Dopamine quinone chemistry: A study of the influence of amide, amidine and guanidine substituents [-NH-CX-Y] on the mode of reaction

Article abstract of DOI:10.1039/b819367c

The influence of N-substituents on the mode of reaction of ortho-quinones generated by oxidation of N-substituted dopamine derivatives 8 has been studied. Ortho-quinones with amide, urea or guanidine side chains are relatively stable, with evidence of rea

Full text of DOI:10.1039/b819367c

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Dopamine quinone chemistry: a study of the influence of amide, amidine and
guanidine substituents [-NH-CX-Y] on the mode of reaction
Edward J. Land,a Almudena Perona,a Christopher A. Ramsden*a and Patrick A. Rileyb
Received 31st October 2008, Accepted 25th November 2008
First published as an Advance Article on the web 14th January 2009
DOI: 10.1039/b819367c
The influence of N-substituents on the mode of reaction of ortho-quinones generated by oxidation
of N-substituted dopamine derivatives 8 has been studied. Ortho-quinones with amide, urea or
guanidine side chains are relatively stable, with evidence of rearrangement to para-quinomethanes.
The N-methylthiourea derivative 11 rapidly cyclises giving a bicyclic product 12. The
trichloromethylamidine derivative 13 also rapidly cyclises but in this case gives a spirocyclic derivative
14. In contrast to the transient formation of spirocyclic products by other ortho-quinones derived from
dopamine derivatives, e.g., 19, the product 14 is stable and has been isolated and fully characterised.
media (e.g. enzymatic oxidation) than in organic solvents7 but we
have observed examples in organic media when intramolecular
Introduction
deprotonation by the amine side chain is favourable.9,11
As part of a continuing interest in the oxidation mechanisms of
tyrosinase1,13 and the suicide inhibition of tyrosinase,14,15 we have
investigated a series of N-substituted dopamine derivatives having
the general structure 8. We have previously described enzyme
and pulse radiolysis studies of some of these compounds,6 and
a preliminary account of our chemical studies has also been
published.16 We now report a full account of our studies of the
chemical oxidation of the catechols 8a-k together with some new
enzyme and pulse radiolysis results on previously undescribed
derivatives.
We have previously described investigations of the reactions of a
variety of dopamine ortho-quinone derivatives 4.1,2 In these studies
we have generated the ortho-quinones 4 by (i) tyrosinase oxidation
of phenols 1 or catechols 2,3–7 (ii) pulse radiolytic oxidation of
catechols 2,3–6,8 and (iii) chemical oxidation of catechols 2.9–11
We have found that cyclisation to the 5-position to give the
bicyclic products 5 (Scheme 1) is usually fast but that faster
transient formation of the less stable spirocyclic compounds
3 is sometimes observed. Cyclisation to the 3-position is not
observed and this aspect has been analysed using quantum
mechanical calculations.12 If cyclisation is not possible then
relatively slower rearrangement to a para-quinomethane 6 is
observed. This rearrangement occurs more easily in aqueous
Results and discussion
The side chains on the catechols 8a-k were chosen to display
a range of properties, particularly nucleophilicity and pKa, in
order to optimise the possibility of observing new ortho-quinone
reactions. Most of the catechols 8 were prepared by demethylation
of the corresponding O,O-dimethylcatechols 7 using either 48%
aqueous HBr or 1M BCl3 in CH2Cl2, and, unless otherwise
stated, are new compounds. The ethers 7 were prepared from
commercial 2-(3,4-dimethoxyphenyl)ethylamine. Relevant details
and any significant features of this methodology are discussed in
the appropriate section of the discussion.
Scheme 1
aLennard-Jones Laboratories, School of Physical and Geographical Sci-
ences, Keele University, Keele, Staffordshire, UK ST5 5BG. E-mail: c.a.
ramsden@chem.keele.ac.uk; Fax: +44 1782 712378; Tel: +44 1782 733045
bTotteridge Institute for Advanced Studies, The Grange, Grange Avenue,
Totteridge, London, UK N20 8AB
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The catechol 8a was prepared by the method of Niederstein
and Peter and had spectroscopic properties identical to those
previously described.17 The urea 8b was prepared by demethylation
of the known ether 7b.18,19 Oxidation of CDCl3/CD3OD solutions
of the acetamide 8a and the N-alkylurea 8b by 2,3-dichloro-
5,6-dicyano-p-benzoquinone (DDQ) was monitored by 1H NMR
spectroscopy. In both cases the three aromatic catechol protons
(d 6.5–6.8) were rapidly replaced by three ortho-quinone protons
(d 6.2–7.1). Under these conditions the ortho-quinones 9 (Y = Me,
NH2) were relatively stable and underwent slow decomposition
over several hours. However, no specific products could be
identified and after twenty four hours complex mixtures had
formed. This contrasts with the behaviour of the same ortho-
quinones 9 (Y = Me, NH2) in aqueous phosphate buffer (pH 7.4)
in which conversion to the 2-hydroxy-para-quinomethanes 10
(Y = Me, NH2), with well-defined isosbestic points, occurs over
several minutes.6 We have described other examples of 4-alkyl-
ortho-quinones that readily rearrange in aqueous buffer but are
relatively stable in organic solvents, with slow formation of
complex mixtures.7 Clearly, the aqueous environment favours
proton transfer and para-quinomethane formation. These results
for ortho-quinones 9 (Y = Me, NH2) therefore show a consistent
patternof behaviour for derivatives inwhich the catecholside chain
is neither sufficiently nucleophilic to cyclise onto the ortho-quinone
ring (Scheme 1) nor sufficiently basic to catalyse rearrangement.
We next turned our attention to amidines (8; X = NH, Y = alkyl,
aryl), which are much stronger bases than ureas and thioureas. The
trichloromethyl hydrochloride salt 8d was prepared by reaction
of trichloroacetonitrile with dopamine.20 The trichloromethyl
derivative was chosen simply because these are the easiest amidines
to prepare directly from primary amines and a nitrile, due to the
enhanced reactivity of trichloroacetonitrile.
