Novel transformation of methoxy tetralones during demethylation with boron trifluoride etherate and acetic anhydride

Article abstract of DOI:10.1081/SCC-200030639

Tetralones 3 and 9 on treatment with a mixture of boron trifluoride etherate and acetic anhydride underwent molecular transformations yielding the fully aromatized products 6 and 10, respectively, Oxidation of dimethoxy tetralin 13 with chromic acid and acetic acid afforded 6-methoxy-α- tetralone 12.

Full text of DOI:10.1081/SCC-200030639

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Novel Transformation of
Methoxy Tetralones During
Demethylation with Boron
Trifluoride Etherate and
Acetic Anhydride
Ajoy K. Banerjee a , Liadis Bedoya a , William J.
Vera a , Carlos Melean a , Henry Mora a , Manuel S.
Laya a & Miguel Alonso b
a Centro de Química, IVIC, Apartado 21827,
Caracas, 1020‐A, Venezuela
b Química Ecológica, Universidad de Los Andes,
Departamento de Química, Facultad de Ciencias,
Mérida, Venezuela
Version of record first published: 10 Jan 2011.
To cite this article: Ajoy K. Banerjee , Liadis Bedoya , William J. Vera , Carlos
Melean , Henry Mora , Manuel S. Laya & Miguel Alonso (2004): Novel Transformation
of Methoxy Tetralones During Demethylation with Boron Trifluoride Etherate and
Acetic Anhydride, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 34:18, 3399-3408
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 18, pp. 3399–3408, 2004
Novel Transformation of Methoxy
Tetralones During Demethylation with
Boron Trifluoride Etherate and Acetic
Anhydride
1
1
Ajoy K. Banerjee,1, Liadis Bedoya, William J. Vera,
Carlos Melean,1 Henry Mora,1 Manuel S. Laya,1
and Miguel Alonso2
*
1
´
Centro de Quımica, IVIC, Caracas, Venezuela
´ ´
Quımica Ecologica, Universidad de Los Andes, Departamento de
2
´ ´
Quımica, Facultad de Ciencias, Merida, Venezuela
ABSTRACT
Tetralones 3 and 9 on treatment with a mixture of boron trifluoride
etherate and acetic anhydride underwent molecular transformations
yielding the fully aromatized products 6 and 10, respectively, Oxidation
of dimethoxy tetralin 13 with chromic acid and acetic acid afforded
6-methoxy-a-tetralone 12.
Key Words: Demethylation; Aromatization; Boron trifluoride etherate;
Acetic anhydride; Chromic acid; Acetic acid; Water; Transformations.
*
´
Correspondence: Ajoy K. Banerjee, Centro de Quımica, IVIC, Apartado 21827,
Caracas 1020-A, Venezuela; E-mail: abanerje@ivic.ve.
3399
DOI: 10.1081/SCC-200030639
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

3400
Banerjee et al.
INTRODUCTION
The use of boron trifluoride etherate and acetic anhydride[1] to cleave
different types of steroidal methyl ethers to the corresponding acetates in
high yield encouraged us to synthesize dimethoxy tetralones 3 and achieve
its transformation to acetoxy tetralone 4, which can easily be converted by
standard organic reactions to 8-methoxy-2-tetralone[2] 5. (Sch. 1). This is an
important starting material in the syntheses of compounds for the study of
dopamine (DA) and serotonin (5-HT) receptors. Tetralone 3, prepared from
the 5-methoxy-a-tetralone 1, underwent an unexpected transformation upon
treatment with boron trifluoride etherate and acetic anhydride. This obser-
vation prompted us to synthesize dimethoxy tetralones 9 and 14 and
examine their reactions with the same reagent. The present paper documents
the results of our investigation.
RESULTS AND DISCUSSIONS
Reduction of commercially available 5-methoxy-a-tetralone 1 (Sch. 1)
with sodium borohydride yielded an alcohol that was methylated by using
sodium hydride and methyl iodide to obtain dimethoxy tetralin 2. This, on
oxidation with chromic acid, acetic acid, and water, afforded tetralone 3 in
35% yield. Unfortunately, attempted oxidation of tetralin 2 under a variety
of conditions (changing the amount of chromic acid and acetic acid, time
period) failed to improve the yield of tetralone 3. It appears that the
Scheme 1.

