O-benzylation of polyphenolics. Preparation of 1,2,4-tribenzyloxybenzene

Article abstract of DOI:10.1081/SCC-100104477

The main phenolic by-products in the O-benzylation of 1,2,4-trihydroxy- or 1,2,4-triacetoxybenzene to 1,2,4-tribenzyloxybenzene have been identified as due to incomplete O-benzylation and C-benzylation. In addition, dibenzyldimethyl-ammonium chloride is f

Full text of DOI:10.1081/SCC-100104477

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O-BENZYLATION OF POLYPHENOLICS. PREPARATION
OF 1,2,4-TRIBENZYLOXYBENZENE
Uffe Anthoni a , Carsten Christophersen b & Per Halfdan Nielsen a
a Marine Chemistry Section, Department of Chemistry , University of Copenhagen,
Universitetsparken 5 , Copenhagen, DK-2100, Denmark
b Marine Chemistry Section, Department of Chemistry , University of Copenhagen,
Universitetsparken 5 , Copenhagen, DK-2100, Denmark
Published online: 09 Nov 2006.
To cite this article: Uffe Anthoni , Carsten Christophersen & Per Halfdan Nielsen (2001) O-BENZYLATION OF
POLYPHENOLICS. PREPARATION OF 1,2,4-TRIBENZYLOXYBENZENE, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 31:14, 2223-2229, DOI: 10.1081/SCC-100104477
To link to this article: http://dx.doi.org/10.1081/SCC-100104477
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SYNTHETIC COMMUNICATIONS, 31(14), 2223–2229 (2001)
O-BENZYLATION OF POLYPHENOLICS.
PREPARATION OF 1,2,4-
TRIBENZYLOXYBENZENE
Uffe Anthoni, Carsten Christophersen,*
and Per Halfdan Nielsen
Marine Chemistry Section, Department of Chemistry,
University of Copenhagen, Universitetsparken 5,
DK-2100 Copenhagen, Denmark
ABSTRACT
The main phenolic by-products in the O-benzylation of
1,2,4-trihydroxy- or 1,2,4-triacetoxybenzene to 1,2,4-tribenzy-
loxybenzene have been identified as due to incomplete O-ben-
zylation and C-benzylation. In addition, dibenzyldimethyl-
ammonium chloride is formed from DMF and benzyl
chloride. The main factors controlling these reactions paths
are discussed. An efficient procedure for preparing 1,2,4-tri-
benzyloxybenzene using NaH in DMF is reported.
1,2,4-Tribenzyloxybenzene (4A), by virtue of the protected hydroxy
groups, is a convenient starting materialfor preparation of compounds
containing the 1,2,4-trihydroxyphenyl group. Examples include metabolites
of dopamine1,2 and models for topaquinone, the cofactor of mammalian
copper amine oxidases.3 The reported method involves benzylation of 1,2,4-
triacetoxybenzene (1b) with benzylcholride and potassium carbonate in
* Corresponding author.
2223
Copyright & 2001 by MarcelDekker, Inc.
www.dekker.com

