A Review on Daphnane-Type Diterpenoids and Their Bioactive Studies

Natural daphnane diterpenoids, mainly distributed in plants of the Thymelaeaceae and Euphorbiaceae families, usually include a 5/7/6-tricyclic ring system with poly-hydroxyl groups located at C-3, C-4, C-5, C-9, C-13, C-14, or C-20, while some special types have a characteristic orthoester motif triaxially connectedat C-9, C-13, and C-14. The daphnane-type diterpenoids can be classified into five types: 6-epoxy daphnane diterpenoids, resiniferonoids, genkwanines, 1-alkyldaphnanes and rediocides, based on the oxygen-containing functions at rings B and C, as well as the substitution pattern of ring A. Up to now, nearly 200 daphnane-type diterpenoids have been isolated and elucidated from the Thymelaeaceae and Euphorbiaceae families. In-vitro and in-vivo experiments of these compounds have shown that they possess a wide range of biological activities, including anti-HIV, anti-cancer, anti-leukemic, neurotrophic, pesticidal and cytotoxic effects. A comprehensive account of the structural diversity is given in this review, along with the cytotoxic activities of daphnane-type diterpenoids, up to April 2019.


Introduction
Since the first daphnane diterpenoid characterized by a macrolactone motif was isolated from Trigonostemon reidioides [1], the daphnane diterpenoids have attracted the interest of many researchers because of their significant bioactive activities. Until now, nearly 200 natural products of daphnane-type diterpenoids have been isolated and identified, and they have shown good biological activities, including anti-HIV, anti-cancer, anti-leukemia, anti-hyperglycemic [2], neurotropic [3], insecticidal and cytotoxic [4] effects. Due to their rich pharmacological activities, especially strong anti-HIV activity and small cytotoxicity, daphnane-type diterpenoids have been employed in a range of clinical applications for a variety of clinical uses [5,6]. Studies have found that the natural daphnane-type diterpenoids usually embrace a 5/7/6-tricyclic ring system with poly-hydroxyl groups located at C-3, C-4, C-5, C-9, C-13, C-14, or C-20, while a special group also have a characteristic orthoester motif connected to C-9, C-13, and C-14. The daphnane-type diterpenoids can be categorized into five types ( Figure 1): 6-epoxy daphnane diterpenoids, resiniferonoids, genkwanines, 1-alkyldaphnanes and rediocides, based on the substitution pattern of ring A and the oxygen-containing functions at rings B and C. Besides, 6-epoxy daphnane diterpenoids usually have a C-6α epoxy structure in ring B; resiniferonoids usually have an α-β unsaturated ketone structure in ring A; genkwanines usually have an α-β saturated ketone structure in ring A, but without a C-6α epoxy structure in ring B; 1-alkyldaphnanes usually have a saturated ring A, and a large ring between the end of the orthoester alkyl chain and C-1 of ring A; and rediocides usually have a 12-carbon macrolide structure between C-3 and C-16, and have a special C-9, C-12, and C-14 orthoester structure. The variety of daphnane-type diterpenoid structures have continued to widen with the discovery of unusual variations with the well-established skeleton. Owing to the unique skeleton and remarkable bioactive activities, daphnane-type diterpenoids have attracted many synthetic endeavors to construct a core structure. However, few papers have reported on the total synthesis of daphnane diterpenoids-isolation from natural plants is still the only source of obtaining daphnane diterpenoids. Considering the extensive interest in daphnane-type diterpenoids, we reviewed the structural and bioactive activities of daphnane-type diterpenoids, with an emphasis on the recent progress in structure identification and bioactive evaluation. containing functions at rings B and C. Besides, 6-epoxy daphnane diterpenoids usually have a C-6α epoxy structure in ring B; resiniferonoids usually have an α-β unsaturated ketone structure in ring A; genkwanines usually have an α-β saturated ketone structure in ring A, but without a C-6α epoxy structure in ring B; 1-alkyldaphnanes usually have a saturated ring A, and a large ring between the end of the orthoester alkyl chain and C-1 of ring A; and rediocides usually have a 12-carbon macrolide structure between C-3 and C-16, and have a special C-9, C-12, and C-14 orthoester structure. The variety of daphnane-type diterpenoid structures have continued to widen with the discovery of unusual variations with the well-established skeleton. Owing to the unique skeleton and remarkable bioactive activities, daphnane-type diterpenoids have attracted many synthetic endeavors to construct a core structure. However, few papers have reported on the total synthesis of daphnane diterpenoids-isolation from natural plants is still the only source of obtaining daphnane diterpenoids. Considering the extensive interest in daphnane-type diterpenoids, we reviewed the structural and bioactive activities of daphnane-type diterpenoids, with an emphasis on the recent progress in structure identification and bioactive evaluation.