Novel Transformation of Methoxy Tetralones During Demethylation
3401
methoxy group at aromatic ring partially inhibited the process of oxidation.
The latter event supports this assumption.
Tetralone 3 upon treatment with boron trifluoride and acetic anhydride at
room temperature underwent rearrangement to afford a solid compound (50%)
whose spectroscopic properties indicated a structure far removed from the
desired product 4. Absorption at 1750 and 1650 cm21 in the infrared (IR)
spectrum indicated the presence of ester and carbonyl group. Its mass spec-
trum displayed parent peak at m/z 258 (Mþ). The 1H NMR spectrum
showed signals at d 2.35 (s, OAc), 2.67 (s, COMe), 3.97 (s, OMe), and
8.82–6.80 ppm (5H, aromatic protons). These spectral data led us to assume
the formation of product 6 during the demethylation of tetralone 3. The pre-
sence of COMe group was revealed in the 13C NMR spectrum by a resonance
at d 170.10 ppm. The rearrangement from 3 to 6 is a mechanistically novel
process, a step-wise description of which is described in Sch. 2.
Scheme 2.

3402
Banerjee et al.
Tetralone 3 forms the carbocation (i) by the elimination of OMe group,
which produces intermediate (iii) by boron trifluoride etherate. This reacts
with acetic anhydride to form the ketone (iv) that leads the formation of
compound 6.
By addition of compound 6, dimethoxy tetralone 3 (12%) whose identity
was confirmed by comparing its spectral data with an authentic specimen
already prepared was recovered.
The demethylation study was then attempted on dimethoxy-a-tetralone 9,
prepared from the commercially available 7-methoxy-a-tetralone 7 by a
procedure similar to that described in Sch. 1. Demethylation of 9 with the same
reagent afforded a crystalline material (60%) that was homogeneous on thin
layer chromotography (TLC) in several systems. It had molecular ion m/z
216 (Mþ) and 173 (Mþ-COMe) in mass spectrum and exhibited absorption
1745 (CO) cm21 in the IR spectrum. The singlet signals at d 3.95 (3H) and
2.62 (3H) indicated the presence of methoxy and acetate group. The signals
at d 7.74 (d, J ¼ 9 Hz), 7.61 (d, J ¼ 8 Hz), (1H), 7.39 (t, J ¼ 7 Hz), (1H),
7.16–7.07 (m, 3H), and 7.08 (s, 1H) ppm were assignable to six aromatic
protons. These spectroscopic data led us to assign the structure 10 for the
solid compound obtained during the demethylation of tetralone 9. The for-
mation of 10 may be rationalized by the reaction path, as depicted in Sch. 3.
Scheme 3.

Novel Transformation of Methoxy Tetralones During Demethylation
3403
The formation of intermediate (ii) from 9 can be explained by the mech-
anism already described in Sch. 2. The intermediate (ii) with acetic anhydride
produces compound 10.
With addition of compound 10, a small amount of oily material was also
obtained, which contained a mixture of products (by TLC). It had a molecular
ion m/z 258 (Mþ) and exhibited in IR spectrum absorption at 1738 and
1680 cm21 (carbonyl). These data led us to believe the presence of product
11 in the reaction mixture. However, no attempt was made to isolate and
characterize compound 11 owing to the small amount of material contami-
nated with other products.
The above findings led us to examine the demethylation of tetralone 14,
which was expected to be obtained easily from 6-methoxy-a-tetralone 12 by
the process already described. Metal hydride reduction of tetralone 12 fol-
lowed by methylation yielded compound 13. An interesting transformation
occurred when oxidation of 13 was attempted with chromic acid and acetic
acid. Instead of expected tetralone 14, 6-methoxy-a-tetralone 12 was obtained
in high yield, whose identity was confirmed by comparison of M.P., mixed,
M.P., NMR, IR, and mass spectral properties as well as TLC mobility with
an authentic specimen (Aldrich) of 6-methoxy-a-tetralone. A probable
interpretation of this unexpected transformation is shown in Sch. 4. It is
likely that the effect of resonance facilitates the formation of diene intermedi-
ate (ii), which is converted to intermediate (iv) by nucleophilic attack of water
to the intermediate (iii). This undergoes further oxidation to yield tetralone 12.
Scheme 4.