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ANTHONI, CHRISTOPHERSEN, AND NIELSEN
aqueous acetone.1,2 However, only a fair yield (around 35%) of pure
isomer-free 4A was obtained. From analysis of the array of by-products it
seemed feasible to minimize the occurrence of side reactions and devise an
improved method for preparation of 4A. The preparation was therefore
repeated and the products separated by column chromatography. In addi-
tion to 4A, by-products were found to arise both from incomplete O-benzy-
lation and from C-benzylation.
Conventionaltheory predicts that the use of the triacetate 1b instead
of 1,2,4-trihydroxybenzene (1a) minimizes oxidative loss of phenol.
Moreover, the formation of dianions (with increased tendency for C-benzy-
lation4) is hindered. Finally, the acetyl group decreases the electron density
of the benzene ring and accordingly the tendency for C-benzylation.5 The
2-OAc group derived from the most acidic hydroxy group (pKA ¼ 9.1 and
11.6 for 2- and 4-OH, respectively, pKA <11.6 for 1-OH.6) has the highest
reactivity while steric approach control favours formation of the 4-OBn
group. Since acetate hydrolysis appears to be slow with potassium carbonate
in aqueous acetone, a stronger base (e.g. NaH in DMF) is essentialfor rapid
formation of 4A. The amount of the C-benzylation is expected to decrease if
the phenolate anion is unsolvated (‘‘naked anion’’) but the cation strongly
solvated. Aqueous acetone is a poor choice because water form strong
hydrogen bonds to the phenolate anion.7 In contrast, the high dielectricity
constant and preference for cation solvatisation makes DMF an attractive
alternative.8
Similar problems have been reported for the benzylation of phloro-
glucinol to 1,3,5-tribenzyloxybenzene.9–12 However, dropwise addition of
water to a mixture of 1,3,5-triacetoxybenzene, benzylcholride and NaH
in DMF is reported to yield 97% of a product devoid of C-benzylated
isomers.5 Under these reaction conditions the OAc-groups are hydrolyzed
stepwise followed by
a fast O-benzylation of the naked anions.
Unfortunately, this procedure applied to 1b furnished only a 45% yield of
pure 4A. Seven phenolic products were identified in the mother liquor
as arising from sequentialbenzyal tion of 1,2,4-triacetoxybenzene as shown
in the scheme. Mono- and di-O-benzylation proceeds with steric approach
controlvia (the acetyalted) 2A to 3A and 3B. The observation of a series
of products arising from incomplete O-benzylation indicates that hydrolysis
of 1,2,4-triacetoxybenzene is the rate determining step even when NaH in
DMF is used. Introduction of further benzylgroups in the O-benzylated 4A,
as expected, affords only small amounts of the C-benzylated 4B and 5.
A substantial amount of dibenzyldimethylammonium chloride (6) was
observed in the aqueous phase after quenching the reaction mixture with
water. Since this may act both as a phase transfer reagent and as a detergent,
1
clean extraction of 4A with EtOAc was not possible. H NMR indicated

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O-BENZYLATION OF POLYPHENOLICS
2225
mixtures of DMF-d7 with benzyl chloride and NaH to be stable for at least
5 h at room temperature although benzyl chloride and DMF when heated
for 6 h at 150ꢀC gives rise to a mixture of benzylmethylammonium chlor-
ides.13 In our experiment, 1b, benzyl chloride and NaH were allowed to
react for only 2.5 h in DMF at room temperature. Accordingly, 6 is prob-
ably formed during the reaction by C–N fission of the tetrahedral DMF-
phenolate addition product assisted by concomitant N-attack of benzyl
chloride.
Based upon these observations an improved method for preparation
of 4A was developed. The reaction rate is increased by substituting the free
phenol 1a for 1b. A 10% excess of NaH, vigorous stirring , and a prolonged
reaction time secures complete conversion of 1a to phenolate in the hetero-
geneous reaction. Addition of NaH in small portions prevents formation of
polyphenolate anions. A 10% excess of benzyl chloride compensates for the
competing formation of 6. This method furnished an excellent yield of a
mixture (88:12) of the tribenzylated 4A and 4B which can be separated by
recrystallization or column chromatography.
EXPERIMENTAL
The 1H and 13C NMR spectra were recorded on a Varian UNITY 400
spectrometer. The position of the OH proton signals were established by
exchange with D2O when necessary. Melting points are uncorrected. Mass
spectra were obtained on a JEOL JMS-HX/HX110A spectrometer. The IR
spectra originate from a Perkin Elmer FT-IR 1760x instrument. The DMF