Occurrence
Natural daphnane-type diterpenoids are mainly distributed in species belonging to the Thymelaeaceae or Euphorbiaceae families (Table 1). These plants grow mainly in tropical and subtropical regions of Asia [7]. Previous chemical investigations on such species have led to the isolation of a number of structurally diverse diterpenoids [8]. Various daphnane-type diterpenoids have been isolated from some parts of the following plants: The twigs and leaves of Trigonostemonthyrsoideum, the roots of Trigonostemonreidioides, the stems of Trigonostemon lii, the twigs and leaves of TrigonostemonchinensisMerr, the stem barks of Daphne giraldii, the air-dried roots of Euphorbia fischeriana, the stems of D. acutiloba, the roots of Lasiosiphonkraussianus, the flower buds of Daphne genkwa, and the roots of MaprouneaafricanaMuell. Arg., Trigonostemonxyphophylloides, Wikstroemiaretusa, Trigonostemonhowii, and Stellerachamaejasme L., and so on [9].

Occurrence
Natural daphnane-type diterpenoids are mainly distributed in species belonging to the Thymelaeaceae or Euphorbiaceae families (Table 1). These plants grow mainly in tropical and subtropical regions of Asia [7]. Previous chemical investigations on such species have led to the isolation of a number of structurally diverse diterpenoids [8]. Various daphnane-type diterpenoids have been isolated from some parts of the following plants: The twigs and leaves of Trigonostemonthyrsoideum, the roots of Trigonostemonreidioides, the stems of Trigonostemon lii, the twigs and leaves of Trigonostemonchinensis Merr, the stem barks of Daphne giraldii, the air-dried roots of Euphorbia fischeriana, the stems of D. acutiloba, the roots of Lasiosiphonkraussianus, the flower buds of Daphne genkwa, and the roots of Maprouneaafricana Muell. Arg., Trigonostemonxyphophylloides, Wikstroemiaretusa, Trigonostemonhowii, and Stellerachamaejasme L., and so on [9].

Euphorbia fischeriana
Generally, the roots and flower budsistheir effective part.

Trigonostemonxyphophylloides
Usually their effective part isroots, stems, twigs and leaves, flower buds.

1-alkyldaphnanes
Wikstroemiachamaedaphne Usually, the flower buds and fresh bark is their effective part.

Trigonostemonthyrsoideum
Generally, their effective part is roots, twigs and leaves.