3404
Banerjee et al.
Tetralins 2 and 8 do not experience a similar type of transformation during
oxidation because of the absence of such resonance effect.
In conclusion, we have described interesting transformations of methoxy
tetralones during demethylation with boron trifluoride etherate and acetic
anhydride. The chemical literature reveals that many organic compounds
undergo molecular transformations[3–5] with boron trifluoride etherate. To
the best of our knowledge, the boron trifluoride-acetic anhydride mediated
transformations during demethylation have not been reported. A novel trans-
formation of methoxy tetralin 13 during oxidation has been reported. It can
also be observed that the yield of tetralone 9 was more satisfactory than tetra-
lone 3. This observation indicates that the absence of 8-methoxy group in
tetralin 8 facilitates in oxidation reaction, whereas its presence causes
partial hindrance in oxidative attack.
EXPERIMENTAL
Unless otherwise stated, IR spectra were taken on Nicolet FT instrument.
1H NMR spectra were recorded on Brucker AM 300 Hz in CDCl3. Mass
spectra were run on gas chromatography Hewlett Packard 5890 Quadrupolar
5972 Serie S. The expression workup indicates that the solution is diluted with
water, extracted with ether, washed with brine, dried (MgSO4), and evapor-
ated under reduced pressure. Column chromatography was performed on
silica gel (Merck grade 60, 70–230 mesh). The spectral and analytical data
of all new compounds have been reported in the Experimental Section. Micro-
analyses were carried out at the Chemistry Department, IVIC, Caracas.
1,5-Dimethoxy Tetralin 2
A mixture of tetralone 1 (1.25 g, 0.006 mol) and sodium borohydride
(645 mg, 0.011 mol) in ethanol (100 mL) was stirred at 08C for 1 hr. The
mixture was diluted with aqueous ammonium chloride (25 mL) and extracted
with ether. The organic extract was washed, dried, and evaporated to give an
alcohol (1.24 g), vmax 3262 cm21 (OH) that, being completely free from tetra-
lone (by TLC), was used for the methylation.
A solution of alcohol (1.24 g) in tetrahydrofuran (25 mL) was added drop-
wise to a suspension of sodium hydride (50% mineral oil, 1.92 g, 0.08 mol) in
tetrahydrofuran (20 mL) under nitrogen. The mixture was warmed for 10 min,
followed by the addition of methyl iodide (14 mL) and then heated under
reflux for 6 hr. The usual workup afforded a viscous oil that, on chromato-
graphic purification (eluant hexane : ether, 9 : 1), afforded the oily tetralin 2

