Facile access to methoxylated 2-phenylnaphthalenes and epoxydibenzocyclooctenes

Article abstract of DOI:10.1016/j.tet.2005.02.091

Methoxylated phenylethanals were treated with concentrated hydrochloric acid in 1,4-dioxane to give methoxylated 2-phenylnaphthalenes or 1,2,9,10-tetrahydro-1,9-epoxydibenzo[a,e]cyclooctenes. Yields in 2-phenylnaphthalenes were quite good and 1,2,9,10-tetrahydro-1,9-epoxydibenzo[a, e]cyclooctenes could be easily isolated. 2-Phenylnaphthalenes were obtained by a tandem aldol condensation-intramolecular Friedel-Crafts cyclisation and 1,2,9,10-tetrahydro-1,9-epoxydibenzo[a,e]cyclooctenes by a O-condensation followed by a double intramolecular Friedel-Crafts alkylation.

Full text of DOI:10.1016/j.tet.2005.02.091

Tetrahedron 61 (2005) 7054–7058
Facile access to methoxylated 2-phenylnaphthalenes
and epoxydibenzocyclooctenes
*
´
Cedric Maurin, Fabrice Bailly and Philippe Cotelle
´
´
Laboratoire de Chimie Organique et Macromoleculaire, UMR CNRS 8009, Universite de Lille 1, 59655 Villeneuve d’Ascq, France
Received 10 January 2005; revised 8 February 2005; accepted 17 February 2005
Available online 3 June 2005
Abstract—Methoxylated phenylethanals were treated with concentrated hydrochloric acid in 1,4-dioxane to give methoxylated
2-phenylnaphthalenes or 1,2,9,10-tetrahydro-1,9-epoxydibenzo[a,e]cyclooctenes. Yields in 2-phenylnaphthalenes were quite good and
1,2,9,10-tetrahydro-1,9-epoxydibenzo[a,e]cyclooctenes could be easily isolated. 2-Phenylnaphthalenes were obtained by a tandem aldol
condensation-intramolecular Friedel–Crafts cyclisation and 1,2,9,10-tetrahydro-1,9-epoxydibenzo[a,e]cyclooctenes by a O-condensation
followed by a double intramolecular Friedel–Crafts alkylation.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Kagan’s ethers 3 are the results of an O-condensation of the
ketal derivatives on the keto forms. The resulting hemi-
acetals cyclize to give benzisopyrans, which cyclize by an
intramolecular Friedel–Crafts reaction to give Kagan’s
ethers (Scheme 1). The balance between the condensations
at O- or C-position depends on the nature of the acid and the
substitution on the aryl ring. The Kagan’s pioneer work1,2
used strong Bro¨nsted acid, that is, fluorosulfonic acid, and
other protic acids (HCl or H2SO4)3,4 in order to promote the
Acid treatment of arylethanals 1 may lead to 2-phenyl-
naphthalenes 2 or 1,2,9,10-tetrahydro-1,9-epoxydibenzo-
[a,e]cyclooctenes 3 (Kagan’s ethers). 2 are obtained by a
C-condensation of the enol form on the keto form. The
resulting aldol condensation product undergoes an intra-
molecular reaction to give, after rearomatisation, the
2-phenylnaphthalenes (Scheme 1).
Scheme 1.
Keywords: 2-Phenylnaphthalenes; Epoxydibenzo[a,e]cyclooctenes; Aldol condensation; Friedel–Crafts reaction.
*
Corresponding author. Tel.: C33 320337231; fax: C33 320336309; e-mail: philippe.cotelle@univ-lille1.fr
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.02.091

C. Maurin et al. / Tetrahedron 61 (2005) 7054–7058
7055
Scheme 2.