Genkwanines
Relative to 6-epoxy daphnane diterpenoids and resiniferonoids, genkwanines have an α-β saturated ketone structure in ring A, but do not possess a C-6α epoxy structure in ring B (Figure 4, Table 4). Compound trigoxyphin H (100) has been isolated from the twigs of Trigonostemonxyphophylloides [33]. The active ingredients trigothysoids A-L (122-124, 96-99, 139-141, 131,128), trigochinins A-E (145-146, 130,  147-148), andtrigonothyrins D, E (143-144) and G (121) have been obtained from the twigs and leaves of Trigonostemonthyrsoideum. These compounds have been evaluated for their anti-HIV activity usingan assay to determine their inhibition of the cytopathic effects of HIV-1 and their cytotoxicity against C8166 cells. Amongst them, trigothysoid A and L exhibited moderate anti-HIV-1 activity; andtrigothysoid C and K andtrigochinins A, B and D expressed weak anti-HIV-1 activity [11]. Trigolins A-G (132-138) and trigonothyrin F (107) have been isolated from the stems of Trigonostemon lii. Trigolins A, G, H, and K have been shown to exhibit modest anti-HIV-1 activity with EC 50 values of 2.04, 9.17, 11.42, and 9.05l µg/mL, respectively [34]. Compound trigochinin F (149) has been obtained from the twigs and leaves of Trigonostemonchinensis Merr, and has shown strong inhibition of HL-60 tumor cell lines [28]. Trigonothyrins A-C (125-127) have been isolated from the stems of Trigonostemonthyrsoideum [6]. Among them, trigonothyrin C has shown significant activity to prevent the cytopathic effects of HIV-1 in C8166 cells, with an EC 50 value of 2.19 µg/mL [35]. Compounds genkwanines F, I, and J (93, 113, 114) have been isolated from the flower buds of Daphne genkwa [14]. Genkwanine H (95) has been obtained from the flower buds of Daphne genkwa, and the compound has been shown to dependently inhibit melanin production [30]. Compounds trigonostempenes A (150) and B (129) have been isolated from the twigs and leaves of Trigonostemonthyrsoideum. Studies have shown that the discovery of these NO inhibitory daphnane diterpenoids-including compound trigonostempene A-which possess IC 50 values comparable topositive controls may have the potential to be developed as anti-neuroinflammatory agents for alzheimer disease (AD) and other related neurological disorders [16]. Most inhibitors of acetylcholinesterase (AchE) are alkaloids that often possess several side effects, whereas these daphnane-type diterpenoids do not belong to the class of alkaloids, and therefore they may constitute novel active AChE inhibitors with fewer side effects. It is important to search for new AChE inhibitors not belonging to this structural class [36,37]. Genkwanines A-E (88-92), G (94), I (113), and K (115) have been obtained from the bud of Daphne genkwa. Among these compounds, genkwanine D has been shown to exhibit strong activity to inhibit the endothelium cell HMEC at IC 50 levels of 2.90-15.0 µM [31]. Compounds trigoxyphins U and W (105-116) have been isolated from the twigs of Trigonostemonxyphophylloides. Trigoxyphin W has shown modest cytotoxicity against BEL-7402, SPCA-1 and SGC-7901, with IC 50 values of 5.62, 16.79 and 17.19 µM, respectively [33]. Trigonosins C-D (106, 142) have been obtained from the roots of Trigonostemonthyrsoideum [21]. Trigoxyphin I (104) has been isolated from the Trigonostemonxyphophylloides [38]. Compounds trigohownins D and E (101-102), and trigohownins A-C (108-110) and F-I (117-120) have been obtained from the Trigonostemonhowii. Among them, trigohownins A and D have been shown to exhibit moderate cytotoxic activity against the HL-60 tumor cellline, with IC 50 values of 17.0 and 9.3 µM, respectively [39]. Trigoxyphins D-F (111-112, 103) have been isolated from Trigonostemonxyphophylloides, with all three compounds found to be inactive against BEL-7402 cells (IC 50 value > 10 µM) [25]. 9 of 16 evaluated for their anti-HIV activity usingan assay to determine their inhibition of the cytopathic effects of HIV-1 and their cytotoxicity against C8166 cells. Amongst them, trigothysoid A and L exhibited moderate anti-HIV-1 activity; andtrigothysoid C and K andtrigochinins A, B and D expressed weak anti-HIV-1 activity [11]. Trigolins A-G (132-138) and trigonothyrin F (107) have been isolated from the stems of Trigonostemon lii. Trigolins A, G, H, and K have been shown to exhibit modest anti-HIV-1 activity with EC50 values of 2.04, 9.17, 11.42, and 9.05l μg/mL, respectively [34]. Compound trigochinin F (149) has been obtained from the twigs and leaves of Trigonostemonchinensis Merr, and has shown strong inhibition of HL-60 tumor cell lines [28]. Trigonothyrins A-C (125-127) have been isolated from the stems of Trigonostemonthyrsoideum [6]. Among them, trigonothyrin C has shown significant activity to prevent the cytopathic effects of HIV-1 in C8166 cells, with an EC50 value of 2.19 μg/mL [35]. Compounds genkwanines F, I, and J (93, 113, 114) have been isolated from the flower buds of Daphne genkwa [14]. Genkwanine H (95) has been obtained from the flower buds of Daphne genkwa, and the compound has been shown to dependently inhibit melanin production [30]. Compounds trigonostempenes A (150) and B (129) have been isolated from the twigs and leaves of Trigonostemonthyrsoideum. Studies have shown that the discovery of these NO inhibitory daphnane diterpenoids-including compound trigonostempene A-which possess IC50 values comparable topositive controls may have the potential to be developed as anti-neuroinflammatory agents for alzheimer disease (AD) and other related neurological disorders [16]. Most inhibitors of acetylcholinesterase (AchE) are alkaloids that often possess several side effects, whereas these daphnane-type diterpenoids do not belong to the class of alkaloids, and therefore they may constitute novel active AChE inhibitors with fewer side effects. It is important to search for new AChE inhibitors not belonging to this structural class [36,37]. Genkwanines A-E (88-92), G (94), I (113), and K (115) have been obtained from the bud of Daphne genkwa. Among these compounds, genkwanine D has been shown to exhibit strong activity to inhibit the endothelium cell HMEC at IC50 levels of 2.90-15.0 μM [31]. Compounds trigoxyphins U and W (105-116) have been isolated from the twigs of Trigonostemonxyphophylloides. Trigoxyphin W has shown modest cytotoxicity against BEL-7402, SPCA-1 and SGC-7901, with IC50 values of 5.62, 16.79 and 17.19 μM, respectively [33]. Trigonosins C-D (106, 142) have been obtained from the roots of Trigonostemonthyrsoideum [21]. Trigoxyphin I (104) has been isolated from the Trigonostemonxyphophylloides [38]. Compounds trigohownins D and E (101-102), and trigohownins A-C (108-110) and F-I (117-120) have been obtained from the Trigonostemonhowii. Among them, trigohownins A and D have been shown to exhibit moderate cytotoxic activity against the HL-60 tumor cellline, with IC50 values of 17.0 and 9.3 μM, respectively [39]. Trigoxyphins D-F (111-112, 103) have been isolated from Trigonostemonxyphophylloides, with all three compounds found to be inactive against BEL-7402 cells (IC50 value > 10 μM) [25].

