Synthesis of [uniformly ring-14C]-labelled 4-hydroxybenzaldehyde, vanillin, and protocatechualdehyde

Article abstract of DOI:10.1002/jlcr.815

[Uniformly ring-¹⁴C]-labelled 4-hydroxybenzaldehyde, vanillin, and protocatechualdehyde were synthesized from [¹⁴C]-labelled phenol, guaiacol, and catechol with methyl dichloromethyl sulfide (CH ₃SCHCl₂) under Friedel-Crafts alkylation conditions in dichloromethane at -78°C for 5 min (in the case of phenol and guaiacol) or at -20°C for 1 min (in the case of catechol), by rapid addition of SnCl ₄ to mixtures of the phenolic compound and CH₃SCHCl ₂, followed by hydrolysis with HCl. Regioselective formylation (para to the -OH group) was achieved. The conversion rates were 96, 81, and 88% for 4-hydroxybenzaldehyde, vanillin, and protocatechualdehyde, respectively, and the yields of the recovered products after work-up amounted to 88, 75, and 83%, respectively. In the case of guaiacol, 17% of isovanillin was obtained as by-product. It was found that the presence of water or ethyl acetate in the reaction mixture, at a molar ratio of 60:1 (water:guaiacol) or 120:1 (ethyl acetate:guaiacol), had little influence on the yields under the reaction conditions. Factors influencing the yields are discussed in the study. Copyright

Full text of DOI:10.1002/jlcr.815

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2004; 47: 209–216.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.815
Research Article
Synthesis of [uniformly ring-14C]-labelled
4-hydroxybenzaldehyde, vanillin, and
protocatechualdehyde
.
Rong Ji* and Andreas Schaffer
Biologie V - Umweltanalytik, RWTH Aachen, Aachen D-52056, Germany
Summary
[Uniformly ring-14C]-labelled 4-hydroxybenzaldehyde, vanillin, and protocatechual-
dehyde were synthesized from [14C]-labelled phenol, guaiacol, and catechol
with methyl dichloromethyl sulfide (CH3SCHCl2) under Friedel–Crafts alkylation
conditions in dichloromethane at ꢀ788C for 5 min (in the case of phenol and guaiacol)
or at ꢀ208C for 1 min (in the case of catechol), by rapid addition of SnCl4 to mixtures
of the phenolic compound and CH3SCHCl2, followed by hydrolysis with HCl.
Regioselective formylation (para to the –OH group) was achieved. The conversion
rates were 96, 81, and 88% for 4-hydroxybenzaldehyde, vanillin, and protocatechual-
dehyde, respectively, and the yields of the recovered products after work-up amounted
to 88, 75, and 83%, respectively. In the case of guaiacol, 17% of isovanillin was
obtained as by-product. It was found that the presence of water or ethyl acetate in the
reaction mixture, at a molar ratio of 60:1 (water:guaiacol) or 120:1 (ethyl
acetate:guaiacol), had little influence on the yields under the reaction conditions.
Factors influencing the yields are discussed in the study. Copyright # 2004 John
Wiley & Sons, Ltd.
Key Words: isotope labelling; Friedel–Crafts alkylation; formylation; 14C-p-hydro-
xybenzaldehyde; 14C-vanillin; 14C-protocatechualdehyde
Introduction
Monomeric phenols, stemming from degradation of plant and animal
residues, microbial synthesis, and rood exudation, are ubiquitous in soil.1,2
In addition to being recognized as allelophathic compounds,3 these naturally
occurring monomeric phenols can be mineralized and stabilized in soil by both
*Correspondence to: R. Ji, Biologie V - Umweltanalytik, RWTH Aachen, Aachen D-52056, Germany.
E-mail: rong.ji@bio5.rwth-aachen.de
Contract/grant sponsor: Deutsche Forschungsgemeinschaft; contract/grant number: Project SPP 1090
Copyright # 2004 John Wiley & Sons, Ltd.
Received 14 November 2003
Revised 14 January 2004
Accepted 20 January 2004

¨
R. JI AND A. SCHAFFER
210
abiotic and biotic soil processes, and regarded as precursors of soil organic
matter.4–8 In order to elucidate these processes, radioactively labelled phenols
are needed to trace the fate of phenols in soil and to identify the binding sites
of the residues in soil matrices. Since the aldehydes 4-hydroxybenzaldehyde,
vanillin, and protocatechualdehyde are intermediates for the chemical
synthesis of isotope-labelled lignin monomers and oligomers, and their
transformation products,9–12 the successful synthesis of these aldehydes is
the prerequisite for obtaining isotope-labelled phenolic compounds, e.g.
p-coumaric acid, p-coumaryl alcohol, ferulic acid, coniferyl alcohol, caffeic
acid, etc.10
The ring-labelling of these aldehydes can be realized by formylation of
commercially available starting compounds (e.g. phenol and catechol).