aldol condensation of arylethanals. The yields in isolated
products were generally poor and could be increased using
trimethylsilyliodide5 or boron tribromide.4 Using these
reagents, high yields in Kagan’s ether may be obtained
and the cyclisations are regioselective. 2-Phenylnaphthalene
can be isolated in modest yield from the reaction of
phenylethanal with boron tribromide indicating that BBr3
reaction conditions are not favorable for the double Friedel–
Crafts cyclisation, which does not occur when the aromatic
ring is not electronically-rich enough. The acid-catalysed
aldol condensation of phenylacetone has previously been
reported by Cort et al.7 They showed the formation of
1-benzyl-3-methylnaphthalene from phenylacetone and
70% sulfuric acid under reflux. Kagan et al.2 have also
submitted phenylacetone to fluorosulfonic acid treatment
and found only an electrophilic substitution of the aromatic
ring ortho and para by a fluorosulfonyl group. They did not
explain the contrast between phenylethanal and phenylace-
tone and simply evoked a steric effect due to the additional
substituent at the carbonyl function. In our hand, we found8
that arylacetones treated with boron tribromide give the 1,3-
dimethyl-2-phenylnaphthalenes in good yields with a
concomitant demethylation when the aromatic ring is
substituted by methoxy group(s). The mechanism is a
tandem aldol condensation–intramolecular cyclisation with
a high regioselectivity. The scope and the limitations of the
reaction of arylacetones with boron tribromide9 were clearly
defined. The cross-condensation, that is, reaction of two
different arylacetones, was carried out using 3,4-dimethox-
yphenylacetone and another variable arylacetone.10 The
objective was to obtain from only one experiment and after
repeated chromatographies at least four different molecules
tested as HIV-1 integrase inhibitors.
1,2,9,10-tetrahydro-1,9-epoxydibenzo[a,e]cyclooctenes.
The reaction of 2-(3,4-dimethoxyphenyl)ethanal with
concentrated HCl in dioxane was reported to give
6,7-dimethoxy-2-(3,4-dimethoxyphenyl)naphthalene in
unsatisfactory low yields (12%,14 20%4). We, therefore,
decided to revisit this reaction with the triple goal to obtain
easily (the simpler purification process), efficiently (the two
condensation products from each reactant if possible) and
rapidly (short reaction time) the products of O- and
C-condensation. With this goal in mind, we rapidly pointed
out that the key parameters were the quality of 1,4-dioxane
and the reagent concentrations. Freshly distilled dioxane on
sodium and benzophenone (in order to avoid free radicals)
was used and the concentration of arylethanal was adjusted
to 0.2 M. Under these conditions, the yields were singularly
improved (Scheme 2).
2. Results and discussion
Arylethanals 1a–f were prepared according to a known
procedure15 and submitted to acidic treatment in aqueous
dioxane during 1 h at room temperature. In all cases, high
yields in crude products were obtained and after purification
2-arylnaphthalenes 2a–f were isolated in satisfactory yields
(32–87%) (Table 1). Four of the six 1,2,9,10-tetrahydro-1,9-
epoxydibenzo[a,e]cyclooctenes 3 were isolated in good
yields (comparatively to their relative proportions in the
crude product) (Table 1). The presence of at least one
methoxy group on position 3 or 5 is absolutely required for
the conversion of 1 into 2 or 3. Under the same reaction
conditions, phenylethanal, 2-methoxyphenylethanal and
4-methoxyphenylethanal gave polymers (data not shown)
indicating that the intramolecular cyclisation required the
presence of a methoxy group ortho or para to the new-
formed C–C bond (Scheme 3). It must be noted that in the
case of 1a, the intramolecular cyclisations after C and
O-condensation are regioselective.
In continuation of our programme dealing with the
discovery of new polyphenolic HIV-1 integrase inhibi-
tors,10–13 we needed to develop a facile and efficient
synthesis of polymethoxylated 2-phenylnaphthalenes and
Table 1. Yields and relative proportions of 2 and 3
R2
R3
R4
R5
2
3
Relative
proportion (%)a
Yield (%)
Relative
proportion (%)a
Yield (%)
a
b
c
d
e
f
H
OMe
H
OMe
OMe
H
OMe
OMe
OMe
H
OMe
OMe
H
H
OMe
H
OMe
OMe
H
H
H
OMe
H
OMe
75
80
90
100
95
67
32
62
87
54
65
53
25
20
10
12
3
X
0
10
Not detected
5
33
15
a The relative proportions in 2 and 3 were measured from the 1H NMR spectra of the crude products obtained by extraction of the reaction mixture even when a
precipitate was observed.

