These compounds were first proposed by the German chemist Kossel in 1891, and they are small molecules synthesized by organisms in response to environmental stress and resistance. They serve as plant protectants that aid in disease resistance and defense against natural enemies
[3][4]. The biological effects of secondary metabolites are the result of species evolution and adaptation to the environment. The secondary metabolites of medicinal orchids can be classified into three groups: terpenoids, phenols and nitrogen compounds, each of which contain tens of thousands of compounds. Furthermore, the production and distribution of these secondary metabolites are species specific and only occur in particular organs, tissues and developmental stages of the orchids
[4]. For instance,
D. nobile has the largest number of dendrobine types, whereas
D. chrysotoxum has the highest content of moscatilin, with stem tissue being the primary medicinal component. The active substances in traditional Chinese medicine are the material basis for their pharmacological effects. Therefore, the identification and analysis of secondary metabolites are necessary prerequisites for the development and application of medicinal orchids’ active substances.
2.1. Alkaloids
Alkaloids are the most commonly found nitrogen-containing compounds in the secondary metabolites of medicinal orchids and an essential source of bioactive compounds. More than 50 species of orchids have yielded the discovery of over 140 alkaloids
[5], which can be classified into various groups based on their structural characteristics, such as sesquiterpenes, indolizines, amides and indoles. In 2019, 52 alkaloid components were detected in 19 orchid species
[6]. However, the alkaloid content in medicinal orchids is typically low, with only five out of thirty-five
Dendrobium plants having an alkaloid content of over 0.1%
[7]. This paper summarizes 27 newly identified alkaloids, whose chemistry structure are shown in
Figure 1. It is worth noting that three new alkaloids were identified from
D. nobile, including a new pair of amide tautomers
[8]. Four new indolizine alkaloids were identified from
D. crepidatum [9][10]. Furthermore, a pair of enantiomers and three new indolizine alkaloids have been discovered from
D. crepidatum [11]. These twenty-seven alkaloids are listed in
Table 1 and include eight sesquiterpene alkaloids (1–8), two amides (9–10), eleven indolizines (11–21), four indoles (22–25), a spiral-shaped alkaloid (26) and a Lycodine-type
Lycopodium alkaloid (27).
2.2. Phenanthrenes
Phenanthrenes are a class of substances with three benzene rings as the parent ring and are commonly found in medicinal orchids. Based on their structural properties, they can be classified into simple phenanthrene, dihydrophenanthrene (DHP), phenanthraquinone (ketone), phenanthrene furan and phenanthrene dimer.
Table 1 summarizes the 39 phenanthrenes that were identified from medicinal orchids between 2018 and 2023 (for their chemical structures, refer to
Figure 2). Notably, a new DHP trimer was identified from
B. striata [12], and silk grass was found to contain six new DHP
[13]. Furthermore, monolithic orchids were discovered to have four optically rotating phenanthraquinones
[14], and two new phenanthraquinones were found in
Dendrobium flowers
[15]. Additionally, three secondary metabolites were identified for the first time from plants in the genus
Bletilla [12]. These thirty-nine phenanthrenes are listed in
Table 1 and include one DHP dimer (28), twenty DHP (29–48), seven simple phenanthrenes (49–55) and nine phenanthraquinones (56–64), as well as furans (65) and biphenanthrene (66).
2.3. Bibenzylates
Bibenzyl is an essential precursor for synthesizing phenanthrene substances, with 1,2-diphenylethane as the parent ring. Among medicinal orchids, the bibenzyls of
Dendrobium have been studied in depth. For instance, eight bibenzyl compounds were isolated from
D. officinale leaves
[16], and its stems contained 15 bibenzyl compounds, including dendrocandin X and 3,4′-dihydroxy-4,5-dimethoxybibenzyl, which were isolated from
D. officinale for the first time
[17]. Additionally, dendrocandin Y, a new bibenzyl derivative, was discovered from the stem extract
[18]. Using UPLC-Q-TOF-MS technology, five bibenzyloids, including densiflorol A, aloifol I and isomoniliformin A, were identified in
D. pendulum for the first time
[19]. The discovery of new benzyl compounds in medicinal orchids serves as a research resource for developing their medicinal value.
Table 1 (67–101) summarizes the information of 35 bibenzyls (for their chemical structures, refer to
Figure 3). In
Table 1, composition (67) was identified as new from
Bletilla, whereas compositions (75, 83, 89) were determined to be new from
Dendrobium. Three new substances (90, 98, 101) were found in
D. plicatile [20],
D. hercoglossum [21] and
D. hancockii [22], respectively, within the genus.
2.4. Other Secondary Metabolites
Medicinal orchids encompass a wide range of species, each characterized by a diverse array of secondary metabolites. These metabolites can be further categorized into various material types. For instance, flavonoids can be further sub-classified into flavanones, flavonols and other categories. These flavonoids are widely distributed in medicinal orchid plants and are a primary source of plant pigments. In fact, 66 flavonoids have been detected in 24 orchid plants
[23], while 34 different species of
Dendrobium have been found to contain flavonoids
[24], including
D. devonianum [25],
D. officinale [26][27][28],
D. fimbriatum [29] and
D. huoshanense [30]. In the family of Orchidaceae, many types of secondary metabolites still exist, and 54 species are summarized in
Table 1 (102–155) (for their chemical structures, refer to
Figure 4).