Stabilizing factors for vanadium(IV) in amavadin

Article abstract of DOI:10.1002/ejoc.200601053

Six secondary N-hydroxy amino acids (amavadin-based ligands) have been prepared through a strategy with nitrone reduction as its key step. Two of these amino acids are the new amavadin ligand analogues 4a and 4b containing either a phenyl or a benzyl grou

Full text of DOI:10.1002/ejoc.200601053

FULL PAPER
DOI: 10.1002/ejoc.200601053
Stabilizing Factors for Vanadium(IV) in Amavadin
Ton Hubregtse,[a] Ulf Hanefeld,[a] and Isabel W. C. E. Arends*[a]
Dedicated to R. A. Sheldon on the occasion of his 65th birthday
Keywords: Amavadin / Vanadium / Reductive hydroxylamination / Hydroxylamines / Nitrones / Mass spectrometry
Six secondary N-hydroxy amino acids (amavadin-based li-
gands) have been prepared through a strategy with nitrone
reduction as its key step. Two of these amino acids are the
new amavadin ligand analogues 4a and 4b containing either
a phenyl or a benzyl group in combination with a small back-
bone substituent. In addition, the monoester 5 of the amava-
din ligand as well as three N-alkylated N-hydroxy amino
acids 6 were prepared. Complexation studies with the new
tri- and bidentate ligands using HRMS revealed that only
amavadin and its two analogues with ligands 4a and 4b are
stable in aqueous and aerobic environments and that the
complexation of vanadium(IV) with ligands 5 and 6 does not
lead to air-stable vanadium(IV) non-oxo compounds. An
interesting spin-off of the synthetic work on nitrones is an
effective and easily applicable method for the synthesis of
racemic N-hydroxy amino acid esters.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
One of the most prominent examples of a vanadium-con-
taining natural compound is amavadin. In 1972, Bayer and
Kneifel isolated this compound from the mushroom Ama-
nita muscaria.[1] The chemical synthesis of amavadin[2,3] and
investigations of its electrochemistry,[4] catalytic proper-
ties[5–8] and structure[9,10] have been reported. The modifica-
tions to amavadin that have been reported mainly concern
the replacement of the natural vanadium. The natural li-
gand 1 or its simplified achiral analogue 2 has been em-
ployed to complex the early transition metals Ti, Zr, Nb,
Ta and Mo.[11] In one particular case, a ligand with ethyl
groups on the backbone was prepared (3) and its complex-
ation with V and Mo studied (Figure 1).
A remarkable feature of amavadin is that it is an air-
stable non-oxo vanadium(IV) complex in which the two N-
hydroxy groups act as η2 ligands. It has previously been
shown that the modified ligands 2 and 3 do not affect the
basic structure of amavadin,[12,13] but that the N-hydroxy
groups are essential for the formation of the non-oxo vana-
dium(IV) nucleus.[14] In order to establish what other fac-
tors are important, we focused our attention on the roles of
the carboxylate groups and the large substituents on the
backbone. With this in mind, we aimed to synthesize li-
gands 4–6 (Figure 2) in which one of the backbone methyl
groups of 1 is replaced by a phenyl or a benzyl group (4)
Figure 1. Amavadin analogues 1–3 reported in the literature.
or in which one of the coordinating groups of 1 is replaced
by a weaker coordinating centre (5) or by a non-coordinat-
ing alkyl group (6).
Figure 2. Amavadin-based ligands 4–6.
[a] Biocatalysis and Organic Chemistry, Delft University of Tech-
nology,
Thus, our two objectives with respect to ligand synthesis
were the introduction of new groups on the ligand back-
bone and the modification of one of the carboxylate groups
Julianalaan 136, 2628 BL Delft, The Netherlands
Fax: +31-152781415
E-mail: i.w.c.e.arends@tudelft.nl
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T. Hubregtse, U. Hanefeld, I. W. C. E. Arends
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of 1. Our synthetic strategy initially focused on the N-alky- comparable to those reported in the literature. However, ex-
lation of a primary hydroxylamine,[15,16] the method on tension of this method to α-bromo acids with an iPr, Ph or
which the traditional synthesis of the amavadin ligand is Bn substituent at the α position was not successful. There-
based.[2,3] As a second route, nitrones were used as interme- fore, we decided to focus on nitrones as intermediates of the
diates.[15,17–19] The synthesis of the monoester 5 and the N- desired amavadin-based ligands.[15,17–19] Our first approach
alkylated N-hydroxy amino acids 6 have no precedent in the comprised the addition of cyanide[34] to nitrones 7, which
literature, although a few reports exist on the synthesis of afforded the α-cyano secondary hydroxylamines
8
other N-alkylated N-hydroxy amino acids.[20–23] Studies on (Scheme 1). However, the hydrolysis of 8 to the correspond-
the interaction of vanadium with N-hydroxy amino com- ing amavadin ligand analogues was unsuccessful. Refluxing
pounds are limited to the inorganic hydroxylamine li- in concentrated HCl, the common procedure for the hydrol-
gand[24,25] (H2NOH) and its N-methyl[26] and N,N-dimethyl ysis of nitriles in the Strecker reaction, resulted in the com-
analogues.[27,28] There are no reports on vanadium com- plete degradation of 8. The application of milder condi-
plexation by N-hydroxy amino acids and derivatives tions[35] or the use of a platinum-based catalyst designed to
thereof.
convert nitriles into amides did not afford the desired prod-
ucts either.[36]
Results and Discussion
Synthesis of Amavadin-Based Ligands
N-Alkylation of Hydroxylamine and Cyanide Addition to
Our second approach to the use of nitrones as intermedi-
ates comprised nitrone reduction.[37–41] We prepared the
nitrones by condensation of an N-hydroxy amino acid ester
with an aldehyde or ketone and in situ reduction using
NaCN·BH3. This method may therefore be called a “re-
ductive hydroxylamination” process, stressing the analogy
with the well-known reductive amination to obtain second-
ary amines.
Nitrones
The N-alkylation of hydroxylamines is useful for the syn-
thesis of N-hydroxy α-amino acids and derivatives
thereof.[15,23,29–31] Although the double SN2 substitution of
hydroxylamine on two molecules of α-bromopropionic acid
did not yield enantiopure 1,[2,13,32,33] the application of tri-
flate as a leaving group did enable the enantioselective syn-
thesis of 1.[3] We attempted SN2 substitutions of α-triflate
esters with an iPr, Ph or Bn substituent at the α position,
but the desired derivatives of 1 were not formed. When re-
Synthesis of the Tridentate Ligands 4
The required nitrones could be generated quickly and ef-
peating the syntheses of 1 and 2 starting from the α-bromo fectively by using α-keto acids (Scheme 2). The first applica-
carboxylic acids, both compounds were isolated in yields tion of the method was the reaction of (racemic) N-hy-
Scheme 1. Attempts to obtain amavadin ligand analogues by the addition of cyanide to nitrones.
Scheme 2. Synthesis of tridentate ligands 4.
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Stabilizing Factors for Vanadium(IV) in Amavadin
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droxyalanine methyl ester (9) with pyruvic acid in order to Synthesis of the Bidentate Ligands 5 and 6
prepare the natural amavadin ligand 1. Subsequent addition
An important feature of the method depicted in
of NaCN·BH3 (instantaneous reaction) and evaporation of
the solvent yielded a white solid, which was then refluxed
in concentrated HCl to hydrolyze the ester. Further workup
was performed according to a procedure reported pre-
viously:[3] the crude ligand was precipitated with 1 equiv. of
[Zn(OAc)2] followed by isolation of the intermediate zinc
salt (49% yield) and release of the pure ligand 1 by ion-
exchange chromatography and lyophilization. The overall
Scheme 2 is that the monoester of the desired ligand is one
of the intermediates. When synthesizing ligand 1 by this
method, we could isolate 5 in a yield of 44% with a dr of
99:1 (NMR) by crystallization from the quenched reaction
mixture after the reduction step. To identify its stereochem-
istry, 5 was hydrolyzed to the amavadin ligand 1. Compari-
son of the NMR spectroscopic data with those of enantio-
pure and racemic 1 revealed 5 to be in the meso form
(Scheme 3).[42] Thus, meso-1 was prepared from rac-5 for
the first time.
The N-alkylated N-hydroxy amino acids 6 could also be
synthesized by NaCN·BH3 reduction of the appropriate
nitrones. The secondary hydroxylamines 12a and 12b thus
obtained could be isolated in high yields. Subsequent hy-
drolysis gave the corresponding N-benzyl-N-hydroxy amino
acids 6a and 6b (Scheme 4). Although the stable nitrones
7a and 7g could be isolated prior to their reduction, other
nitrones can often only be prepared in situ and therefore
require the “reductive hydroxylamination” sequence. Ac-
cordingly, we generated the nitrone of acetaldehyde and 9
and reduced this in situ to secondary hydroxylamine 12c.
Subsequent hydrolysis yielded N-ethyl-N-hydroxyalanine
(6c).
