Angelic acid

August 21, 2023
My name is heavenly, but I can be nasty.
What molecule am I?
Image of Angelic acid 3D Image of Angelic acid

Angelic acid (cis-2-methyl-2-butenoic acid) is an unsaturated aliphatic carboxylic acid that was discovered by Munich pharmacist Ludwig Andreas Buchner in 1842. Buchner isolated it from the carrot-like roots of the herb Angelica archangelica (garden angelica). According to lore, an archangel revealed the herb’s medicinal properties, hence the heavenly name.

Since its discovery, angelic acid and its esters have been found in other plants, such as Roman chamomile (Anthemis nobilis), sabadilla (Schoenocaulon officinale), and lovage (Levisticum officinale). It is often found together with its more thermodynamically stable trans isomer, tiglic acid1.

In 1949, Robert E. Buckles* and Gene V. Mock at Iowa State University (Ames) reported the synthesis of tiglic acid by treating 2-hydroxy-2-methylbutyronitrile2 with sulfuric acid, followed by hydrolysis. They then converted tiglic acid to angelic acid in a three-step process: (1) addition of bromine across the double bond; (2) reaction with potassium hydroxide to form 3-bromoangelic acid3; and (3) reduction with sodium amalgam to produce angelic acid. Six years later, Buckles, Mock, and Louis Locatell, Jr., wrote an extensive review of the chemistry of both isomers.

Angelic and tiglic acids are used primarily as their esters in such products as perfumes and flavorings. The angelic ester petasin4, originally discovered in the common butterbur plant (Petasites hybridus), is a sesquiterpene with potential medical uses.

Despite its name, angelic acid has a spicy—even pungent—odor and a biting, acidic taste. As shown in the hazard information table, it must be handled with care.

1. CAS Reg. No. 80-59-1.
2. CAS Reg. No. 4111-08-4.
3. CAS Reg. No. 35057-99-9.
4. CAS Reg. No. 26577-85-5.

Angelic acid hazard information*

Hazard class**GHS code and hazard statement
Corrosive to metals, category 1H290—May be corrosive to metalsChemical Safety Warning
Skin corrosion/irritation, category 1CH314—Causes severe skin burns and eye damageChemical Safety Warning
Serious eye damage/eye irritation, category 1H318—Causes serious eye damageChemical Safety Warning
Specific target organ toxicity, single exposure, respiratory tract irritation, category 3H335—May cause respiratory irritationChemical Safety Warning

*Compilation of multiple safety data sheets.
**Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms.

Molecule of the Future

Pedrolide1 is a diterpenoid that contains what had previously been an unprecedented 5–5–6–6–3 carbon skeleton. In 2021, Maria-José U. Ferreira* and collaborators at the University of Lisbon, the University of Szeged (Hungary), Uppsala University (Sweden), and the University of Porto (Portugal) isolated it from the flowering shrub Euphorbia pedroi that grows only in the Arrábida Natural Park in Portugal.

Molecule of  the Future

Ferreira et al. deduced its structure, proposed a possible biosynthetic pathway, and established that it, like other terpenoids in E. pedroi, exhibits multidrug-resistance reversal properties, which are crucial weapons in cancer treatments. This past April, Marlene Fadel and Erick M. Carreira* at ETH Zurich reported a 20-step enantioselective total synthesis of pedrolide.

1. CAS Reg. No. 2701553-64-0.

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Angelic acid fast facts

CAS Reg. No.565-63-9
SciFinder nomenclature2-Butenoic acid, 2-methyl-, (2Z)-
Empirical formulaC5H8O2
Molar mass100.21 g/mol
AppearanceWhite crystals or powder or colorless liquid
Melting point45 °C
Boiling point185 °C
Water solubilitySlight
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