The chemical introduction of an aldehyde group on phenols can be performed
by many methods,13,14 such as condensation reaction (with glyoxylic acid or
Reimer–Tiemann reaction) and Friedel–Crafts reaction (Vilsmeier reaction
or Gattermann reaction). Condensation of glyoxylic acid with phenol,
guaiacol, or catechol followed by an oxidation process can lead to
4-hydroxybenzaldehyde, vanillin, or protocatechualdehyde in relatively good
yields (e.g. Kalikar et al., 1986; Bjorsvik et al., 2000)15,16 and is currently the
main route for vanillin manufacture (see Li and Frost, 1998).17 Bjorsvik et al.16
applied a statistical experimental design and multivariate modelling to the
synthesis of protocatechualdehyde from catechol leading to 90.5% of the
regioselectivity under the optimized experiment conditions, with a conversion
of 78.4%.
The introduction of an aldehyde group using methyl dichloromethyl sulfide
by Friedel–Crafts reaction18 was many times applied in the literature for the
synthesis of radioactively labelled 4-hydroxybenzaldehyde and vanillin9,11,19
for the reason that this method was superior to other methods (e.g. Vilsmeier,
Gattermann, or Sandmeyer reaction) in terms of yield and simplicity.9
However, under the reaction conditions used in the literature this synthesis
gave 50% yield for the 4-hydroxybenzaldehyde synthesis,9 and a maximal
yield of 63% for vanillin,11 because the formylation is not strictly
regioselective under the reaction conditions and other formylation by-
products were formed. In addition, vanillin formed in the reaction mixture
can react with CH3SH to give dimethyl mercaptan and other by-products, e.g.
di- and triphenyl methane.11
In this study, we optimized the reaction conditions for the Gross–Matthey
method18 for the synthesis of [uniformly ring-14C]-labelled 4-hydroxybenzal-
dehyde, vanillin, and protocatechualdehyde to give yields of 88, 75, and 83%,
respectively. Except for the formylated products at the para-position to the
–OH group (in the case of guaiacol, also to the –OCH3 group), hardly any by-
products were formed under the modified reaction conditions.
Copyright # 2004 John Wiley & Sons, Ltd.
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SYNTHESIS OF [UNIFORMLY RING-14C]-
211
Results and discussion
[Uniformly ring-14C]-labelled 4-hydroxybenzaldehyde (2), vanillin (4), and
protocatechualdehyde (6) were synthesized by Friedel–Crafts alkylation in
solvent CH2Cl2 from [14C]-labelled phenol (1), guaiacol (3), and catechol (5)
with CH3SCHCl2 and catalyzed by SnCl4 at low temperatures, followed by
hydrolysis with HCl (Figure 1). The synthesis of 4-hydroxybenzaldehyde and
vanillin were conducted at ꢀ788C and protocatechualdehyde at ꢀ208C. The
conversion rates, yields, and recoveries of radioactivity are summarized in
Table 1.
CHO
H+
CH3SCHCl2 + SnCl4
*
OH
*
CH2Cl2, -78˚C, 5 min
OH
2
1
CHO
CHO
H+
CH3SCHCl2 + SnCl4
CH2Cl2, -78˚C, 5 min
+
*
OH
*
OH
*
OMe
OMe
OH
OMe
7
4
3
CHO
H+
CH3SCHCl2 + SnCl4
CH2Cl2, -20˚C, 1 min
*
OH
*
OH
OH
OH
5
6
Figure 1. Synthesis of [uniformly ring-14C]-labelled 4-hydroxybenzaldehyde,
vanillin, and protocatechualdehyde by Friedel–Crafts alkylation
Table 1. Analytical data for the synthesis of [uniformly ring-14C]-labelled
4-hydroxybenzaldehyde, vanillin, and protocatechualdehyde
Synthesis
Conversion ratea
(%)
Recovered yieldb
(%)
Radiochemical purity
(%)
1 ! 2
3 ! 4+7
96
81
17
88
88
75
17
83
97
99
96
99
4
7
5 ! 6
a Percentage of 14C in product relative to the amount of 14C compound used (as determined from
autoradiography).
b Percentage of 14C in the purified product.
