New insight into the oxidative chemistry of noradrenaline: Competitive o-quinone cyclisation and chain fission routes leading to an unusual 4-[bis-(1H-5,6-dihydroxyindol-2-yl)methyl]-1,2-dihydroxybenzene derivative

Article abstract of DOI:10.1016/j.tet.2005.02.030

Oxidation of 5×10^-3 M noradrenaline in aqueous phosphate buffer, pH 7.4, with K₃Fe(CN)₆, NaIO₄ or Fe²⁺/EDTA/H₂O₂ followed by extraction with ethyl acetate and acetylation with Ac₂O/Pyr led to a main reaction product which was isolated and identified as 4-[bis-(1H-5,6-diacetoxyindol-2-yl) methyl]-1,2-diacetoxybenzene, an unprecedented [bis-(indol-2-yl)methyl]-benzene derivative unsubstituted on the 3-position of the indole rings. This product was also obtained in 40% yield by reaction of 5,6-dihydroxyindole with 3,4-dihydroxybenzaldehyde. Other components of the oxidation mixture were 1-acetyl-3,5,6-triacetoxyindole, derived from noradrenolutin, and 5,6-diacetoxyindole, originating from cyclisation/dehydration of the o-quinone of noradrenaline, along with some 3,4-diacetoxybenzaldehyde. Inspection of the aqueous phase revealed the presence of 3,4-dihydroxymandelic acid and 3,4-dihydroxybenzaldehyde, derived from oxidative breakdown of the 2-amino-1-hydroxyethyl chain via a p-quinomethane intermediate. These results disclose new aspects of the oxidative chemistry of noradrenaline beyond the aminochrome stage and provide a route to novel [bis-(indol-2-yl)methyl]-benzene derivatives of potential pharmacological interest.

Full text of DOI:10.1016/j.tet.2005.02.030

Tetrahedron 61 (2005) 4075–4080
New insight into the oxidative chemistry of noradrenaline:
competitive o-quinone cyclisation and chain fission routes leading
to an unusual 4-[bis-(1H-5,6-dihydroxyindol-2-yl)methyl]-
1
,2-dihydroxybenzene derivative
Paola Manini, Alessandro Pezzella, Lucia Panzella, Alessandra Napolitano
and Marco d’Ischia
*
Department of Organic Chemistry and Biochemistry, University of Naples ‘Federico II’, Complesso Universitario Monte S. Angelo,
Via Cinthia 4, I-80126 Naples, Italy
Received 29 November 2004; revised 25 January 2005; accepted 10 February 2005
K3
2C
Abstract—Oxidation of 5!10 M noradrenaline in aqueous phosphate buffer, pH 7.4, with K Fe(CN) , NaIO or Fe /EDTA/H O
2 2
3
6
4
followed by extraction with ethyl acetate and acetylation with Ac O/Pyr led to a main reaction product which was isolated and identified as
2
4
-[bis-(1H-5,6-diacetoxyindol-2-yl)methyl]-1,2-diacetoxybenzene, an unprecedented [bis-(indol-2-yl)methyl]-benzene derivative
unsubstituted on the 3-position of the indole rings. This product was also obtained in 40% yield by reaction of 5,6-dihydroxyindole with
,4-dihydroxybenzaldehyde. Other components of the oxidation mixture were 1-acetyl-3,5,6-triacetoxyindole, derived from noradrenolutin,
3
and 5,6-diacetoxyindole, originating from cyclisation/dehydration of the o-quinone of noradrenaline, along with some 3,4-
diacetoxybenzaldehyde. Inspection of the aqueous phase revealed the presence of 3,4-dihydroxymandelic acid and 3,4-dihydroxybenzalde-
hyde, derived from oxidative breakdown of the 2-amino-1-hydroxyethyl chain via a p-quinomethane intermediate. These results disclose
new aspects of the oxidative chemistry of noradrenaline beyond the aminochrome stage and provide a route to novel [bis-(indol-2-
yl)methyl]-benzene derivatives of potential pharmacological interest.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
knowledge of the oxidative chemistry of noradrenaline
norepinephrine, 1) has remained surprisingly scanty. This
(
The elucidation of the oxidative pathways of catechol-
amines involving the spontaneous cyclisation and/or
tautomerism of their o-quinones has traditionally been an
neurotransmitter is produced mainly in the locus coeruleus,
one of the putative candidates for the brain’s ‘pleasure’
centre implicated in physical and mental arousal and
elevated mood.
