to this band is free, like in the purpurin
molecule).
A comparison of the ranges in which the ν(OH)
frequencies of the compounds of the mompain (2, 4, 6,
12, and 14) and the isomompain series (1, 3, 5, 8, and
13) vary demonstrated that these intervals do not overlap
(Fig. 1). For the above-mentioned series of compounds,
the difference between the boundaries of the intervals in
which the stretching vibration frequencies of the
The revealed characteristic features of the UV and IR
spectra of substituted 2,6- and 2,7-dihydroxynaphth-
azarins were used for investigation of the structure of
hydroxylated bisnaphthazarin, viz., cuculoquinone, which
has been previously isolated from lichen Cetraria
cucullata.11 It should be mentioned that the amphi(2,6)-
quinoid structure, viz., the structure of 7,7´-bis(3-ethyl-
1,4,5,8-tetrahydroxynaphthalene-2,6-dione) (17), has
been assigned to cuculoquinone previously.11 This struc-
ture was subjected to question in the study2 in which this
natural product was identified as 7,7´-bis(3-ethyl-2,5,6,8-
tetrahydroxynaphthalene-1,4-dione) (18), i.e., the
1,4-naphthoquinoid structure was assigned to this com-
pound. However, based on the 1,4-naphthoquinone
structure of cuculoquinone, the structure of 6,6´-bis(3-
ethyl-2,5,7,8-tetrahydroxynaphthalene-1,4-dione) (19)
may be proposed as an alternative, all the more so since
the spectral methods used did not allow the authors of
the cited study to unambiguously choose between this
structure and structure 18.11
β-hydroxy groups at the C(2) atom vary is no less than
–1
19 cm
and the corresponding differences for the
β-hydroxy groups at the C(6) and C(7) atoms are no less
–1
than 11 cm . Therefore, the regularity revealed for the
ν(OH) frequencies in the IR spectra enables one to
reliably identify substituted 2,6- and 2,7-dihydroxy-
naphthazarins.
Low concentrations of the compounds under study
in chloroform, which are used for recording the IR
spectra (see Experimental), preclude the formation of
noticeable amounts of intermolecular associates. This
was confirmed by our experiments on dilution of solu-
tions and variation of the solvents. The frequency of the
ν(OH) absorption band in the IR spectrum of naphtho-
The UV spectrum of a methanolic solution of
cuculoquinone has an intense band at 239 nm and a
lower-intensity band at 270 nm, which fall into the
regions found for substituted 2,7-dihydroxynaphthazarins
2, 4, 6, 12, and 14 (Fig. 2). In the UV spectrum of an
alkaline methanolic solution of cuculoquinone, the low-
intensity long-wavelength band is shifted to 305 nm
(Fig. 3) and its absorption increases by a factor of 1.4,
which is typical of the spectra of 2,7-dihydroxy-
naphthazarin and its derivatives. In the visible region of
1
2
–1
3550
3500
3450
3400
ν/cm
Fig. 1. Ranges of stretching vibration frequencies of the
β-hydroxy group in the IR spectra of chloroform solutions of
2,7-dihydroxynaphthazarins 2, 4, 6, 12, and 14 (1) and
2,6-dihydroxynaphthazarins 1, 3, 5, 8, and 13 (2).

92
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 1, January, 2001
Glazunov et al.
A
OH OH
Et
O
OH OH
305
0.8
0.6
0.4
0.2
1
O
O
OH OH
O
Et
OH OH
17
2
3
O
HO
HO
O
Et
OH
571
OH
OH
O
O
OH
Et
HO
O
HO
O
O
HO
HO
18
HO
Et
OH
238
294
385
556
λ/nm
OH
OH
Fig. 3. Absorption spectra (A) of alkaline methanolic solutions
of cuculoquinone (1), 3-ethyl-2,7-dihydroxynaphthazarin (4)
(2), and 3-ethyl-2,6-dihydroxynaphthazarin (3) (3).
Et
HO
OH
O
and 14 (see Table 2). In addition, the observed ratio of
the peak intensities of the ν(OH) bands in the IR
spectrum of this compound is analogous to the ratio of
the corresponding intensities for 2,7-dihydroxynaphth-
azarins 2, 4, 6, 12, and 14.