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ANTHONI, CHRISTOPHERSEN, AND NIELSEN
was dried for one week with activated molecular sieves. Benzyl chloride was
freshly distilled before use.
1,2,4-Tribenzyloxybenzene (4A) from 1,2,4-triacetoxybenzene (1b) and
benzyl chloride. The reported procedure1,2 using K2CO3 in aqueous acetone
was followed, however, recrystallization and/or purification by VLC only
afforded pure 4A in low yield. The remanence was separated by LC (Si-60
column, EtOAc/heptane 15:85) and shown (NMR, TLC) to contain mainly
2,4-dibenzyloxy-5-benzylphenol (4B) together with smaller amounts of
mono- and di-O-benzylderivatives of 1,2,4-trihydroxybenzene.
An improved yield was obtained using NaH in DMF. A solution of
benzylchol ride (18.4 g, 146 mmo)l, 1b (10.4 g, 40 mmol) and NaH (60% in
mineral oil, 12.2 g, 304 mmol) in dry DMF (200 ml) was flushed with nitro-
gen for a few minutes. With efficient stirring, water (2.2 ml, 120 mmol) was
added via a syringe over a period of ca. 0.5 h. The reaction, monitored by
formation of hydrogen, was complete in a further 2 h. When the almost
colourless mixture was quenched with water (200 ml) a dark brown colour
developed. Extraction with EtOAc (2 Â 300 ml), washing the extract with
water, drying with MgSO4 and evaporation to dryness left the crude, dark
brown product (10 g). Recrystallization from hexane/toluene (95/5) furn-
ished pure 4A (7 g, 45% yield). Mass spectrometry indicated that small
amounts of a tetrabenzylic compound, probably 5, was removed by this
procedure.
The aqueous phase contained a considerable amount (ca 1g) of 6. The
mother liquor from recrystallization of 4A was purified by VLC (EtOH/
H2O) and, when necessary, further purified by LC. The first three fractions
contained 2A in addition to smaller amounts of other O- and C-monoben-
zylated products. The following fraction consisted of a mixture of almost
equalamounts of the dibenzylated 3A and 3B. The finalfractions contained
4A with small amounts of 4B.
1,2,4-Tribenzyloxybenzene (4A) from 1,2,4-trihydroxybenzene (1a),
benzyl chloride and NaH in DMF. A solution of benzyl chloride (16.6 g,
132 mmol) and 1a (5.0 g, 40 mmol) in dry DMF (100 ml) was carefully
flushed with nitrogen at room temperature. 60% NaH in mineraloil
(5.3 g, corresponding to 3.2 g pure NaH, 132 mmol) is added in small por-
tions with vigorous stirring over a period of 3.5 h. The slightly exothermic
reaction monitored by evolution of hydrogen was complete in a further 1.5 h
leaving a dark brown reaction mixture. After dilution with water (250 ml)
extraction with EtOAc (250 ml, then 100 ml) was feasible. The EtOAc
extracts were combined, filtered, and washed with water (2 Â 100 ml) to
remove the dark brown water-soluble by-products. Drying with MgSO4
and evaporation to dryness followed by freeze-drying (to remove DMF)
1
left a light brown solid (13.9 g) shown by H NMR to consist solely of