When the amidine 8d was oxidised using one equivalent of DDQ
an unexpected result was obtained. 1H NMR examination of the
reaction solution revealed the quantitative formation of a single
product that on superficial examination appeared to be the ortho-
quinone 13. However, in contrast to the ortho-quinones 9 (Y = Me,
NH2), this product remained stable in solution over several days
with no evidence of decomposition. The reaction was repeated on
a preparative scale, and the product isolated as colourless needles,
mp 144 ◦C (decomp.), and identified as the spirocyclic compound
1
14. The H NMR spectrum shows two methylene groups (d 2.20
and 3.85). The signal at d 2.20 is at too high a field to be a benzylic
type signal (~d 2.65 in 9 and ~d 2.75 in 7 or 8) but is entirely
consistent with a spirocyclic CH2 attached to a quaternary carbon
atom. In addition, this signal (d 2.20) appears as a pseudo-quartet,
which can be attributed to the non-equivalence of the protons.
The second methylene signal (d 3.85) is entirely consistent with
the NCH2 group. The CH protons of the dienone fragment in
compound 14 appear at d 5.95, 6.35 and 7.00 with mutual coupling
constants of 3 and 10 Hz. The signals resemble those recorded for
the ortho-quinones 9 (Y = Me, NH2) except that the ‘enolic’ CH
(d 5.95) is at significantly higher field than the corresponding
protons in the ortho-quinones 9 (~d 6.25). The 13C NMR spectrum
also fully supports the spirocyclic structure 14 with notable signals
=
=
at d 55.3 (spiro C), 159.0 (N C-N) and 182.0 (C O).
We have not previously isolated a spirocyclic product in our
ortho-quinone studies. However, we have detected the transient
formation of the very short-lived species 3 (t1/2 ~0.1 s) using pulse
radiolysis.9 The UV spectrum of the product 14 [lmax 244 (e 7415)
and 320sh (e 1440) nm] is comparable to those of the transient
species 3 [lmax 250 and 310 nm],9 and is attributable to the dienone
fragment.
In contrast to the urea 8b, DDQ oxidation of the thiourea
8c rapidly gave a single product that showed only two aromatic
protons in the 1H NMR spectrum. This was identified as a 5-thia-
7-aza-benzocycloheptene and isolated as its hydrochloride salt 12.
Cyclisation was so rapid that the ortho-quinone precursor 11 was
not observed by 1H NMR spectroscopy. This rapid cyclisation is
consistent with the results of a pulse radiolysis study in which
the cyclisation 11 → 12 was complete after approximately three
seconds in phosphate buffer.6 This ring formation is in accord with
the side chain being a powerful S-nucleophile. The chloride 12
was isolated as a crystalline solid (mp 227–228 ◦C) (59%) and the
bicyclic structure is fully supported by its spectroscopic properties.
The mechanism of irreversible formation of the derivative 14
merits some discussion. We propose that the initially formed ortho-
quinone 13 undergoes spirocyclisation to give the kinetic product
15. In contrast to other spirocyclic species 3 that we have generated,
the 1-proto cation 15 can rapidly tautomerise to the resonance
stabilised 3-proto cation 14, which is much more stable. Since the
tautomerisation 15 → 14 is irreversible, the amidinium chloride
14 can no longer equilibrate with the ortho-quinone precursor 13.
Because the trichloromethyl substituent is atypical, we de-
cided to investigate the benzamidinium derivative 8e. Attempts
to prepare the precursor 7e by reaction of benzonitrile with
2-(3,4-dimethoxyphenyl)ethylamine in the presence of aluminium
trichloride, using the method of Brodrick and Short,21 were
unsuccessful. Using this procedure we obtained a product that
was identified as the N1,N2-bis-alkylamidinium chloride 16, mp
138 ◦C (26%). This product had insufficient aromatic protons
relative to the methylene and methyl protons for it to be the
amidine 7e. The mass spectrum showed a strong ion at m/z 450
corresponding to the cation and this product (16) is presumably
formed by further reaction of the desired amidine 7e with excess
amine. We therefore prepared the derivative 7e by an alternative
1
The H NMR spectrum shows two uncoupled aromatic protons
at d 6.59 and 6.66, corresponding to a 4,5-disubstituted catechol,
and this is confirmed by the 13C NMR spectrum, which shows
=
two aromatic CH signals (d 118.6 and 119.8), and also a C N
signal (d 167.3). The UV spectrum is consistent with a catechol
structure [lmax 287 (e 2800)] and the mass spectrum confirmed
the constitution and shows a strong fragment ion at m/z 168
∫
corresponding to elimination of MeNH.C N.
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255 and 135 (100%)) indicating that they are closely related, and
isomeric with the major product and ortho-quinone precursor 17.
The UV spectra suggest that they are catechol derivatives and the
most likely structures are the (E)- and (Z)-isomers 21.
route involving reaction of 2-(3,4-dimethoxyphenyl)ethylamine
with ethyl benzamidate in methanol. Recrystallisation of the crude
product gave a 73% yield of the amidinium chloride 7e. Demethy-
lation of this product using 48% aqueous hydrobromic acid gave
the bromide 8f and demethylation using boron trichloride in
methylene chloride gave the chloride 8e. The bromide 8f was then
oxidised using pulse radiolysis, tyrosinase and DDQ.
In a 1H NMR study, DDQ oxidation of the catechol 8f in
CD3OD gave the ortho-quinone 17 [d 7.17 (d, J 10 Hz), 6.41 (d,
J 10 Hz) and 6.33 (s)] which was stable in solution (>1 h). In
contrast to the derivative 13, which under the same conditions very
rapidly formed the spirocyclic product 14, there was no evidence of
cyclisation. In particular, there were no signals in the region d 1.5–
2.5 which would be characteristic of a spirocyclic CCH2. Similar
NMR studies using D2O as solvent, with and without the presence
of base, also failed to show any evidence of spirocyclisation.
The difference in behaviour of the amidinium ortho-quinones
13 (CCl3) and 17 (Ph) is of some interest. Assuming that the
spirocyclic product 20 is the main product of tyrosinase oxidation
in phosphate buffer, it forms significantly more slowly than
product 14 based on the relative stabilities of the ortho-quinones
13 and 17 in phosphate buffer measured by pulse radiolysis. This
difference in behaviour may be related to base strength: the weaker
base 13 may be a more effective N-nucleophile.
∑
-
In a pulse radiolysis study, Br2 oxidation of the bromide 8f
gave the semiquinone (k 1.7 ¥ 108 M-1 s-1) (lmax 310 and 350sh
nm). This disproportionated to the ortho-quinone 17 (lmax 400 nm;
e 1000 M-1 cm-1), which was stable for at least ten seconds. Under
the same conditions the trichloromethylamidine 8d gave the short-
lived ortho-quinone 13 which rapidly cyclised (t1/2 ~0.003 s) to the
spiro-product 14.6
Tyrosinase oxidation of the benzamidinium bromide 8f showed
no spectral evidence of ortho-quinone formation (lmax 400 nm),
despite clear oximetric evidence of tyrosinase-catalysed oxidation.
