Synthesis of a non-conjugated alkenone by Knoevenagel condensation; 1-phenylpent-1-en-4-one

Andrew McCarroll ([email protected]),
A contribution from the Walton Group, St Andrews

Chemicals Used
Pentane-2,4-dione (as received), phenylacetaldehyde (as received), piperidine (as received), dichloromethane (as received) zinc acetate dihydrate (as received)

Procedure
Pentane-2,4-dione (10.01 g; 0.1 mol), phenylacetaldehyde (12.0 g; 0.1 mol) and piperidine (0.1 g) were stirred for 24 hours. DCM (100 cm³) was added, and the mixture washed with 5% HCl (50 cm³) and water (50 cm³), then dried and concentrated to give 3-styrylpentane-2,4-dione. Column chromatography (PE/EtOAc) gave pure product (3.80 g; 19%). 3-Styrylpentane-2,4-dione (0.80 g; 3.96 mmol) and zinc acetate dihydrate (0.02 g) were refluxed in methanol (2 cm³) for 24 hours. Bulb to bulb distillation yielded 1-phenylpent-1-en-4-one (60 ^oC @ 0.04 mmHg) as a pale yellow oil (0.41 g; 65%).

Author's Comments
The procedure is very simple, and was chosen despite the poor yields as other routes involved unpleasant and expensive reagents. The method was reported to work for a variety of substrates. The product, unlike other non-conjugated alkenones (see [[SyntheticPage 165] ]), was stable with respect to its conjugated isomer.

Data
¹H nmr, CDCl₃: 3-Styrylpentane-2,4-dione: 1.73 (6H, s), 6.42 (1H, d, J = 16.2 Hz), 6.75 (1H, d, J = 16.2 Hz), 7.26-7.45 (5H, m) 1-phenylpent-1-en-4-one: 2.24 (3H, s), 3.35 (2H, d), 6.25-6.65 (2H, m), 7.18 (5H, m)

Lead Reference
K. Uehara, F. Kitamura, M. Tanaka, Bull. Chem. Soc. Jpn, 1976, 49, 493.

Other References
R. Verhe, N. de Kimpe, D. Courtheyn, L. de Buyck, N. Schamp, Tetrahedron, 1982, 38, 3649. A. J. McCarroll, PhD Thesis, University of St. Andrews, 2000.

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