2. Structural Assignment and Stereochemistry Determination
A total of 97 metabolites have been separated from various fungal source extracts using different chromatographic techniques and characterized by NMR, MS, and IR spectral analyses as well as chemical derivatization. The relative configuration of these metabolites was established using NOESY or ROESY spectral analyses. Various studies reported the assigning of their absolute stereochemistry using total synthesis
[13][14], Mosher’s method
[15], X-ray diffraction, chemical conversion
[16][17][18], and ECD analyses
[19]. The reported metabolites have been categorized into bi-, tri-, and tetracyclic derivatives.
3. Biological Activities of Bergamotane Sesquiterpenoids
Various reported studies revealed the assessment of bergamotane sesquiterpenoids for diverse bioactivities, including plant growth regulation, phototoxic, antimicrobial, anti-HIV, cytotoxic, pancreatic lipase inhibition, antidiabetic, anti-inflammatory, and immunosuppressive
NO (nitric oxide) is a substantial pro-inflammatory mediator, and its excessive production is accompanied with various inflammatory illnesses; therefore, it possesses a remarkable role for regulating immune responses and inflammation
[20]. NO production inhibitors may represent the potential capacity for treating various inflammatory disorders. Thus, further research for fungal metabolites must be conducted to discover novel anti-inflammation agents.
The epigenetic chemical manipulation of
Eutypella sp. derived from deep-sea hydrothermal sulfide deposit by co-treatment with SBHA (histonedeacetylase inhibitor, suberohydroxamic acid) and 5-Aza (DNA methyltransferase inhibitor, 5-azacytidine) was shown to activate a biosynthetic sesquiterpene-linked gene cluster
[21]. From elicitor-treated cultures EtOAc extract, eutypeterpenes A–F (
18–
22 and
28) along with xylariterpenoids A (
16) and B (
17) were purified using SiO
2/RP-18/HPLC that were identified by spectral analyses, as well as by using chemical conversion, X-ray diffraction, ECD, and calculated NMR for configuration assignments.
Eutypeterpene A (
28) is the first bergamotene sesquiterpene incorporating a dioxolanone moiety. These metabolites were assessed for their NO production inhibitory capacity induced by LPS-(lipopolysaccharide) in RAW 264.7 macrophages
[21]. The results indicated thatcompound
18 and
19 (IC
50 13.4 and 16.8 μM, respectively) displayed more effectiveness than quercetin (IC
50 of 17.0 μM), whereas other metabolites had noticeable potentials (IC
50 values ranged from 18.7 to 24.3 μM) with weak cytotoxic capacities (IC
50 > 100 μM). A structure–activity study revealed that the analog with a triol unit (
18) at the side chain was more effective than compound
16,
17, and
19 with a diol unit, which were more potent than compound
20,
21, and
28 with one hydroxy group. Furthermore, the α,β-unsaturated ketone unit (as in compound
21 and
22) and the OH-linked carbon configuration also affected the activities (
16 versus
17)
[21] (
Figure 1).
Figure 1. Structures of bicyclic bergamotane sesquiterpenoids (1–17).
Biogenetically, compounds
18–
22 are derived from FPP that performs a 1,6-cyclization to produce bisabolane (
A). The 4,7-cyclization of
A generates bergamotane (
B), which further generates
18–
22 via diverse oxidation and reduction processes. Additionally, compound
28 is formed from
18 by carbonate incorporation
[21] (Scheme 1).
Scheme 1. Biosynthetic pathway of eutypeterpenes A–F (compounds
18–
22 and
28)
[21].
The deep-sea-isolated
Graphostroma sp. MCCC3A00421 associated with the Atlantic Ocean hydrothermal sulfide deposits biosynthesized new bergamotane sesquiterpenoids: (10S)-xylariterpenoid A (
23), (10R)-xylariterpenoid B (
24), xylariterpenoid E (
25), xylariterpenoid F (
26), and xylariterpenoid G (
27), which were purified using SiO
2/OSD/Sephadex LH-20/RP-18 CC and preparative TLC. They were characterized by extensive spectral data, and their absolute configuration was established by ECD, Cu-Ka-single-crystal X-ray diffraction, and modified Mosher’s method analyses. Compound
25 is trinor-bergamotane. Compounds
23,
26, and
27 revealed moderate inhibition potentials (IC
50s of 86, 85, and 85 μM, respectively) of NO production in LPS-stimulated RAW264.7 macrophages compared with aminoguanidine (IC
50 of 23 μM). It was noted that bergamotane moiety’s 10S configuration obviously boosted the activity as in compound
23 (10S, IC
50 of 85 μM) versus compound
24 (10R, of IC
50 230 μM) (
Figure 2)
[16].
Figure 2. Structures of bicyclic bergamotane sesquiterpenoids (compounds 18–28).
