Chemical Diversity of Plant Cyanogenic Glycosides: An Overview of Reported Natural Products

Cyanogenic glycosides are an important and widespread class of plant natural products, which are however structurally less diverse than many other classes of natural products. So far, 112 naturally occurring cyanogenic glycosides have been described in the phytochemical literature. Currently, these unique compounds have been reported from more than 2500 plant species. Natural cyanogenic glycosides show variations regarding both the aglycone and the sugar part of the molecules. The predominant sugar moiety is glucose but many substitution patterns of this glucose moiety exist in nature. Regarding the aglycone moiety, four different basic classes can be distinguished, aliphatic, cyclic, aromatic, and heterocyclic aglycones. Our overview covers all cyanogenic glycosides isolated from plants and includes 33 compounds with a non-cyclic aglycone, 20 cyclopentane derivatives, 55 natural products with an aromatic aglycone, and four dihydropyridone derivatives. In the following sections, we will provide an overview about the chemical diversity known so far and mention the first source from which the respective compounds had been isolated. This review will serve as a first reference for researchers trying to find new cyanogenic glycosides and highlights some gaps in the knowledge about the exact structures of already described compounds.


Introduction
Like many other plant natural products, cyanogenic glycosides serve as defense agents against herbivores, in this case by releasing toxic hydrogen cyanide after tissue damage. Some plant species are deadly for humans due to their high content of cyanogenic glycosides. For other species, used as staple foods, the content of cyanogenic glycosides requires special modes of preparation in order to detoxify the plants before human consumption. For a third group of plants, the moderate content (or the additional/consumption in moderate amounts) of cyanogenic glycoside makes them highly praised aroma plants (such as almonds in the production of marzipan).
The first cyanogenic glycoside that was isolated from a plant source was amygdalin (65) which was obtained from bitter almonds [Prunus dulcis (Mill.) D.Webb var. amara (DC.) H.Moore] in 1830 [1]. Currently, more than 2500 plant species are known to contain cyanogenic glycosides [2]. To the best of our knowledge, the latest addition to the list of naturally occurring cyanogenic glycosides is prunasin methacrylate (71), which was isolated from Centaurea microcarpa Coss. & Durieu ex Batt. & Trab. (Asteraceae) in 2018 [3]. In total there are now 112 cyanogenic glycosides reported; 68 cyanogenic glycosides were reported before the year 2000 and 44 cyanogenic glycosides have been reported from 2000 until today.
Cyanogenic glycosides or α-hydroxynitrile glycosides are a unique class of natural products featuring a nitrile moiety, which after enzymatic degradation of the genuine natural product can release hydrogen cyanide (prussic acid). Cyanogenic glycosides consist of two main parts, an aglycone and a sugar moiety. The general structure of cyanogenic glycosides is displayed in Figure 1: R 1 represents the aglycone part; R 2 , R 3 , R 4 , and R 5 represent the possible positions of substituents attached to the glucose moiety. The aglycone part consists of a nitrile group linked to an aliphatic, cyclic, aromatic, or heterocyclic moiety. Aglycones of cyanogenic glycosides are biosynthesized starting from one of the following amino acids: phenylalanine, tyrosine, valine, isoleucine, leucine, 2-(2cyclopentenyl)-glycine, and 2-(2 -hydroxy-3 cyclopentenyl)-glycine [4]. Additionally, some cyanogenic glycosides (discussed in groups A and B) also feature sulfate groups in their structures. The sugar moiety consists usually of glucose or a substituted glucose moiety. Besides monoglycosides, the most common sugar moieties in cyanogenic glycoside, also di-and triglycosides occur in some compounds. Additional substitutions of all hydroxyl groups of the sugar moiety also exist and thus add to the variety of natural products. In addition to glucose, the following sugars have been reported as parts of cyanogenic glycosides: allose, apiose, arabinose, rhamnose, and xylose. A sugar moiety is connected to an aglycone via the oxygen atom (relative to the nitrile group). Specific enzymes (β-glucosida the resulting structures after tissue damage in the plant. In vitro, ble by diluted acids or bases. After splitting-off the sugar moiet hydrolysis of the β-glycosidic bond, the aglycone can be further de either spontaneously (in vitro) or facilitated by an additional s droxynitrile lyase. This phenomenon is called cyanogenesis, a t Henry and Dunstan in 1905 [5].
Some natural cyanogenic glycosides are enantiomers, differ try of the aglycone, but not the sugar part. Example of such com tralin (R) (5)/epilotaustralin (S) (11); volkenin (1R, 4R) (36)/epivo raktophyllin (1R,4S) (38)/tetraphyllin B (1S,4S) (45); suberin A (1 B (1S,2S,3S,4S) (51) and prunasin (R) (54)/sambunigrin (S) (55) [6 Cyanogenic glycosides are fascinating natural products, bec a vital role for the plants producing them, but also for other livin role of cyanogenic glycosides in protecting the plants producing cial at the early stages of plant development. Accordingly, the con glycosides is often higher in seedlings and young leaves than in A sugar moiety is connected to an aglycone via the oxygen linked to the α-carbon atom (relative to the nitrile group). Specific enzymes (β-glucosidases) can easily hydrolyse the resulting structures after tissue damage in the plant. In vitro, hydrolysis is also possible by diluted acids or bases. After splitting-off the sugar moiety from the aglycone by hydrolysis of the β-glycosidic bond, the aglycone can be further degraded, releasing HCN, either spontaneously (in vitro) or facilitated by an additional specific enzyme, (S)-hydroxynitrile lyase. This phenomenon is called cyanogenesis, a term first introduced by Henry and Dunstan in 1905 [5].
Cyanogenic glycosides are fascinating natural products, because they have not only a vital role for the plants producing them, but also for other living organisms. The crucial role of cyanogenic glycosides in protecting the plants producing them is particularly crucial at the early stages of plant development. Accordingly, the concentration of cyanogenic glycosides is often higher in seedlings and young leaves than in mature plants [8]. In agriculture, cyanogenic glycosides can also be employed to protect non-source plants from herbivory by spraying preparations containing cyanogenic glycosides (e.g., cassava wastewater obtained in the process of reducing linamarin (1) content in cassava, in order to make it safe for human consumption [9]). Moreover, recent results have indicated a potential neuroprotective action of prunasin 2 ,3 ,4 ,6 -tetra-O-gallate (83) [10].