1
yield of 1 was 43%, and H NMR spectroscopy showed a
slight preference for the formation of the meso isomer (de ≈
20%). When starting from racemic N-hydroxyphenylglycine
methyl ester (10) and pyruvic acid, the acid-catalyzed ester
hydrolysis caused complete degradation of the product
(smell of benzaldehyde), but on switching to alkaline condi-
tions compound 4a was isolated in 47% yield as a mixture
of two diastereomers in a ratio of 1:5. Compound 4a was
also isolated when the reaction sequence was performed
with (racemic) N-hydroxyalanine methyl ester (9) and phen-
ylglyoxylic acid. A benzyl group could be introduced at the
ligand backbone by starting with N-hydroxyphenylalanine
methyl ester (11) and glyoxylic acid. After hydrolysis of the
ester under reflux in 1  HCl, the desired product 4b was
isolated in 41% yield. The use of optically pure 11 did not
result in optically pure 4b. Racemization of the chiral centre
is likely to take place in the nitrone intermediate, although
The reductive hydroxylamination method strongly de-
refluxing in hydrochloric acid could also induce such a race- pends on the availability of the appropriate hydroxylamine.
mization.
We prepared the racemic N-hydroxy amino acid esters 9
Scheme 3. Synthesis of bidentate ligand 5 and meso-1.
Scheme 4. Synthesis of bidentate ligands 6.
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T. Hubregtse, U. Hanefeld, I. W. C. E. Arends
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and 10 by N-alkylation of (Z)-benzaldoxime[43,44] and Vanadium Complexation by Bidentate Ligands 5 and 6
enantiopure 11 by indirect N-oxidation of the enantiopure
In compound 5 one of the carboxylate groups of ligand
1 is protected as the methyl ester; it is thus an ideal com-
pound to study whether all the hydroxy groups in 1 need to
be deprotonated in order to complex vanadium(IV) as a
non-oxo complex. The complexation was performed by
mixing 5 and [VO(acac)2] in a 2:1 molar ratio in methanol
phenylalanine methyl ester.[45,46] At a later stage, we could
improve the first procedure by using the cheap and com-
mercially available acetaldoxime instead of (Z)-benzaldox-
ime, which resulted in a convenient three-step one-pot pro-
cedure towards 9 and 10.
after which a brown solid was isolated. The molecular mass
(ESI HRMS) corresponded to a non-oxo vanadium com-
Vanadium Complexation by Amavadin-Based Ligands
plex with two ligands of 5. This was tentatively assigned to
a formal dimethyl ester of amavadin[48] such as 14 with a
Vanadium Complexation by Tridentate Ligands 4
hexacoordinate vanadium (Scheme 6). No significant
changes were seen in the MS when larger ligand/vanadium
The in situ complexation of ligand analogues 4a and 4b
to [VO(acac)2] in methanol gave solutions with an intense
blue colour reminiscent of the blue solutions of amavadin.
HRMS (ESI) confirmed the presence of the amavadin ana-
ratios were used. The transient intermediate 14 was oxid-
ized to complex 15,[49] which can also be obtained directly
by mixing the VV precursor [VO(OPr)3] with 5. However,
logues 13a and 13b (Scheme 5). Both complexes were stable
15 was not stable either; it slowly degraded and was there-
in air and no oxidation occurred. At the start of the project,
fore not fully characterized. 51V NMR spectroscopy of the
we anticipated that large R groups would improve the solu-
unstable product gave a large resonance at δ = –651 ppm,
bility of these complexes in organic solvents, but this ap-
accompanied by smaller resonances at δ = –605, –623 and
pears not to be the case. The VV analogues of 13a and 13b
1
–704 ppm. Similarly, the H and 13C NMR spectra showed
were isolated as the PPh4 salts after oxidation with
several species but did not allow the exact identification of
[(NH4)2Ce(NO3)6][3] and 1H, 13C and 51V NMR spec-
the complexes. Evidently, 5 does not form an amavadin-like
troscopy of these complexes confirmed their great similari-
complex with vanadium(IV).
ties with amavadin. The complexes with ligands 1, 2, 3, 4a
The complexation of 6a–6c with VIV and VV nuclei was
and 4b all have vanadium shifts between δ = –230 and
performed by mixing the ligand with either [VO(acac)2] or
1
–290 ppm, while the distinct resonances observed in the H
[VO(OPr)3] in different molar ratios (2:1–4:1) in methanol.
and 13C NMR spectra could be assigned to the ligands un-
Given the results described above, it is not surprising that
the vanadium was immediately oxidized and a complex
ambiguously.[47] This demonstrates that ligands 4a and 4b
behave similarly to 1, 2 and 3 and that the presence of larger
mixture was obtained. Thus, ligands 6 are not able to stabi-
backbone substituents still leads to an air-stable non-oxo
lize the VIV nucleus either.
vanadium(IV) nucleus.
Conclusions
The aim of this study on the non-oxo vanadium(IV) nu-
cleus in amavadin was to investigate if large ligand back-
bone substituents can be accommodated and to what extent
the carboxylate groups are essential. The complexation
studies showed that increasing the size of the methyl back-
bone substituents with a phenyl or a benzyl group still gives
non-oxo VIV complexes that are analogous to amavadin. In
contrast, the protection of one carboxylate group per ligand
does not give a stable non-oxo vanadium(IV) compound.
Scheme 5. Formation of the amavadin analogues 13a and 13b from
ligands 4a and 4b.
Scheme 6. Schematic representation of the tentative VIV and VV complexes formed from ligand 5.
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Stabilizing Factors for Vanadium(IV) in Amavadin
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started and the pH of the reaction mixture was kept at 7.2 by the
controlled addition of a 4  NaOH solution with the temperature
being kept at 30–35 °C. When the reaction was complete, the pH
of the mixture was lowered to 4.4 with dilute HCl. A solution of
[Zn(OAc)2·2H2O] in 40 mL of water was added to precipitate the
ligand. The milk-white suspension was washed with water (a cycle
of centrifugation, decanting the supernatant and suspending the
residue in water was performed three times) and dried in vacuo,
after which the zinc complex was obtained as a fine white powder.
It was then dissolved in 20 mL of 6  HCl (or the minimum
Instead, the initially obtained complex is slowly oxidized in
air. Complete absence of one of the carboxylic acid groups
results in immediate oxidation. In conclusion, these results
show that in addition to the secondary N-hydroxy group,
two unprotected carboxylate groups are also necessary to
obtain an air-stable non-oxo vanadium(IV) nucleus in ama-
vadin analogues.
We have also demonstrated that reductive hydroxylamin-
ation is a versatile method for preparing secondary N-hy-
droxy amino acid ligands. A one-pot three-step non-stereo- amount that was necessary for complete dissolution) and the
solution was added to a column of H+-loaded DOWEX-50
selective synthesis has been developed to prepare the natu-
ion-exchange resin that had previously been washed with
ral ligand and two of its derivatives with a phenyl or a ben-
water until a neutral pH was obtained. Elution with water first gave
zyl backbone substituent. Moreover, the amavadin ligand
an eluate with a pH Ͻ 1, then the pH was increased to 3.4, and
monoester and N-alkylated N-hydroxy amino acids are also
when the pH was decreased again the eluate was collected.
accessible by this strategy.
After concentration and lyophilization, a white powder was ob-
tained.
Ligand 1: From (R)-2-bromopropionate (19.122 g, 125 mmol), hy-
droxylamine hydrochloride (3.474 g, 50 mmol) and [Zn(OAc)2·
Experimental Section
Methods and Materials: Column chromatography was carried out
with silica gel 0.060–0.200 mm, pore diameter approx. 6 nm (Fluka
silica gel 60). TLC was performed on 0.20 mm silica gel aluminium
sheets (Merck silica gel 60 F254). Elution was carried out with mix-
tures of petroleum ether (PE; boiling range 40–65 °C) and ethyl
acetate (EtOAc). The plates were developed by dipping them into
either a ninhydrin bath (1.5 g of ninhydrin in 100 mL of ethanol)
in which the hydroxylamines gave pale red to yellow spots or an
iodine/silica gel bath (prepared as a mixture of 5 g of iodine and
300 g of silica gel). Ion-exchange chromatography was carried out
with DOWEX 50WX8-200 resin. 1H and 13C NMR spectra were
recorded with a Varian VXR400S (400 MHz) or a Varian Unity
Inova 300 (300 MHz) instrument. Chemical shifts of the 1H and
13C nuclei are expressed in ppm (δ) relative to tetramethylsilane (in
CDCl3 or DMSO) or relative to tBuOH (in H2O). Chemical shifts
of the 51V nuclei are expressed in ppm (δ) relative to the external
reference VOCl3. Coupling constants (J) are expressed in Hz. The
abbreviations used are as follows: s (singlet), d (doublet), t (triplet)
and q (quartet). Low-resolution mass spectrometry was performed
with a Micromass Quattro LC-MS spectrometer (ESI). High-reso-
lution mass spectrometry was performed with a Thermo LTQ Or-
bitrap spectrometer; resolution 50000; mass accuracy Ͻ 2 ppm;
samples were diluted with a 1:1 methanol/water solution; injection
volume: 10 µL; flow rate: 50 µL/min; eluent: 1:1 methanol/water.