Novel Transformation of Methoxy Tetralones During Demethylation
3405
(1.18 g, 87%); m/z 192 (Mþ) and 160 (Mþ-MeOH); d 7.14 (m, 1H), 6.96
(d, J ¼ 8 Hz, 1H), 6.96 (d, J ¼ 8 Hz, 1H) (aromatic protons), 4.28 (m, 1H),
3.79 (s, 3H, OMe), 3.47 (s, 3H, OMe), 1.72–2.53 (m, 6H, methylene
protons) (Found: C, 75.23; H, 8.57. C12H16O2 requires C, 74.97; H, 8.39%).
4,8-Dimethoxy-a-tetralone 3
To a solution of tetralin 2 (500 mg, 0.002 mol) in acetic acid (5 mL),
cooled to 08C, was added a solution of chromic acid (1 g, 0.01 mol) in
water (1 mL) and acetic acid (10 mL). The reaction mixture was stirred at
room temperature, and the progress of the reaction was monitored by tlc.
After this mixture had been stirred for 5 hr, water (20 mL) was added and
extracted with dichloromethane. The dried extract on evaporation yielded a
dark red material that was chromatographed. Hexane : ether (6 : 4) elute
yielded tetralone 3 (187 mg, 35%); m/z 206 (Mþ) and 162 (C10H10O2);
vmax 1680 cm21 (CO), d 7.47 (m, 1H), 7.02 (d, J ¼ 7 Hz, 1H), 6.95
(d, J ¼ 8 Hz, 1H) (aromatic protons); 4.34 (m, 1H), 3.89 (s, 3H) (OMe),
3.36 (s, 3H, OMe), 2.91–2.23 (m, 6H, methylene protons); 13C NMR: d
196.17 (C-1), 159.54 (C-8), 144.66 (C-10), 133.51 (C-6), 120.77 (C-9),
119.70 (C-5), 111.80 (C-7), 76.39 (C-4), 55.90 (C-12), 55.67 (C-11), 34.94
(C-2), and 26.79 (C-3) (Found: C, 70.11; H, 6.96. C12H14O3 requires C,
69.88; H, 6.84%).
1-Acetyl-4-ketomethyl-8-methoxy Naphthalene 6
A solution of tetralone 3 (185 mg, 0.0008 mol) in acetic anhydride (8 mL)
was cooled to 08C and to this added freshly distilled boron trifluoride etherate
(1.5 mL), which had been previously cooled to 08C. The reaction mixture was
stirred (15 hr) till no further progress was observed in tlc monitoring and then
poured into ice cold water, stirred for 1 hr, and then extracted with ether. The
ether extract was washed, dried, and evaporated. Purification of the residue
from hexane : ether (7 : 3) afforded methoxy naphthalene 6 (1.16 mg, 50%);
M.P. 1388C–1408C (from ether); m/z 258 (Mþ), vmax 1750, 1650 cm21
(CO); d 8.82 (dd, J ¼ 1 Hz), 8.12 (dd, J ¼ 1 Hz), (1H), 7.94 (d, J ¼ 8 Hz,
1H), 7.55 (dd, J ¼ 7 Hz) 7.53 (dd, J ¼ 7 Hz) (1H), 7.11 (dd, J ¼ 1 Hz), 7.10
(dd, J ¼ 1 Hz) (1H), 6. 80 (d, J ¼ 8 Hz, 1H) (aromatic protons), 3.97 (s, 3H,
OMe), 2.67 (s, 3H, COMe), and 2.35 (s, 3H, OCOMe), 13C NMR: d 200.17
(C-5), 170.10 (C-4), 158.78 (C-7), 146.57 (C-6), 134.19 (C-8), 131.19 (C-
12), 128.24 (C-10, C-14), 124.01 (C-15), 120.29 (C-11), 119.51 (C-9),
104.06 (C-13), 56.25 (C-3), 29.68 (C-2), and 20.91 (C-1), (Found: C, 70.01;