This suggests that a rapid reaction of the initially formed ortho-
quinone 17 takes place. The spectral scans showed generation of
a product(s) with a well-defined absorption at 305 nm, which is
not consistent with formation of the para-quinomethane 18. An
HPLC study, using mass spectral and UV-vis analysis, showed
one major product (rt 2.89 min) and two minor products (rt 1.82
and 2.07 min), together with the precursor 8f (rt 2.68 min). The
major product showed a strong molecular ion (m/z 255, 100%)
and UV absorption at lmax 234 and 305 nm. This UV spectrum
is comparable to that of the trichloromethyl spiro-derivative 14
and similar short-lived species, e.g. 19, lmax 250 and 305 nm.9
Since the spectrum of this product corresponds closely to that of
the original spectral scan (lmax 305 nm), we conclude that this is
the major product of tyrosinase oxidation and propose that this
product has the spirocyclic structure 20. The mass spectrum shows
two weak fragment ions at m/z 226 (loss of CH2NH or COH) and
m/z 151 (loss of PhCNH2). The minor products show virtually
identical UV spectra (lmax 232 and 300 nm) and mass spectra (m/z
We next turned our attention to guanidine derivatives. Because
guanidines are strong bases, we investigated the unsubstituted salts
8h,i and two N-substituted derivatives 8j,k (X = NCOCH3, NCN)
selected to be weaker bases. The parent guanidinium bromide 8h
was prepared from the known sulfate 7g by demethylation using
aqueous HBr. DDQ oxidation of the catechol 8h (pKa ~13) in
1
CD3OD gave a H NMR spectrum [d 7.30 (d, J 7 Hz), 6.57 (d,
J 7 Hz) and 6.50 (s)] consistent with formation of the ortho-
quinone 22. Over a period of four hours the ortho-quinone 22
equilibrates to an equimolar mixture of the quinone 22 and the
catechol precursor 8h. The partial regeneration of the catechol
∑
-
8h can be attributed to bromide reduction (2Br- → Br2 → Br2)
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of the quinone to the semi-quinone (Q → Q∑-) which rapidly
disproportionates (2Q∑- → Q + Q2-). Similar behaviour was not
observed using the more electronegative chloride salt 8i.
Pulse radiolytic oxidation of the catechol 8h gave the semi-
quinone (lmax 310 and 350sh nm); this disproportionates to the
ortho-quinone 22 (lmax 400 nm; e 1400 M-1 cm-1) which is stable
for at least ten seconds. Tyrosinase oxidation of the catechol 8h
showed evidence of ortho-quinone formation (lmax ~400 nm) after
30 and 60 seconds but in later scans the main product absorbs at
~480 nm, which can be attributed to rapid rearrangement to the
para-quinomethane 23. This is comparable to the behaviour of
the ortho-quinones 9 (Y = Me, NH2),6 and is consistent with our
previous observations of rapid conversion to a quinomethane in
aqueous buffer.6,7
Finally, we investigated the N-cyanoguanidine 8k on the
basis that it should be an even weaker base (pKa ~0) than
the N-acetyl derivative. The 3,4-dimethoxy derivative 7k was
prepared from 2-(3,4-dimethoxyphenyl)ethylamine and dimethyl
=
N-cyanothioiminocarbonate [(MeS)2C N.CN] using standard
procedures,23 and demethylated (aq. HBr) to give the catechol
8k. In a pulse radiolysis study of the oxidation of the catechol 8k
the cyano substituent appeared to interfere with the one-electron
∑
-
oxidation by Br2 and gave a radical of unknown structure that
did not decay to the ortho-quinone. This study therefore provided
no useful information on the stability of the ortho-quinone 26.
Tyrosinase oxidation of catechol 8k did not show evidence (lmax
400 nm) of the ortho-quinone 26 but the final spectrum was
very similar to that shown by the N-acetyl derivative 8j with
a very broad peak at lmax ~470 nm. This suggests formation of
the para-quinomethane 27 that is too fast for the ortho-quinone
to be observed. When the catechol 8k was oxidised using one
The N-acetyl derivative 8j can be expected to be a significantly
weaker base (pKa ~8) than the parent guanidine (pKa ~13).22
Prolonged heating of the sulfate 7g with acetic anhydride and
triethylamine gave the N,N ¢-diacetylguanidine 7l in poor yield.
This was demethylated using BCl3 to give the catechol 8j. The 13
C
NMR spectrum of compound 7l shows two carbonyl signals at
=
d 172.8 and 186.5 which we assign to the groups –NH.C O and
=
=
N.C O, respectively. An alternative route to catechol 8j involved
1
equivalent of DDQ in CD3OD, the H NMR spectrum showed
a shorter reaction time with acetic anhydride and triethylamine to
give the N-acetylguanidine 7j which was demethylated (BCl3) to
give the desired guanidine 8j. The product 8j was isolated as the
free base and all spectral details were in accord with the proposed
structure. The 13C NMR spectrum showed a carbonyl signal at
d 173.7 and, on the basis of the 13C NMR spectrum of compound
immediate formation of the ortho-quinone [d 6.27 (s), 6.40 (d)
and 7.15 (d)]. The ortho-quinone decayed over five hours: after
three hours catecholic protons (d 6.5–6.8) dominated the spectrum
but the product was clearly a mixture. Attempts to identify or
isolate products on a preparative scale were unsuccessful.
=
7l, we conclude that this product is the –NH.C O tautomer, as
shown in structures 24 and 25.
Conclusions
DDQ oxidation of the catechol 8j in CD3OD gave the ortho-
quinone 24 [d 6.30 (s), 6.50 (d) and 7.15 (d)] which slowly
decayed over a period of twenty four hours. A complex set
of signals develops as the ortho-quinone decays. There is some
evidence to suggest that the para-quinomethane 25 is an initial
product, and formation is possibly assisted by intramolecular
deprotonation by the basic guanidine function. After two hours
new signals appeared in the catechol region (d 6.6–6.9) suggesting
intramolecular cyclisation by the para-quinomethane side chain.