Copyright # 2004 John Wiley & Sons, Ltd.
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R. JI AND A. SCHAFFER
212
Compared to the reported yields in the literature, our modified procedure
gives higher yields for all three aldehydes. The formylation is almost
quantitatively regiospecific at the para-positions to the active groups. In the
case of phenol (with only one –OH group) and catechol (with two chemically
identical –OH groups), the para-products accounted for nearly 100% of the
products. In the case of guaiacol, two para-products (vanillin and isovanillin)
were obtained, and the para-position ratio of the –OH group to the –OCH3
group is about 81:17 (Table 1). Ortho-products of phenol and guaiacol were
not observed and therefore the purification procedures were quite simple. In
our experiments, we used a ten-fold excess of CH3SCHCl2 or SnCl4, due to the
volatility of the chemicals and the low volume applied (ml range), even though
the preliminary experiments showed that 5% more than the stoichiometric
amount is actually enough.
One of the modifications in our method is the lower reaction temperatures.
At low temperature, the formation of dimethylmercaptal or di- and
triphenylmethane, which was observed at 08C by Kratzl and Vierhapper,11
is suppressed. It was shown that with decreasing reaction temperature, the
conversion rate of vanillin increases and that of isovanillin decreases slightly,
e.g. at 08C the conversion rates of vanillin and isovanillin amounted to
73.2 ꢁ 0.2%, and 21.5 ꢁ 1.0% (n=5), respectively, while at ꢀ208C, the figures
were 75.7 ꢁ 0.6% and 20.3 ꢁ 1.0% (n=6), respectively. At T5ꢀ608C, the
conversion rates of vanillin and isovanillin were 81.0 ꢁ 0.7 and 17.2 ꢁ 0.7%
(n=6), respectively.
Besides the reaction temperature, the rate of addition of the catalyst SnCl4
to the mixture of phenol and guaiacol is another influential factor in
determining the product yields. The addition of SnCl4 should be rapid.
Experiments showed that at ꢀ188C, the yields of vanillin and isovanillin
amounted to 54 and 27%, respectively, when SnCl4 was added to guaiacol
(0.4 mmol) within 20 s, and the yield of 4-hydroxybenzaldeyhde amounted to
30%, when SnCl4 was added to phenol (0.4 mmol) within 4 min. However, the
yield of protocatechualdehyde seemed to be independent of the addition rate.
As an alternative to the addition of catalyst to the reactants, the phenols can
be added dropwise to the mixture of CH3SCHCl2 and SnCl4, and the yields
remained almost the same.
This Friedel–Crafts reaction is very fast and even at ꢀ788C phenol
and guaiacol can be completely converted within 5 min. Longer reaction
time leads to lower yields. In the case of guaiacol, it was shown that the
yields of vanillin and isovanillin decreased from 80.7 to 69.0% and from
17.7 to 14.5%, respectively, when the reaction time was prolonged from
5 to 60 min. In the case of catechol, 1 min reaction time was suitable, even
though the conversion of catechol was not complete (about 5% catechol
remained). When the reaction time was prolonged to 5 min, the yield of
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SYNTHESIS OF [UNIFORMLY RING-14C]-
213
protocatechualdehyde did not increase and high amounts of by-products were
formed (ca. 10%).
As an alternative to CH2Cl2 as solvent, petrol ether can be used for synthesis
of 4-hydroxybenzaldehyde and vanillin. It was also shown that acetonitrile can
be used as a solvent for the synthesis of protocatechualdehyde; however, the
yield was not determined. We found that the presence of water and ethyl
acetate (EtOAc) in the reaction mixture had little influence on the yield of
vanillin. When H2O was added to the reaction mixture at a molar ratio of 60:1
(H2O:guaiacol) before reaction at ꢀ788C for 5 min, the yields of vanillin and
isovanillin amounted to 74.5 and 14.8%, respectively. In the case of EtOAc,
with a molar ratio of 120:1 (EtOAc:guaiacol), the yields of vanillin and
isovanillin amounted to 73.0 and 14.8%, respectively. The molar ratio of
guaiacol:CH3SCHCl2:SnCl4 in these experiments was 1:10:10.
Purification of products can be performed by preparative thin-layer
chromatography (TLC) or column chromatography. During the work-up,
especially, the reduction of mixture volume by rotary evaporation, some
compounds may be formed, which, however, can be divided again into
products by boiling the mixture in water for 30–60 min. Pure 4-hydroxyben-
zaldehyde can also be obtained by sublimating the raw product under
0.04 mbar at 1058C for 3 h. Good separation of vanillin and isovanillin by
preparative TLC needs several times development of the plate in the eluent (n-
pentane:EtOAc=3:1/v:v, containing 0.5% HCOOH).
