1 Common Structural Basis of Phenylpropenes

1.1 The C6–C3 skeleton is the structural basis for their classification as phenylpropanoids

Simple phenylpropanoids generally have a C6–C3 unit, composed of a six-carbon aromatic ring and a three-carbon side chain, as their basic structure. In plants, these structures are derived mainly from the phenylpropanoid metabolic pathway and share a common biogenetic origin with phenylpropanoic acids, phenylpropanals, phenylpropanols, and related compounds.[1]

If Ar represents a substituted aromatic ring, the three-carbon side chain of phenylpropenes can be summarized as:

Ar–C7–C8–C9

The side chains of the representative natural phenylpropenes discussed in this article occur mainly in the following two forms:

① Allyl type: Ar–CH₂–CH=CH₂

② Propenyl type: Ar–CH=CH–CH₃

These two structures contain the same three side-chain carbon atoms, but the double bond is located at a different position.[2–3]

1.2 Phenylpropenes are not equivalent to volatile terpenoid constituents

Many phenylpropenes occur in plant essential oils and have pronounced aromas, making them easy to confuse with volatile constituents such as monoterpenes and sesquiterpenes.

However, their biogenetic origins and carbon skeletons are different. Terpenoids are biosynthetically based on isoprene units, whereas phenylpropenes belong to the phenylpropanoid system and are characterized by a C6–C3 structure consisting of an aromatic ring linked to a three-carbon side chain.

2 Double-Bond Position Is the First Key Factor in Understanding Structural Differences Among Phenylpropenes

One of the most important structural variations within the phenylpropene class occurs in the three-carbon side chain. As shown in Figure 1 below, the double bond in allyl-type phenylpropenes is located at C8=C9, whereas that in propenyl-type (iso-type) phenylpropenes is located at C7=C8. A change in double-bond position alters the conjugation relationship between the side chain and the aromatic ring. In addition, substituents such as hydroxy, methoxy, and methylenedioxy groups on the aromatic ring further expand the structural diversity of phenylpropenes.[4]

Figure 1. Differences in the side-chain structures of phenylpropenes and the biosynthetic relationship of eugenol

A, basic structures of allyl-type and iso-type phenylpropenes and their aromatic-ring substitution patterns; B, simplified biosynthetic route from L-phenylalanine to eugenol via coniferyl alcohol and coniferyl acetate. Original figure reproduced from Louie GV, Baiga TJ, Bowman ME, et al. PLoS ONE, 2007, Figure 1 [4]

2.1 Allyl type: the double bond is separated from the aromatic ring by a methylene group

The allyl side chain can be represented as:

Ar–CH₂–CH=CH₂

According to the C7, C8, and C9 numbering of the phenylpropanoid side chain, the double bond is located at C8=C9.

C7, which is directly attached to the aromatic ring, is an sp³-hybridized —CH₂— group. The aromatic ring and the C=C double bond are therefore separated by a methylene group and cannot form a continuous π-conjugated system.

Representative members include:

· Eugenol

· Estragole, also known as methyl chavicol

· Methyleugenol

· Safrole

· Myristicin

2.2 Propenyl type: the double bond is directly attached to the aromatic ring

The propenyl side chain is:

Ar–CH=CH–CH₃

The double bond is located at C7=C8.

Unlike the allyl type, C7 itself participates in the double bond and is directly attached to the aromatic ring, thereby forming a continuous π system: aromatic ring–C=C.

Representative members include:

· Isoeugenol

· Anethole (trans-anethole is common in natural sources)

Isoeugenol is a side-chain double-bond positional isomer of eugenol; in its structure, the allyl group is replaced by a prop-1-enyl group. Trans-anethole has a structure in which a para-methoxyphenyl ring is directly attached to a prop-1-enyl group.[3,9]

2.3 Two representative pairs of isomers reveal the significance of double-bond position

Eugenol and isoeugenol have the same molecular formula, C₁₀H₁₂O₂; estragole and anethole likewise both have the molecular formula C₁₀H₁₂O.[9] The difference between each pair lies in a one-position shift of the C=C double bond within the same three-carbon side chain. Although this change is small, it simultaneously alters the conjugation relationship, the possibility of geometric isomerism, and some aspects of chemical reactivity.