However, the 1H NMR was complex and all attempts on a
preparative scale to isolate products were unsuccessful. In a similar
manner to compound 8h, pulse radiolytic oxidation of the catechol
8j gave the ortho-quinone 24 (lmax 400 nm; e 1600 M-1 cm-1)
which was stable for more than ten seconds. Tyrosinase oxidation
showed a clear 400 nm absorbance indicating initial formation
of the ortho-quinone 24. This converts, with isosbestic points
at 372 and 428 nm, to a product absorbing at longer wave-
lengths (~500 nm) which is presumed to be the para-quino-
methane 25.
The purpose of this investigation was to explore the diversity of
4-substituted ortho-quinone chemistry by oxidation of a series
of catechols 8 in which the side chains were chosen to have a
wide variety of properties (e.g., pKa, nucleophilicity). In addition
to extending our knowledge of ortho-quinone chemistry, new
reactions of synthetic interest or of use for pro-drug activation
by tyrosinase are a potential outcome. The formation of bicyclic
products, e.g., 12, from thioureas provides access to novel het-
erocyclic derivatives and a wider variety of thiourea derivatives
merits further investigation. In addition, the formation of the
spirocyclic product 14 and the difference in behaviour of the
corresponding phenyl precursor 17 is unexplained, and a more
extensive investigation of amidine derivatives would be of interest.
Experimental
1H and 13C NMR spectra were recorded using a Bruker Advance
DPX300 NMR spectrometer. IR spectra were recorded on a
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Perkin-Elmer Paragon 1000 FT-IR spectrometer or a Thermo
Nicolet Avatar 320 FT-IR spectrometer. IR spectra were measured
as thin films (liquids) or potassium bromide discs (solids). NMR
spectra were measured in CDCl3 with tetramethylsilane as internal
standard unless otherwise stated. Only significant bands for the
IR spectra are quoted. Mass spectrometry was carried out by the
EPSRC National Mass Spectrometry Service, Swansea. Melting
points were determined on a Kofler block or a Bibby Stuart
Scientific SMP3 melting point apparatus and are uncorrected.
Column chromatography was carried out using BDH silica gel
(particle size 33–70 mm) and chromatotron chromatography was
carried out using a LDC Analytical Consta Metric 3200 solvent
delivery system and plates made using Merck silica gel 60 PF254
containing gypsum. New compounds were shown to be pure by
both tlc and NMR spectroscopy.
1517, 1473, 1263, 1235, 1157, 1140, 1020, 850, 815, 766 and 611;
dH (CDCl3) 2.07 (3H, s, CH3.CO), 2.10 (1H, br s, NH), 2.77 (2H,
t, J 6.9 Hz, ArCH2), 3.60 (2H, m, NCH2), 3.80 (3H, s, OCH3),
3.82 (3H, s, OCH3), 6.71 (3H, m, aromatic H), 8.99 (1H, br s,
=
C NH) and 13.07 (1H, br s, NH.Ac); dC (CDCl3) 25.2 (CH3),
28.8 (ArCH2), 35.0 (NCH2), 56.0 (2 ¥ OCH3), 111.4 (CH), 112.0
(CH), 120.8 (CH), 131.1 (C), 147.9 (COH), 149.1 (COH), 155.6
=
=
(C N) and 172.6 (NH.C O). This material was converted into
compound 8j without further characterisation.
N,N -Diacetyl-N¢-[2-(3,4-dimethoxyphenyl)ethyl]guanidine 7l.
A suspension of N-[2-(3,4-dimethoxyphenyl)ethyl]-guanidinium
sulfate 7g (10.0 g, 37.0 mmol) in CH2Cl2 (100 mL) was stirred with
triethylamine (3.74 g, 37.0 mmol) (12 h). Acetic anhydride (3.77 g,
37.0 mmol) was then added and the mixture was heated under
reflux (7 d). After cooling, the mixture was filtered and washed
(¥ 3) with H2O. Concentration gave a hygroscopic solid that was
recrystallised from CH2Cl2–hexane and identified as compound
7l (1.14 g, 10%), colourless prisms, mp 92–93 ◦C; dH (CDCl3) 2.13
(3H, s, CO.CH3), 2.14 (3H, s, CO.CH3), 2.83 (2H, t, J 7.0 Hz,
ArCH2), 3.65 (2H, m, CH2N), 3.86 (3H, s, OCH3), 3.88 (3H, s,
OCH3), 6.80 (3H, m, aromatic H), 9.05 (1H, br s, NH) and 13.14
(1H, br s, NH); dC (CDCl3) 25.4 (CO.CH3), 29.1 (CO.CH3), 35.2
(ArCH2), 42.9 (CH2N), 56.2 (OCH3), 56.3 (OCH3), 111.7 (CH),
112.3 (CH), 121.1 (CH), 131.4 (C), 148.2 (C), 149.4 (C), 155.8
Preparation of O,O-dimethylcatechols 7
Compounds 7a,24 7b,18,19 7g,25 and 7k23 were prepared according
to literature procedures.
N1-[2-(3,4-Dimethoxyphenyl)ethyl]-N2-methylthiourea
mixture of 2-(3,4-dimethoxyphenyl)ethylamine (1.81 g,
7c.
A
10.0 mmol) and methyl isothiocyanate (0.88 g, 12.0 mmol) in
EtOH (25 mL) was heated under reflux (1 h). The solution
was concentrated to give compound 7c (2.4 g, 93%), colourless
needles, mp 95 ◦C; nmax/cm-1 3345, 3221, 3019, 2996, 2961, 2936,
1563, 1514, 1466, 1259, 1234, 1193, 1158, 1138 and 1026; dH
(CDCl3) 2.80 (5H, m), 3.65 (2H, m), 3.79 (6H, s, 2 ¥ OCH3), 5.70
(1H, br s, NH), 6.00 (1H, br s, NH) and 6.70 (3H, m, arom. H);
dC (CDCl3) 31.0 (ArCH2), 35.2 (CH3N), 46.1 (CH2N), 56.3 (2 ¥
OCH3), 111.8 (CH), 112.3 (CH), 121.1 (CH), 131.5 (C), 148.0
=
=
=
=
(C N), 172.8 (NH.C O) and 186.5 ( N.C O). This material was
converted to compound 8j without further characterisation.