Therefore, the data of electronic absorption and IR
spectroscopy of bisnaphthazarin isolated from lichen
Cetraria cucullata unambiguously indicate that this com-
pound belongs to the series of substituted 2,7-dihydroxy-
naphthazarins. This fact gives grounds to revise the
structure of 7,7´-bis(3-ethyl-2,5,6,8-tetrahydroxy-
naphthalene-1,4-dione) (18), which has been assigned
19
the spectrum of cuculoquinone, the absorption band is
shifted bathochromically to 571 nm on going from a
neutral to alkaline methanolic solution, which is also in
agreement with the above-mentioned characteristic fea-
tures of the spectra of 2,7-dihydroxynaphthazarins.
The IR spectrum of a solution of cuculoquinone in
chloroform has two ν(OH) absorption bands at 3527 and
–1
3420 cm (Fig. 4), which fall in the ranges found by us
for 2,7-dihydroxynaphthazarin derivatives 2, 4, 6, 12,
A
Absorption/rel.unit
1.0
3531
0.20
3512
3430
1
0.8
270
3391
0.15
2
0.6
3
0.10
0.4
0.2
0.05
1
3
2
3600
–1
3500
3400
3300 ν/cm
238
294
385
556
λ/nm
Fig. 2. Absorption spectra (A) of methanolic solutions of
cuculoquinone (1), 3-ethyl-2,7-dihydroxynaphthazarin (4) (2),
and 3-ethyl-2,6-dihydroxynaphthazarin (3) (3).
Fig. 4. Fragments of the IR spectra of chloroform solutions of
cuculoquinone (1), 3-ethyl-2,7-dihydroxynaphthazarin (4) (2),
and 3-ethyl-2,6-dihydroxynaphthazarin (3) (3).

2,6(7)-Dihydroxynaphthazarins
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 1, January, 2001
93
and kept for 15 h. The precipitate that formed was filtered off,
washed with water (3½2 mL), and dried under reduced pressure
over P2O5. Elution with the 2 : 1 hexane—acetone system on a
column with SiO2 (Chemapol L 40/100 µm) afforded com-
pound 8 in a yield of 132 mg (51%), m.p. 227—230 °C (with
decomp.). Found (%): C, 47.02; H, 2.02. C10H5ClO6. Calcu-
lated (%): C, 46.81; H, 1.96. 1H NMR (acetone-d6), δ: 6.62 (s,
1 H, H(3)); 10.52 (br.s, 1 H, β-OH); 12.31 and 13.03 (both s,
1 H each, α-OH). MS (70 eV), m/z (Irel (%)): 256/258 [M]+
(100), 228/230 (42), 200 (5), 193 (12), 186 (17).
to this compound previously, in favor of 6,6´-bis(3-
ethyl-2,5,7,8-tetrahydroxynaphthalene-1,4-dione) (19).
In addition to cuculoquinone, its monomer was also
isolated from lichen Cetraria cucullata. Structure 3 was
assigned to this monomer based on the formal synthesis
of cuculoquinone by oxidative coupling of dimethyl
ether of this monomer.11 Since structure 19 was pro-
posed for cuculoquinone, its monomer should be identi-
fied as 3-ethyl-2,7-dihydroxynaphthazarin (4), which is
one of the metabolites of urchins Echinothrix isolated
previously.12 Bisnaphthazarin 19 has been found in abys-
sal sea cucumbers Psychopotes longicauda, Benthodytes
typica, and B. lingua2 and, more recently, in lichen
Cetraria islandica.13 The well-studied mechanism of bio-
synthesis of compounds analogous to spinochromes14
can be considered as circumstantial evidence in favor of
structure 19. The structures of all natural naphtho-
quinone derivatives containing the 2,7-dihydroxynaphth-
azarin fragment as a subgroup, which have been isolated
in recent years,13,15 are consistent with the above
mechanism.
6,7-Dichloro-5,8-dihydroxy-2-methoxy-1,4-naphthoquinone
(10). Compound 10 was prepared in quantitative yield by
careful treatment of substrate 9 with a solution of diazomethane
in diethyl ether.18 M.p. 216—218 °C. Found (%): C, 46.02;
H, 2.17. C11H6Cl2O5. Calculated (%): C, 45.70; H, 2.09.
