Isolation and synthesis of acmellonate, a new unsaturated long chain 2-ketol ester from Spilanthes acmella.

Paracress (Spilanthes acmella) is commonly used as a spice, e.g., in Madagascar. To get more information about the taste active compounds, Jambu oleoresin produced from Spilanthes acmella and extracts prepared directly from the plant were fractionated and the fractions were analysed and evaluated by tasting. In addition to the long known tingling compounds (2E,6Z,8E)-deca-2,6,8-trienoic acid N-isobutyl amide (spilanthol) and (2E,6Z,8E)-deca-2,6,8-trienoic acid N-(2-methylbutyl) amide, as a minor constituent also the new 2-ketol ester (7Z,9E)-2-oxo-undeca-7,9-dienyl 3-methylbut-2-enoate (acmellonate) was isolated. The structure of the latter compound was verified by total synthesis of the ketol ester via the reaction of tris(trimethylsilyloxy)ethene and (6Z,8E)-deca-6,8-dienoic acid chloride and subsequent classical esterification. Acmellonate shows a weak tingling and numbing effect on the tongue and contributes only to a small extend to the overall flavour of Jambu.

Hydroxy- or methoxy-substituted benzaldoximes and benzaldehyde-O-alkyloximes as tyrosinase inhibitors.

Several benzaldoximes, benzaldehyde-O-ethyloximes, and acetophenonoximes were synthesized and evaluated as tyrosinase inhibitors by an assay based on tyrosinase catalyzed L-DOPA oxidation. Whereas benzaldoxime itself is only a weak inhibitor, its derivatives with one or two hydroxy or methoxy moieties in para and meta positions depress tyrosinase activity. Acetophenonoximes and trisubstituted benzaldoximes show no inhibitory activity. The IC(50) of 3,4-dihydroxybenzaldehyde-O-ethyloxime (0.3 +/- 0.1 micromol L(-1)) is of the same magnitude as tropolone (0.13 +/- 0.08 micromol L(-1)), one of the best tyrosinase inhibitors known so far.