N1,N2-Bis-[2-(3,4-dimethoxyphenyl)ethyl]benzamidinium chlo-
ride 16. Powdered anhydrous AlCl3 (10.4 g, 80 mmol) was added
(over 20–30 s) to a stirred mixture of benzonitrile (8.8 g, 85 mmol)
and 2-(3,4-dimethoxyphenyl)ethylamine (14.3 g, 80 mmol) and
heated at (200 ◦C)(1 h). After cooling, the thick orange oil
was stirred with water (150 mL). The aqueous component
was filtered and extracted with ether (3 ¥ 50 mL) to remove
unreacted benzonitrile. The aqueous solution was then extracted
with CH2Cl2 (3 ¥ 50 mL) and the combined extracts were dried
(Na2SO4). Concentration under vacuum gave a light brown solid
that was identified as compound 16 (5.0 g, 26%). A small sample
was recr◦ystallised from CH2Cl2–Et2O to give colourless crystals,
mp 138 C; nmax/cm-1 3173, 2936, 1638, 1516, 1448, 1420, 1262,
1237, 1157, 1141, 1025, 764 and 703; dH (CDCl3) 2.70 (4H, t,
J 6.5 Hz, ArCH2), 3.08 (4H, m, NHCH2), 3.78 (6H, s, OCH3),
3.82 (6H, s, OCH3), 6.3–7.4 (11H, m, aromatic H) and 10.54 (2H,
t, J 2.0 Hz, NH); dC (CDCl3) 35.7 (ArCH2), 46.7 (NCH2), 56.0
(2 ¥ OCH3), 111.2 (CH), 112.4 (CH), 121.2 (CH), 125.0 (CH),
=
(C), 149.3 (C) and 182.6 (C S). This product was converted to
the catechol 8c without further purification.
N-[2-(3,4-Dimethoxyphenyl)ethyl]benzamidinium chloride 7e.
2-(3,4-Dimethoxyphenyl)ethylamine (2.83 g, 15.6 mmol) and ethyl
benzimidate hydrochloride (2.90 g, 15.6 mmol) in MeOH (50 mL)
were stirred overnight under an N2 atmosphere. After evaporation,
the crude product was recrystallised from MeOH–Et2O to give
compound 7e (3.8 g, 73%), colourless plates, mp 201–202 ◦C
(lit.21 200.5–201.5 ◦C); nmax/cm-1 3176, 3018, 1672, 1614, 1581,
1516, 1467, 1259, 1240, 1159, 1143, 1026 and 748; dH (CD3OD)
2.99 (2H, t, J 7.0 Hz, ArCH2), 3.70 (2H, t, J 7.0 Hz, NHCH2),
3.80 (6H, s, 2 ¥ OCH3), 6.9 (3H, m, aromatic H), and 7.6 (5H,
m, C6H5); dC (CDCl3) 33.3 (ArCH2), 44.5 (NCH2), 55.5 (2 ¥
OCH3), 112.3 (CH), 112.9 (CH), 121.4 (CH), 127.8 (CH), 129.4
=
127.0 (CH), 129.2 (CH), 129.6 (C.C N), 131.7 (CCH2), 147.8
=
(CH), 129.7 (CH), 130.8 (C.C N), 133.7 (CCH2), 148.6 (C.OMe),
=
(C.OMe), 149.0 (C.OMe) and 168.0 (C N); m/z (Electrospray)
+
=
149.7 (C.OMe) and 164.9 (C N); m/z (Electrospray) 285 (M ,
450 [M - Cl+](50%), 449 (100), 299 (5), 285 (5), 165 (75) and
cation)(25%), 165 (40), 150 (50), 135 (75), 119 (55), 107 (77),
103 (80), 91 (100); Found: M+ (cation), m/z 285.1599; Calc. for
C17H21N2O2; 285.1598.
150 (5).
Preparation of N-substituted dopamine derivatives 8. Com-
pounds 8a17 and 8d20 were prepared according to literature
procedures.
N-Acetyl-N¢-[2-(3,4-dimethoxyphenyl)ethyl]guanidine 7j. N-
[2-(3,4-Dimethoxyphenyl)ethyl]guanidinium sulfate 7g (2.0 g,
7.4 mmol), triethylamine (0.74 g, 7.4 mmol) and acetic anhydride
(0.76 g, 7.4 mmol) in CH2Cl2 (25 mL) were heated under reflux
(5 h) and then stirred at room temperature (48 h). The solution
was filtered and concentrated under vacuum to give a solid product
that was identified as compound 7j (0.7 g, 36%), cream prisms, mp
101–103 ◦C; nmax/cm-1 3253, 3083, 2992, 2928, 2840, 1634, 1566,
N-[2-(3,4-Dihydroxyphenyl)ethyl]guanidinium bromide 8h.
The guanidinium sulfate 7g (2.0 g, 7.4 mmol) was dissolved in
48% aq HBr (50 mL) and under an N2 atmosphere was heated
at 90 ◦C (16 h). Most of the HBr was removed by evaporation
under reduced pressure to afford a brown solid. This residue was
extracted with EtOH and the solution filtered. Concentration
948 | Org. Biomol. Chem., 2009, 7, 944–950
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under reduced pressure gave a hygroscopic solid, which rapidly
became an oil, that was identified as the bromide 8h (0.6 g,
27%), brown oil; nmax/cm-1 3346, 1653, 1527, 1446, 1358, 1197,
1116, 1059, 1002, 870, 815, 781; dH (CDCl3) 2.73 (2H, d, J
7.0 Hz, ArCH2), 3.52 (2H, m, CH2N), 6.57 (1H, dd, J 2.1 and
8.0 Hz, aromatic 6H), 6.68 (1H, d, J 2.1 Hz, aromatic 2H),
6.71 (1H, d, J 8.0 Hz, aromatic 5H); dC (CDCl3) 35.4 (ArCH2),
44.1 (CH2N), 116.6 (CH), 117.0 (CH), 121.2 (CH), 130.8 (C),
stirred at room temperature (56 h). The reaction mixture was then
quenched with H2O (10.0 mL) and the aqueous fraction extracted
with CH2Cl2 (3 ¥ 10 mL). The combined CH2Cl2 solutions were
dried (Na2SO4) and concentrated to give a solid that was identified
as compound 8j (0.16 g, 34%), hygroscopic solid, mp ill-defined;
n
max/cm-1 3157, 1721, 1688, 1624, 1594, 1517, 1443, 1371, 1247,
1199, 1153, 1116 and 1039; dH (CD3OD) 2.16 (3H, s, CH3), 2.80
(2H, t, J 7.0 Hz, ArCH2), 3.52 (2H, t, J 7.0 Hz, NCH2), 6.60 (1H,
dd, J 2.0 and 7.0 Hz, aromatic 6H), 6.71 (1H, d, J 2.0 Hz, aromatic
2H) and 6.72 (1H, d, J 7.0 Hz, aromatic 5H); dC (CD3OD) 23.5
(CH3), 33.6 (ArCH2), 43.1 (NCH2), 115.6 (CH), 115.9 (CH), 120.2
=
145.2 (C.OH), 146.5 (C.OH), 158.6 (C N); m/z (Electrospray)
196 [M - Br+](27%), 137(100), 119 (33), 91(53), 60 (92); Found:
M - Br+, m/z 196.1081; Calc. for C9H14N3O2; 196.1081.