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C. Maurin et al. / Tetrahedron 61 (2005) 7054–7058
Scheme 3.
2,4-Dimethoxyphenylethanal also gave a dark material from
which no organic compound could be isolated confirming
the requirement of a methoxy group in position 3 or 5. Good
yields in isolated products were obtained from the reaction
of 1b–1f possibly due to the presence of two or three
methoxy groups. In the case of 1d, 3d was not observed,
whereas in the case of 1e, the product of O-condensation 3e
could not be isolated.
Table 2. Spectroscopic data of compounds 2 and 3
Product
1H NMR (300 MHz, CDCl3) d, ppm (J, Hz)
13C NMR (75 MHz, CDCl3) d, ppm
2a
3.90 (s, 3H), 3.94 (s, 3H), 6.92 (ddd, 1H, 3JZ8.0 Hz, 4JZ2.7, 1.0 Hz),
7.17 (br s, 1H), 7.18 (dd, 1H, 3JZ8.8 Hz, 4JZ2.4 Hz), 7.25 (m, 1H),
7.30 (dm, 1H, 3JZ8.0 Hz), 7.40 (t, 1H, 3JZ8.0 Hz), 7.71 (dd, 1H,
55.3 (2CH3), 105.6 (CH), 112.5 (CH), 113.0 (CH), 119.2 (CH), 119.8
(CH), 125.7 (CH), 126.0 (CH), 127.2 (CH), 129.1 (C), 129.7 (CH),
129.8 (CH), 133.9 (C), 136.3 (C), 142.8 (C), 157.8 (C), 160.0 (C)
4
3JZ8.3 Hz, JZ1.7 Hz), 7.80 (d, 1H, 3JZ8.8 Hz), 7.81 (d, 1H,
3JZ8.3 Hz), 7.98 (d, 1H, 4JZ1.7 Hz)
3a
2b
2.69 (d, 2H, 2JZ16.1 Hz), 3.50 (dd, 2H, 2JZ16.1 Hz, 3JZ5.7 Hz),
3.70 (s, 6H), 5.23 (d, 2H, 3JZ5.7 Hz), 6.50 (d, 2H, 4JZ2.2 Hz), 6.69
36.6 (2CH2), 55.1 (2CH3), 69.0 (2CH), 112.5 (2CH), 113.5 (2CH),
126.1 (2CH), 129.9 (2C), 132.9 (2C), 158.3 (2C)
3
(dd, 2H, JZ8.55 Hz, 4JZ2.2 Hz), 6.99 (d, 2H, 3JZ8.55 Hz)
3.60 (s, 3H), 3.94 (s, 3H), 4.02 (s, 3H), 4.05 (s, 3H), 6.96 (dd, 1H,
55.9 (CH3), 56.9 (CH3), 60.5 (CH3), 61.1 (CH3), 111.5 (CH), 115.4
(CH), 120.9 (CH), 122.8 (CH), 124.1 (CH), 124.5 (CH), 127.7 (CH),
128.0 (CH), 128.1 (C), 129.7 (C), 134.1 (C), 135.8 (C), 143.0 (C),
146.8 (C), 148.4 (C), 153.2 (C)
4
4
3JZ7.6 Hz, JZ1.9 Hz), 7.07 (dd, 1H, 3JZ7.6 Hz, JZ1.9 Hz), 7.14
(t, 1H, 3JZ7.6 Hz), 7.32 (d, 1H, 3JZ8.9 Hz), 7.65 (d, 1H, 3JZ8.9 Hz),
7.74 (dd, 1H, 3JZ8.75 Hz, 4JZ1.6 Hz), 7.97 (d, 1H, 4JZ1.6 Hz), 8.18
(d, 1H, 3JZ8.75 Hz)
3b
2.92 (d, 2H, 2JZ16.6 Hz), 3.29 (dd, 2H, 2JZ16.6 Hz, 3JZ5.9 Hz),
3.74 (s, 6H), 3.79 (s, 6H), 5.29 (d, 2H, 3JZ5.9 Hz), 6.74 (d, 2H,
3JZ8.3 Hz), 6.83 (d, 2H, 3JZ8.3 Hz)
