Synthesis of Some Novel Coumarinolignans: Newer Catalyst for Phenolic Oxidative Coupling
A newer catalyst was investigated as a catalyst for oxidative coupling of coumarin with propenyl phenols and alkene substrates. This resulted in dimerization of the two through C-O-C linkage yielding some novel coumarinolignanoids. These were characterized by elemental analysis, FT-IR, 1H NMR spectral studies. The rate of reaction was enhanced significantly there by decreasing the overall time of reaction and good product yield was also obtained.
The reaction mixture was cooled and poured over crushed ice. The brown solid
obtained was filtered, washed thoroughly with ice-cold water and dried. It was
re crystallized from ethanol as yellow needles, Yield 9 g. M.Pt. 269°C (Lit
Preparation of Ferulic Acid Ester
Prepared molecular sodium from 0.04 mole of clean sodium dried xylene contained
in a three-necked flask fitted with mechanical stirrer and reflux condenser.
When cold xylene was poured off as completely as possible. 0.016 mole of ethyl
acetate containing 0.5 mL of absolute alcohol in a flask was cooled rapidly
to 0°C and 0.032 moles of verteraldehyde was added slowly (during 90 min)
from dropping funnel while the mixture was stirred. The temperature was kept
between 0-5°C and was not allowed to rise above 10°C. The stirring was
continued when practically all the sodium had reacted. 0.58 mL of glacial acetic
acid was added followed by an equal amount of water. The layer of ester was
separated and the aqueous layer was extracted with 25 mL of ethyl acetate. The
combined organic layer was washed with 150 mL of 1:1 hydrochloric acid and dried
with magnesium sulphate. Ethyl acetate was distilled off on water bath and residue
distilled Sunder diminished pressure. Yield 75°C. M.Pt. 77°C (Lit 77-78°C).
Preparation of Coniferyl Alcohol
Dry bowl was set up to serve later as cooling bath in a fume cupboard. A
three-necked flask with stirrer, dropping funnel and a double surface condenser
attached with guard tube containing calcium chloride to open ends. The mechanical
stirrer should be powerful one. It must be emphasized that all operations, including
weighing with solid lithium aluminium hydride must be conducted in fume cupboard
during weighing etc., the front of fume chamber is pulled down so that there
is narrow opening to allow hands to enter.
The dropping funnel was removed from the flask neck and replaced by a funnel with a very short wide stem to introduce 0.090 g of lithium aluminium hydride into the flask and 5 mL of sodium dried ether to transfer last traces. The dropping funnel and guard tube were replaced and the stirrer was set in motion and solution of 0.5 g of ferulic acid ester in 15 mL of diethyl ether in dropping funnel. After stirring for 10 min the ferulic acid ester solution was added so that ether refluxed gently; the reaction mixture became viscous and four 25 mL portion of ether added during reduction to facilitate stirring. The stirring was continued for 10 min.
Excess of lithium aluminium hydride was decomposed by drop wise addition 10
mL ethyl acetate. The reaction product was filtered from sludge through sintered
glass funnel. The ethereal solution was dried with magnesium sulphate and the
ether was distilled off on rotary evaporator. The sludge remaining in filter
funnel was dissolved in 20% sulphuric acid; the resulting solution was extracted
with ether. The ether was removed by means of rotary evaporator. The residue
was crystallized on cooling. Yield 70%. M.Pt. 79°C (Lit = 78-82°C).
Preparation of Coumarinolignans
The synthesis of lignans were done according to a reported procedure (Merlini
and Zanarotti, 1980). Accordingly, coniferyl alcohol (0.6 g; 3 mmol), ferulic
acid ester (0.66 g; 3 mmol) , vinyl acetate (0.258 g; 3 mmol) and acrylic acid
(0.216 g; 3 mmol) were dissolved in dry benzene-methanol (18:5 V/V; 150 mL)
separately each for synthesizing [Compd-1, Compd-2, Compd-3 and Compd-4 respectively.
To these added esculetin 0.534 g (3 mmol) and TiO2 (3 mmol) and suspension
was stirred at room temperature. After 30 min of stirring added 3-4 mL hydrogen
peroxide and stirring was continued until Thin Layer Chromatography (TLC) showed
no starting material. The mixture was filtered and solvent evaporated. Product
was recrystallized with methanol/water/ethyl acetate.
Results and Discussion
In the present study, titanium dioxide was analyzed as catalyst in the presence of hydrogen peroxide. The hydrogen peroxide has only been used in the presence of hoersradish peroxidase for the synthesis of coumarinolignans. The investigation is aimed to study the utility of hydrogen peroxide in the presence of transition metal oxide i.e. titanium oxide for the phenolic oxidative coupling. Titanium oxide acts as the most suitable semiconductor for catalytic and photo catalytic process, as it is chemically inert, non-photo corrosive and non-toxic in nature. TiO2 absorbs light energy or photons that lead to charge separation or photo excitation with energies equal to or greater than band gap energy. An electron is therefore transferred to conduction band leaving a positive hole in the valence band and these holes have high affinity for electron (Davis et al., 1994). It has also been reported that with an increase in chemically adsorbed -OH on the surface, the polar properties and hydrophilicity of the surface is enhanced (Yu et al., 2002). Since hydrogen peroxide is able to disintegrate at room temperature in the presence of catalysts belonging to transition elements, hence there is probability of OH* radical .