=
In a similar manner, the following derivatives were prepared
from the O,O-dimethylcatechols 7b, 7e and 7k.
(CH), 129.1 (C), 144.4 (COH), 145.6 (COH), 154.4 (C N) and
+
=
173.7 (NH.C O); m/z (Electrospray) 238 [MH ](30%), 196 (100),
N-[2-(3,4-dihydroxyphenyl)ethyl]urea 8b (0.46 g, 89%), mp
133–135 ◦C (with softening at 78 ◦C); nmax/cm-1 3317, 1687,
1639, 1570, 1521, 1468, 1332, 1257, 1185, 1115, and 786; dH
(CDCl3/CD3OD) 2.69 (2H, t, J 7.0 Hz, ArCH2) 3.37 (2H, t, J
7.0 Hz, CH2N), 6.55 (1H, dd, J 8.0 and 1.7 Hz, aromatic 6H),
6.68 (1H, d, J 1.7 Hz, aromatic 2H) and 6.72 (1H, d, J 8.0 Hz,
aromatic 5H); dC (CD3OD) 38.9 (ArCH2), 46.5 (NCH2), 119.5
(CH), 120.0 (CH), 124.2 (CH), 134.4 (C), 147.8 (COH), 149.2
137 (20), 105 (25), 74 (25), Found: MH+, m/z 238.1187; Calc. for
C11H16N3O3; 238.1186.
In a similar manner to Method 2, the following derivatives was
prepared from the O,O-dimethylcatechols 7c and 7e.
N1 -[2-(3,4-Dihydroxyphenyl)ethyl]-N2 -methylthiourea
8c
(0.35 g, 76%), tiny crystals, mp 103 ◦C; nmax/cm-1 3290, 1576,
1521, 1452, 1355, 1282, 1224, 1200, 1111, 951 and 790; dH
(CD3OD) 2.69 (2H, t, J 7.0 Hz, ArCH2), 2.91 (3H, br s, NCH3),
3.55 (2H, m, NCH2), 6.55 (1H, dd, J 8.0 and 1.7 Hz, aromatic
6H), 6.68 (1H, d, J 1.7 Hz, aromatic 2H) and 6.72 (1H, d, J
8.0 Hz, aromatic 5H); dC (CD3OD) 29.6 (ArCH2), 33.7 (CH3N),
45.6 (CH2N), 114.7 (CH), 115.3 (CH), 119.6 (CH), 129.6 (C),
∑
+
=
(COH) and 165.5 (C O); m/z (EI) 196 [M ](5%), 136 (100), 124
(20), 123 (50), 77 (20), Found (Electrospray): MH+, m/z 197.0921;
Calc. for C9H13N2O3; 197.0921.
N-[2-(3,4-Dihydroxyphenyl)ethyl]benzamidinium bromide 8f
(0.65 g, 72%), colourless powder, mp 111–112 ◦C; nmax/cm-1 3141,
1666, 1622, 1581, 1526, 1446, 1354, 1284, 1256, 1196, 1115 and
781; dH (CD3OD) 2.89 (2H, t, J 7.0 Hz, ArCH2), 3.66 (2H, m,
CH2NH), 6.62 (1H, dd, J 2.0 and 8.0 Hz, aromatic 6H), 6.73 (2H,
m, aromatic 2H and 5H), 7.62 (5H, m, C6H5), 8.76 (1H, br s,
NH), 9.29 (1H, br s, NH) and 9.53 (1H, br s, NH); dC (CD3OD)
33.2 (ArCH2), 44.8 (CH2N), 115.6 (CH), 116.0 (CH), 120.2 (CH),
∑
+
=
143.0 (C), 144.4 (C) and 174.9 (C S); m/z (EI) 226 [M ](80%),
195 (40), 136 (90), 123 (90), 107 (40) and 91 (100), Found: MH+,
m/z (Electrospray) 227.0847; Calc. for C10H14N2SO2; 227.0849.
N-[2-(3,4-dihydroxyphenyl)ethyl]benzamidinium chloride 8e
(0.11 g, 40%), hygroscopic solid; dH (CD3OD) 2.92 (2H, t, J 7.0 Hz,
ArCH2), 3.68 (2H, m, CH2N), 6.76 (3H, m, aromatic H), 7.63 (5H,
m, C6H5), 8.80 (1H, br s, NH), 9.31 (1H, br s, NH) and 9.56 (1H,
br s, NH); dC (CD3OD) 33.3 (ArCH2), 44.7 (CH2N), 115.6 (CH),
116.0 (CH), 120.2 (CH), 127.8 (CH), 129.4 (2 ¥ CH), 129.9 (C),
127.9 (CH), 129.4 (CH), 129.5 (CH), 129.9 (C), 133.6 (C), 144.3
+
=
(C), 145.6 (C) and 165.1 (C N); m/z (Electrospray) 257 (M ,
cation)(100%), 121 (5); Found: M+ (cation), m/z 257.1282; Calc.
133.6 (C), 145.6 (C), 148.2 (C) and 165.1 (C N): the sample was
=
for C15H17O2N2; 257.1285.
N1 -Cyano-N2 -[2-(3,4-dihydroxyphenyl)ethyl]guanidine
spectroscopically almost identical to the bromide 8f and was used
without further analysis.
