3.2. Phytochemicals

The major chemical constituents from Aquilaria plants are sesquiterpenoids and chromones (Table 2, Table 3 and Table 4). These are divided into two categories: (A) Volatile compounds of agarwood and (B) Non-volatile compounds of agarwood.

Aromatic compounds from agarwood.

Chemical structure of 2-(2-phenylethyl) chromones in agarwood.

Chemical structure of terpenoids from agarwood.

Aromatic group

Aromatic volatile compounds are another important component in agarwood. For example, benzylacetone, which has been widely studied, is regarded as a landmark substance in aromatic components. Chen et al. [20] used the GC-MS and found that chloroform could effectively extract and separate aromatic compounds. Zhang et al. [21] isolated a large number of aromatic compounds from the volatile oil, in which the mass fraction of benzylacetone was the highest, reaching 19.51%. There are great differences in the mass fraction of aromatic compounds in different species. Tajuddin and Yusoff [22] found that the proportion of aromatic compounds in Malaysia agarwood volatile oil is larger than that of other species, of which 4-phenyl-2-butanone accounts for 32.1% of the volatile oil. Mei et al. [23] also isolated some aromatic compounds from agarwood, such as benzylacetone, 2,4-di-tert-butylphenol, 3,5-di-tert-butylphenol, and 4-methoxyphenylacetone (Table 2). These chemical structures were isolated on the basis of spectroscopic evidence with no cytotoxic activity against different cell lines [23].

Fatty acids

Lin et al. [24] used GC-MS to extract the agarwood produced by the nailing method, burrowing method, and cutting method with ether, and it was concluded that the agarwood in the cutting method was mainly fatty acid. Bhardwaj [25] found stearic acid, oleic acid, linoleic acid, and palmitic acid in the agarwood volatile oil by GC-MS. Mei et al. [26] isolated many kinds of fatty acids, such as hexadecenoic acid, tridecanoic acid, octadecenoic acid, and so on.

Chromones

These compounds mainly exist in the Aquilaria, and they are volatile constituents of agarwood (Table 3). The dimeric 2-(2-phenylethyl) chromones of agarwood lead to the identification by structure. Liu et al. [27] isolated different kinds of 2-(2-phenylethyl) chromones from agarwood. Mei et al. [28] analyzed the extracts by GC-MS after extracting high-quality agarwood with ether, and found that the relative content of 2-(2-phenylethyl) chromone compounds was 60% in high-quality agarwood. Xia et al. [29] and Yang et al. [30] used a spectrophotometer to evaluate the quality of agarwood and determine the absorbance value of 2-(2-phenylethyl) chromones at the wavelengths of 230 nm and 250 nm. Zhang et al. [31] obtained various derivatives of 2-(2-phenylethyl) chromones by co-fermentation of endophytic strains with agarwood. Thirteen 2-(2-phenylethyl) chromone derivatives were obtained by ethanol extraction, elution, and purification, and then bioassay for antibacterial activity. The results showed that some of 2-(2-phenylethyl) chromone derivatives had good inhibitory effect on Staphylococcus aureus [32]. Yang Lin also used a spectrophotometer to isolate different chromones, such as 2-[2-(4-hydroxyphenyl)ethyl]chromone, 5,6,7,8,-tetrahydroxy-5,6,7,8-tetrahydro-2-[2-(4-methoxyphenyl)ethy]-chromone, Rel-(1AR,2R,3R,7bS)-1a,2,3,7b-tetrahydro-2,3-hidydroxy-5[2-(4-methoxyphenyl)ethy]-7H-oxireno[f][1]benzophran-7-one and oxidoagarchromones (Table 3), which showed the mechanism of analgesia and sedation related to the regulation of gene expression of the GABAA receptor, GABAA receptor function, and promotion of Cl⁻¹ influx.

Terpenoids

Terpenoids are compounds derived from mevalonic acid that have two or more isoprene units in their basic carbon frame. The major components of agarwood are terpenoids, which include sesquiterpenes and diterpenes. One of the criteria used to evaluate the quality of agarwood is the content of sesquiterpenes and triterpenes (Table 4). Sukkaew et al. [36] isolated and identified volatile compounds from agarwood by GC-MS. Yang et al. [37] isolated and identified a monoterpene derivative [(-)-bornyl ferulate]. According to the chemical structure, sesquiterpenes isolated from agarwood are divided into different categories including agarofurans, eudesmanes, eremophilanes, guaianes, agarospirols, and cadinanes [38,39]. Several kinds of components with a high content (white caryophyllus, white caryophyllenol, and α-white fragrant alkene, etc.) were calculated by the normalization method [40], extracting sesquiterpenes from the “whole-tree agarwood-inducing“ technology in artificial agarwood. Jin et al. [41] have found that the structures of sesquiterpenes are similar, so it is difficult to separate and purify sesquiterpenes by conventional separation methods, such as chromatographic column separation and alcohol extraction. Ismail et al. [42] also found that all the volatile oil chemical structures were mainly composed of sesquiterpenes, such as α-agarofuran, (5S,7S,10S)-(-)selina-3, (+)-(4S,5R)-dihydrokaranone, α-guaiene, agarospirol, and 8-β-H-dihydrogmelofuran (Table 4).

By using silica gel column chromatography to isolate a variety of compounds from traditional Chinese medicinal agarwood, triterpenes (e.g., Ivy sapogenin and 3-oxo-22-hydroxyhopane, etc.) were isolated by recrystallization [43]. Various triterpenoids (approximately 14) were separated and purified from agarwood, including the first discovery of hydroxy-domperidone [44]. Tian and Haigang [45] used petroleum ether and ethanol to extract 10 diterpenoids from agarwood.

Steriods

Some steroids, such as β-sitosterol and 12 flavonoids, were isolated [48].

Flavonoids

Qi et al. [49] isolated 31 flavonoids, including flavonoid glycosides with anti-inflammatory activity. Chen et al. [50] isolated lignans and phenylpropanoids from agarwood, while Hendra et al. [51] and Li et al. [52] isolated several types of phenolic acids including p-methoxyphenylpropionic acid, 4-hydroxyphenylpropionic acid, and others.

Alkaloids

Chen et al. [53] isolated alkaloids from the wood of agarwood. Qi et al. [49] isolated benzophenones from agarwood and found the compound iriflophene, which can inhibit the respiratory burst of neutrophils.