1H NMR (CDCl3), δ: 3.98 (s, 3 H, OMe); 6.29 (s, 1 H, H(3));
12.77 and 13.28 (both s, 1 H each, α-OH). MS (18 eV),
m/z (Irel (%)): 288/290/292 [M]+ (100), 287/289/291 [M – 1]+
(88), 273/275/277 (23), 272/274/276 (44), 270/272/274 (75),
269/271/273 (59), 258 (10), 256 (10), 223 (14), 222 (10).
7-Chloro-2,5,7,8-tetrahydroxy-1,4-naphthoquinone (12). A
mixture of dry 6,7-dichloro-2-methoxynaphthazarin (10) (85 mg,
0.29 mmol), freshly calcinated CsF (400 mg, 2.6 mmol), and
freshly calcinated Al2O3 (360 mg, 3.5 mmol) in anhydrous
MeOH* (10 mL) was refluxed with stirring for 10 h. Then the
reaction mixture was cooled and the precipitate was filtered off
and washed successively with 5% HCl (0.5 mL) and acetone
(3 mL). The combined acidic filtrates were concentrated to
∼1 mL under reduced pressure, diluted with H2O (20 mL), and
extracted with chloroform (3½10 mL). The extract was worked
up according to a standard procedure and the residue was
refluxed in concentrated HBr (15 mL) for 15 min. The reaction
mixture was diluted with H2O (50 mL) and extracted with ethyl
acetate (3½10 mL). The extract was concentrated and com-
pound 12 was isolated in a yield of 32 mg (43%) by preparative
TLC, Rf 0.28, m.p. 257—261 °C. Found (%): C, 46.60; H, 2.05.
C10H5ClO6. Calculated (%): C, 46.81; H, 1.96. 1H NMR
(acetone-d6), δ: 6.65 (s, 1 H, H(3)); 10.16 (br.s, 1 H, β-OH);
12.08 and 13.00 (both s, 1 H each, α-OH). MS (18 eV),
m/z (Irel (%)): 257/259 (40), 256/258 [M]+ (100), 255/257 (11),
228/230 (13), 224 (9), 223 (30), 222 (75). In addition to
compound 12, compound 8 was isolated from the reaction
mixture by preparative TLC in a yield of 19 mg (25%), Rf 0.33.
Radical alkylation of 2,6(7)-dihydroxynaphthazarins 1, 2,
8, and 12 with propionyl peroxide. Propionyl peroxide19 was
added dropwise to a boiling solution of the substrate (0.5 mmol)
in ButOH (30 mL). The course of the reaction was monitored
by TLC. The reaction was terminated when the degree of
conversion reached ∼60%. The reaction mixture was concen-
trated and the residue was chromatographed by preparative TLC.
Ethylation of mompain 2 (110 mg) afforded a fraction with
Rf 0.43, viz., 3,6-diethyl-2,5,7,8-tetrahydroxy-1,4-naphtho-
quinone (6) in a yield of 5.2 mg (6% with respect to consumed
mompain 2), m.p. 185—187 °C. Found (%): C, 61.02; H, 4.98.
C14H14O6. Calculated (%): C, 60.43; H, 5.07. 1H NMR
(CDCl3), δ: 1.19 (t, 6 H, 2 CH3, J = 7.7 Hz); 2.72 (q, 4 H,
2 CH2, J = 7.7 Hz); 6.73 (br.s, 2 H, 2 β-OH); 11.79 and 13.54
(both s, 1 H each, α-OH). MS (70 eV), m/z (Irel (%)):
278 [M]+ (99), 277 [M – 1]+ (100), 263 (22), 262 (18),
247 (13), 235 (18).
Experimental
The melting points were determined on a Boetius heating
stage and were not corrected. The UV spectra were recorded on
a Specord M 40 spectrophotometer in quartz tubes (the thick-
ness of the layer was 1.0 mm). The concentrations of
methanolic solutions of compounds 1—6, 8, and 12—14 were
(4.70—5.40)•10–4 mol L–1. Alkaline methanolic solutions were
prepared by the addition of a 0.3 M NaOH solution (50 µL) to a
methanolic solution of the substrate (1.5 mL). The IR spectra
were measured on a Bruker Vector 22 Fourier spectrometer (the
resolution was 2.0 cm–1) in CDCl3 using cells with CaF2
windows and polyethylene gaskets (the thickness of the layer
was from 0.40 to 1.50 mm). The concentrations of solutions of
–1
compounds 1—6, 8, and 12—14 were (2.5—5.0)•10–3 mol L
.