3 Why Does Shifting the Double Bond by One Position Produce Important Structural Differences?

3.1 First change: the conjugation relationship between the aromatic ring and the double bond is altered

In the allyl type, the sp³-hybridized C7 interrupts the π system between the aromatic ring and the C8=C9 double bond. In the propenyl type, the C7=C8 double bond is directly attached to the aromatic ring, forming a continuous conjugated system. The resulting increase in electron delocalization further affects the spectroscopic and photochemical behavior of the molecule.

For example, trans-anethole exhibits pronounced ultraviolet absorption, with an absorption maximum at approximately 258 nm in isooctane; isoeugenol and eugenol also exhibit different photochemical behavior under ultraviolet irradiation.[5,9] The specific absorption wavelength is additionally influenced by aromatic-ring substituents such as phenolic hydroxy and methoxy groups, as well as by solvent conditions.

3.2 Second change: the propenyl type can form E/Z geometric isomers

The terminal portion of the allyl-type structure is:

—CH=CH₂

The terminal carbon of the double bond bears two hydrogen atoms and therefore does not meet the substitution requirements for E/Z geometric isomerism.

In contrast, in the propenyl type:

Ar–CH=CH–CH₃

each of the two double-bonded carbon atoms is attached to two different substituents, allowing E and Z geometric isomers to form.

Isoeugenol occurs in cis and trans isomeric forms, and anethole likewise has corresponding geometric isomers. Ultraviolet photochemical studies have confirmed that isoeugenol can undergo E ↔ Z photoisomerization.[5]

3.3 Double-bond position also has a clear enzymatic basis in plants

Studies have shown that eugenol synthase (EGS) acts on coniferyl alcohol ester precursors to form eugenol with an allyl side chain, whereas the related isoeugenol synthase forms propenyl-type products. Studies in anise have also identified trans-anol/isoeugenol synthase and an O-methyltransferase with pronounced substrate selectivity for a C7=C8 propenyl side chain.[3–4]

This demonstrates that the difference between C8=C9 and C7=C8 is also an actual outcome of branching in plant phenylpropene biosynthesis.

4 Aromatic-Ring Substitution Is the Second Structural Variable That Determines the Properties of Phenylpropenes

The same allyl or propenyl side chain can be combined with differently substituted aromatic rings. Common substituents mainly include:

· Phenolic hydroxy group (—OH)

· Methoxy group (—OCH₃)

· Methylenedioxy group (—O—CH₂—O—)

The structure of a phenylpropene should therefore be examined from two perspectives simultaneously: the double-bond position in the side chain and the form of oxygen-containing substitution on the aromatic ring.

4.1 Structural comparison of representative phenylpropenes

5 Understanding the Principal Physicochemical Properties of Phenylpropenes from Their Structures

5.1 Most exhibit aromatic character, lipophilicity, and a degree of volatility

Representative phenylpropenes have relatively low molecular weights and generally contain only one aromatic ring, a short unsaturated three-carbon side chain, and a limited number of oxygen-containing functional groups. Many members are therefore distinctly hydrophobic and can occur as volatile aroma constituents in plants.

Eugenol, estragole, anethole, methyleugenol, and safrole can all occur in plant essential oils.[2,6]

Their solubility in water is generally limited, whereas their solubility in ethanol, ethers, and other organic media is better. For example, estragole is poorly soluble in water but soluble in alcohol, whereas trans-anethole is only slightly soluble in water and is compatible with organic solvents such as chloroform and ether.[9]

Eugenol and estragole are generally liquids at room temperature, whereas the melting point of trans-anethole is close to room temperature, allowing it to occur in a crystalline state at lower room temperatures.[9]

5.2 A free phenolic hydroxy group on the aromatic ring significantly alters polarity and acid–base properties

Eugenol and isoeugenol contain a free phenolic hydroxy group, which can act as a hydrogen-bond donor and also exhibits the weak acidity characteristic of phenols.

The reported pKa of eugenol is approximately 10.2.[9] This means that it remains predominantly in its nonionized form in ordinary neutral aqueous solutions, but can form a phenolate under sufficiently alkaline conditions.

In contrast, estragole, anethole, and methyleugenol lack a free phenolic hydroxy group and cannot undergo the same phenolic-hydroxy-group ionization.

5.3 The double bond and phenolic hydroxy group jointly affect oxidative stability

Phenylpropenes contain a C=C double bond, while eugenol, isoeugenol, and related compounds additionally contain electron-rich phenolic structures. They may therefore undergo oxidation, isomerization, or further structural transformation under exposure to light, air, and oxidizing conditions.