1
–6
important goal in organic chemistry.
The biological
significance of these reactions is that they appear to be the
mechanism of formation of a series of reactive indolic
species and/or quinomethane (quinone methide) inter-
mediates involved in the synthesis of neuromelanin and
7
–9
10–13
related pigments,
sclerotisation of insect cuticle, and in the autoactivation of
in neuronal degeneration,
1
in the
4
1
5
tyrosinase. Catecholamine oxidation represents also a
convenient entry to 5,6-dihydroxyindoles and related
systems of biological and pharmacological relevance.
1
6,17
Whereas several insights have been gained into the
1
oxidative pathways of dopamine and adrenaline,
–6,18,19
Keywords: Noradrenaline; Noradrenolutin; Oxidation; Quinomethane;
Bis-(indol-2-yl)methyl]-benzene.
Oxidation of 1 leads to the formation of the unstable
o-quinone. This undergoes cyclisation and oxidation to give
noradrenochrome (2) via an indoline intermediate
[
*
Corresponding author. Tel.: C39 081 674133; fax: C39 081 674393;
e-mail: alesnapo@unina.it
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.02.030

4
076
P. Manini et al. / Tetrahedron 61 (2005) 4075–4080
0,21
2
C C
671 ([MCH] ), with peaks at 693 ([MCNa] ) and 709
commonly referred to as leuconoradrenochrome. The
subsequent steps of the pathway are believed to involve
isomerisation of 2 to a fluorescent indoxyl derivative,
known as noradrenolutin (3a). Whereas the analogous
oxidation product of adrenaline, adrenolutin (3,5,6-tri-
hydroxy-1-methylindole), was isolated and characterised
as early as 1949, compound 3a has never been obtained by
direct oxidation of 1, and has become available only by a
synthetic approach. Apart from 2, no other product of
C
1
([MCK] ). The H NMR spectrum displayed a broad
singlet (2H) at d 10.20 for N–H protons, three apparent
singlets at d 6.86, 7.14 and 7.28 (2H each), the signals (1H
each) for an ABX spin system at d 7.17 (d, JZ8.0 Hz), 7.21
(d, JZ2.0 Hz) and 7.31 (dd, JZ8.0, 2.0 Hz), and a singlet
(1H) at d 5.91. The signals for six acetyl groups completed
2
2
2
3
1
1
1
the H NMR spectrum. The H, H COSY spectrum revealed
correlations between the signal at d 10.20 and those at d 6.86
and 7.14, ascribable to the H-3 and H-7 protons of a 5,6-
2
4
noradrenaline oxidation has so far been identified.
2
5
diacetoxyindole system, respectively.
In an attempt to fill this gap, we have re-examined the
oxidation of 1, and have succeeded in isolating a major
product whose spectral features are different from those of
the known oxidation products of catecholamines. We report
herein the isolation and structural characterisation of this
new product and the elucidation of new aspects of the
oxidative chemistry of 1.
1
The C NMR spectrum showed the presence of fourteen
3
2
signals in the sp region between d 144.0 and 105.9 and one
signal at d 40.7 correlating with the proton resonance at d
5.91. This latter displayed multibond proton–carbon con-
2
nectivities with several sp carbons, including indolic ones,
suggesting a methine group linking two indole rings and a
disubstituted phenyl moiety. Overall, these data were
consistent with the structure of the unusual 4-[bis-(1H-5,6-
diacetoxyindol-2-yl)methyl]-1,2-diacetoxybenzene (4b). A
complete assignment of the proton and carbon resonances,
2
. Results and discussion
1
13
1
13
K3
as deduced from H, C HMQC and H, C HMBC
experiments, is reported in Table 1.
In preliminary experiments, the oxidation of 1 (5!10 M)
was investigated using a range of oxidising systems,
including potassium ferricyanide, sodium periodate and
2
C
the Fe /EDTA/H O system (Fenton reagent) in 0.1 M
2
2
phosphate buffer, pH 7.4. In all cases, the reaction mixture
became red in colour and then turned to purplish and dark
brown. Ethyl acetate extraction, followed by acetylation
with Ac O/Pyr to prevent oxidation of the phenolic
2
components, afforded four main products (HPLC). When
little sodium dithionite was added prior to extraction, to halt
the reaction and prevent degradation of oxidisable species, a
modest increase in the yield of extractable material was
observed without, however, appreciable changes in product
distribution. Accordingly, this precaution was included in
all work-up procedures for preparative purposes.