Optical rotations were obtained using a PerkinElmer 241 polarime-
ter. Melting points were measured with a Büchi 510 apparatus and
are uncorrected. The CHN elemental analyses were performed at
the Dr. Verwey Chemical Laboratory in Rotterdam, The Nether-
lands. The α-bromo acids,[50,51] α-hydroxy acid esters,[52,53] N-hy-
droxyglycine ethyl ester[54] and (R)-N-phenylalanine methyl ester
(11)[46] were prepared according to literature procedures. We have
previously reported the synthesis of 13b and the NMR spectro-
scopic data of its PPh4 salt after oxidation to VV.[42] All other chem-
icals and dry solvents were obtained from Aldrich or Merck and
were used as received. Other solvents were purchased from Baker.
Petroleum ether (PE) with a boiling range of 40–65 °C was used.
Ultrapure water (MilliQ) was used.
2H2O] (10.975 g, 50 mmol), the zinc complex (6.841 g, 28.44 mmol,
57%) was obtained. The reaction was carried out in water (110 mL)
and was complete after 40 h. Ion-exchange chromatography yielded
4.312 g (24.34 mmol, 49% overall yield) of 1. M.p. 137–138 °C.
[α]2D0 = +21.3 (c = 1, H2O). MS: m/z = 178.3 (ES+, [M + 1]), 176.3
(ES–, [M – 1]). NMR spectroscopy showed that the ratio of meso-
1/1 was approximately 1:4 (de = 60%). The 1H and 13C NMR spec-
tra of 1 are identical to those of 1 described in the literature.[3] The
1H and 13C NMR data for meso-1 are identical to those of meso-1
described below.
Ligand 2: From bromoacetic acid (34.738 g, 250 mmol), hydroxyl-
amine hydrochloride (6.949 g, 100 mmol) and [Zn(OAc)2·2H2O]
(21.950 g, 100 mmol), the zinc complex (11.600 g, 54.6 mmol, 55%)
was obtained. The reaction was carried out in water (150 mL) at
room temp. instead of 30–35 °C and was complete after 24 h. The
free ligand was released batchwise; ligand 2 (1.180 g, 7.91 mmol,
79%) was obtained as a white solid (43% starting from bromoace-
tic acid and hydroxylamine hydrochloride) from the zinc complex
(2.125 g, 10 mmol). M.p. 137 °C. 1H NMR (D2O, 300 MHz,
25 °C): δ = 3.80 (s, 4 H, 2 ϫ NCH2) ppm. 13C NMR (D2O,
75 MHz, 25 °C): δ = 173.33 (2 C, 2 ϫ COOH), 61.49 (2 C, 2 ϫ
NCH2) ppm. MS: m/z = 150.3 (ES+, [M + 1]), 148.3 (ES–, [M –
1]). C4H7NO5 (149.10): calcd. C 32.22, H 4.73, N 9.39; found C
32.30, H 4.59, N 9.28.
Synthesis of Nitrones 7: The nitrones 7a, 7b and 7g were synthesized
and isolated according to literature procedures.[43,54]
Nitrone 7a: Yield: 2.14 g (48%), white crystals. M.p. 112 °C. 1H
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.28 (t, J = 7.1 Hz, 3 H,
CH2CH3), 1.78 (d, J = 7.0 Hz, 3 H, CHCH3), 4.26 (m, J = 7.1 Hz,
2 H, CH2CH3), 4.75 (q, J = 7.0 Hz, 1 H, CHCH3), 7.42 (m, 3 H,
C6H5), 7.48 (s, 1 H, N=CH), 8.25 (m, 2 H, C6H5) ppm. 13C NMR
(CDCl3, 75 MHz, 25 °C): δ = 167.96 (1 C, COOEt), 134.95 (1 C,
N=CH), 130.72, 130.27, 128.86, 128.54 (6 C, C6H5), 73.30 (1 C,
CHCH3), 62.26 (1 C, OCH2CH3), 15.61 (1 C, OCH2CH3), 14.03 (1
C, CHCH3) ppm.
Synthesis of Amavadin Ligands 1 and 2 by Substitution of α-Bromo Nitrone 7b: Yield: 643 mg (42%), white crystals. M.p. 114 °C. 1H
Acids: The synthesis was performed according to a literature pro-
cedure with modifications.[33] A pH-controlled automatic burette
was charged with a 4  NaOH solution. With the aid of this setup,
a stirred solution of hydroxylamine hydrochloride and the α-bromo
acid in water was brought to pH = 7.2. At this point, the reaction
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.30 (t, J = 7.1 Hz, 3 H,
CH2CH3), 4.33 (m, 2 H, CH2CH3), 5.82 (s, 1 H, CHC6H5), 7.39
(m, 8 H, 2 ϫ C6H5), 7.49 (s, 1 H, N=CH), 8.17 (m, 2 H, C6H5)
ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 167.10 (1 C, CO-
OEt), 135.76 (1 C, N=CH), 131.12, 130.98, 130.25, 129.85, 129.71,
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T. Hubregtse, U. Hanefeld, I. W. C. E. Arends
129.43, 129.18, 128.44 (12 C, C6H5), 81.71 (1 C, CHC6H5), 62.53 dried with Na2SO4 and traces of toluene needed to be removed by
FULL PAPER
(1 C, OCH2CH3), 14.03 (1 C, OCH2CH3) ppm.
stripping with dichloromethane.
Nitrone 7g: Yield: 3.45 g (60%), white crystals. M.p. 30 °C. 1H
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.31 (t, J = 7.2 Hz, 3 H,
CH2CH3), 4.28 (q, J = 7.2 Hz, 2 H, CH2CH3), 4.71 (s, 2 H, NCH2),
7.42 (m, 3 H, C6H5), 7.43 (s, 1 H, N=CH), 8.24 (m, 2 H, C6H5)
ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 165.70 (1 C, CO-
OEt), 137.13 (1 C, N=CH), 130.96, 130.08, 128.84, 128.54 (6 C,
C6H5), 68.14 (1 C, NCH2), 62.34 (1 C, OCH2CH3), 14.07 (1 C,
OCH2CH3) ppm.
Hydroxylamine 8a: From 7a (1.921 g, 8.68 mmol) and a 1 
AlEt2CN toluene solution (9.12 mL, 9.12 mmol), hydroxylamine 8a
(2.069 g, 8.33 mmol, 96%) was obtained as a yellow oil. H NMR
1
(CDCl3, 300 MHz, 25 °C): δ = 1.29, 1.30 (t, J = 7.1 Hz, 3 H,
CH2CH3), 1.45, 1.47 (d, J = 7.0 Hz, 3 H, CHCH3), 3.73, 3.74 (q,
J = 7.1 Hz, 1 H, CHCH3), 4.20, 4.22 (q, J = 7.0 Hz, 2 H, CH2CH3),
5.12, 5.13 (s, 1 H, CHC6H5), 5.63, 5.68 (s, 1 H, NOH), 7.42 (m, 3
H, C6H5), 7.56 (m, 2 H, C6H5) ppm. 13C NMR (CDCl3, 75 MHz,
25 °C): δ = 172.04, 171.91 (1 C, COOEt), 132.54, 132.31, 129.47,
129.38, 128.97, 128.91, 128.77, 128.69, (6 C, C6H5), 116.40, 115.73
(1 C, NϵC), 62.96, 62.29, 61.40, 61.29, 61.19, 59.84 (3 C, CHCH3,
OCH2CH3, CHC6H5), 15.25, 14.73 (1 C, CHCH3, OCH2CH3),
14.12 (1 C, CHCH3, OCH2CH3) ppm. C13H16N2O3 (248.28): calcd.
C 62.89, H 6.50, N 11.28; found C 62.79, H 6.32, N 11.38.
Nitrones 7c–7f were generated in situ prior to further reaction and
were not subjected to further analysis. Their formation from N-
hydroxy amino acid esters could be monitored by TLC as they
generally have a very strong UV absorption and Rf values of
Ͻ 0.05, whereas the N-hydroxy amino acid esters are readily visual-
ized with iodine and ninhydrin at higher Rf values. The exact pro-
cedures for the in situ generation of the nitrones is described in
the procedures of the respective reductive hydroxylamination (see
below).
Hydroxylamine 8b: From 7b (629 mg, 2.22 mmol) and a 1 
AlEt2CN toluene solution (2.33 mL, 2.33 mmol), hydroxylamine 8b
(659 mg, 2.12 mmol, 96%) was obtained as white crystals. M.p.