3406
Banerjee et al.
H, 5.60. C15H14O4 requires C, 69.75; H, 5.46%). Hexane : ether (6 : 4) elute
furnished tetralone (3) (27 mg 12%) whose identity was confirmed by compar-
ing (spectral data and tlc) with an authentic specimen of (3) whose synthesis
has already been described.
1,6-Dimethoxy Tetralin 8
A mixture of tetralone 7 (1.52 g, 0.008 mol) and sodium borohydride
(755 mg, 0.022 mol) in ethanol (100 mL) was stirred at 08C for 1 hr. The
mixture was diluted with aqueous solution of ammonium chloride (25 mL)
and extracted with ether. The extract was washed, dried, and evaporated to
afford an alcohol (1.55 g), vmax 3420 cm21 (OH) that was completely free
from tetralone 7, as evidenced by TLC, and therefore was directly used
without further purification.
A solution of alcohol (1.53 g) in tetrahydrofuran (30 mL) was added
drop-wise to a suspension of sodium hydride (50% mineral oil, 2.02 g) in tetra-
hydrofuran (25 mL) under nitrogen. The mixture was warmed for 10 min,
followed by the addition of methyl iodide and then heated under reflux for
6 hr. The reaction mixture was cooled, diluted with water, and extracted
with ether. The dried extract was evaporated and the resulting residue was
chromatographed; elution with hexane : ether (9 : 1) afforded the oily tetralin
8 (1.45 g, 88%); m/z 192 (Mþ) and 160 (Mþ-MeOH); d 7.03 (d, J ¼ 8 Hz,
1H), 6.93 (d, J ¼ 2.6 Hz, 1H), 6.78 (d, J ¼ 8.4 Hz, 1H) (aromatic protons),
4.31 (m, 1H), 3.82 (s, 3H, OMe), 3.47 (s, 3H, OMe), 1.74–2.76 (m, 6H,
methylene protons), (Found: C, 75.18; H, 8.55. C12H16O2 requires C, 74.97;
H, 8.39%).
4,6-Dimethoxy-a-tetralone 9
To a solution of tetralin 8 (850 mg, 0.004 mol) in acetic acid (10 mL),
cooled to 08C, was added a solution of chromic acid (2 g, 0.002 mol) in
water (3 mL) and acetic acid (15 mL). The reaction mixture was stirred at
room temperature and the progress of the reaction was monitored by TLC.
After this mixture had been stirred for 6 hr, water (25 mL) was added and
extracted with dichloromethane. The dried extract was evaporated and the
residual oil on chromatographic purification (eluant hexane : ether, 6 : 4)
yielded tetralone 9 (546 mg, 60%); m/z 206 (Mþ) and 162 (C10H10O2);
vmax 1685 cm21 (CO); d 8.04 (d, J ¼ 8.7 Hz, 1H), 7.01 (d, J ¼ 8.6 Hz, 1H),
6.95 (m, 1H) (aromatic protons), 4.44 (m, 1H), 3.92 (s, 3H, OMe), 3.51
(s, 3H, 1H), 2.95–2.24 (m, 6H, methylene protons); 13C NMR: d 196.32