These hydroxyl radicals in turn would abstracts proton from the two reactants
giving rise to phenoxyl radicals and thus O-β coupling mechanism.
Initially the coupling reactions were carried out in the presence of titanium dioxide to observe the course of reaction. The attempts were feebly successful. The reactions were carried out for up to 36 h in some cases and changing temperature up to 60°C however did not produce any good yields of products. The reaction was able to proceed when the catalytic amount of hydrogen peroxide was added. This was able to initiate the reaction soon and 8-12 h was the overall reaction of time. The yields of the product were quite high as compared to previous attempts of synthesis of lignans.
The FT-IR spectra of the synthesized compounds (Compd 1-4) in KBr were made and spectra were recorded in the range of γmax 4000 - 400 cm-1. The C-O-C group peaks observed in esculetin with coniferyl alcohol, ferulic acid ester, vinyl acetate and acrylic acid are observed in the range of γmax 1270-1230 cm-1 and shifts in the range of γmax 1282-1224 cm-1 confirming the coupling of the synthesized compounds.
Brownish-Yellow solid; Yield (52%); FTIR (KBr), 3650, 3610.5, 3421.1, 1691,
1564.2, 1443,1282.7, 1228.5 cm-1; δH (300 MHz; d6-DMSO)
6.37 (1H, d, H-3,), 7.66 (1H , d, H-4), 7.23 (H-5, s, Ar), 7.1 (H-8, s, Ar,),
7.51 (H-2, s, Ar), 3.785 (H-3, s, OCH3), 11.31 (H-4,
d, OH), 7.3 (H-5, t, Ar), 7.32 (H-6, d, Ar), 5.5 (H-7, d,
-CH), 4.491 (H-8, q, -CH), 3.9 (H-9, t, -CH2OH), ; Calc.
for C19H16O7 : C , 64.04; H, 4.49; O, 31.46
Found : C, 67.09; H, 4.42; O, 31.41.
Buff solid; Yield (60%); FTIR (KBr) 3610.2, 3332.1, 1745.3, 1691.9, 1565,
1536.1, 1281, 1228.5 cm-1; δH (300 MHz; d6-DMSO)
6.39 (1H, d, H-3), 7.62 (1H , d, H-4), 7.27 (H-5, s, Ar), 7.3 (H-8, s, Ar,),
7.52 (H-2, s, Ar), 3.71 (H-3, s, OCH3), 11.31 (H-4,
d, OH), 7.3 (H-5, t, Ar), 7.33 (H-6, d, Ar), 5.8 (H-7, d,
-CH), 3.81 (H-8, d, -CH), 3.85 (H-9a, q, -CH3CH2COO),
3.85 (H-9b, t, -CH3CH2COO) ; Calc.
for C21H18O8 : C , 63.31; H, 4.52; O, 32.16
Found : C, 63.39; H, 4.50; O, 32.11.
Orange-Brown solid; Yield (61%); FTIR (KBr) 3333.6, 1742.6, 1684, 1542,
1277.4, cm-1; δH (300 MHz; d6-DMSO) 6.33
(1H, d, H-3), 7.61 (1H, d, H-4), 7.3 (H-5, s, Ar), 7.16 (H-8, s, Ar,) , 7.5
(H-1, d, -CH2), 2.36 (H-2, t, -CH), 2.05 (H-3,
s, OCOCH3); Calc. for C13H10O6
: C , 59.54; H, 4.52; O, 32.16. Found : C, 59.51; H, 4.56; O, 32.19.
Orange-Yellow solid; Yield (62%); FTIR (KBr) 3333.2, 1760, 1699, 1686, 1599,
1522, 1291, cm-1; δH (300 MHz; d6-DMSO)
6.37 (1H, d, H-3), 7.66 (1H, d, H-4), 7.20 (H-5, s, Ar), 7.2 (H-8, s, Ar,) ,
3.5 (H-1, d, -CH2), 3.36 (H-2, t, -CH), 11.6 (H-3,
d, -COOH) Calc. for C12H8O6 : C, 58.06;
H, 3.22; O, 38.71 Found : C, 58.02; H, 3.21; O, 38.75.
In summary titanium dioxide in the presence of hydrogen peroxide is a viable method for carrying out synthesis of lignans through phenolic oxidative coupling. The coarse of reaction was significantly reduced when compared to previous attempts. Titanium oxide itself did not produce any good results. It suggests that the use of oxides of transition metals could further be explored to obtain a better yield of lignans." class="btn btn-success" target="_blank">View Fulltext