The most abundant of the four products, which was
relatively more retained on reverse phase, displayed in the
ESI(C)-MS spectrum a pseudomolecular ion peak at m/z
Consistent with the proposed formulation, a ROESY
experiment revealed contacts of the methine singlet at d
5.91 with the protons of the catechol moiety at d 7.21 and
3 2
Table 1. NMR spectroscopic data of compound 4b ((CD ) CO)
13
C
1
d H (mult, J Hz)
1
1
H, H COSY
1
13
H, C HMBC
d
C–H
40.7
5.91 (bs)
6.86, 10.20
118.8, 120.3, 123.5, 124.1,
26.3, 127.1
1
—
C-1
C-2
C-3
C-4
C-5
C-6
142.0
144.0
124.1
127.1
123.5
124.1
120.3
126.3
118.8
105.9
136.3
138.6
112.9
134.1
—
—
—
—
7.31
—
7.17, 7.21
7.31
—
5.91, 7.14, 10.20
—
7.14
—
—
6.86, 7.28, 10.20
—
5.91, 6.86, 7.14
—
—
—
—
—
40.7, 142.0
—
40.7, 124.1, 142.0
127.1, 144.0
—
118.8, 127.1, 134.1
—
126.3, 136.3, 138.6
—
—
118.8, 134.1, 136.3, 138.6
—
—
169.5
169.8
—
7.21 (d, 2.0)
—
7.31 (dd, 8.0, 2.0)
7.17 (d, 8.0)
—
6.86 (bs)
—
7.28 (s)
—
—
7.14 (s)
—
10.20 (s)
2.20 (s)
2.24 (s)
—
0
0
C-2
C-3
0
0
C-3a
0
C-4
C-5
C-6
C-7
0
0
0
C-7a
N–H
COOCH
COOCH
COOCH
COOCH
3
3
3
3
21.3
21.3
169.5
169.8
—
—

P. Manini et al. / Tetrahedron 61 (2005) 4075–4080
4077
carefully controlled conditions ensuring rigorous exclusion
of oxygen. As obtained, the compound was rapidly
converted into a species exhibiting a reddish purple
chromophore (lmaxZ476, 553 nm) denoting an oxidation
product. This, however, proved to be too unstable to be
isolated and characterised by spectroscopic techniques,
despite several attempts.
The fourth product isolated from the oxidation mixture of 1
was formulated as 1-acetyl-3,5,6-triacetoxyindole (3b). It
exhibited a pseudomolecular ion peak ([MCH] ) in the
C
C
ESI(C)-MS spectrum at m/z 334, with ([MCNa] ) and
C
[MCK] ) peaks at m/z 356 and 372. The H NMR
1
(
spectrum showed three singlets (1H each) in the aromatic
region, two of which shifted downfield at d 7.87 and 8.28,
and signals for four acetyl groups. The C NMR spectrum
1
3
consistently showed eight signals in the range between d
141.0 and 111.4, besides the signals for four acetyl groups.
Figure 1. Energy minimised (MMC) structure of compound 4b.
Highlighted is the distance between the H-3 and the NH proton of two
different indole units.
The overall yields of the isolated products 3b–6b were less
than 10% of the starting material. The remainder of the
oxidation mixture was accounted for by significant amounts
of insoluble dark polymeric materials together with a series
of products all in very small amounts, which could not be
isolated for spectroscopic characterisation. The poor mass
balance was apparently due to the relative instability of the
products which do not tend to accumulate in the reaction
mixture but, as generated, are rapidly degraded to polymeric
materials.
7
.31 and between the N–H proton of one indole unit with the
H-3 proton of the other indole unit. In accord with this
conclusion, brief inspection of the geometry optimised
structure of 4b (MMC) indicated a low energy confor-
mation in which the planes of the indole units form a
dihedral angle of about 898 and the distance between the NH
of one indole unit and the H-3 proton of the other one was
˚
1
.9 A, that is, low enough to account for a well detectable
contact (Fig. 1). In addition, a distinct cross peak in the
ROESY spectrum between the signals at d 6.86 and 7.28, the
latter due to the indolic H-4 proton, provided further support
to the 2-substitution of the indole rings, consistent with
The formation of structure 4a by oxidation of 1 was
remarkable, since it incorporated the 5,6-dihydroxyindole
system which is at a lower oxidation state compared to 2. As
mentioned earlier, the possibility that its incorporation into
the structure of 4a could be due to the reduction of 2
2
6,27
previous reports.