1
142–143 °C. H NMR (CDCl3, 300 MHz, 25 °C): δ = 1.17 (t, J =
Synthesis of N-Hydroxy Amino Acid Esters 9 and 10 by the “Acet-
aldoxime Route”: Sodium was dissolved in dry methanol (50 mL)
under nitrogen. After addition of the acetaldoxime and the appro-
priate α-bromo methyl ester, the mixture was stirred overnight. By
then the pH of the solution had decreased to almost neutral, as
indicated by wet pH paper. After addition of the hydroxylamine
hydrochloride, TLC (PE/EtOAc, 1:1) showed complete and imme-
diate conversion of the nitrone. The solvent was evaporated, the
residue was dissolved in 1  HCl (30 mL) and was then washed
with diethyl ether (5ϫ30 mL). Saturated NaHCO3 solution was
added to the aqueous layer and the product was then extracted
with diethyl ether until TLC could not detect any further product
in the aqueous layer. The combined layers were dried with Na2SO4,
after which the N-hydroxy amino acid methyl ester was isolated.
7.1 Hz, 3 H, CH2CH3), 4.10 (m, 2 H, CH2CH3), 4.59, 4.80 (2 ϫ s,
2 H, CHC6H5), 5.81 (s, 1 H, NOH), 7.36–7.47 (m, 8 H, 2 ϫ C6H5),
7.59–7.62 (m, 2 H, 2 ϫ C6H5) ppm. 13C NMR (CDCl3, 75 MHz,
25 °C): δ = 170.09 (1 C, COOEt), 133.57, 132.80, 130.01, 129.72,
129.22, 128.84, 128.53 (12 C, 2 ϫ C6H5), 115.05 (1 C, NϵC), 74.30,
(1 C, NϵCCH), 61.78, 59.41 (2 C, OCH2CH3, CHC6H5), 14.13 (1
C, OCH2CH3) ppm. C18H18N2O3 (310.35): calcd. C 69.66, H 5.85,
N 9.03; found C 69.57, H 5.71, N 8.81.
Synthesis of α-Cyano Secondary Hydroxylamine 8c: Sodium
(0.644 g, 28.00 mmol) was dissolved in dry methanol (50 mL) under
nitrogen. After the addition of acetaldoxime (1.654 g, 28.00 mmol)
and methyl 2-bromopropionate (4.911 g, 29.41 mmol), the mixture
was stirred overnight. By then the pH of the solution had decreased
to almost neutral, as indicated by wet pH paper. Because of the
instability of the nitrone, subsequent steps were performed quickly.
The methanol was evaporated and CH2Cl2 was added to precipitate
the NaBr, which was then removed by filtration. Concentration
yielded a mixture of the nitrone and the oxime ether as a yellow
oil. Petroleum ether was added to form a biphasic system, and the
oxime ether was separated from the nitrone by removing the
colourless upper layer using a pipette. The remaining yellow oil was
dissolved in CH2Cl2 (50 mL) under nitrogen, and a 1  Et2AlCN
solution in toluene (8 mL, 8 mmol) was added in one portion. TLC
(PE/EtOAc, 8:2) showed a complete and immediate conversion of
the nitrone (Rf Ͻ 0.05; strong UV absorption) and formation of a
new product (Rf = 0.4; ninhydrin). After the addition of saturated
NaHCO3 (50 mL) and 10 min of fast stirring (viscous suspension),
the aqueous layer was extracted with CH2Cl2 (3ϫ30 mL). The col-
lected extracts were dried with Na2SO4 and traces of toluene and
the oxime ether were removed by stripping with CH2Cl2. The re-
sulting yellow oil became a solid at –18 °C and after washing with a
mixture of petroleum ether/diethyl ether hydroxylamine 8c (1.363 g,
7.92 mmol, 28%) was obtained as white crystals. Recrystallization
gave 0.605 g (3.51 mmol, 13%) of 8c as a single diastereoisomer.
M.p. 107–108 °C. 1H NMR (CDCl3, 300 MHz): δ = 1.35 (d, J =
7.0 Hz, 3 H, NCHCH3), 1.61 (d, J = 6.9 Hz, 3 H, NCHCH3), 3.76
(q, J = 6.9 Hz, 1 H, NCHCH3), 3.77 (s, 3 H, OCH3), 3.94 (q, J =
7.0 Hz, 1 H, NCHCH3), 6.23 (s, 1 H, NOH) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 172.84 (C, COOEt), 116.85 (1 C, NϵC),
64.34 (1 C, NCHCH3), 52.36 (1 C, OCH3), 49.99 (1 C, NCHCH3),
17.12, 15.03 (2 C, NCHCH3) ppm. C7H12N2O3 (172.18): calcd. C
48.83, H 7.02, N 16.27; found C 48.87, H 6.99, N 16.18.
N-Hydroxy Amino Acid Ester 9: Because of the high solubility of
the product in water, the extraction with diethyl ether after the
addition of NaHCO3 was difficult. Instead, the water was evapo-
rated, the residue suspended in diethyl ether and dried with
Na2SO4.
From
methyl
2-bromopropionate
(19.674 g,
117.80 mmol), acetaldoxime (6.627 g, 112.19 mmol), sodium
(2.579 g, 112.19 mmol) and hydroxylamine hydrochloride (3.898 g,
56.10 mmol), compound 9 (4.174 g, 35.04 mmol, 31%) was ob-
tained as white crystals. The analytic data match the values re-
ported in the literature.[46] M.p. 29–30 °C. 1H NMR (H2O,
300 MHz, 25 °C): δ = 1.27 (d, J = 7.2 Hz, 3 H, CHCH3), 3.75 (q,
J = 7.2 Hz, 1 H, CHCH3), 3.77 (s, 3 H, OCH3), 6.04 (br., 2 H,
NHOH) ppm.
N-Hydroxy Amino Acid Ester 10: From methyl 2-bromo-2-phenyl-
acetate (5.249 g, 22.91 mmol), acetaldoxime (1.301 g, 22.03 mmol),
sodium (0.507 g, 22.03 mmol) and hydroxylamine hydrochloride
(1.531 g, 22.03 mmol), compound 10 (1.275 g, 7.04 mmol, 32%)
was obtained as a white solid. The analytic data match the values
reported in the literature.[55] M.p. 71 °C. 1H NMR (H2O, 300 MHz,
25 °C): δ = 3.75 (s, 3 H, OCH3), 4.78 (s, 1 H, CHC6H5), 5.89 (br.,
2 H, NHOH), 7.34 (s, 5 H, C6H5) ppm.
Synthesis of α-Cyano Secondary Hydroxylamines 8a and 8b: A 1 
Et2AlCN solution was added to a solution of the nitrone in dry
CH2Cl2 (40 mL) under nitrogen. The mixture was stirred for 10 min
after which TLC (PE/EtOAc, 8:2) showed full conversion of the
nitrone. After the addition of saturated NaHCO3 (50 mL) and
10 min of fast stirring (viscous suspension), the aqueous layer was
extracted with CH2Cl2 (3ϫ30 mL). The collected extracts were
2418
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Stabilizing Factors for Vanadium(IV) in Amavadin
FULL PAPER
Synthesis of α-Cyano Secondary Hydroxylamine 8d: The same pro-
cedure as used for the synthesis of 8c was applied. By allowing
in methanol until TLC (PE/EtOAc, 1:1) showed full conversion of
the N-hydroxy amino acid ester. After the addition of NaCN·BH3,
sodium (0.841 g, 36.60 mmol), methyl bromoacetate (5.821 g, TLC showed a complete and immediate conversion of the nitrone
38.05 mmol) and acetone oxime (2.649 g, 36.24 mmol) to react, the
formation of the intermediate nitrone was complete within 2 h. Af-
ter its isolation as a yellow oil and the immediate addition of a 1 
Et2AlCN solution in toluene (4.5 mL, 4.5 mmol), hydroxylamine
(Rf Ͻ 0.05; strong UV absorption). Evaporation of the methanol
yielded a white solid, which was then hydrolyzed under acidic (1
and 4b) or basic (4a) conditions. [Zn(OAc)2·2H2O] was added to
precipitate the ligand, and the resulting milk-white suspension was
washed with water (a cycle of centrifugation, decanting the super-
8d (319 mg, 1.85 mmol, 5%) was isolated as white crystals. M.p.
1
95 °C. H NMR (CDCl3, 300 MHz): δ = 1.56 (s, 6 H, NC(CH3)2), natant and suspending the residue in water was performed three
3.68 (s, 2 H, NCH2), 3.78 (s, 3 H, OCH3), 6.48 (s, 1 H, NOH) ppm. times) and dried in vacuo. It was then dissolved in 3  HCl (using
13C NMR (CDCl3, 75 MHz): δ = 170.34 (C, COOMe), 119.90 (1
the minimum amount that was necessary for complete dissolution)
C, NϵC), 59.53, 56.19 [2 C, NC(CH3)2, NCH2], 52.51 (1 C, OCH3), and added to a column of H+-loaded DOWEX-50 ion-exchange
25.41 [2 C, N(CH3)2] ppm. C7H12N2O3 (172.18): calcd. C 48.83, H
7.02, N 16.27; found C 48.91, H 6.99, N 16.12.
resin that had previously been washed with water until neutral pH.