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O-BENZYLATION OF POLYPHENOLICS
2227
4A (88%) and 4B (12%). On extraction of the combined aqueous washings
with 100 mlEtOAc further material(1.4 g) with simialr composition was
secured.
Boiling the combined raw materials (15.3 g) with heptane/EtOAc
(95/5) left a dark insoluble material (0.4 g) and gave a light tan solution.
Evaporating the solvent and removing the mineral oil (ca. 1 g) left a crystal-
line residue (12.2 g, 77% yield). Recrystallization once from hexane/toluene
(95/5) using active carbon furnished almost colourless 4A (10.8 g, 69% yield,
mp 79–80ꢀC). Alternatively, 4A and 4B could be separated by column chro-
matography (SiO2, EtOAc/heptane 1/6).
1,2,4-Tribenzyloxybenzene (4A). Mp 81–82ꢀC (lit 181–82ꢀC, 1480–
81ꢀC, 282ꢀC). TLC (SiO2, EtOAc/heptane 3:7) Rf ¼ 0.50. EIMS m/z (relative
%): 396 [Mþ](24), 305 [Mþ-benzyl](14), 181 (11), 91 [benzylþ](100). 1H
NMR (CDCl3, 400 MHz, d): 4.96, 5.09, and 5.12 (CH2, s), 6.46 (H5, dd,
J ¼ 8.7, 2.9 Hz), 6.66 (H3, d, J ¼ 2.9 Hz), 6.86 (H6, d, J¼ 8.7 Hz), 7.2–7.5 (15
1
Ar-H, m), H NMR(DMSO-d6, 400 MHz, d): 5.01, 5.03, and 5.13 (CH2, s),
6.51 (H5, dd, J ¼ 8.7, 2.7 Hz), 6.79 (H3, d, J ¼ 2.8 Hz), 6.94 (H6, d,
J ¼ 8.8 Hz), 7.3–7.5 (15 Ar-H, m). 13C NMR (CDCl3, 100 MHz, d): 70.4,
71.0, and 72.5 (CH2), 103.7 (C3), 105.6 (C5), 116.9 (C6), 127.2–128.4
(16C), 136.9, 137.5, 143.1, 150.1, 154.0. IR (KBr, cmꢁ1): 1609 m, 1594 m,
and 1514 vs (aryl), 1227 vs (aryl-O), 1007 s (O-CH2). Anal. Calc. for
C27H24O3: C 81.79; H 6.10. Found: C 81.60; H 6.03.
2,4-Dibenzyloxy-5-benzylphenol (4B). Mp 107–108ꢀC. TLC (SiO2,
EtOAc/heptane 3:7) Rf ¼ 0.44. EIMS m/z (relative %): 396 [Mþ](9), 305
[Mþ-benzyl](4), 181 (8), 91 [benzylþ](100). 1H NMR (CDCl3, 400 MHz,
d): 4.71 and 5.04 (O-CH2, s), 4.93 (C-CH2, s), 6.57 (H3, s), 6.86 (H6, s),
1
7.10–7.42 (15 Ar-H, m), 7.4 (OH). H NMR (DMSO-d6, 400 MHz, d): 5.16
(O-CH2, s), 4.95 and 4.93 (CH2, s), 6.92 and 6.94 (H3 and H6, s), 7.20–7.47
(15 Ar-H and OH, m) 13C NMR (CDCl3, 100 MHz, d): 71.6, 71.7, and 72.2
(CH2), 103.8 (C3), 119.5 (C6), 120.8, 127.0–128.4 (15C), 137.2, 137.4, 137.5,
143.0, 148.6, 150.8. IR (KBr, cmꢁ1): 1601 m and 1507 s (aryl), 1215 vs (aryl-
O), 1024 s (O-CH2). Anal. Calc. for C27H24O3: C 81.79; H 6.10. Found C
82.23; H 6.12. H3 and H6 in CDCl3 were assigned by comparison with 3B
and 4A. The regiochemistry could now be inferred from cross peaks in the
ROESY spectrum (CDCl3, mixing time 200 ms) between H3 and the O-
benzylic methylene protons (assigned to positions 2 and 4) in combination
with a cross peak between H6 and the C-benzylic methylene protons known
to be located at position 5.
2,5-Dibenzyloxyphenol (3A). TLC (SiO2, EtOAc/heptane 3:7) Rf ¼ 0.38.
1H NMR (DMSO-d6, 400 MHz, d): 4.96 and 5.00 (CH2, s), 6.34 (H4, dd,
J ¼ 8.7, 3.0 Hz), 6.50 (H6, d, J ¼ 3.0 Hz), 6.85 (H3, d, J ¼ 8.9 Hz), 7.25–7.50
1
(Ar-H, m), 9.12 (OH, s). H NMR (CDCl3, 400 MHz, d): 3–5 (OH, very