8k
(0.26 g, 45%), dark oil, after column chromatography [silica gel:
eluent CH2Cl2–CH3OH (4:1)]; nmax/cm-1 3333, 2476, 1626, 1524,
1440, 1348, 1283, 1203, 1112 and 973; dH (CD3OD) 2.72 (2H, t,
J 7.0 Hz, ArCH2), 3.37 (2H, t, J 7.0 Hz, CH2N), 6.55 (1H, d, J
8.0 Hz, aromatic 6H), 6.66 (1H, s, aromatic 2H), 6.69 (1H, d, J
8.0 Hz, aromatic 5H); dC (CD3OD) 34.4 (ArCH2), 43.4 (CH2N),
115.9 (CH), 116.3 (CH), 121.0 (CH), 130.4 (C), 143.5 (COH),
Oxidation of catechols 8
6-Methylamino-8,9-dihydro-5-thia-7-azabenzocycloheptene-2,3-
diol hydrochloride 12. The thiourea 8c (0.25 g, 1.1 mmol) in
MeOH–CHCl3 (1:9) (10 mL) was treated with 2,3-dichloro-5,6-
dicyanobenzoquinone (0.25 g, 1.1 mmol). The precipitate that
immediately formed was collected and dissolved in ethanolic
HCl (5.0 mL). The solution was concentrated to give a solid
that was recrystallised from ethanolic HCl and identified as the
hydrochloride 12 (0.17 g, 59%), colourless prisms, mp 227–228 ◦C;
=
144.7 (COH), 157.4 and 157.5 (C N and C∫N).
N-Acetyl-N¢-[2-(3,4-dihydroxyphenyl)ethyl]guanidine 8j.
Method 1. Under an N2 atmosphere, 1M BCl3 in CH2Cl2
(56.0 mL, 56 mmol)) was slowly added to a solution of compound
7l (1.74 g, 5.65 mmol) in CH2Cl2 (25.0 mL) and the mixture was
stirred at room temperature (72 h). The reaction mixture was then
quenched with H2O (50 mL) and the aqueous fraction extracted
with CH2Cl2 (3 ¥ 10 mL). The combined CH2Cl2 solutions were
dried (Na2SO4) and concentrated to give a solid that was identified
as compound 8j (1.30 g, 95%), identical with a sample prepared
by Method 2.
n
max/cm-1 3132, 1637, 1511, 1441, 1367, 1245, 1173, 875, 812 and
724; lmax (0.1 M phosphate buffer): pH 7.4, 287 (e 2800) nm; dH
(CD3OD) 2.78 (3H, s, NCH3), 3.07 (2H, t, J 6.0 Hz, ArCH2),
3.69 (2H, t, J 6.0 Hz, CH2N), 6.59 (1H, s, aromatic H) and 6.66
(1H, s, aromatic H); dC (CD3OD) 31.3 (ArCH2), 32.4 (CH3N), 46.8
(CH2N), 115.1 (C), 118.6 (CH), 119.8 (CH), 133.4 (C), 146.2 (C),
∑
+
=
149.3 (C) and 167.3 (C N); m/z (EI) 224 [M ](25%), 168 (100),
167 (80), 149 (30), 121 (30), Found: MH+, m/z (Electrospray)
225.0691; Calc. for C10H13N2O2S; 225.0692.
Method 2. Under an N2 atmosphere, 1M BCl3 in CH2Cl2
(4.0 mL, 4 mmol) was slowly added to a solution of compound
7j (0.5 g, 2 mmol) in CH2Cl2 (5.0 mL) and the mixture was
8-Hydroxy-2-trichloromethyl-1-aza-spiro[5.5]undeca-2,7,10-
trien-9-one hydrochloride 14. The amidine 8d (0.37 g, 1.1 mmol)
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in MeOH–CHCl3 (1:9) (10 mL) was treated with 2,3-dichloro-
5,6-dicyanobenzoquinone (0.25 g, 1.1 mmol). The solution was
concentrated and the mixture separated by chromatotron chro-
matography (silica gel: ethyl acetate). The crude product was re-
crystallised from ethanolic HCl and identified as the hydrochloride
14 (0.28 g, 76%), colourless rods, mp 144 ◦C (decomp.); nmax/cm-1
2940, 1670, 1637, 1570, 1437, 1351, 1259, 1185, 1087, 1044, 858,
831 and 653; lmax (0.1 M phosphate buffer): pH 7.4, 220 (e 7000),
244 (e 7415) and 320sh (e 1440); dH (CD3OD) 2.20 (2H, m, CCH2),
3.85 (2H, t, J 6.0 Hz, CH2N), 5.95 (1H, d, J 3.0 Hz, C7-H), 6.35
(1H, d, J 10.0 Hz, C10-H) and 7.00 (1H, dd, J 3.0 and 10.0 Hz,
C11-H); dC (CD3OD) 29.1 (CCH2), 38.5 (CH2N), 55.3 (spiro-C),
Acknowledgements
We thank the EPSRC National Mass Spectrometry Service Centre
for mass spectra, Mr J. Clews (Keele) for technical assistance,
Dr M. R. L. Stratford (Gray Institute, University of Oxford) for
HPLC analysis, and the Ministerio de Educacion y Ciencia, Spain
for an EPI Grant (to A. P.). The pulse radiolysis experiments were
carried out at the Free Radical Research Facility (Station 0.1) in
the Synchrotron Radiation Department of the STFC Daresbury
Laboratory, Warrington, UK. We thank Drs S. Navaratnam and
R. Edge for expert assistance in conducting these experiments.
87.9 (CCl3), 115.1 (C7), 128.8 (C10), 147.5 (C11), 149.4 (C8), 159.0
+
References
=
=
(C N) and 182.0 (C O); m/z (Electrospray) 395 [M - Cl] (10%),
161 (45%) and 135 (100%), Found: [M - Cl]+, m/z (Electrospray)
294.9802 (35Cl); Calc. for C10H10O2N2Cl3; 294.9802.
1 E. J. Land, C. A. Ramsden and P. A. Riley, Acc. Chem. Res., 2003, 36,
300.
2 E. J. Land, C. A. Ramsden and P. A. Riley, Arkivoc, 2007, (xi), 23.
3 C. J. Cooksey, P. J. Garratt, E. J. Land, S. Pavel, C. A. Ramsden, P. A.
Riley and N. P. M. Smit, J. Biol. Chem., 1997, 272, 26226.
4 C. J. Cooksey, P. J. Garratt, E. J. Land, C. A. Ramsden and P. A. Riley,
Biochem. J., 1998, 333, 685.
5 E. J. Land, C. A. Ramsden, P. A. Riley and G. Yoganathan, Pigment
Cell Res., 2003, 16, 397.
6 J. Borovansky, R. Edge, E. J. Land, S. Navaratnam, S. Pavel, C. A.
Ramsden, P. A. Riley and N. P. M. Smit, Pigment Cell Res., 2006, 19,
170.