Elution with water first gave an eluate with a pH Ͻ 1, after which
the pH increased again. When the pH started to decrease for the
second time, the eluate was collected. After concentration, the solu-
tion was either lyophilized (1) or concentrated to dryness and
stripped with CH2Cl2 (4a and 4b; both are slightly soluble in
CH2Cl2).
Synthesis of α-Cyano Secondary Hydroxylamines 8e and 8f: The
N-hydroxyglycine ethyl ester and the aldehyde were condensed in
CH2Cl2 (20 mL) under nitrogen with the aid of Na2SO4 as drying
agent. When TLC (PE/EtOAc, 1:1) showed full conversion of the
ester to the intermediate nitrone (Rf Ͻ 0.05; strong UV absorp-
tion), a 1  Et2AlCN solution in toluene was added in one portion.
TLC showed a complete and immediate conversion of the nitrone
and formation of a new product. After the addition of saturated
NaHCO3 (50 mL) and 10 min of fast stirring (viscous suspension),
the aqueous layer was extracted with CH2Cl2 ( 3ϫ30 mL). The
collected extracts were dried with Na2SO4 and traces of toluene
were removed by stripping with petroleum ether. The resulting yel-
low oil became a solid at –18 °C, which was purified by recrystalli-
zation from a mixture of petroleum ether/diethyl ether.
Ligand 1: The synthesis was performed by using 9 (357 mg,
3.00 mmol), pyruvic acid (280 mg, 3.18 mmol), NaCN·BH3
(211 mg, 3.36 mmol) and [Zn(OAc)2·2H2O] (659 mg, 3.00 mmol).
The hydrolysis step was performed by stirring the mixture in 10 
HCl (11 mL) at 110 °C for 6 h. After evaporation of the aqueous
HCl, the mixture was dissolved in water and the pH was adjusted
to 4.4 with aqueous NH4OH before adding [Zn(OAc)2·2H2O]. The
zinc complex was obtained as a fine white powder in a yield of
350 mg (1.46 mmol, 49%), from which the free ligand 1 was ob-
tained in an overall yield of 229 mg (1.29 mmol, 43%). M.p. 120 °C
(decomp.). 1H and 13C NMR show the presence of two dia-
stereomers in a ratio of A/B = 6:4, with A being the meso com-
pound. In all other respects, the 1H NMR, 13C NMR, and mass
spectra are identical to those of 1 obtained by substitution on 2-
bromopropionate as described above.
Hydroxylamine 8e: From N-hydroxyglycine ethyl ester (0.350 g,
2.94 mmol), acetaldehyde (0.142 g, 3.23 mmol) and a 1  Et2AlCN
solution in toluene (3.4 mL, 3.4 mmol), hydroxylamine 8e (430 mg,
2.50 mmol, 85%) was obtained as white crystals. During the con-
densation, the mixture was cooled with ice to prevent evaporation
of the acetaldehyde. Initially, the condensation reaction proceeded
slowly, but it could be promoted by the addition of one drop of
Ligand 4a (Starting from 10): The synthesis was performed by using
10 (336 mg, 1.85 mmol), pyruvic acid (171 mg, 1.94 mmol),
NaCN·BH3 (128 mg, 2.04 mmol) and [Zn(OAc)2·2H2O] (407 mg,
1.85 mmol). The hydrolysis step was performed by stirring in 0.5 
NaOH at room temp. for 5 h. The pH was then adjusted to 1.80
with aqueous HCl after which the mixture was concentrated. Pre-
cipitation with [Zn(OAc)2·2H2O] yielded 265 mg (0.88 mmol, 47%)
of the zinc complex as a fine white powder, from which 4a was
obtained as a white solid in an overall yield of 172 mg (0.72 mmol,
47%). NMR spectra showed the presence of 4a as two isomers in
a ratio of A/B = 1:5. The assignment of the 1H and 13C resonances
was based on this ratio. M.p. 115 °C (decomp.). 1H NMR (DMSO,
300 MHz, 25 °C): δ = 1.13B, 1.35A (d, J = 6.9 Hz, 3 H, CHCH3),
3.09B, 3.24A (q, J = 6.9 Hz, 1 H, CHCH3), 4.54A, 4.99B (s, 1 H,
CHC6H5), 7.33–7.50A+B (m, 5 H, C6H5) ppm. 13C NMR (DMSO,
75 MHz, 25 °C): δ = 173.12B, 172.83B (2 C, 2 ϫ COOH), 136.84B,
129.47B, 129.21B, 129.09B (6 C, C6H5), 73.23B (1 C, CHC6H5),
59.11B (1 C, CHCH3), 16.66B (1 C, CHCH3) ppm. HRMS (ESI):
calcd. for C11H14NO5 [M + H]+ 240.0867; found 240.0863. It was
possible to obtain 4a as one diastereoisomer analogously to the
synthesis of meso-1: one diastereoisomer of the intermediate mono-
ester was selectively crystallized and then hydrolyzed to the diacid.
The NMR spectra correspond to isomer B of 4a. This dia-
stereomerically pure sample of 4a was used for the complexation
to vanadium.
1
dilute HCl and was complete after 4 h. M.p. 35–36 °C. H NMR
(CDCl3, 300 MHz, 25 °C): δ = 1.30 (t, J = 7.2 Hz, 3 H, CH2CH3),
1.56 (d, J = 7.2 Hz, 3 H, CHCH3), 3.55 (J = 16.5 Hz, 1 H, NCH2),
3.75 (J = 16.5 Hz, 1 H, NCH2), 3.90 (q, J = 7.2 Hz, 1 H, CHCH3),
4.23 (q, J = 7.2 Hz, 2 H, CH2CH3), 6.73 (s, 1 H, NOH) ppm. 13C
NMR (CDCl3, 75 MHz, 25 °C): δ = 169.48 (1 C, COOEt), 117.39
(1 C, NϵC), 61.69 (1 C, OCH2CH3), 58.70, 54.30 (2 C, NCH2,
NCHCN), 17.02 (1 C, CHCH3), 14.34 (1 C, OCH2CH3) ppm.
C7H12N2O3 (172.18): calcd. C 48.83, H 7.02, N 16.27; found C
48.94, H 6.99, N 16.02.
Hydroxylamine 8f: From N-hydroxyglycine ethyl ester (0.119 g,
1.00 mmol), freshly distilled valeraldehyde (0.095 g, 1.10 mmol)
and a 1  Et2AlCN solution in toluene (1.15 mL, 1.15 mmol), hy-
droxylamine 8f (173 mg, 0.81 mmol, 81%) was obtained as white
1
crystals. The condensation was complete after 1 h. M.p. 87 °C. H
NMR (CDCl3, 300 MHz, 25 °C): δ = 0.96, 0.99 [2 ϫ d, J = 7.1 Hz,
6 H, CH(CH3)2], 1.30 (t, J = 7.2 Hz, 3 H, CH2CH3), 1.69–1.97 [m,
3 H, NCCH2, CH(CH3)2], 3.55 (d, J = 16.5 Hz, 1 H, NCH2), 3.76
(d, J = 16.5 Hz, 1 H, NCH2), 3.82 (dd, J = 7.1 Hz, J = 7.2 Hz, 1
H, NCH), 4.23 (q, J = 7.2 Hz, 2 H, OCH2), 6.40 (s, 1 H, NOH)
ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 169.27 (1 C, CO-
OEt), 116.70 (1 C, NϵC), 61.40 (1 C, OCH2CH3), 58.60, 57.76 (2
C, NCH2, NCHCN), 39.48, 24.84, 22.52, 21.91 [4 C, CH2CH-
(CH3)2], 14.13 (1 C, OCH2CH3) ppm. C10H18N2O3 (214.26): calcd.
C 56.06, H 8.47, N 13.07; found C 56.04, H 8.44, N 12.95.
Ligand 4a (Starting from 9): The procedure was identical to the
Synthesis of Ligands 1, 4a and 4b by Reductive Hydroxylamination: one used for the synthesis of 4a starting from 10, but during the
The N-hydroxy amino acid ester and the α-keto acid were stirred condensation step the use of MgSO4 as a drying agent appeared to
Eur. J. Org. Chem. 2007, 2413–2422
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2419

T. Hubregtse, U. Hanefeld, I. W. C. E. Arends
FULL PAPER
be necessary. The methanol was therefore substituted by the low-
boiling dichloromethane. When the condensation was complete, the
reaction mixture was filtered, concentrated and the reduction was
performed after the addition of methanol. From 9 (380 mg,
3.19 mmol), phenylglyoxylic acid (508 mg, 3.38 mmol), NaCN·BH3
(225 mg, 3.57 mmol) and [Zn(OAc)2·2H2O] (701 mg, 3.19 mmol),
4a was obtained as a white solid in an overall yield of 161 mg
(0.67 mmol, 21%). M.p. 118 °C (decomp.). 1H NMR, 13C NMR
and the mass spectra are identical to those of 4a obtained by start-
ing from 10.