The stretching vibration frequencies of the absorption bands of
the β-hydroxy groups were measured after "smoothing" of the
spectrum. The reproducibility of the results of measurements
1
was not worse than ±0.5 cm–1. The H NMR spectra were
recorded on a Bruker AC-250 spectrometer (250.13 MHz) in
CDCl3 and acetone-d6 (Me4Si as the internal standard). The
mass spectra (EI) were obtained on an LKB-9000S instrument
with direct introduction of the samples; the energies of ionizing
electrons were 18 and 70 eV.
The course of the reactions and the purities of the com-
pounds were monitored by TLC on Merñk 60F-254 plates using
a 2 : 1 hexane—acetone system as the eluent. The individual
compounds were isolated from mixtures of the reaction prod-
ucts by preparative TLC on plates (20½20 cm) with a nonfixed
silica gel layer (H+ form;5 5—40 µm) in the 2 : 1 hexane—ace-
tone system. The yields of the resulting compounds were not
optimized. Compounds 1 and 2 were prepared according to a
procedure reported previously.5 The conditions of conversion of
2,3-dichloronaphthazarin (7)16 into 6,7-dichloro-2-hydroxy-
naphthazarin (9) have been reported previously.17
The major product with Rf 0.36, viz., 3-ethyl-2,5,7,8-
tetrahydroxy-1,4-naphthoquinone (4), was obtained in a yield
of 39.8 mg (51% with respect to consumed compound 2),
m.p. 183—186 °C (sublim.) (cf. lit. data:13 m.p. 182—188 °C).
7-Chloro-2,5,6,8-tetrahydroxy-1,4-naphthoquinone (8). A
mixture of 2,3-dichloronaphthazarin (7) (259 mg, 1 mmol),
boric acid (280 mg), and concentrated H2SO4 (3 mL) was
heated on a molten metal bath at 200—210 °C for 1 h. The
mixture was cooled to ∼20 °C, poured into ice water (25 mL),
* Reagents were prepared according to a known procedure.3

94
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 1, January, 2001
Glazunov et al.
Found (%): C, 58.04; H, 4.25. C12H10O6. Calculated (%):
C, 57.60; H, 4.03. H NMR (CDCl3), δ: 1.19 (t, 3 H, CH3,
This work was partially supported by the Russian
Foundation for Basic Research (Project No. 96-15-
97316), the USA Civilian Research and Development
Foundation (Grant REC-003), and the Ministry of Edu-
cation of the Russian Federation.
1
J = 7.7 Hz); 2.68 (q, 2 H, CH2, J = 7.7 Hz); 6.65 and 6.90
(both br.s, 1 H each, β-OH); 11.70 and 13.11 (both s, 1 H each,
α-OH). MS (70 eV), m/z (Irel (%)): 250 [M]+ (100), 249
[M – 1]+ (48), 235 (8), 207 (29), 206 (13). The fraction with
Rf 0.29 contained the initial mompain 2, 40.7 mg (37%).
Ethylation of isomompain 1 (110 mg) afforded a fraction
with Rf 0.51, viz., 3,7-diethyl-2,5,6,8-tetrahydroxy-1,4-naph-
thoquinone (5), in a yield of 28 mg (29% with respect to
consumed isomompain 1), m.p. 228—232 °C. Found (%):
C, 59.63; H, 5.23. C14H14O6. Calculated (%): C, 60.43; H, 5.07.
1H NMR (acetone-d6), δ: 1.13 (t, 6 H, 2 CH3, J = 7.9 Hz);
2.65 (q, 4 H, 2 CH2, J = 7.9 Hz); 9.74 (br.s, 2 H, 2 β-OH);
13.11 (s, 2 H, 2 α-OH). MS (70 ýÂ), m/z (Irel (%)): 278 [M]+
(97), 277 [M – 1]+ (100), 263 (22), 262 (22), 250 (64), 249
(14). 3-Ethyl-2,5,6,8-tetrahydroxy-1,4-naphthoquinone (3) was
obtained as the major product in a yield of 60.7 mg (70%),
Rf 0.40, m.p. 227—229 °C. Found (%): C, 57.42; H, 4.40.