Rediocides
Rediocides usually have a 12-carbon macrolide structure between C-3 and C-16, and have a special C-9, C-12, and C-14 orthoester structure ( Figure 6, Table 6). The active compounds trigothysoids N-P (182-184), rediocides A, C, and F (176-177, 179), and trigonosin F (181) have been obtained from the twigs and leaves of Trigonostemonthyrsoideum. Amongst them, compounds trigothysoid N, rediocides A, C, and F, and trigonosins F have shownpotent anti-HIV-1 activity, with EC 50 values ranging from 0.001 to 0.015 nM. Additionally, trigothysoid O has been shown to exhibit moderate anti-HIV-1 activity [11], while rediocide A has shown potent activities against mosquito larvae in an in-vitro assay study and against fleas (Ctenocephalides felis) in an artificial membrane feeding system, exhibiting LD 90 values of 1 and 0.25 ppm, respectively [39]. Trigochilides A and B (175, 186) have been isolated from the twigs and leaves of Trigonostemonchinensis Merr. Trigochilide A has shown modest cytotoxicity against HL-60 (human leukemia) and BEL-7402 (human hepatoma), with demonstrated IC 50 values of 3.68 and 8.22 µM, respectively, whereas compound trigochilide B has only been shown to exhibit weak cytotoxicity against two tumor cell lines, with IC 50 values of 33.35 and 54.85 µM [1]. Compound rediocide E (178) has been obtained from the roots of Trigonostemonreidioides, and has shown significant acaricidal activity on D. pteronyssinus [40]. Trigonosin E (180) and trigonostempene D (185) have beenisolated from the twigs and leaves of Trigonostemonthyrsoideum [16,21]. Rediocides B, G, and D (187-189) have been isolated from the Trigonostemonreidioides, and have been evaluated for their insecticidal properties in an anti-flea artificial membrane feeding assay (as detailed earlier). In this assay, rediocides B and D exhibited LD 90 values of 0.25 and 0.5 ppm, respectively, and thus were equipotent with rediocide A (LD 90 0.25 ppm) [41]. 13 of 16 has only been shown to exhibit weak cytotoxicity against two tumor cell lines, with IC50 values of 33.35 and 54.85 μM [40]. Compound rediocide E (178) has been obtained from the roots of Trigonostemonreidioides, and has shown significant acaricidal activity on D. pteronyssinus [41]. Trigonosin E (180) and trigonostempene D (185) have beenisolated from the twigs and leaves of Trigonostemonthyrsoideum [16,21]. Rediocides B, G, and D (187-189) have been isolated from the Trigonostemonreidioides, and have been evaluated for their insecticidal properties in an anti-flea artificial membrane feeding assay (as detailed earlier). In this assay, rediocides B and D exhibited LD90 values of 0.25 and 0.5 ppm, respectively, and thus were equipotent with rediocide A (LD90 0.25 ppm) [42].   Figure 6. Five types (Y1-Y5) of rediocides skeletons.

Conclusions
It can be concluded that the bioactive activities of daphnane-type diterpenoids is obviously related to structure types. The most important points of them are the following: (1) The orthoester groups at C-9, C-13 and C-14 are essential to the cytotoxic activity. Daphnane-type diterpenoids with orthoester groups at C-9, C-13, and C-14 usually have stronger activity than daphnane-type diterpenoids with orthoester groups at C-9, C-12, and C-14 or C-12, C-13 and C-14. The absence of the orthoester group is unhelpful to the cytotoxic activity. (2) Specific to the 6-epoxyl groups, free 20-hydroxyl and 3-carbonyl are important for their activities. (3) Side chains at C-10 are crucial for cytotoxic activities. Generally speaking, long C-10 alkyl chains are more important than phenyl at C-10. Interestingly, the structure with macro-lactones exhibited much stronger activity than the others. Due to the rich activities of daphnane-type diterpenoids, researchers have not stopped exploring and researching such compounds and their bioactive activities from plants.