31.4 (2CH2), 55.7 (2CH3), 59.9 (2CH3), 68.5 (2CH), 110.5 (2CH),
120.7 (2CH), 126.0 (2C), 131.0 (2C), 146.3 (2C), 150.9 (2C)
2ca
3.96 (s, 3H), 4.01 (s, 3H), 4.038 (s, 3H), 4.045 (s, 3H), 6.99 (d, 1H,
55.9 (2CH3), 56.05 (CH3), 56.08 (CH3), 106.2 (CH), 106.6 (CH), 110.6
(CH), 111.7 (CH), 119.5 (CH), 123.8 (CH), 123.9 (CH), 126.8 (CH),
128.2 (C), 129.6 (C), 134.5 (C), 136.9 (C), 148.6 (C), 149.3 (C), 149.5
(C), 149.9 (C)
35.5 (2CH2), 55.7 (2CH3), 56.0 (2CH3), 69.2 (2CH), 108.0 (2CH),
111.5 (2CH), 123.4 (2C), 129.5 (2C), 147.4 (2C), 148.0 (2C)
3JZ8.6 Hz), 7.16 (s, 1H), 7.21 (s, 1H), 7.26 (d, 1H, 4JZ1.6 Hz), 7.27
3
4
(dd, 1H, JZ8.6 Hz, 4JZ1.6 Hz), 7.60 (dd, 1H, 3JZ8.3 Hz, JZ1.
9 Hz), 7.76 (d, 1H, 3JZ8.3 Hz), 7.79 (d, 1H, 4JZ1.9 Hz)
2.67 (d, 2H, 2JZ15.9 Hz), 3.46 (dd, 2H, 2JZ15.9 Hz, 3JZ5.9 Hz),
3.78 (s, 6H), 3.84 (s, 6H), 5.20 (d, 2H, 3JZ5.9 Hz), 6.48 (s, 2H), 6.57
(s, 2H)
3cb
2d
3.76 (s, 3H), 3.84 (s, 3H), 3.96 (s, 3H), 3.97 (s, 3H), 6.71 (s, 2H), 6.89
3
55.78 (CH3), 55.80 (CH3), 55.9 (CH3), 56.5 (CH3), 103.4 (CH), 103.5
(CH), 113.0 (CH), 113.2 (CH), 117.1 (CH), 121.3 (CH), 122.1 (CH),
125.3 (C), 126.4 (C), 127.9 (CH), 132.1 (C), 136.2 (C), 149.5 (C),
149.8 (C), 151.1 (C), 153.9 (C)
(dd, 1H, JZ9.0 Hz, 4JZ2.9 Hz), 6.96 (d, 1H, 3JZ9.0 Hz), 7.06 (d,
3
1H, 4JZ2.9 Hz), 7.75 (dd, 1H, JZ8.8 Hz, 4JZ1.8 Hz), 8.25 (d, 1H,
3JZ8.8 Hz), 8.36 (d, 1H, 4JZ1.8 Hz)
2e
2f
3f
3.66 (s, 3H), 3.91 (s, 3H), 3.96 (s, 3H), 3.97 (s, 3H), 3.98 (s, 3H), 4.07
(s, 3H), 6.77 (d, 1H, 3JZ8.6 Hz), 6.99 (s, 1H), 7.12 (d, 1H,
3JZ8.6 Hz), 7.54 (dd, 1H, 3JZ8.6 Hz,
55.9 (CH3), 56.1 (CH3), 61.0 (CH3), 61.1 (CH3), 61.2 (CH3), 61.5
(CH3), 102.6 (CH), 107.6 (CH), 121.3 (CH), 123.2 (C), 125.0 (CH),
125.6 (CH), 126.3 (CH), 128.7 (C), 130.8 (C), 135.7 (C), 140.8 (C),
142.6 (C), 147.9 (C), 151.6 (C), 152.1 (C), 153.2 (C)
55.8 (CH3), 56.2 (2CH3), 60.9 (CH3), 61.1 (CH3), 61.5 (CH3), 102.1
(CH), 104.7 (2CH), 119.4 (CH), 124.4 (C), 125.4 (CH), 127.0 (CH),
129.9 (C), 136.7 (C), 137.52 (C), 137.55 (C), 141.2 (C), 148.0 (C),
153.1 (C), 153.5 (2C)
4JZ1.6 Hz), 7.82 (d, 1H, 4JZ1.6 Hz), 8.06 (d, 1H, 3JZ8.6 Hz)
3.92 (s, 3H), 3.97 (s, 6H), 3.997 (s, 3H), 4.000 (s, 3H), 4.08 (s, 3H), 6.