References
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1
C12H10O6. Calculated (%): C, 57.60; H, 4.03. H NMR (ac-
etone-d6), δ: 1.12 (t, 3 H, CH3, J = 7.9 Hz); 2.61 (q, 2 H, CH2,
J = 7.9 Hz); 6.51 (s, 1 H, H(7)); 9.79 and 10.13 (both br.s,
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236 (8), 235 (11), 234 (7), 222 (8), 207 (7). The fraction with
Rf 0.32 contained the initial isomompain 1, 33 mg (30%).
Ethylation of 6-chloro-2,7-dihydroxynaphthazarin (12)
(128 mg) afforded a complex mixture of products from which
the fraction with Rf 0.36, viz., 6-chloro-3-ethyl-2,5,7,8-
tetrahydroxy-1,4-naphthoquinone (14), was isolated by pre-
parative TLC in a yield of 3 mg (2%), m.p. 218—222 °C.
1H NMR (CDCl3), δ: 1.18 (t, 3 H, CH3, J = 7.5 Hz); 2.71 (q,
2 H, CH2, J = 7.5 Hz); 11.76 (br.s, 1 H, α-OH); 13.37 (s, 1 H,
α-OH). 1H NMR (acetone-d6), δ: 1.14 (t, 3 H, CH3,
J = 7.5 Hz); 2.70 (q, 2 H, CH2, J = 7.5 Hz); 9.77 (br.s, 1 H,
β-OH); 12.11 (br.s, 1 H, α-OH); 13.58 (s, 1 H, α-OH). MS
(70 eV), m/z (Irel (%)): 284/286 [M]+ (23), 283/285 [M – 1]+
(100), 249 (9), 241 (9).
Ethylation of 7-chloro-2,6-dihydroxynaphthazarin (8)
(128 mg) afforded a complex mixture of products from which
the fraction with Rf 0.42, viz., 7-chloro-3-ethyl-2,5,6,8-
tetrahydroxy-1,4-naphthoquinone (13), was isolated by pre-
parative TLC in a yield of 5 mg (3%), m.p. 221—224 °C.
1H NMR (CDCl3), δ: 1.17 (t, 3 H, CH3, J = 7.6 Hz); 2.70 (q,
2 H, CH2, J = 7.6 Hz); 12.58 and 12.78 (both s, 1 H each,
α-OH). 1H NMR (acetone-d6), δ: 1.14 (t, 3 H, CH3,
J = 7.5 Hz); 2.69 (q, 2 H, CH2, J = 7.5 Hz); 9.97 (br.s, 1 H,
β-OH); 12.92 and 12.95 (both s, 1 H each, α-OH). MS (70 eV),
m/z (Irel (%)): 284/286 [M]+ (14), 283/285 [M – 1]+ (100),
269/271 (16), 268/270 (12), 241/243 (41).
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6,6´-Bis(3-ethyl-2,5,7,8-tetrahydroxynaphthalene-1,4-
dione) (19). UV (MeOH), λmax/nm (lg ε): 216 (4.00), 239
(4.10), 270 (3.81), 329 (3.61), 545 sh (3.39), 485 (3.45), 518
(3.51), 556 (3.34). UV (MeOH+NaOH), λmax/nm (lg ε): 229
(4.00), 258 (3.99), 305 (4.18), 385 (3.45), 463 sh (3.47), 495
(3.56), 571 (3.42). IR (CDCl3), ν/cm–1: 3527 m, 3420 m,
1631 sh.m, 1602 s, 1586 sh.s.
We thank L. S. Stepanenko (the Laboratory of Chem-
istry of Quinoid Compounds of the Pacific Institute of
Bioorganic Chemistry of the Far-Eastern Branch of the
Russian Academy of Sciences) for providing a sample of
natural cuculoquinone.
Received April 10, 2000;
in revised form July 26, 2000