Umbelliferone

Umbelliferone or 7-hydroxycoumarin is a widespread natural product of the coumarin family. It occurs in many familiar plants from the Apiaceae (Umbelliferae) family such as carrot, coriander and garden angelica, as well plants from other families such as the mouse-ear hawkweed. It is a yellowish-white crystalline solid which has a slight solubility in hot water, but high solubility in ethanol. It absorbs ultraviolet light strongly at several wavelengths, leading to its use in sunscreen creams and lotions.

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Chemical synthesis

Umbelliferone is traditionally synthesized using the Pechmann condensation, from resorcinol and formylacetic acid (generated from malic acid in situ).

A newer synthesis uses methyl propiolate and a palladium catalyst.

Ultraviolet fluorescence

Umbelliferone absorbs strongly at 300, 305 and 325 nm, with log ε values of 3.9, 3.95 and 4.15 respectively, and it fluoresces blue in both ultraviolet and visible light. The powerful absorption at three different wavelengths, coupled with the fact that the energy is dissipated safely as visible light, make umbelliferone a useful sunscreen agent. The absorption changes in alkaline solution, since the phenolic hydroxyl group is deprotonated (pK_a = 7.7).

Uses

The ultraviolet activity of umbelliferone lead to its use as a sunscreen agent, and an optical brightener for textiles. It has also been used as a gain medium for dye lasers. Umbelliferone can be used as a fluorescence indicator for metal ions such as copper and calcium. It acts as a pH indicator in the range 6.5-8.9.

Derivatives of umbelliferone

Umbelliferone is the parent compound for a large number of natural products. Herniarin or 7-O-methylumbelliferone (7-methoxycoumarin) occurs in the leaves of water hemp (Eupatorium ayapana) and rupturewort. O-glycosylated derivatives such as skimmin (7-O-β-D-glucopyranosylumbelliferone) occur naturally and are used for the fluorimetric determination of glycoside hydrolase enzymes. Isoprenylated derivatives are also widespread, such as marmin (found in grapefruit skin and in the bark of the Bael tree) and furocoumarins such as marmesin, angelicin and psoralen.

References

1. F. M. Dean Naturally Occurring Oxygen Ring Compounds, Butterworths, London, 1963.
2. J. A. Joule, K. Mills Heterocyclic Chemistry, 4th edition, Blackwell Science, Oxford, UK, 2000.
3. Comprehensive Natural Products Chemistry, Volume 2, p 677, D. H. R. Barton, K. Nakanishi, O. Meth-Cohn, editors, Elsevier, Oxford, UK, 1999.