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Thursday, 10 September 2015


Preparation of 1,5-bis(4'-methoxyphenyl)-1,4-pentadien-3-one (Product B)

A solution of p-anisaldehyde (0.61mL, 5.1mmol) and freshly recrystallized 4-(4'-methoxyphenyl)-3-buten- 2-one (0.60g, 3.4mmol) in ethanol (15mL) is prepared in a 100mL round bottom flask. A potassium hydroxide (0.8g) solution in water (20mL) is prepared in a separate beaker. The potassium hydroxide solution is then gradually added to the solution in the round bottom flask over two minutes with continuous stirring for 30 minutes. The solid is extracted by vacuum filtration, followed by water washing. The resulting solid is then dried and recrystallized from ethanol (Figure 6).
Figure 6. Recrystallized 1,5-bis(4'-methoxyphenyl)penta-1,4-dien-3-one (Product B).
Figure 7 depicts the 1H NMR spectrum of 1,5-bis(4'-methoxyphenyl)penta-1,4-dien-3-one, confirming the loss of the reactive methyl centre in Product A following the reaction with p-anisaldehyde. In this case also, the 2D J-resolved and COSY experiments are used to interpret the downfield resonances between 6.82-8.02ppm (Figures 8 and 9).
Figure 7. 1H NMR spectrum of 1,5-bis(4'-methoxyphenyl)-1,4-pentadien- 3-one (Product B) in CDCI3.
The J-resolved spectrum (Figure 8) differentiates 8.6Hz doublet and 16.4Hz doublet, centered at 6.90ppm. It also identifies a 16.0 Hz doublet at 7.73ppm and an 8.9Hz doublet at 7.57ppm further downfield. This data is used to determine the two spin systems in the COSY spectrum (Figure 9), one between 6.90-7.57ppm and the other between 6.90-7.73ppm. The alkene protons on C- 1/C-5 and C-2/C-4 can be allocated as 7.73ppm and 6.90ppm, respectively, while the promatic protons on C-2' and C-3' can be assigned as 7.57ppm and 6.90ppm, respectively. The magnitude of the coupling constants of H-1/H-5 and H-2/H-4 confirms the alkene’s E-geometry.
Figure 8. 2D J-resolved spectrum of 1,5-bis(4'-methoxyphenyl)-1,4-pentadien-3-one (Product B) in CDCI3.
Figure 9. COSY spectrum of 1,5-bis (4'-methoxyphenyl)-1,4-pentadien-3-one (Product B) in CDCI3.

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