Synthesis of Secondary Hydroxylamines 12a and 12b: NaCN·BH3
(1.1 equiv.) and acetic acid (1.1 equiv.) were added to a solution of
the nitrone 7 in methanol (20 mL). The reaction was complete after
24 h (TLC). The solvent was then evaporated, saturated aqueous
NaHCO3 was added, the product was extracted with CH2Cl2 (3 ϫ
20 mL) and dried with MgSO4.
Hydroxylamine 12a: From nitrone 7a (574 mg, 2.59 mmol),
NaCN·BH3 (179 mg, 2.85 mmol) and acetic acid (171 mg,
2.85 mmol), hydroxylamine 12a (557 mg, 2.49 mmol, 96%) was ob-
1
tained as a colourless oil. H NMR (CDCl3, 300 MHz, 25 °C): δ =
1.29 (t, J = 7.2 Hz, 3 H, CH2CH3), 1.41 (d, J = 6.9 Hz, 3 H,
CHCH3), 3.55 (q, J = 7.0 Hz, 2 H, CH2CH3), 3.86 (J = 13.2 Hz, 1
H, NCH2), 3.99 (J = 13.2 Hz, 1 H, NCH2), 4.20 (q, J = 7.2 Hz, 1
H, CHCH3), 5.75 (s, 1 H, NOH), 7.26–7.36 (m, 5 H, C6H5) ppm.
13C NMR (CDCl3, 75 MHz, 25 °C): δ = 173.09 (1 C, COOEt),
137.23, 129.48, 128.33, 127. 44 (6 C, C6H5), 63.87, 60.80, 60.71 (3
C, NCH, NCH2, OCH2CH3), 14.22, 14.12 (2 C, OCH2CH3,
CHCH3) ppm. HRMS (ESI): calcd. for C12H18NO3 [M + H]+
224.1282; found 224.1277.
Ligand 4b: The synthesis was performed by using (R)-N-hydroxy-
phenylalanine methyl ester (408 mg, 2.09 mmol), glyoxylic
acid·H2O (202 mg, 2.19 mmol), NaCN·BH3 (144 mg, 2.29 mmol)
and [Zn(OAc)2·2H2O] (459 mg, 2.09 mmol). The hydrolysis step
was performed by refluxing in 1  HCl (15 mL) for 1 h. After evap-
oration of the aqueous HCl, the mixture was dissolved in water
and the pH was then adjusted to 1.80 with aqueous NH4OH after
which the mixture was concentrated. After precipitation with
[Zn(OAc)2·2H2O], 4b was obtained as a white solid in an overall
yield of 206 mg (0.86 mmol, 41%). M.p. 130 °C (decomp.). [α]2D0
=
Hydroxylamine 12b: From nitrone 7g (574 mg, 2.77 mmol),
NaCN·BH3 (191 mg, 3.05 mmol) and acetic acid (183 mg,
3.05 mmol), hydroxylamine 12b (531 mg, 2.54 mmol, 92%) was ob-
–5.0 (c = 1, EtOH). 1H NMR (DMSO, 300 MHz, 25 °C): δ = 2.92–
2.97 (m, 2 H, CH2C6H5), 3.60–3.66 (m, 3 H, NCH, NCH2), 7.16–
7.29 (m, 5 H, C6H5) ppm. 13C NMR (DMSO, 75 MHz, 25 °C): δ
= 172.51, 171.69 (2 C, 2 ϫ COOH), 139.04, 129.92, 128.81, 126.83
(6 C, C6H5), 71.56 (1 C, NCH), 59.04 (1 C, NCH2), 35.73 (1 C,
CH2C6H5) ppm. HRMS (ESI): calcd. for C11H14NO5 [M + H]+
240.0867; found 240.0864.
1
tained as white crystals. M.p. 73 °C. H NMR (CDCl3, 300 MHz,
25 °C): δ = 1.26 (t, J = 7.1 Hz, 3 H, CH2CH3), 3.50 (s, 2 H,
NCH2COOEt), 3.92 (s, 2 H, NCH2C6H5), 4.18 (q, J = 7.1 Hz, 1
H, CH2CH3), 6.49 (s, 1 H, NOH), 7.25–7.38 (m, 5 H, C6H5) ppm.
13C NMR (CDCl3, 75 MHz, 25 °C): δ = 170.03 (1 C, COOEt),
136.38, 129.68, 128.39, 127.66 (6 C, C6H5), 63.86, 60.85, 60.11 (3
C, NCH2C6H5, NCH2COOEt, OCH2CH3), 14.16 (1 C, OCH2CH3)
ppm. C11H15NO3 (209.24): calcd. C 63.14, H 7.23, N 6.69; found
C 62.95, H 7.16, N 6.48. HRMS (ESI): calcd. for C11H16NO3 [M
+ H]+ 210.1125; found 210.1121.
Synthesis of Ligand rac-5: N-Hydroxy amino acid ester 9 (580 mg,
4.87 mmol) and pyruvic acid (455 mg, 5.16 mmol) were stirred in
methanol until TLC (PE/EtOAc, 1:1) showed full conversion of 9.
After the addition of NaCN·BH3 (343 mg, 5.45 mmol), TLC
showed a complete and immediate conversion of the nitrone (Rf Ͻ
0.05; strong UV absorption). Evaporation of the methanol yielded
a white solid which was then dissolved in water (5 mL). To remove
the cyanide, aqueous HCl (1–2 equiv.) was added and the mixture
stirred in the open vessel at 60 °C in the fumehood for 20 min. The
pH of the mixture was brought to 1.9 using NH4OH solution, it
was concentrated and left to crystallize at 0 °C after the addition
of a small amount of ethanol. This yielded 330 mg (2.13 mmol,
44%) of rac-5 as white crystals. M.p. 137–138 °C (decomp.). 1H
NMR (H2O, 300 MHz, 25 °C): δ = 1.34, 1.35 (2 ϫ d, J = 6.9 Hz,
6 H, 2 ϫ CHCH3), 3.74 (s, 3 H, OCH3), 3.87, 4.01 (2 ϫ q, J =
6.9 Hz, 2 H, 2 ϫ CHCH3) ppm. 13C NMR (H2O, 75 MHz, 25 °C):
δ = 176.00, 174.01 (2 C, 2 ϫ COOH, COOMe), 62.28, 61.88 (2 C,
2 ϫ NCH), 52.73 (1 C, OCH3), 14.51, 14.09 (2 C, 2 ϫ NCH3) ppm.
HRMS (ESI): calcd. for C7H14NO5 [M + H]+ 192.0867; found
192.0863. HRMS (ESI): calcd. for C7H13NO5Na [M + Na]+
214.0686; found 214.0680.
Synthesis of Secondary Hydroxylamine 12c: N-Hydroxy amino acid
ester 9 (385 mg, 3.23 mmol), acetaldehyde (157 mg, 3.55 mmol) and
acetic acid (213 mg, 3.55 mmol) were stirred in CH2Cl2 (20 mL)
under nitrogen at 0 °C using Na2SO4 as a drying agent. When TLC
(PE/EtOAc, 1:1) showed full conversion of the ester to the interme-
diate nitrone (Rf Ͻ 0.05; strong UV absorption), NaCN·BH3
(223 mg, 3.55 mmol) was added. The reaction was complete after
1 h (TLC). The solvent was then evaporated, saturated aqueous
NaHCO3 was added, the product was extracted with CH2Cl2
(3ϫ20 mL) and dried with MgSO4. Compound 12c was isolated
in a yield of 385 mg (2.62 mmol, 81%) as a colourless oil. 1H NMR
(CDCl3, 300 MHz, 25 °C): δ = 1.18 (t, J = 7.0 Hz, 3 H, NCH2CH3),
1.36 (d, J = 6.9 Hz, 3 H, CHCH3), 2.70 (dq, 1 H, NCHHCH3),
2.88 (dq, 1 H, NCHHCH3), 3.56 (q, J = 7.0 Hz, 1 H, NCHCH3),
3.75 (s, 3 H, OCH3), 5.91 (br., 1 H, NOH) ppm. 13C NMR (CDCl3,
75 MHz, 25 °C): δ = 173. 73 (1 C, COOMe), 64.81 (1 C, NCH),
51.91, 50.60 (2 C, OCH3, NCH2), 14.02, 12.16 (2 C, CHCH3,
CH2CH3) ppm. HRMS (ESI): calcd. for C6H14NO3 [M + H]+
148.0969; found 148.0966.