Experimental
TLC was performed on silica gel 60 with fluorescence indicator (Sil G-25
.
UV254, 0.25 mm; Macherey-Nagel, Duren, Germany) and viewed under UV
light (254 nm). Preparative TLC was performed on plates with 2 mm of silica
gel 60 layer, which were pre-treated by eluting the plates in EtOAc. Column
chromatography was conducted with silica gel 60 (particle diameter 0.064–
0.200 mm, Merck, Germany). For autoradiography of the TLC plate, a
bioimaging analyzer (Fujifilm BAS-1000; Tokyo, Japan) was used. Quantita-
tive determination of radioactivity was performed with a liquid scintillation
counter LS6500 (Beckman Coulter; CA, USA) using the cocktail Lumasafe
Plus (Lumac LSC; Groningen, The Netherlands). Product identification was
conducted by TLC and gas chromatography coupled to a mass spectrometer.
[U-14C]-phenol (1), with a specific activity of 2.70 ꢂ 109 Bq mmolꢀ1 and 97%
radiochemical purity, was supplied as a solution in petrol ether by Hartmann
Analytic (Braunschweig, Germany), its radiochemical purity decreased to 95%
during storage. [U-14C]-phenol, without any purification, was diluted with
non-labelled phenol, resulting in
108 Bq mmolꢀ1. [U-ring-14C]-guaiacol (3, 4.11 ꢂ 108 Bq mmolꢀ1, 98.1% radio-
chemical purity) and [U-14C]-catechol (5, 7.28 ꢂ 107 Bq mmolꢀ1, 99.7% of
a specific radioactivity of 4.11 ꢂ
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R. JI AND A. SCHAFFER
214
radiochemical purity) were synthesized in our laboratory from [U-14C]-phenol
according to Kratzl and Vierhapper20 and Ji and Schaffer. The remaining
21
.
chemicals were purchased from commercial sources.
[Uniformly ring-14C]-4-hydroxybenzaldehyde (2)
(
To a 5-ml flask containing 1 ml of CH2Cl2, five pieces of molecular sieves 4 A
(diameter 2 mm), and a magnetic stirring bar were added 50 ml of [U-14C]-
phenol solution (1) (2.60 ꢂ 106 Bq) and 7 ml of CH3SCHCl2 at room
temperature. The mixture was cooled to ꢀ788C in a dry ice-acetone bath
and 7 ml of SnCl4–CH2Cl2 solution (1:1/v:v, ꢀ788C) were added within 1 s with
vigorous stirring. The color of the mixture immediately became yellow. After
stirring for 5 min, 2 ml of 2 M HCl were added to the mixture. The reaction
flask was heated to room temperature and stirred for 20 min. The mixture was
extracted with EtOAc (3 times, each 10 ml). The EtOAc extract was dried over
anhydrous Na2SO4, rotary evaporated at 308C to ca. 2 ml, further reduced to
0.5 ml by N2 passage, and transferred onto
a
silica gel plate
(20 cm ꢂ 20 cm ꢂ 2.0 mm). The plate was developed in n-hexane:EtOAc (3:1/
v:v) containing 0.5% HCOOH. Autoradiography showed that 90.8% of the
total radioactivity was located on the spot corresponding to 4-hydroxyben-
zaldehyde (2) (Rf of phenol, para-, meta-, and ortho-hydroxybenzaldehyde:
0.48, 0.23, 0.28, and 0.58, respectively). The product (2) band was separated
and extracted with EtOAc (6 times, each 15 ml). Rotary evaporation of solvent
resulted in the yellow product (2) with 97% radiochemical purity
(2.16 ꢂ 106 Bq, yield 87.6% with respect to phenol).
[Uniformly ring-14C]-vanillin (4)
To mixture of [U-ring-14C]-guaiacol (3) (3.74 ꢂ 107 Bq) and CH3SCHCl2
(131 ml) in a 25-ml round bottom flask containing 4 ml petrol ether, 5 ml of
(
CH2Cl2, 10 pieces of molecular sieves 4 A (diameter 2 mm), and a magnetic
stirring bar were added 308 ml of SnCl4–CH2Cl2 solution (1:1/v:v, ꢀ788C)
within 1 s with vigorous stirring at ꢀ788C. The color of the mixture turned
from yellow–orange to red–brown within several seconds. After a further
5 min stirring, 2 ml of 2 M HCl (room temperature) were added and ice and red
solid substances were immediately formed in the mixture. The bath was
removed and the slightly orange mixture was transformed to give two phases.
