Synthesis of E,E-dibenzylidene acetone (dba) and aryl substituted derivatives; enones

Dr. Ian J. S. Fairlamb ([email protected]),
A contribution from the Fairlamb Group, York

Chemicals Used
Aldehydes were purchased from Lancaster or Aldrich. Acetone and ethanol were purchased from Merck. We used all materials as recieved - no special purification was employed

Procedure
To a 500ml conical flask equipped with a magnetic stirrer was added a solution of sodium hydroxide (4.72 g, 0.118 mol, 2.5 eqv.) in water (47 mL). Ethanol (33 mL) was added with stirring and the solution was cooled by an ice-water bath. To the resultant solution, benzaldehyde (5 g, 0.047 mol, 1 eqv.) and analytical grade acetone (1.36 g, 0.023 mol, 0.5 eqv.) were added slowly over 15 minutes. On complete addition, the mixture was stirred for one hour at room temperature. A yellow solid appeared slowly, and after a further hour it was filtered in vacuo and then washed with ether (3 x 50 mL). The solid material could be purified by flash chromatography using petroleum ether 40-60°C/ethyl acetate (4/1, v/v) to afford dibenzylidene acetone as a yellow solid (3.98 g, 74 %) or recrystallised from CH₂Cl₂.

Author's Comments
The alpha,beta-unsaturated carbonyl compound, E,E-dibenzylidene acetone (dba or 1,5-bis-penta-1E,4E-dien-3-one), is readily synthesised by simple base-mediated aldol condensation of two equivalents of benzaldehyde with acetone in the presence of sodium hydroxide in a water/ethanol solvent medium at room temperature (ref. 1). A literature survey reveals that other methods have been described. For example, a titanium tetralkoxide-induced aldol condensation has been reported for the synthesis of dba and derivatives (containing alkenic substitution) (ref. 2). Although this reaction occurs under neutral conditions, it requires up to 4 days to reach completion. An alternative procedure employs a partially dehydrated barium hydroxide catalyst (C-200, heterogeneous) for aldol condensation, under reflux with ethanol, to give dba in yields >95% in 1 h, although no substituted dba derivatives were reported (ref. 3). An indium trichloride mediated reaction of two equivalents of benzaldehyde with acetone provides dba in 88% yield, but requires 16 h reaction time at 110 degrees celcius in a sealed tube (ref. 4). A microwave assisted variant, mediated by KF-Al2O3, has further been reported from dba (ref. 5). We have established that the best practical method is the classic way using the simple base-mediated aldol condensation reaction. The KEY point is in the due care and attention required in the purification of these derivatives. Compounds were purified according to the following key: A: The product precipitates. Filtered and washed with ether. Filtrate concentrated in vacuo, then recrystallised from CH2Cl2. B: Extracted into CH2Cl2, washed with water, then sat. aq. NaCl, dried (MgSO4) and concentrated in vacuo. Purified by flash chromatography using ethyl acetate/hexane mixtures. C: As for B, but then recrystallization with THF. D: As for A, but taken up in toluene (aldehyde insoluble in this). Filter, concentrate filtrate and recrystallize from hexane. R = 4-CH3, (Method A); R = 4-t-Bu, (Method C); R = 4-OMe, (Method A);R = 2-OMe, (Method A); R = 4-CF3, (Method A); R = 3-CF3, (Method A); R = 2-CF3, (Method A); R = 2,4,6-Me, (Method A); R = 2,4,6-OMe, (Method B); R = 4-F, (Method A); R = 4-Cl, (Method A); R = 4-Br, (Method D); R = 3-NO2, (Method C); R = 4-NO2, (Method C); R = 4-NMe2, (Method B); R = 3,4,5-OMe, (Method B); R = 3,5-OMe, (Method B). The isolated yields in the vast majority of cases are very good, on a relatively large scale (5-50 g). If you are wondering what to do with all these dba derivatives, see ref. 6. A complete (non refereed paper has been prepared by our group and available at: http://www-users.york.ac.uk/%7Eijsf1/dba-derivatives.pdf).