Synthesis of meso-1: A solution of rac-5 (146 mg, 0.76 mmol) in 1 
HCl (3 mL) was refluxed for 1 h. The pH of the mixture was
brought to 4.2 with a 0.5  NaOH solution and the product was
precipitated with [Zn(OAc)2·2H2O] (168 mg, 0.76 mmol). Com-
pound meso-1 was released by ion-exchange chromatography ac-
cording to the procedures reported above for 1 and was isolated as
a white solid in a yield of 48 mg (0.27 mmol, 35%). 1H and 13C
NMR spectra showed the presence of one diastereomer which was
identified as the meso-form of 1 after comparison with the NMR
spectroscopic data of 1 obtained by substitution of (R)-2-bromo-
propionate as described above. 1H NMR (D2O, 400 MHz): δ = 1.43
Synthesis of Ligands 6a–6c: A solution of the appropriate methyl
ester (12a–12c) in 1  HCl (20 mL) was stirred at 125 °C for 1 h,
the reaction was monitored by TLC (PE/EtOAc, 4:1). The solvent
was then evaporated and the residue stripped twice with water to
remove residual HCl. After dissolving the residue in water (10 mL),
the pH of the solution was brought to 2.4 using NH4OH solution.
The solution was concentrated and left to crystallize at 0 °C after
the addition of a small amount of ethanol.
(d, J = 6.0 Hz, 6 H, CHCH3), 4.08 (q, J = 6.0 Hz, 1 H, CHCH3) Ligand 6a: The hydrolysis required 3  HCl instead of 1 . From
ppm. 13C NMR (D2O, 100 MHz): δ = 175.91 (2 C, COOH), 64.58 12a (465 mg, 2.08 mmol), ligand 6a (189 mg, 1.04 mmol, 50%) was
(2 C, CH), 14.76 (2 C, CH3) ppm.
2420
obtained as white crystals. M.p. 147 °C (decomp.). 1H NMR (D2O,
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Eur. J. Org. Chem. 2007, 2413–2422

Stabilizing Factors for Vanadium(IV) in Amavadin
FULL PAPER
Table 1. HRMS (ESI) data for amavadin analogues 13a and 13b using ligands 4a and 4b (= L).
Product
Molecular ion
[VO(acac)2] + 2 L
Ionization
Empirical formula
Calcd. M
Found M
13a
[M + H]+
C22H23N2O10
C22H21N2O10
V
V
526.0787
524.0641
526.0788
524.0639
[M – H]–
13b
[M + H]+
C22H23N2O10
C22H21N2O10
V
V
526.0787
524.0641
526.0786
524.0639
[M – H]–
Table 2. HRMS (ESI) data for tentative complexes 14 and 15 using ligand 5 (= L).
Product
Molecular ion
Empirical formula
[VO(acac)2] + 2 L
[VO(OPr)3] + 2 L
Ionization
Calcd. M
Found M
Found M
14
[M + H]+
C14H23N2O10V
C14H21N2O10V
430.0787
428.0641
430.0787
n.a.
[M – H]–
–
15
[M + H]+
C14H24N2O11V
C14H22N2O11V
447.0815
445.0668
447.0813
445.0663
447.0814
445.0663
[M – H]–
300 MHz, 25 °C): δ = 1.53 (d, J = 7.2 Hz, 3 H, CHCH3), 3.91 (q,
NMR (CDCl3, 300 MHz, 25 °C): δ = 0.946 [4 ϫ d (overlapping),
J = 7.1 Hz, 1 H, CHCH3), 4.46 (s, 2 H, NCH2) 7.43–7.51 (m, 5 H, 6 H, CHCH3], 4.88, 4.89, 5.11, 5.13 [4 ϫ q (overlapping), 2 H,
C6H5) ppm. 13C NMR (D2O, 75 MHz, 25 °C): δ = 175.59 (1 C, CHCH3], 5.82, 5.83, 6.07, 6.10 (4 ϫ s, 2 H, CHC6H5), 7.30–7.46
COOH), 132.89, 131.49, 130.86, 130.57 (6 C, C6H5), 68.23, 62.46 (m, 10 H, C6H5) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ =
(2 C, NCH, NCH2), 14.17 (1 C, CHCH3) ppm. HRMS (ESI):
174.09, 173.97, 172.81, 171.98, 171.91, 170.72 (4 C, COOH),
136.95, 136.80, 136.76, 129.86, 129.70, 129.03 (12 C, C6H5), 80.63,
80.49, 79.70, 79.51 (2 C, CHC6H5), 72.96, 72.80, 72.38, 72.17 (2 C,
CHCH3), 18.87 (2 C, CHCH3) ppm. 51V NMR (79 MHz, CDCl3,
25 °C): δ = –243 ppm.
calcd. for C10H14NO3 [M + H]+ 196.0969; found 196.0967.
Ligand 6b: From 12b (209 mg, 1.00 mmol), ligand 6b (150 mg,
0.83 mmol, 83%) was obtained as white crystals. M.p. 135 °C (de-
comp.). 1H NMR (CDCl3, 300 MHz, 25 °C): δ = 3.74 (s, 2 H,
NCH2C6H5), 4.35 (s, 2 H, NCH2COOH), 7.33–7.38, 7.47–7.51 (m,
5 H, C6H5), 12.47 (s, 2 H, COOH, NOH) ppm. 13C NMR (CDCl3,
75 MHz, 25 °C): δ = 171.09 (1 C, COOH), 131.62, 129.70, 129.54,
128.62 (6 C, C6H5), 62.84, 60.14 (2 C, NCH2C6H5, NCH2COOH)
ppm. HRMS (ESI): calcd. for C9H12NO3 [M + H]+ 182.0812;
found 182.0811.
Complexation of Ligands 5and 6 to [VO(acac)2]. Solutions for
HRMS Measurements: [VO(acac)2] (0.010 mmol) was added to a
solution of the ligand (0.021 mmol) in methanol (0.8 mL). The mix-
tures were stirred in an open flask for 1 h, after which they were
analyzed by electrospray HRMS.
Complexation of Ligands 5 and 6 to [VO(OPr)3]. Solutions for
HRMS Measurements: [VO(OPr)3] (0.035 mmol) was added to a
solution of the ligand (0.074 mmol) in methanol (2 mL). The mix-
tures were stirred in an open flask for 1 h, after which they were
analyzed by electrospray HRMS.
Ligand 6c: The hydrolysis step required 1.5 h instead of 1 h. As
crystals could not be obtained (oily precipitate), the solution was
concentrated to dryness and the raw product was then extracted
with THF. The product was then crystallized from a water/ethanol
mixture. From 12c (335 mg, 2.28 mmol), ligand 6c (140 mg,
1.05 mmol, 46%) was obtained as white crystals. M.p. 146 °C (de-
comp.). 1H NMR (DMSO, 300 MHz, 25 °C): δ = 1.03 (t, J =
7.1 Hz, 3 H, CH2CH3), 1.17 (d, J = 6.6 Hz, 3 H, CHCH3), 2.61
(dq, 1 H, NCHHCH3), 2.71 (dq, 1 H, NCHHCH3), 3.29 (q, J =
6.6 Hz, 1 H, CHCH3), 7–12 (br., 2 H, COOH, NOH) ppm. 13C
NMR (CDCl3, 75 MHz, 25 °C): δ = 174.40 (1 C, COOH), 65.61
(1 C, NCHCH3), 51.02 (1 C, NCH2), 14.76, 13.07 (2 C, CHCH3,
CH2CH3) ppm. HRMS (ESI): calcd. for C5H12NO3 [M + H]+
134.0812; found 134.0811.
High-Resolution Electrospray MS Data: The HRMS (ESI) data re-
corded for the vanadium complexes are shown in Table 1 (triden-
tate ligands 4a and 4b) and Table 2 (bidentate ligand 5).
Acknowledgments
We gratefully acknowledge TNO in Zeist, The Netherlands, for the
high-resolution mass spectrometry measurements. T. H. thanks the
Netherlands Research School Combination Catalysis (NRSC-C)
for financial support.
Synthesis of Vanadium(IV) Complex 13a: [VO(acac)2] (22 mg,
0.084 mmol) was added to a solution of 4a (44 mg, 0.184 mmol) in
methanol (15 mL). The mixture immediately turned deep brown
for a few seconds. After stirring for 20 min, the mixture became
clear blue-purple and the smell of Hacac was noticed. All volatiles
were removed by rotary evaporation and the residue was stripped
twice with methanol. The dry residue was then washed twice with
dichloromethane, yielding 42 mg (0.080 mmol, 96%) of 13a as a
glittering blue solid. M.p. 200 °C (decomp.). HRMS (ESI): calcd.
for C22H23N2O10 [M + H]+ 526.0787; found 526.0786. HRMS
(ESI): calcd. for C22H21N2O10 [M – H]– 524.0641; found 524.0636.
[1] E. Bayer, H. Kneifel, Z. Naturforsch. Teil B 1972, 27, 207.
[2] H. Kneifel, E. Bayer, J. Am. Chem. Soc. 1986, 108, 3075–3077.
[3] T. Hubregtse, E. Neeleman, Th. Maschmeyer, R. A. Sheldon,
U. Hanefeld, I. W. C. E. Arends, J. Inorg. Biochem. 2005, 99,
1264–1267.
[4] M. A. Nawi, T. L. Riechel, Inorg. Chim. Acta 1987, 136, 33–
39.
[5] P. M. Reis, J. A. L. Silva, J. J. R. F. da Silva, A. J. L. Pombeiro,
Chem. Commun. 2000, 1845–1846.