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ANTHONI, CHRISTOPHERSEN, AND NIELSEN
broad), 5.00 and 5.05 (CH2, s), 6.44 (H4, dd, J ¼ 8.7, 3.0 Hz), 6.65 (H6, d,
J ¼ 3.0 Hz), 6.83 (H3, d, J ¼ 8.8 Hz), 7.3–7.5 (10 Ar-H, m). 13C NMR
(DMSO-d6, 100 MHz, d): 69.5 and 70.9 (CH2), 103.8 (C6), 104.2 (C4), 115.9
(C3), 127.6–128.4 (10C), 137.4, 137.7, 141.0 (C2), 148.1 (C1), 153.5 (C5). The
regiochemistry was established (HSQCR, HMQCR optimized for J ¼ 7 Hz)
by long-range coupling from the OH proton to C1, C2, and C6 in DMSO-
d6. The compound has been reported previously, however, without any
NMR data.15
2,4-Dibenzyloxyphenol (3B). TLC (SiO2, EtOAc/heptane 3:7)
1
Rf ¼ 0.38. H NMR (DMSO-d6, 400 MHz, d): 4.97 and 5.09 (CH2, s), 6.43
(H5, dd, J ¼ 8.6, 3.0 Hz), 6.69 (H6, d, J ¼ 8.4 Hz), 6.69 (H3, d, J ¼ 3.0 Hz),
7.25–7.50 (Ar-H, m), 8.51 (OH, s). 1H NMR (CDCl3, 400 MHz, d): 3–
5 (OH, very broad), 4.99 and 5.06 (CH2, s), 6.50 (H5, dd, J ¼ 8.6, 2.7 Hz),
6.65 (H3, d, J ¼ 2.8 Hz), 6.85 (H6, d, J ¼ 8.6 Hz), 7.3–7.5 (10Ar-H, m). 13C
NMR (DMSO-d6, 100 MHz, d): 69.8 and 69.9 (CH2), 103.1 (C3), 106.1 (C5),
115.7 (C6), 127.6–128.4 (10C), 137.4, 137.5, 141.1 (C1), 147.1 (C2), 151.5
(C4). The regiochemistry was established (HSQCR, HMQCR optimized for
J ¼ 7 Hz) by long-range coupling from the OH proton to C1, C2, and C6
1
in DMSO-d6. The H NMR data in CDCl3 concur with those previously
published.16
4-Benzyloxycatechol (2A). TLC (SiO2, EtOAc/heptane 3:7) Rf ¼ 0.12.
1
EIMS m/z (relative%): 216 [Mþ](30), 91 [benzylþ](100). H NMR (DMSO-
d6, 400 MHz, d): 4.94 (CH2, s), 6.07 (H5, dd, J ¼ 8.6, 2.8 Hz), 6.33 (H3, d,
J ¼ 2.8 Hz), 6.71 (H6, d, J ¼ 8.6 Hz), 7.4–7.6 (15 Ar-H, m). 13C NMR
(DMSO-d6, 100 MHz, d): 71.3 (CH2), 103.9 (C3), 105.0 (C5), 116.8 (C6),
127.5–129.0 (6C), 139.4 (C1), 148.2 (C2), 152.4 (C4). The regiochemistry
was established (HSQCR, HMQCR optimized for J ¼ 7 Hz) by long-range
coupling from the CH2 protons to C4.
Dibenzyldimethylammonium chloride (6). An authentic sample was
prepared according to the directions given by Kantor and Hauser.17 Mp
94–95ꢀC (litP 94–95ꢀC). FABMS (relative %): m/z 226 [M]þ(100), 488 þ 490
1
[MCl þ M]þ(25). H NMR (DMSO-d6, 400 MHz, d): 2.90 (2CH3, s), 4.75
(2CH2, s), 7.50–7.54 (6Ar-H, m), 7.61–7.65 (4Ar-H, m). 13C NMR (DMSO-
d6, 100 MHz, d): 48.0 (CH3), 66.8 (CH2), 128.1, 128.8, 130.2, 133.1. IR (KBr,
cmꢁ1): 1621 m, 1598 m, 1584 m and 1494 s (phenyl), 1479 vs and 1455 vs
(CH2Nþ/CH3Nþ). Elemental analysis indicated a monohydrate: Calc. for
C16H22NOCl: C 68.68; H 7.92; N 5.01. Found: C 68.54; H 7.87; N 4.85.
In order to clarify the origin of 6 observed in the reaction mixture from
1b, benzylchol ride and NaH in DMF, 1H NMR spectra were recorded of
a solution of benzyl chloride (70 ml) in DMF-d7 (650 ml) at room tempera-
ture. Since the spectrum was unchanged for 48 h, NaH in mineraloil(30 mg)
1
was added. During the first 5 h no changes were detected in the H NMR

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O-BENZYLATION OF POLYPHENOLICS
2229
spectrum, but after a further 72 h complete conversion had occurred to a
complicated mixture containing ca 25% 6.
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Received in the UK August 14, 2000

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