Under prolonged light irradiation and air exposure, isoeugenol can undergo oxidative degradation and form oxidative dimerization products; eugenol may also gradually change color during storage.[9,11]

During analysis, phenylpropene reference standards and essential oils containing high levels of phenylpropenes therefore generally require protection from light, sealed storage, and minimization of prolonged exposure to air. These considerations concern not only sample preservation but also the reliability of chromatographic quantitation.

6 Common Methods for the Detection and Identification of Phenylpropenes

In practice, detection and identification should follow this sequence:

Preliminary separation → Select a chromatographic method according to volatility → Compare with reference standards → Perform spectroscopic confirmation when necessary

6.1 Thin-layer chromatography: suitable for rapid comparison, but unable to confirm structure on its own

Thin-layer chromatography (TLC) can be used for the preliminary qualitative analysis of phenylpropenes.

In a suitable developing system, the test sample and reference standard can be compared in terms of:

① Rf value;

② characteristics observed under ultraviolet light;

③ spot position and color after visualization.

The use of TLC for identifying eugenol has been reported.[8]

For members containing a free phenolic hydroxy group, such as eugenol and isoeugenol, visualization reactions associated with phenolic hydroxy groups can assist in assessment.

Broad-spectrum visualization systems such as anisaldehyde–sulfuric acid and vanillin–sulfuric acid can visualize many classes of volatile natural products, but their results are not specific to phenylpropenes.

6.2 GC and GC–MS are the principal methods for analyzing free phenylpropenes

Because many phenylpropenes have low molecular weights, acceptable thermal stability, and a degree of volatility, gas chromatography (GC) is highly suitable for their separation.

Coupling GC with mass spectrometry gives gas chromatography–mass spectrometry (GC–MS).

Its advantages include the simultaneous acquisition of:

1. Chromatographic retention information;

2. Electron-impact mass-spectral fragmentation information.

GC–MS has been used to simultaneously separate and determine several phenylpropenes, including anethole, estragole, eugenol, methyleugenol, safrole, and myristicin.[6]

6.3 Confirmation of positional isomers requires retention information + mass spectrometry + reference standards

Although GC–MS is highly suitable for phenylpropenes, particular attention must be paid to isomeric pairs such as eugenol/isoeugenol and estragole/anethole. The same molecular formula means that they have the same molecular mass, while their similar structures may also produce some similar mass-spectral fragments.

A more reliable determination generally requires a combination of:

① Reference-standard retention time;

② Retention index;

③ Complete electron-impact mass-spectral fragmentation pattern;

④ A validated standard mass-spectral library;

⑤ Different stationary phases or further spectroscopic analysis when necessary.

For compounds such as isoeugenol that exhibit E/Z isomerism, it is also necessary to verify whether the chromatographic conditions used can separate the geometric isomers. Determination of only “total isoeugenol” does not establish the E/Z composition of the sample.

6.4 HPLC is an important complement to GC

High-performance liquid chromatography (HPLC) is another option for samples in different states.

Liquid chromatography may be considered, for example, in the following situations:

① The sample matrix is unsuitable for direct gas-chromatographic analysis;

② Free phenylpropenes and their low-volatility or nonvolatile bound precursors need to be analyzed simultaneously;

③ The target analytes or their derivatives are suitable for ultraviolet, fluorescence, or mass-spectrometric detection;

④ Specific isomers or complex components need to be separated under liquid-phase conditions;

HPLC has been used for the qualitative and quantitative analysis of eugenol, and an HPLC–fluorescence method has also been established for eugenol, methyleugenol, and related constituents.[8,10]

6.5 NMR is suitable for resolving the confirmatory question of “which double-bond structure is actually present”

Nuclear magnetic resonance (NMR) spectroscopy is important for distinguishing the allyl type from the propenyl type.

The allyl type, Ar–CH₂–CH=CH₂, shows the following signals in the ¹H NMR spectrum:

① A benzylic —CH₂— signal directly attached to the aromatic ring;

② One vinylic —CH= signal;

③ Two proton signals from the terminal =CH₂ group.

By contrast, the propenyl type, Ar–CH=CH–CH₃, exhibits:

① Two vinylic protons;

② A methyl signal from the group attached to the double bond;

③ No benzylic Ar–CH₂— group characteristic of the allyl type.