The alternative substitution through
the 3-position, that is, with the dZ6.86 H being H-2 rather
than H-3, is untenable on the basis of the observed through-
space contacts and the chemical shift values reported in the
literature for the H-2 protons of 5,6-dihydroxyindoles,
3
0
following treatment with dithionite was ruled out by a
careful analysis of the ethyl acetate extractable fraction
obtained omitting treatment with dithionite. Under these
conditions, acetylation of the extract gave a more complex
mixture in which 4b was clearly present, along with some
2
7–29
which never fall at d!7.0.
Scrutiny of the parent structure 4a shows that it conceivably
arises by coupling of two 5,6-dihydroxyindole (5a) units
through the 2-position with one molecule of 3,4-dihydroxy-
benzaldehyde (6a). This mechanism was supported by the
identification of 5,6-diacetoxyindole (5b) and 3,4-
diacetoxybenzaldehyde (6b) among the four main products
obtained by oxidation of 1, and was definitively secured by
reaction of 5a with 6a in 0.1 M phosphate buffer at pH 7.4
under a vigorous stream of argon. After extraction and
acetylation it was possible to isolate by PLC fractionation a
product identical in all respects with 4b (40% yield).
5b, indicating the presence of 5a in the crude mixture. On
this basis, it is argued that 5a is formed by dehydration of
leuconoradrenochrome, a process that would favourably
compete with the usual oxidative route to 2. To the best of
our knowledge, this is the first study demonstrating the
direct formation of 5a by oxidation of 1 under physiologi-
3
0
cally relevant conditions without a reductive step.
The mechanism of formation of 6a is likewise worthy of
note, as it reflects the oxidative breakdown of the side chain
of 1. This would likely proceed through the intermediacy of
3,4-dihydroxymandelic acid, which had previously been
proposed as the direct precursor of 6a under oxidative
3
1,32
conditions.
Consistent with this prediction, analysis of
the oxidation mixture of 1 showed the presence of
detectable amounts of 3,4-dihydroxymandelic acid. More-
over, when reacted under the above conditions, the acid was
converted to 6a.
In separate experiments, attempts were made to detect the
bis-(indol-2-yl)methyl]-benzene derivative 4a in the oxi-
dation mixture from 1 by direct HPLC analysis. To this aim,
compound 4b was deacetylated with 1 M NaOH under
On this basis, a possible mechanistic scheme for the
oxidation of 1 is outlined in Scheme 1. Formation of 3,4-
dihydroxymandelic acid from 1 would involve oxidation of
the catecholamine to the o-quinone followed by rapid
[

4078
P. Manini et al. / Tetrahedron 61 (2005) 4075–4080
Scheme 2. Proposed mechanism of formation of compound 4a.
catecholamines. Other highlights of this study include the
hitherto unrecognised conversion of 1 quinone to 3,4-
dihydroxymandelic acid under non-enzymatic conditions;
the isolation of 3a as the acetylated derivative by direct
oxidation of 1; and the identification of 5a among the
products spontaneously formed by oxidation of 1 without
reductive treatment of the mixture.
The chemistry described in this paper may also hint at
practical routes to novel [bis-(indol-2-yl)methyl]-benzene
compounds. [Bis-(indolyl)methyl]-benzene derivatives are
the focus of increasing interest because of their biological
activities, as exemplified by their antibacterial, anticarcino-
Scheme 1. Proposed mechanism of formation of compounds 3a, 5a and 6a
by oxidation of 1.
3
7–39
genic, antiinflammatory and toxic properties.
Generally, these compounds are prepared by protic or
Lewis acid catalysed condensations of indoles with
tautomerism leading to a p-quinomethane intermediate and
then to 3,4-dihydroxymandelic aldehyde, which would then
be readily oxidised to the carboxylic acid. Quinomethane
formation from catecholamine quinones is usually much
slower than cyclisation and only occurs when the latter
4
0–42
aldehydes.
In all cases, however, [bis-(indol-3-
yl)methyl]-benzene derivatives are obtained, unless the
4
3
3
-position is substituted, for example, by an alkyl residue,
3
3
and no straightforward route to [bis-(indol-2-yl)methyl]-
benzenes unsubstituted on the 3-position is available.
process is unfavourable. This is consistent with the notion
that o-quinone of 1 undergoes cyclisation at a relatively
3
4
slower rate and is therefore susceptible to tautomerism, a
process favoured by the hydroxyl group enhancing the
acidity of the b-proton.