Results
The keyword "cyanogenic glycosides" was used to search the literature for references to this particular group of compounds; in this way, we found numerous articles, book chapters, and seminar proceedings, some dating back to the 19th century. These publications were then screened for those, which discussed the isolation and elucidation of cyanogenic glycosides compounds.
In the following sections, all cyanogenic natural products found in the literature are mentioned and displayed in Figures 2-18. In Figure 19, abbreviations used in the other figures are explained. In Table 1, the trivial and semi-trivial names (if at all existing) are indicated, along with the compound numbers and the number of the respective figure in which the chemical structure of the compound is presented. The 112 individual natural products retrieved from the literature, were divided into four groups, based on their aglycones. Group A comprises 33 cyanogenic glycosides with a non-cyclic aliphatic aglycone, group B 20 cyanogenic glycosides featuring cyclopentene or cyclopentane in the aglycone, group C contains 55 cyanogenic glycosides with an aromatic aglycone (some of which are the derivatives of prunasin), and group D consists of four cyanogenic glycosides with a heterocyclic aglycone (pyridinone derivatives). The references cited in this review are, whenever possible, the first articles that reported the isolation of a particular cyanogenic glycoside.
Within each group, compounds have been ordered first by the respective aglycone and then according to the substitution pattern of the respective aglycones and their sugar moieties. Here, compounds with ether bound substituents have been queued before compounds with additional sugar moieties, and these before compounds with acyl-substituents. The (R)-and (S)-series (if at all applicable or known) of otherwise identical aglycones/compounds have been ordered separately.
Gynocardin (48) (Figure 9), the so far only cyclopentene derivative f hydroxy moieties in the cyclopentene part of the molecule, was isolated fro odorata R.Br. (Achariaceae) [37]. Similar to the situation described above, compounds from Passiflo depicted in Figure 10 were also isolated in a brief period of time by two comp As again, the first description is not very reliable regarding stereochemistry the publication from Jaroszewski and his team [7] and only mention tha Seigler [52] were probably the first to isolate compounds 49 and 51 (in passisuberosin and epipassisuberosin). However, here we follow Jarosze workers [7] and name the compounds (which were first fully elucidated in Similar to the situation described above, compounds from Passiflora suberosa L. depicted in Figure 10 were also isolated in a brief period of time by two competing groups. As again, the first description is not very reliable regarding stereochemistry, we adhere to the publication from Jaroszewski and his team [7] and only mention that Spencer and Seigler [52] were probably the first to isolate compounds 49 and 51 (initially named passisuberosin and epipassisuberosin). However, here we follow Jaroszewski and co-workers [7] and name the compounds (which were first fully elucidated in reference [7]) suberin A (49) and B (51). The β-D-gentiobiosides additionally described by Spencer and Seigler are tentatively assigned structures 50 and 52. These should be named 6 -O-β-D-glucopyranosylsuberin A (50) and 6 -O-β-D-glucopyranosylsuberin B (52), instead of passisuberosin diglycoside and epipassisuberosin diglycoside, respectively. As a number of compounds allegedly containg rhamnose, isolated by the same authors, proved later to contain other rare sugars, a re-investigation of this compound using modern two-dimensional NMR experiments seems warranted. Passiguatemalin (53) (Figure 10), which could be envisaged as a ring-opened epoxide, was isolated from Passiflora hahnii (E.Fourn.) Mast. [as Passiflora guatemalensis S.Watson] (Passifloraceae) [38]. Though a full set of spectral data was provided, the stereochemistry of this compound has not been established yet [38]. Passiguatemalin (53) (Figure 10), which could be envisaged as a ring-opened epoxide, was isolated from Passiflora hahnii (E.Fourn.) Mast. [as Passiflora guatemalensis S.Watson] (Passifloraceae) [38]. Though a full set of spectral data was provided, the stereochemistry of this compound has not been established yet [38].