[6] P. M. Reis, J. A. L. Silva, A. F. Palavra, J. J. R. F. da Silva, T.
Kitamura, Y. Fujiwara, A. J. L. Pombeiro, Angew. Chem. Int.
Ed. 2003, 42, 821–823.
[7] P. M. Reis, J. A. L. Silva, J. J. R. F. da Silva, A. J. L. Pombeiro,
J. Mol. Catal. A 2004, 224, 189–195.
Oxidation of 13a to the Vanadium(V) Complex [VL2]–[PPh4]+ (L =
Ligand): The oxidation and isolation processes were carried out
according to the procedure reported earlier for amavadin.[3] 1H
Eur. J. Org. Chem. 2007, 2413–2422
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2421

T. Hubregtse, U. Hanefeld, I. W. C. E. Arends
FULL PAPER
[8] P. M. Reis, J. A. L. Silva, A. F. Palavra, J. J. R. F. da Silva,
A. J. L. Pombeiro, J. Catal. 2005, 235, 333–340.
[9] E. M. Armstrong, R. L. Beddoes, L. J. Calviou, J. M. Char-
nock, D. Collison, N. Ertok, J. H. Naismith, C. D. Garner, J.
Am. Chem. Soc. 1993, 115, 807–808.
[10] R. E. Berry, E. M. Armstrong, R. L. Beddoes, D. Collison,
S. N. Ertok, M. Helliwell, C. D. Garner, Angew. Chem. Int. Ed.
1999, 38, 795–797.
[11] P. D. Smith, J. J. A. Cooney, E. J. L. McInnes, R. L. Beddoes,
D. Collison, S. M. Harben, M. Helliwell, F. E. Mabbs, A. Man-
del, A. K. Powell, C. D. Garner, J. Chem. Soc., Dalton Trans.
2001, 3108–3114, and references therein.
[12] M. A. A. F. de C. T. Carrondo, M. T. L. S. Duarte, J. C. Pessoa,
J. A. L. Silva, J. J. R. F. da Silva, M. C. T. A. Vaz, L. F. Vilas-
Boas, J. Chem. Soc., Chem. Commun. 1988, 1158–1159.
[13] P. D. Smith, R. E. Berry, S. M. Harben, R. L. Beddoes, M. Hel-
liwell, D. Collison, C. D. Garner, J. Chem. Soc., Dalton Trans.
1997, 4509–4516.
[33] E. Koch, H. Kneifel, E. Bayer, Z. Naturforsch., Teil B 1986,
41, 359–362.
[34] P. Merino, T. Tejero, J. Revuelta, P. Romero, S. Cicchi, V. Man-
nucci, A. Brandi, A. Goti, Tetrahedron: Asymmetry 2003, 14,
367–379.
[35] S.-I. Murahashi, T. Shiota, Tetrahedron Lett. 1987, 28, 6469–
6472.
[36] X.-b. Jiang, A. J. Minnaard, B. L. Feringa, J. G. de Vries, J.
Org. Chem. 2004, 69, 2327–2331.
[37] S.-I. Murahashi, S. Watanabe, T. Shiota, J. Chem. Soc., Chem.
Commun. 1994, 725–726.
[38] S.-I. Murahashi, T. Tsuji, S. Ito, Chem. Commun. 2000, 409–
410.
[39] M. L. M. Pennings, D. N. Reinhoudt, S. Harkema, G. J.
van Hummel, J. Org. Chem. 1982, 47, 4419–4425.
[40] R. Bonnett, R. F. C. Brown, V. M. Clark, I. O. Sutherland, A.
Todd Sr., J. Chem. Soc. 1959, 2094–2102.
[41] C. P. Sár, T. Kálai, N. M. Bárácz, G. Jerkovich, K. Hideg,
Synth. Commun. 1995, 25, 2929–2940.
[42] T. Hubregtse, H. Kooijman, A. L. Spek, Th. Maschmeyer,
R. A. Sheldon, I. W. C. E. Arends, U. Hanefeld, J. Inorg. Bio-
chem., DOI: 10.1016/j.jinorgbio.2007.02.006.
[43] E. Buehler, G. B. Brown, J. Org. Chem. 1967, 32, 265–267.
[44] T. Polon´ski, A. Chimiak, J. Org. Chem. 1976, 41, 2092–2095.
[45] T. Polon´ski, A. Chimiak, Tetrahedron Lett. 1974, 28, 2453–
2456.
[46] G. Grundke, W. Keese, M. Rimpler, Synthesis 1987, 1115–1116.
[47] It has previously been shown that only minimal structural
changes take place upon the one-electron oxidation to vanadi-
um(V) when the amavadin ligand 1[9,10] or its analogues 2[9,12]
and 3[13] are complexed to vanadium. It has thus been demon-
strated that the NMR data of vanadium(V) complexes are
valuable for discussing the properties of the vanadium(IV)
complexes.
[14] E. Bayer, E. Koch, G. Anderegg, Angew. Chem. 1987, 99, 570–
572; Angew. Chem. Int. Ed. Engl. 1987, 26, 545–546.
[15] A. R. Katritzky, R. J. K. Taylor, C. Ramsden, Comprehensive
Organic Functional Group Transformations II, Elsevier/Perga-
mon, Amsterdam, 2005.
[16] H. C. J. Ottenheijm, J. D. M. Herscheid, Chem. Rev. 1986, 86,
697–707.
[17] R. Bloch, Chem. Rev. 1998, 98, 1407–1438.
[18] K. B. G. Torssel, Nitrile Oxides, Nitrones, and Nitronates in Or-
ganic Synthesis, VCH, Weinheim, 1988.
[19] P. Merino, Science of Synthesis, Thieme Medical Publishers,
Stuttgart, 2006, vol. 27, chapter 13, p. 511.
[20] H. T. Nagasawa, J. G. Kohlhoff, P. S. Fraser, A. A. Mikhail, J.
Med. Chem. 1972, 15, 483.
[21] J. F. W. Keana, S. Pou, J. Org. Chem. 1989, 54, 2417–2420.
[22] D. Naskar, A. Roy, W. L. Seibel, D. E. Portlock, Tetrahedron
Lett. 2003, 44, 8865–8868.
[48] Attempts were made to prepare 14 through the reaction of nat-
ural amavadin with diazomethane, but no reaction was ob-
served.
[23] L. W. Jones, R. T. Major, J. Am. Chem. Soc. 1930, 52, 1078–
1085.
[24] A. D. Keramidas, S. M. Miller, O. P. Anderson, D. C. Crans, J.
Am. Chem. Soc. 1997, 119, 8901–8915.
[49] The empirical formula of 15 requires one of the ligand’s oxygen
atoms to be protonated; the exact location of the proton as
drawn is speculative.
[50] T. Sierra, J. L. Serrano, M. B. Ros, A. Ezcurra, J. Zubía, J. Am.
Chem. Soc. 1992, 114, 7645–7651.
[51] R. S. Ward, A. Pelter, D. Goubet, M. C. Pritchard, Tetrahe-
dron: Asymmetry 1995, 6, 469–498.
[52] S. Kano, Y. Yuasa, T. Yokomatsu, S. Shibuya, J. Org. Chem.
1988, 53, 3865–3868.
[53] Organikum, VEB Deutscher Verlag der Wissenschaften, Berlin,
1984, p. 499
[54] S. Wolfe, C. Akuche, S. Ro, M.-C. Wilson, C.-K. Kim, Z. Shi,
Can. J. Chem. 2003, 81, 915–936.
[55] F. Lehr, J. Gonnermann, D. Seebach, Helv. Chim. Acta 1979,
[25] K. Wieghardt, U. Quilitzsch, B. Nuber, J. Weiss, Angew. Chem.
Int. Ed. Engl. 1978, 17, 351–352.
[26] K. Wieghardt, U. Quilitzsch, Z. Anorg. Allg. Chem. 1979, 457,
75–83.
[27] L. Saussine, H. Mimoun, A. Mitschler, J. Fisher, New J. Chem.
1980, 4, 235–237.
[28] S. J. Angus-Dunne, P. C. Paul, A. S. Tracey, Can. J. Chem.
1997, 75, 1002–1010.
[29] R. W. Feenstra, E. H. M. Stokkingreef, R. J. F. Nivard, H. C. J.
Ottenheijm, Tetrahedron 1988, 44, 5583–5595.
[30] R. W. Feenstra, E. H. M. Stokkingreef, R. J. F. Nivard, H. C. J.
Ottenheijm, Tetrahedron Lett. 1987, 28, 1215–1218.
[31] T. Kolasa, Can. J. Chem. 1985, 63, 2139–2142.
[32] J. Felcman, M. C. T. A. Vaz, J. J. R. F. da Silva, Inorg. Chim.
Acta 1984, 93, 101–108.
62, 2258–2275.
Received: December 5, 2006
Published Online: April 4, 2007
2422
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Eur. J. Org. Chem. 2007, 2413–2422