3. Experimental
3
.1. General methods
As shown in Scheme 1, cyclisation and tautomerism of the
o-quinone of 1 represent two competing routes leading to 5a
and 6a, respectively. These latter, as confirmed by the
synthetic experiment described before, would then concur to
the formation of 4a through coupling processes reminiscent
of the reaction of indoles with the Ehrlich reagent
ESI(C)-MS spectra were recorded with a Waters ZQ
quadrupole mass spectrometer. High resolution EI-MS
spectra were obtained at 70 eV and 230 8C using a Kratos
1
13
MS 50 spectrometer. H and C NMR spectra were
recorded at 400 and 100 MHz, respectively, using a Bruker
WM 400 spectrometer. H, H COSY, H, C HMQC,
3
5
(
Scheme 2).
1
1
1
13
1
13
H, C HMBC, and ROESY experiments were run at
00 MHz using standard pulse programs from the Bruker
library.
The unusual regiochemical course of the coupling reaction
would be dictated by frontier orbital interactions, as argued
4
3
6
by the high HOMO coefficient on the C-2 position of 5a,
due to the electron-releasing effect of the hydroxyl group on
the 6-position of the indole ring.
UV spectra were performed with a Beckmann DU 640
spectrophotometer. Analytical and preparative HPLC were
carried out on a Gilson apparatus equipped with a UV
detector set at 280 nm using a Sphereclone ODS (5 mm,
4.6!250 mm) or Econosil (10 mm, 22!250 mm) column,
respectively. For analytical runs, 0.02 M acetic acid/aceto-
nitrile 60:40 (v/v) was used as the eluant, at a flow rate of
In conclusion, the identification of a novel [bis-(indol-2-
yl)methyl]-benzene derivative provides the first detailed
insight into the oxidative pathway of 1 beyond the
aminochrome stage and fills a gap in the chemistry of

P. Manini et al. / Tetrahedron 61 (2005) 4075–4080
4079
C
([MCK] ); HREI-MS for C H N O : calcd 670.1799,
35 30 2 12
found 670.1823.
1
0
1
mL/min. In preparative runs elution conditions were
.02 M acetic acid/acetonitrile 50:50 (v/v), at a flow rate of
5 mL/min. TLC and PLC was carried out on silica gel
plates (0.25 and 0.50 mm, respectively) from Merck.
Noradrenaline hydrochloride was from Sigma and 3,4-
dihydroxybenzaldehyde was from Fluka. 3,4-Dihydroxy-
mandelic acid, hydrogen peroxide (30% solution in water),
NaIO and K Fe(CN) were from Aldrich. 5,6-Dihydroxy-
indole was prepared as reported. Molecular mechanics
calculations were carried out with Hyperchem 6.01 package
produced by Hypercube, Inc. (Waterloo, Ont., Canada)
Compound 5b. FT-IR (CHCl ) n
3
1765, 1509, 1468, 1371,
1325, 1304; d (CD OD), 2.26 (6H, s, CH ), 6.48 (1H, d JZ
max
H
3
3
3.2 Hz, H-3), 7.28 (1H, s, H-7), 7.36 (1H, s, H-4), 7.38 (1H,
d JZ3.2 Hz, H-2).
4
5
4
3
6
4
4
Compound 6b. FT-IR (CHCl ) n
3
1766, 1690, 1600, 1490,
max
1450, 1420, 1365, 1250; d (CD OD), 2.26 (6H, s, CH ),
3
H
3
7.44 (1H, d JZ8.0 Hz, H-5), 7.77 (1H, d JZ2.0 Hz, H-2),
7.84 (1H, dd JZ8.0, 2.0 Hz, H-6), 9.94 (1H, s, CHO).
4
6
2
000.
3
.2. Oxidation of noradrenaline (1)
3.4. Synthesis of 4-[bis-(1H-5,6-diacetoxyindol-2-
yl)methyl]-1,2-diacetoxybenzene (4b)
A solution of 1 (60 mg, 0.29 mmol) in 0.1 M phosphate
buffer, pH 7.4 (60 mL), was treated with K Fe(CN)
3
6
Compound 5a (100 mg, 0.67 mmol) was added to 0.1 M
phosphate buffer, pH 7.4 (134 mL), previously purged with
an argon stream, and treated with 6a (464 mg, 3.4 mmol).