Even though eugenol and isoeugenol have the same molecular formula, their ¹H NMR side-chain signals provide clear structural distinctions. A 2024 study used ¹H NMR for the quantitative analysis of isoeugenol and simultaneously observed characteristic signals from phenylpropanoids including eugenol, methyleugenol, estragole, and safrole. This demonstrates that NMR is suitable not only for confirming pure compounds but also for analyzing complex spice and essential-oil samples.[7]

7 What Questions Are Addressed by the Different Identification Methods?

Reliable identification of a phenylpropene should answer three questions simultaneously: whether the side-chain double bond is located at C7=C8 or C8=C9, whether a propenyl side chain has the E or Z configuration, and what types and positions of oxygen-containing substituents are present on the aromatic ring. GC–MS is suitable for screening and quantitation, whereas final confirmation of positional or geometric isomers should still combine reference-standard retention information with structural evidence from NMR and other techniques.

8 Representative Products Related to Side-Chain Double-Bond Position, Oxygenated Aromatic-Ring Substitution, and Geometric Configuration of Phenylpropenes

Table 1. Phenylpropenes Containing a Phenolic Hydroxy Group, One Methoxy Group, or Two Methoxy Groups

Table 2. Trimethoxyphenylpropenes

Table 3. Methoxy- and Methylenedioxy-Substituted Allylbenzenes

Note: The products listed above are representative Aladdin products related to scientific research. Their specific uses should be determined in conjunction with the product specifications, batch COA, and the intended reaction or evaluation system. Further information on product specifications, grades, and COAs can be retrieved from the Aladdin website using the product name, CAS number, or catalog number.

References

[1] Zhu Z, Chen R, Zhang L. Simple phenylpropanoids: recent advances in biological activities, biosynthetic pathways, and microbial production. Natural Product Reports, 2024, 41(1): 6–24. doi:10.1039/D3NP00012E.

[2] Atkinson RG. Phenylpropenes: occurrence, distribution, and biosynthesis in fruit. Journal of Agricultural and Food Chemistry, 2018, 66(10): 2259–2272. doi:10.1021/acs.jafc.6b04696.

[3] Koeduka T, Baiga TJ, Noel JP, Pichersky E. Biosynthesis of t-anethole in anise: characterization of t-anol/isoeugenol synthase and an O-methyltransferase specific for a C7-C8 propenyl side chain. Plant Physiology, 2009, 149(1): 384–394. doi:10.1104/pp.108.128066.

[4] Louie GV, Baiga TJ, Bowman ME, et al. Structure and reaction mechanism of basil eugenol synthase. PLoS ONE, 2007, 2(10): e993. doi:10.1371/journal.pone.0000993.

[5] Krupa J, Olbert-Majkut A, Reva I, Fausto R, Wierzejewska M. Ultraviolet-tunable laser induced phototransformations of matrix isolated isoeugenol and eugenol. The Journal of Physical Chemistry B, 2012, 116(36): 11148–11158. doi:10.1021/jp306339g.

[6] Tsai CJ, Li JH, Feng CH. Dual dispersive liquid-liquid microextraction for determination of phenylpropenes in oils by gas chromatography-mass spectrometry. Journal of Chromatography A, 2015, 1410: 60–67. doi:10.1016/j.chroma.2015.07.095.

[7] Fabry P, Weber S, Teipel J, Richling E, Walch SG, Lachenmeier DW. Quantitative NMR spectrometry of phenylpropanoids, including isoeugenol in herbs, spices, and essential oils. Foods, 2024, 13(5): 720. doi:10.3390/foods13050720.

[8] Shormanov VK, Sukhomlinova EA. Isolation of eugenol from biological material. Sudebno-Meditsinskaia Ekspertiza, 2008, 51(3): 17–21.

[9] National Center for Biotechnology Information. PubChem Compound Summary: Eugenol (CID 3314), Isoeugenol (CID 853433), Estragole (CID 8815), Anethole (CID 637563), Methyleugenol (CID 7127), Safrole (CID 5144). U.S. National Library of Medicine.

[10] Takada M, Sakamoto M, Yamada H, et al. HPLC fluorescence method for eugenols in basil products derivatized with DIBI. Chemical and Pharmaceutical Bulletin, 2022, 70(1): 37–42. doi:10.1248/cpb.c21-00575.

[11] Ahn J, Avonto C, Pandey P, et al. Chemistry of Isoeugenol and Its Oxidation Products: Mechanism and Kinetics of Isoeugenol as a Skin Sensitizer. Chemical Research in Toxicology. 2023;36(5):747–756. doi:10.1021/acs.chemrestox.2c00407.

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