Group C: Cyanogenic Glycosides 54-108 Featuring an Aromatic Aglycone
A total of 55 cyanogenic glycosides containing an aromatic aglycone have been described. Amygdalin (65) isolated from bitter almond in 1830 was the first cyanogenic glycoside with an aromatic aglycone to be reported [1]. The latest compound is a derivative of prunasin, prunasin methacrylate (71), reported in 2018 [3]. An experiment by Fischer in 1895 yielded a derivative compound of amygdalin and was named Fisher's glycoside [53]. Later on, this glycoside was also isolated from other plant species, such as Prunus padus L. (Rosaceae), as prulaurasin (a mixture of prunasin and sambunigrin) [54] and Prunus serotina Ehrh. (Rosaceae) [55]. The name prunasin was then introduced in 1912 for compound 54 [56].

Bioactivity
Cyanogenic glycosides are foremost toxins, protecting the plant producing them from herbivores [2]. Recent studies on the role of cyanogenic glycosides in plant development have in addition revealed a function of cyanogenic glycosides as a nitrogen source for developmental processes and in playing a role in the adaption to environmental challenges [4].
Due to their presence in numerous edible plants, there are many reports dealing with the bioactivities of linamarin (1) and amygdalin (65). For decades, amygdalin (65) has been investigated for its potential application in cancer treatment [101,102]. Targeted cancer therapy, such as suicide gene therapy, antibody-directed enzyme prodrug therapy (ADEPT), and nanoporous imprinted polymers (nanoMIPs) gave particularly promising results [103]. While amygdalin (65) has mainly been investigated as a potential anticancer agent, studies focusing on linamarin (1) are mainly related to agriculture. Here, linamarin (1), a side product from making cassava safe for human consumption, has been tested as herbicides and bio-pesticides [104].

Materials and Methods
Literature was searched for cyanogenic glycosides using Google Scholar, PubChem, Reaxys, and SciFinder. Keywords were "cyanogenic glycosides", "cyanogenesis" as well as individual names of known cyanogenic glycosides. Names of the known cyanogenic glycosides compounds often led to articles reporting the isolation of related structures. All of the references in this review were then accessed from the homepages of their respective journals and for older articles, published between the years 1800-1930, from the Biodiversity Heritage Library website (https://www.biodiversitylibrary.org). After collecting the articles, structures of individual cyanogenic glycosides were then sorted as described in the results section, not only based on the precursor in the respective biosynthetic pathways [108], but also based on superficial chemical similarity (e.g., group A6).

Conclusions
The overview provided above shows that currently, 112 distinct cyanogenic glycosides are known from the plant kingdom. This is considerably more than the current literature estimate of about 60 different compounds [109]. Cyanogenic glycosides with an aromatic aglycone are the most diverse group with 55 individual natural products. The most complex structure containing a cyanogenic glycoside moiety is canthium glycoside (87) from Psydrax schimperiana (A.Rich.) Bridson (as Canthium schimperianum A.Rich.) (Rubiaceae). This compound, which also features an iridoid moiety, has a molecular formula of C 43 H 51 NO 21 . In contrast linamarin (1) as the simplest known cyanogenic glycoside has a molecular formula of only C 10 H 17 NO 6 . Many of the natural products compiled above, have so far only been reported form a single source, while others are unusually widely distributed in the plant kingdom. Some of the rare compounds have been isolated in times, when structure elucidation of complex natural products was more difficult than today and some re-assignments regarding exact positions of sugar moieties and stereochemistry seem inevitable, when these compounds will be re-investigated. Looking at the enormous possibilities how cyanogenic glycosides, such as e.g., prunasin (54) can be incorporated into more complex natural products, makes it intuitively clear that many more natural products encompassing a cyanogenic glycoside moiety still can be discovered.
This review is intended as a guide to get a quick overview, which compounds have already been described and where in the plant kingdom to look for potentially new natural products. The lack of detailed bioactivity reports on any other cyanogenic glycosides other than amygdalin and linamarin are also potential research topics.