Chemospecific and Regioselective Ethereal Methyl-Oxygen Bond Cleavage Behavior of Aroylated Dimethoxynaphthalenes by Combined Action of AlCl 3 and Aroyl Group

AlCl3-mediated cleavage of ethereal methyl-oxygen bond in aroylated 2,7-dimethoxynaphthalene compounds proceeds chemospecifically and regioselectively. The ethereal bond at the β(2)-position of 1-monoaroylated 2,7-dimeth-oxynaphthalene is cleaved readily and predominantly against the β(7)-position, whereas scission of β-ethereal bonds of 1,8-diaroylated 2,7-dimethoxynaphthalene hardly undergoes like the non-aroylated mother frame compound of 2,7dimethoxynaphthalene.


Introduction
Congested molecular units having non-coplanar alignment of aromatic rings such as biphenyl, binaphthyl, and other poly (aromatic rings) compounds have been in the limelight for building block of functional molecules and polymers [1][2][3][4].Recently, the authors' group has found that 2,7-dialkoxynaphthalenes readily undertake acidmediated diaroylation with high peri-regioselectivity to give 1-aroyl and 1,8-diaroyl-2,7-dialkoxynaphthalenes in satisfactory yields [5,6].In crystal, the aroyl group of the resulting molecules attaches to the naphthalene ring as nearly perpendicular manner and for diaroylated derivatives two aroyl groups are situated in opposite directions (anti-orientation) [7][8][9].The authors' group has integrated the naphthalene-1,8-bis(carbonylarylene) unit into poly(arylene ether ketone) backbone and reported unique solubility tendency to organic solvents and characteristic thermal behavior of the resulting polymers with interpretation in relation to the spatial organization of the repeating unit [10].In addition, some curious reactions of the 2,7-dimethoxynaphthalene and its derivatives such as reversible aroylation depending on Brønsted acid strength [5] and dual aroylation mediated by Lewis acid [6] are also revealed during the studies on aroylation of naphthalene derivatives.
In this article, the authors introduce Methyl-oxygen bond cleavage behavior of β-ethereal substituent in aroylated dimethoxynaphthalenes by the combined action of AlCl 3 and aroyl group and discuss the regioselectivity and the chemospecificity based on the comparison of the corresponding reaction behaviors among the homologous and analogous dialkoxynaphthalene molecules.
As well known methyl aryl ether generally resists acid-mediated Methyl-oxygen bond cleavage.So deprotection of methyl group from methoxyarenes needs specific reagents such as iodotrimethylsilane with Lewis acids, or BBr 3 in place of AlCl 3 , which has almost no potential for this purpose [11][12][13][14].Furthermore, the BBr 3 -mediated cleavage often suffers from low regioselectivity.Therefore, effective and regioselective cleavage of methyl aryl ether by AlCl 3 is of interest.
β-ethereal substituent of 7-methoxy group unchanged was achieved even when ten equimolar amounts of AlCl 3 were employed against substrate 1aa (Entry 6).

Table 3. Ethereal Methyl-oxygen bond cleavagereaction of 1,8-diaroylnaphthalene 7aa by
The distinct behavior that contrary to complete cleavage of β(2)-methyl ether bond of 1-monoaroyl-2,7-dimethoxynaphthalene 1aa, the β-methyl ether bonds of 1,8-diaroyl-2,7-dimethoxynaphthalene 7aa were essentially unchanged indicates the plausible origination of the chemospecificity from the facility of formation of the required conformation for methyl ether bond cleavage.The required conformation still remains indeterminable, however, the probable situation of AlCl 3 between ketonic carbonyl oxygen atom and ether oxygen atom might promote the scission of methyl-oxygen bond.In the case of 1,8-diaroyl-2,7-dimethoxynaphthalene 7aa, the formation of the required conformation is presumably obstructed sterically compared to 1-and 3-monoaroy-lated naphthalene derivatives.
These data indicate that steric hindrance around ketonic carbonyl group increases in the order of monoaroyldimethoxy derivative 1aa<diaroyl-monohydroxymonomethoxy compound 8ba<diaroyl-dimethoxy compound 7aa.Furthermore, it led us to conjecture that the sterically fixed aroyl group is difficult to promote the methyl ether-cleavage.In other words, rotation ability of aroyl group presumably enables to cleave the methyl-oxygen bond by formation of the suitable arrangement of the intervening species.

Experimental
All reagents were of commercial quality and were used as received.Solvents were dried and purified using standard techniques.

Measurement
1 H NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (300 MHz) and a JEOL ECX400 spectrometer (400 MHz).Chemical shifts are expressed in ppm relative to internal standard of Me 4 Si (δ 0.00). 13C NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (75 MHz) and a JEOL ECX400 spectrometer (100 MHz).Chemical shifts are expressed in ppm relative to internal standard of CDCl 3 (δ 77.0).IR spectra were recorded on a JASCO FT/IR-4100 spectrometer.Elemental analyses were performed on a Yanaco CHN CORDER MT-5 analyzer.High-resolution FAB mass spectra were recorded on a JEOL MStation (MS700) ion trap mass spectrometer in positive ion mode.

Typical Procedure of Methyl-OxygenBond Cleavage Reaction Mediated by AlCl 3
To a solution of 1-(4-chorobenzoyl)-2,7-dimetoxynaphthalene (1aa, 0.1 mmol, 32.7 mg) and in dichloromethane (0.25 ml), AlCl 3 (0.5 mmol, 66.7 mg) was added by portions at ambient temperature under nitrogen atmosphere.After the reaction mixture was stirred in the refluxing solution for 30 min, it was poured into iced water (20 ml) and the mixture was extracted with CHCl 3 (15 ml × 3).The combined extracts were washed with sat.NaCl aq. and dried over anhydrous sodium sulfate.The solvent was removed under reduced pressure to give solid.The crude product was purified by recrystallization (2ca, hexane, isolated yield 75%).
Other ether cleavage reactions were undertaken by essentially the same procedure as above.

6 )
manifest the essentially similar reaction behaviors to those of 2,7-dimethoxynaphthalene