Novel Transformation of Methoxy Tetralones During Demethylation
3407
(C-1), 163.76 (C-6), 145.35 (C-10), 129.77 (C-8), 125.18 (C-9), 114.39 (C-7),
112.04 (C-5), 77.16 (C-4), 56.49 (C-11), 55.49 (C-12), 34.43 (C-2), and 27.90
(C-3), (Found: C, 70.14; H, 6.95. C12H14O3 requires C, 69.88; H, 6.84%).
1-Acetyl-6-methoxy Naphthalene 10
A solution of tetralone 9 (512 mg, 0.002 mol) in acetic anhydride (22 mL)
was cooled to 08C and to this added freshly distilled boron trifluoride etherate
(3.8 mL), which had been previously cooled to 08C. The reaction mixture was
stirred (18 hr) till no further progress was observed in tlc monitoring and then
poured into ice cold water, stirred for 1 hr, and then extracted with ether. The
ether extract was washed, dried, and evaporated to yield an oily residue, which
was chromatographed. Hexane : ether (8 : 2) elute contained compound 10
(321 mg, 60%), M.P. 728C–738C (from ether); m/z 216 (Mþ) and 173
(Mþ-MeCO); vmax 1745 cm21 (CO); d 7.74 (d, J ¼ 9 Hz, 1H), 7.61
(d, J ¼ 8 Hz, 1H), 7.39 (t, J ¼ 7 Hz, 1H), 7.16–7.07 (m, 3H), and 7.08
(s, 1H) (aromatic protons), 3.95 (s, 3H, OMe) and 2.62 (s, 3H, OCOMe);
13C NMR: d 169.47 (CO), 158.02 (C-7), 146.75 (C-4), 136.05 (C-10),
126.13 (C-2), 124.81 (C-1), 122.77 (C-9), 122.09 (C-5), 119.33 (C-6),
115.81 (C-3), and 105.89 (C-8), (Found: C, 72.42; H, 5.75. C13H12O3 requires
C, 72.21; H, 5.59%).
1,6-Dimethoxy Tetralin 13
To tretalone 12 (302 mg, 0.001 mol), in ethanol (12 mL), sodium boro-
hydride (170 mg, 0.006 mol) was added portionwise and stirred for 4 hr at
room temperature. The mixture was diluted with water and thoroughly
extracted with ether. The combined extracts were washed, dried, and evapora-
ted to yield alcohol (304 mg); vmax 3257 cm21 (OH). This was completely free
from tetralone 12 (by TLC) and IR spectrum and thus used for methylation
without purification.
A solution of alcohol (302 mg) in tetrahydrofuran (10 mL) was added
drop-wise to a stirred suspension of sodium hydride (50% mineral oil, 410 mg)
in tetrahydrofuran (10 mL) under nitrogen. The mixture was warmed for
10 min, followed by the addition of methyl iodide (5 mL) and then heated
under reflux for 6 hr. The reaction mixture was cooled, diluted with water,
extracted with ether, dried, and evaporated. The oily residue on chromato-
graphic purification (hexane : ether, 9 : 1) yielded dimethoxy tetralin 13
(289 mg; 88%); m/z 192 (Mþ) and 160 (Mþ-MeOH); d 7.23 (s, 1H), 6.72
(d, 1H, J ¼ 8 Hz), 6.59 (d, 1H, J ¼ 2 Hz) (aromatic protons), 4.24 (m, 1H,

3408
Banerjee et al.
1-H), 2.68–1.71 (m, 6H), 3.86 (s, 3H, OMe), and 3.75 (s, 3H, OMe), (Found:
C, 75.24; H, 8.58. C12H16O2 requires C, 74.97; H, 8.39%).
6-Methoxy-a-tetralone 12
To a solution of tetralin 13 (305 mg, 0.001 mol) in acetic acid (2 mL),
cooled to 08C, was added a solution of water (1 mL) and chromic acid
(1.51 g, 0.015 mol). The reaction mixture was stirred (6 hr) till no further pro-
gress was observed in TLC monitoring, diluted with water, and extracted with
dichloromethane. The dried extract on evaporation yielded a semisolid
material that, on crystallization with ether, afforded 6-methoxy-1-tetralone
12 (245 mg, 88%); M.P. 788–798C (778C–798C) (lit. values from Aldrich
Chemical Company), m/z 176 (Mþ); vmax 1670 cm21 (CO); d 8.48 (d, 1H,
J ¼ 8 Hz), 6.79 (d, J ¼ 8 Hz), 6.78 (d, J ¼ 3 Hz), (1H), 6.65 (d, J ¼ 3 Hz,
1H), (aromatic protons), 2.91 (t, 2H, J ¼ 2.5), 2.58 (t, 2H, J ¼ 2.9), 2.13–
2.04 (m, 1H); 13C NMR: d 197.21 (C-1), 163.52 (C-6), 146.94 (C-10),
129.63 (C-8), 126.32 (C-9), 113.01 (C-5), 112.60 (C-7), 55.39 (C-11) 38.88
(C-2), 30.15 (C-4), and 23.35 (C-3), (Found: C, 75.21; H, 6.69. C11H12O2
requires C, 74.97; H, 6.86%).
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Received in the USA June 2, 2004