After 24 h, the reaction mixture was acidified to pH 5 and
worked up as above. After acetylation the residue was
subjected to PLC (eluant chloroform/ethyl acetate 1:1 plus
(
97 mg, 0.29 mmol). After 5 min the reaction mixture was
reduced with sodium dithionite (2 mg/mL), acidified to pH 5
with HCl, and extracted with ethyl acetate (2!30 mL). The
organic layers were collected, dried over anhydrous sodium
sulphate, evaporated under reduced pressure, and the
residue was treated with acetic anhydride (0.500 mL) and
pyridine (0.025 mL) overnight. The acetylated mixture was
evaporated under reduced pressure and the residue was
taken in ethyl acetate (5 mL) and subjected to HPLC
analysis. Similar experiments were carried out by treating 1
1% acetic acid) to afford 4b (90 mg, 40%, R Z0.37).
f
Acknowledgements
(
with Fe(NH ) (SO ) !6H O (95 mg, 0.29 mmol), EDTA
60 mg, 0.29 mmol) with NaIO (63 mg, 0.29 mmol) or
4
This work was carried out in the frame of research programs
on antitumor agents, sponsored by MIUR (PRIN 2003) and
on the mechanisms of aging and photoaging (IFO 2003)
sponsored by the Ministry of Health. We thank the ‘Centro
Interdipartimentale di Metodologie Chimico-Fisiche’
4
2
4 2
2
(
90 mg, 0.29 mmol) and H O (0.025 mL, 0.29 mmol). All
2 2
the experiments were carried out also by omitting treatment
with sodium dithionite.
(
CIMCF, University of Naples Federico II) for NMR and
3
4
.3. Isolation of 1-acetyl-3,5,6-triacetoxyindole (3b),
-[bis-(1H-5,6-diacetoxyindol-2-yl)methyl]-1,2-
mass facilities. We thank Mrs Silvana Corsani for technical
assistance.
diacetoxybenzene (4b), 5,6-diacetoxyindole (5b) and 3,4-
diacetoxybenzaldehyde (6b)
A solution of 1 (600 mg, 2.9 mmol) in 0.1 M phosphate
buffer, pH 7.4 (600 mL), was treated with NaIO (624 mg,
References and notes
4
2
.9 mmol). After 5 min, the reaction mixture was worked up
as above. After acetylation, the mixture was subjected to
preparative HPLC to give compounds 3b (29 mg, 3% yield,
t Z11.9 min), 4b (45 mg, 7% yield, t Z23.0 min), 5b
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R
R
(
7 mg, 1% yield, t Z7.2 min), and 6b (7 mg, 1% yield, t Z
R
R
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determined by H NMR.
3. Manini, P.; Panzella, L.; Napolitano, A.; d’Ischia, M.
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1
4
. Napolitano, A.; Crescenzi, O.; Pezzella, A.; Prota, G. J. Med.
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3
1
386, 1371, 1298; d ((CD ) CO), 2.30 (3H, s, CH ), 2.32
H
5. d’Ischia, M.; Palumbo, A.; Prota, G. Tetrahedron 1988, 44,
6441–6446.
3 2
3
(3H, s, CH ), 2.37 (3H, s, CH ), 2.67 (3H, s, CH ), 7.39 (1H,
3 3 3
s, H-4), 7.87 (1H, s, H-2), 8.28 (1H, s, H-7); d ((CD ) CO),
1
6. Palumbo, A.; d’Ischia, M.; Misuraca, G.; Prota, G. Biochim.
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C
3 2
9.6 (2!CH ), 19.8 (CH ), 22.8 (CH ), 111.4 (C-7), 111.7
3 3 3
(
1
C-4), 115.9 (C-2), 121.6 (C-3a), 129.9 (C-7a), 133.6 (C-3),
39.4 (C-5), 141.0 (C-6), 167.6 (CH COO–), 168.0 (2!
CH COO–), 168.9 (CH COO–); ESI(C)-MS: m/z 334
7. Prota, G. Melanins and Melanogenesis; Academic: San Diego,
1992.
3
8. Double, K. L.; Zecca, L.; Costi, P.; Mauer, M.; Griesinger, C.;
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Riederer, P.; Gerlach, M. J. Neurochem. 2000, 75, 2583–2589.
3
3
C
C
C
([MCH] ), 356 ([MCNa] ), 372 ([MCK] ); HREI-MS
for C H NO : calcd 333.0848, found 333.0935.
16
15
7
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