90 (s, 2H), 6.98 (s, 1H), 7.61 (dd, 1H, 3JZ8.5 Hz, 4JZ1.9 Hz), 7.76 (d,
1H, 3JZ8.5 Hz), 8.19 (d, 1H, 4JZ1.9 Hz)
2.76 (d, 2H, 2JZ16.5 Hz), 3.39 (dd, 2H, 2JZ16.5 Hz, 3JZ6.2 Hz),
3.81 (s, 6H), 3.85 (s, 6H), 4.00 (s, 6H), 5.34 (d, 2H, 3JZ6.2 Hz), 6.35
(s, 2H)
34.1 (2CH2), 55.8 (2CH3), 60.6 (2CH3), 60.7 (2CH3), 65.8 (2CH),
107.3 (2CH), 122.9 (2C), 128.0 (2C), 139.8 (2C), 149.2 (2C), 152.7
(2C)
a IR (cmK1): 2934w; 2836w; 1606m; 1506s; 1462m; 1256s; 1241s; 1166s; 1138s; 1023m; 857m.
b IR (cmK1): 2995w; 2919m; 2833w; 1610m; 1517s; 1465m; 1358m; 1249s; 1120s; 1016m; 848m.

C. Maurin et al. / Tetrahedron 61 (2005) 7054–7058
7057
Table 3. Physical, analytical and mass spectroscopic data for compounds 2a–f, 3a–b and 3fa
Product
Mp (8C)
Elemental analyses
MS (IE)
3a
115–118
Anal. Calcd for C18H18O3 (282.33): C, 76.57; H, 6.43. Found C,
76.86%; H, 6.31%
m/z (%)Z283 (26), 282 ([MC], 100), 267 (27), 254 (45), 253
(42), 251 (28), 249 (29), 239 (45), 224 (22), 223 (32), 208 (25),
179 (30), 165 (38), 151 (27), 122 (82)
3b
159–161
Anal. Calcd for C20H22O5 (342.39): C, 70.16; H, 6.48. Found: C,
69.98; H, 6.39
m/z (%)Z342 ([MC], 100), 314 (45), 313 (29), 311 (43), 299
(62), 284 (28), 283 (59), 280 (27), 268 (28), 252 (22), 165 (23),
152 (74), 137 (25)
2e
3f
102–104
181–182
Anal. Calcd for C22H24O6 (384.42): C, 68.74; H, 6.29. Found: C,
68.42; H, 6.47
Anal. Calcd for C22H26O7 (402.44): C, 65.66; H, 6.51. Found: C,
65.95; H, 6.39
m/z (%)Z385 (36), 384 ([MC], 100)
m/z (%)Z402 ([MC], 100), 374 (12), 373 (15), 359 (10), 343
(10), 182 (19)
a Known products: 2a, yellow powder, mp 90–92 8C (lit.16 mp 92 8C); 2b, white powder, mp 63–65 8C (lit.16 mp 68–69 8C); 2c, yellow powder, mp 177–179 8C
(lit.17 mp 179–180 8C); 3c, yellow powder, mp 162–164 8C (lit.6 mp 163–164 8C); 2d, white powder, mp 97–99 8C (lit.16 mp 99 8C); 2f white powder, mp
164–166 8C (lit.24 mp 165.5–166 8C).
Whatever the number and the position of the methoxy
groups, a pronounced preference for the C-condensation and
the formation of 2 was observed attested by the relative
proportions of 2 and 3 calculated from the 1H NMR spectra
of the crude product. Higher proportions in 3 were observed
when a hydrogen atom substituted the position 2 (Tables 2
and 3).
dioxane mixture was extracted with diethyl ether. The
combined organic layers were dried over Na2SO4 and
evaporated in vacuum.