Data
selected data: 1,5-Bis-penta-1E,4E-dien-3-one Mp 120-121 oC; 1H 7.75 (d, 2H, 3J=16.0, H1), 7.61 (4H, m, H2', H6'), 7.42 (6H, m, H3', H5', H4'), 7.09 (2H, d, 3J=16.0, H2); 13C 189.0, 143.4, 134.9, 130.6, 129.0, 128.5, 125.5.IR (CH2Cl2, cm-1) 1657m, 1651m (C=O), 1627vs (C=C), 1591w (C=C aromatic), 1574w (C=C aromatic), 983m (CH trans); UV (THF nm) 233 (pi-pi*), 321 (n-pi*); LRMS (EI) m/z 234 (M+, 100), 205 (10), 131 (55), 103 (75), 91 (30), 77 (75), 51 (40); HRMS (EI) m/z C17H14O calculated mass of 234.1102, found 234.1103. 1,5-Bis-(3',5'-dimethoxyphenyl)penta-1E,4E-dien-3-one Mp 132-133 oC; 1H 7.62 (2H, d, 3J=8.2, H4'), 7.01 (2H, d, 3J=15.7, H1), 6.71 (4H, t, 3J=7.6, 7.9, H6'), 6.48 (2H, d, 3J=15.7, H2), 3.70 (12H, s); 13C 188.7, 160.9, 143.2, 136.5, 125.5, 106.1, 102.7, 55.3. IR (CH2Cl2, cm-1) 1652m (C=O), 1621vs (C=C), 1596w (C=C aromatic), 1572w (C=C aromatic), 989m (CH trans); LRMS (EI) m/z 354 (M+, 100), 203 (10); HRMS (EI) m/z C21H22O5 calcd. mass of 354.1467, found 354.1485. 1,5-Bis-(3',4',5'-trimethoxyphenyl)penta-1E,4E-dien-3-one Mp 155-156 oC; 1H 7.55 (1H, d, 3J=15.8, H1), 6.94 (1H, d, 3J=15.8, H2), 6.77 (2H, s, 3J=7.8, H2', H6'); 3.78 (12H, s, 4xOCH3(meta)), 3.72 (s, 2xOCH3(para)); 13C 188.5, 153.9, 143.2, 140.7, 130.6, 125.2, 105.8, 60.9, 56.4. IR (CH2Cl2, cm-1): 1648m (C=O), 1617vs (C=C), 1583w (C=C aromatic), 1504w (C=C aromatic), 1000m (CH trans); LRMS (EI) m/z: 414 (M+, 45), 399 (15), 383 (15), 181 (100); HRMS (EI) m/z: C23H26O7 calcd. mass of 414.1678, C23H26O7 requires 414.1679. 1,5-Bis-(2',4',6'-trimethoxyphenyl)penta-1E,4E-dien-3-one Mp 202-204 oC; 1H 8.01 (2H, d, 3J=16.1, H1), 7.37 (2H, d, 3J=16.1, H2), 6.12 (4H, s, 3J=8.3, H3��, H5��); 3.87 (12H, s, 4xOCH3(ortho)), 3.80 (6H, s, 2xOCH3(para)); 13C 189.7, 160.8, 159.4, 130.7, 124.4, 104.2, 88.5, 53.5, 53.2. IR (CH2Cl2, cm-1): 1620m (C=O), 1598vs (C=C), 1572w (C=C aromatic), 997m (CH trans); LRMS (EI) m/z: 414 (M+, 15), 399 (10), 383 (100); HRMS (EI) m/z: C23H26O7 calcd. mass of 414.1678, C23H26O7 requires 414.1687. 1,5-Bis-(3'-nitroophenyl)penta-1E,4E-dien-3-one Mp 147-148 oC; 1H 8.47 (2H, t, 4J=1.8, H2'), 8.21 (2H, ddd, 3J=8.2, 4J=2.2, 0.9, H4'), 7.91 (2H, dd, 3J=7.9, 4J=0.9, H6'), 7.73 (2H, d, 3J=15.9, H1), 7.59 (2H, t, 3J=7.9, H5'), 7.17 (2H, d, 3J=5.9, H2); 13C 206.8, 175.7, 141.1, 134.5, 130.5, 129.0, 127.9, 125.1, 122.9. IR (KBr, cm-1) 1677m (C=O), 1633vs (C=C), 1600w (C=C aromatic), 1573w (C=C aromatic), 1510s (NO2 asym.), 1350s (NO2 sym.), 992m (CH trans); LRMS (EI) m/z 324 (M+, 48), 307 (47), 202 (19), 176 (67), 102 (100); HRMS (EI) m/z C17H11O5N2 calcd. mass of 324.0732, found 324.0744.

Lead Reference
ref 1. Conard, C. R.; Dolliver, M. A. Org. Synth. 1943, 2, 167.

Other References
ref 2. Mahrwald, R.; Schick, H. Synthesis 1990, 592; ref 3. Sinisterra, J. V.; Garcia-Raso, A.; Cabello, J. A.; Marinas, J. M. Synthesis 1984, 502; ref 4. Deng, G.; Ren, T. Synth. Commun. 2003, 33, 2995; ref 5. Yadav, J. S.; Sub Reddy, B. V.; Nagaraju, A.; Sarma, J. A. R. P. Synth. Commun. 2002, 32, 893; ref 6. Fairlamb, I. J. S.; Kapdi, A. R.; Lee, A. F. Org. Lett. 2004, 6, 4435.

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