The organic phase was carefully separated. The aqueous phase was mixed with
5 ml of H2O and extracted with CH2Cl2 (5 times, each 4 ml). The organic phase
and CH2Cl2 extracts were combined and dried over anhydrous Na2SO4.
Autoradiography, which was conducted by developing the plate firstly in
n-pentane:EtOAc (3:1/v:v) once, then in CHCl3 twice, showed that 79 and
17% of the radioactivity was located on the spots corresponding to vanillin
Copyright # 2004 John Wiley & Sons, Ltd.
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SYNTHESIS OF [UNIFORMLY RING-14C]-
215
(4) and isovanillin (7). The extract was rotary evaporated to a volume of ca.
10 ml and transferred onto a silica gel column (350 ml, diameter 4 cm). The
products (4 and 7) were sequentially eluted using a mixture of n-pentane and
EtOAc with the following ratios (n-pentane:EtOAc/v:v): 1:0 (400 ml), 9:1
(100 ml), 8:1 (100 ml), 7:1 (100 ml), 6:1 (600 ml), 5:1 (200 ml), 4:1 (200 ml), 3:1
(200 ml), 2:1 (100 ml) and 1:1 (100 ml). Every 20 ml of eluate were collected as
one fraction. Fractions of vanillin and isovanillin were identified by TLC,
separately combined, and rotary evaporated to dryness, resulting in
2.75 ꢂ 107 Bq of vanillin (4) (yield 75.1% with respect to guaiacol) and
6.11 ꢂ 106 Bq of isovanillin (7) (yield 16.7% with respect to guaiacol).
Autoradiography (n-pentane:EtOAc=3:1/v:v, containing 0.5% of HCOOH)
showed that the vanillin had a radiochemical purity of 99.2% and the
isovanillin 96%, containing 2.4% vanillin (Rf of 4 and 7: 0.32 and 0.25,
respectively).
[Uniformly ring-14C]-protocatechualdehyde (6)
To mixture of [U-14C]-catechol (5) (6.22 ꢂ 106 Bq) and CH3SCHCl2 (0.10 ml)
in a 25-ml round bottom flask containing 4 ml of CH2Cl2 and 10 pieces of
(
molecular sieves (diameter 2 mm, 4 A) were added 50 ml of SnCl4–CH2Cl2
solution (1:1/v:v) within 1 s with strong stirring at ꢀ208C. The color of the
mixture became immediately red–orange and a precipitate formed. After 1 min
stirring, 2 ml of 2 M HCl was added to terminate the reaction. The mixture was
warmed to room temperature and stirred for a further 30 min. After addition
of 10 ml of n-pentane, the mixture was extracted with 2 M HCl (4 times, each
5 ml). The product in the aqueous extracts was separated by extraction with
EtOAc (3 times, each 8 ml). The EtOAc extracts were combined, dried over
anhydrous Na2SO4, and rotary evaporated at 408C to ca. 1 ml. The product in
the EtOAc solution was separated by preparative TLC using n-hexane:EtOAc
(2:1/v:v) containing 0.5% HCOOH as eluent. Autoradiography showed that
91 and 4.7% of the radioactivity was located on the spots corresponding to
protocatechualdehyde (6) and catechol (5) (Rf of 6 and 5: 0.22 and 0.37,
respectively). The protocatechualdehyde band was separated and extracted
with mixture of EtOAc and n-pentane (1:1/v:v). After rotary evaporation of
the organic solvents, a brown–red solid was obtained (5.2 ꢂ 106 Bq) in 83%
yield (with respect to catechol). Autoradiography (n-hexane:EtOAc=2:1/v:v,
containing 0.5% HCOOH) showed the radiochemical purity of the product to
be 99.2%.
Conclusion
Friedel–Crafts formylation of guaiacol, phenol, and catechol using
CH3SCHCl2 at low temperature and rapid addition of the catalyst gives the
required products in good yields and high regiospecificity (para to –OH
Copyright # 2004 John Wiley & Sons, Ltd.
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R. JI AND A. SCHAFFER
216
group). The reported yields are consistently higher than those reported
previously.
Acknowledgements
This study was supported by the Deutsche Forschungsgemeinschaft (project
SPP 1090). We thank Zai-Xin Chen, John Niederer, and Konrad Haider for
their assistance.
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