Purification of 2a and 3a. The crude product was purified by
column chromatography on silica gel using a mixture of
hexane/ethyl acetate 80:20 as eluent to give 2a and 3a in 32
and 10% yield, respectively.
As a conclusion, we reported in this paper the facile
synthesis and isolation of six methoxylated 2-phenyl-
naphthalenes and four 1,2,9,10-tetrahydro-1,9-epoxydibenzo-
[a,e]cyclooctenes. The yields in 2-phenylnaphthalenes are
quite good and suffer the comparison with the literature16
except for the serendipitously synthesis of 2c.17 Amongst
the four 1,2,9,10-tetrahydro-1,9-epoxydibenzo[a,e]cyclo-
octenes, only 3c has been previously obtained from 1c and
trimethylsilyl iodide in high yield.6
Purification of 2b and 3b. The crude product was purified by
column chromatography on silica gel using a mixture of
hexane/ethyl acetate 70:30 as eluent to give 2b and 3b in 62
and 12% yield, respectively.
Purification of 2c and 3c. In this case, a precipitate was
obtained. Its spectroscopic data were in accordance with the
structure of 2c (87% yield). The crude product obtained by
extraction was purified by column chromatography on silica
gel using a mixture of hexane/ethyl acetate 70:30 as eluant.
3c was obtained in 3% yield.
3. Experimental
3.1. General
Purification of 2d. The crude product was purified by
column chromatography on silica gel using a mixture of
hexane/ethyl acetate 70:30 as eluant to give 2d in 54% yield.
Arylethanals 1 were synthesized according to a known
procedure.15 Arylethanals 1a–d and 1f were previously
described.18–23 Mps were determined on a Reichert
Thermopan apparatus, equipped with a microscope and
are uncorrected. NMR spectra were obtained on an AC 300
Bruker spectrometer in CDCl3 with TMS as internal
reference. Mass spectra were recorded on a Thermo-
Finnigan PolarisQ mass spectrometer (70 eV, Electronic
Impact). Elemental analyses were performed by CNRS
laboratories (Vernaison). Infra-red spectra were obtained on
a Perkin-Elmer 881 spectrometer on KBr paths.
Purification of 2e. The crude product was purified by
column chromatography on silica gel using a mixture of
dichloromethane/methanol 97:3 as eluant to give 2e in 65%
yield. Whereas 3e has been identified on the 1H NMR
spectrum, it could not be isolated in a pure form.
Purification of 2f and 3f. Compounds 2f and 3f were
precipitated by addition of water. The yield was almost
quantitative. The solid mixture was triturated twice in hot
diethyl ether and rapidly filtered to give pure 2f (53% yield).
The filtrate was maintained at 0 8C during 2 h and 3f
crystallized. It was obtained in 15% yield.
3.1.1. 2,3,4-Trimethoxyphenylethanal 1e. Yellow oil,
85% yield, 1H NMR (CDCl3, 300 MHz): 3.57 (d, 2H,
3JZ2.0 Hz), 3.82 (s, 3H), 3.84 (s, 3H), 3.84 (s, 3H), 6.62
3
3
(d, 1H, JZ8.3 Hz), 6.79 (d, 1H, JZ8.3 Hz), 9.66 (t, 1H,
3JZ2.0 Hz).
Acknowledgements
3.1.2. Reaction of 1 in HCl/dioxane—General procedure.
1 (5 mmol) was dissolved in freshly distilled 1,4-dioxane
(10 mL) and HCl 12 M (15 mL) was added. The mixture
was stirred for 1 h and water (10 mL) was added. When a
precipitate was obtained, it was filtered and washed with
water and then with diethyl ether. Otherwise, the aqueous
This work was financially supported by grants from le
Centre National de la Recherche Scientifique (CNRS) and
l’Agence Nationale de la Recherche contre le SIDA
(ANRS).

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13. Maurin, C.; Bailly, F.; Mbemba, G.; Buisine, E.; Vezin, H.;
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