Oxidative pulse radiolysis studies
The pulse radiolysis experiments were carried out on the 12 MeV
linear accelerator at the Daresbury Laboratory, using the Free
Radical Research Facility.26–28 This accelerator provides pulse
lengths between 0.2 and 2 msec with doses up to 30 Gy. Absorbed
∑
-
doses were determined from the transient (SCN)2 formation in
air-saturated KCNS solutions (10 mM) as described by Adams
and co-workers,29 but using the updated Ge value of 2.59 ¥
10-4 m2J-1 obtained by Buxton and Stuart,30 G being the radiation
7 E. J. Land, C. A. Ramsden, P. A. Riley and M. R. L. Stratford, Arkivoc,
2008, (ii), 258.
8 E. J. Land, C. A. Ramsden and P. A. Riley, J. Photochem. & Photobiol.
B: Biology, 2001, 64, 123.
9 J. Clews, C. J. Cooksey, P. J. Garratt, E. J. Land, C. A. Ramsden and
P. A. Riley, J. Chem. Soc., Perkin Trans. 1, 2000, 4306.
10 E. J. Land, C. A. Ramsden, P. A. Riley and G. Yoganathan, Tetrahedron,
2003, 59, 9547.
∑
chemical yield of (SCN)2 - and e its molar absorption coefficient at
∑
-
475 nm. Generation of the single-electron oxidising species Br2
was carried out by irradiating nitrous oxide-saturated buffered
aqueous solutions of 0.1 M KBr.5
11 E. J. Land, C. A. Ramsden, P. A. Riley and G. Yoganathan, Org. Biomol.
Chem., 2003, 1, 3120.
12 E. J. Land, C. A. Ramsden and P. A. Riley, Tetrahedron, 2006, 62, 4884.
13 E. J. Land, C. A. Ramsden and P. A. Riley, in ‘The Pigmentary System:
Physiology and Pathophysiology’ (eds.: J. J. Nordlund, R. E. Boissy,
V. J. Hearing, R. A. King, W. S. Oetting and J.-P. Ortonne ) 2nd Edition,
pp. 354–394, Blackwell Publishing Ltd, Oxford, 2006.
14 E. J. Land, C. A. Ramsden and P. A. Riley, Tohoku J. Exp. Med., 2007,
212, 341.
15 E. J. Land, C. A. Ramsden, P. A. Riley and M. R. L. Stratford,
Tohoku J. Exp. Med., 2008, 216, 231.
16 E. J. Land, A. Perona, C. A. Ramsden and P. A. Riley, Org. Biomol.
Chem., 2005, 3, 2387.
Enzymatic oxidation studies using oximetry and
spectrophotometry
These investigations were carried out using the apparatus previ-
ously described,3 consisting of a 1 cm light path quartz spectropho-
tometer cuvette of 3.65 mL capacity adapted to hold a Clark-
type polarimetric electrode (Yellow Springs Instruments, Yellow
Springs, Ohio, Model 5300) with the tip located orthogonal to the
light path of a Hewlett-Packard diode-array spectrophotometer
(Model 8452A). The reaction mixture was stirred by a vertically
mounted magnetic stirrer and the cell closed with a capillary
stopper through which additions were made. The e◦xperiments were
made in 0.1 M phosphate buffer (pH 7.4) at 24 C to which the
dopamine substrates were added. Stock solutions of the catechols
were made up freshly in glass distilled water. A standard solution
of tyrosinase, consisting of 300 Sigma units mL-1 made up in 0.1 M
phosphate buffer (pH 7.4), and stored at -20 ◦C, was used in the
experiments. Following enzyme addition the spectral changes were
followed at 30 second scanning intervals using the kinetic mode of
a software program (UV-Vis Chemstation A0801(66) from Agilent
Technologies, Hannover, Germany). The oximetric readings were
continuously recorded on a chart recorder and converted to
nanomoles of oxygen utilised. The oximeter was calibrated using
dithionite as previously described.3 Unless otherwise stated, the
amount of enzyme used was 30 Sigma units (100 ml), giving an
enzyme concentration in the reaction mixture of approximately
8.2 units mL-1.
17 Y. Niederstein and M. G. Peter, Liebigs Ann. Chem., 1989, 1189.
18 J. S. Buck, J. Am. Chem. Soc., 1934, 56, 1607.
19 A. Ishii, T. Kotani, Y. Nagaki, Y. Shibayama, Y. Toyomaki, N.
Okukado, K. Ienaga and K. Okamoto, J. Med. Chem., 1996, 39, 1924.
20 W. S. Saari, M. B. Freedman, J. R. Huff, S. W. King, A. W. Raab, S. J.
Bergstrand and E. L. Engelhardt, J. Med. Chem., 1978, 21, 1283.
21 C. I. Brodrick and W. F. Short, J. Chem. Soc., 1951, 1343.
22 K. Matsumoto and H. Rapoport, J. Org. Chem., 1968, 33, 552.
23 G. J. Durant, J. C. Emmett, C. R. Ganellin, P. D. Miles, M. E. Parsons,
H. D. Prain and G. R. White, J. Med. Chem., 1977, 20, 901.
24 M. R. Pitts, J. R. Harrison and C. J. Moody, J. Chem. Soc., Perkin
Trans. 1, 2001, 955.
25 J. H. Short, U. Biermacher, D. A. Dunnigan and T. D. Leth, J. Med.
Chem., 1963, 6, 275.
26 J. P. Keene, J. Sci. Instrum., 1964, 41, 493.
27 J. Butler, B. W. Hodgson, B. M. Hoey, E. J. Land, J. S. Lea, E. J. Lindley,
F. A. P. Rushton and A. J. Swallow, Radiat. Phys. Chem., 1989, 34, 633.
28 D. J. Holder, D. Allan, E. J. Land and S. Navaratnam, Proc. 8th Eur.
Particle Accelerator Conference, Paris, 2002, pp. 2804–2806.
29 G. E. Adams, J. W. Boag, J. Currant and B. D. Michael, in Pulse
Radiolysis, M. Ebert, J. P. Keene, A. J. Swallow and J. H. Baxendale,
eds., Academic Press, London, 1965, pp. 117–129.
30 G. V. Buxton and C. R. Stuart, J. Chem. Soc., Faraday Trans., 1995, 91,
279.
950 | Org. Biomol. Chem., 2009, 7, 944–950
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©