3.2. Phytotoxic Activity
Prehelminthosporol (
38) and dihydroprehelminthosporol (
34) along with compounds
35–
37,
39,
40, and
43 were separated by SiO
2, flash CC, and preparative TLC from the EtOAc extract of the
Bipolaris species, which is a
Sorghum halepnse (Johnson grass) pathogen (
Figure 3). These metabolites were assessed for their phytotoxic potential towards
Sorghum bicolor (Sorghum) and
Sorghum balepense (Johnson grass) in leaf spot assays
[22][23]. Compounds
34,
38, and
39 produced similar lesions to those caused by the fungus in the field. The lesions appeared as a reddish-brown area (0.3–0.5 cm diameter) surrounded by a black circle with an outer chlorotic zone. Compounds
34 and
38 (concentration of 25 μg/5 μL) had comparable toxic effectiveness, while compound
38 maintained its effect at a lower concentration of 2.5 μg/5 μL; meanwhile, the other compounds were non-toxic
[22][23]. Victoxinine was also toxic to cereals in the order of oats > rye and barley > wheat > sorghum in a root inhibition assay
[23]. The phytotoxic influence of compounds
34 and
38–
40 versus sorghum, corn, bent-grass, sickle-pod, and morning glory was also assessed in leaf spot assays. Moreover, victoxinine caused a water-soaked translucent appearance with defined irregular necrotic edges. It is worth mentioning that 3-deoxyanthocyanidins are sorghum stress response metabolites (phytoalexins), which were accountable for the red wound response. Compounds
34,
38, and
39 were elicitors of a very strong reddening compared with the wounding-produced reddening, but compound
40 did not elicit a sorghum phytoalexin response. In bent grass and corn, compounds
34 and
38–
40 produced a light-brown area limited by a chlorotic region, whereas in sickle pod and morning glory, they showed necrotic lesions that extended at high concentrations beyond the under-drop area. It is noteworthy that compound
38 was the most toxic compound versus all tested plants except for the morning glory
[23].
Figure 3. Structures of the bicyclic bergamotane sesquiterpenoids (compounds 29–37).
Helminthosporium victoriae, the causative agent of oats Victoria blight disease yielded phytotoxins, victoxinine (
40) and victoxinine α-glycerophosphate (
41), which were separated from its diethyl ether extract using Sephadex LH-20 and SiO
2 CC and detected on the TLC plate by giving a blue color with 5% vanillin:H
2SO
4 [24] (
Figure 4). The existence of α-glycerophosphate moiety was established by coupling between the phosphorous and carbon. Compound
40 completely prohibited the root growth of toxin-susceptible and toxin-resistant oats (concentration of 2.5 × 10
−4 M); it was ≈ 7500 times more toxic for susceptible plants on a weight basis, while its toxicity for resistant plants was nearly similar, suggesting a role of the victoxinine moiety on the toxicity
[24][25][26]. On the other side, compound
41 (concentration of 100 µg/mL) demonstrated little or no growth inhibition effectiveness on either susceptible or resistant oats
[24].
Figure 4. Structures of tricyclic bergamotane sesquiterpenoids (compounds 38–43).
3.3. Anti-HIV Activity
From Paraconiothyrium brasiliense, new tricyclic sesquiterpenoids, brasilamides A–D (45–48) and the formerly reported pinthunamide (44), were separated from the culture’s EtOAc extract utilizing SiO2/Sephadex LH-20 CC and HPLC. Their structures were established using NMR and X-ray analyses (Figure 5). Compounds 45 and 46 are rare metabolites having a 4-oxatricyclo[3.3.1.02,7]nonane moiety with a tetrahydro-2H-pyrone or a tetrahydro-2H-pyran linked with bicyclo[3.1.1]heptane ring at C-5 and C-2, whereas compounds 47 and 48 are analogs of 44, possessing an unprecedented 9-oxatricyclo[4.3.0.04,7]-nonane core.
Figure 5. Structures of tricyclic bergamotane sesquiterpenoids (44–55).
The differences of the above-mentioned compounds from
44 were the existence of a tetrahydrofuran moiety connected to the bicycle[3.1.1]heptane unit instead of γ-lactone ring, as well as different C-10 substituents. Compounds
45–
48 demonstrated inhibitory effectiveness (EC
50s of 108.8, 57.4, and 48.3 µM, respectively) versus HIV-1 replication in C8166 cells compared with indinavir sulfate (EC
50 of 8.2 nM)
[17]. Biogenetically, they were derived from the mevalonate/
trans-
cis-farnesol/bisabolene/bergamotane pathway (Scheme 2).
Scheme 2. Biosynthetic pathways of brasilamides A–D (
45–
48)
[17].
3.4. Immunosuppressive Activity
Immunosuppressants are drugs that prohibit body immunity and are principally utilized in organ transplantation to overcome rejection and in auto-immune illnesses
[27]. Currently, many immunosuppressive agents act by prohibiting T-cell proliferation; however, there is no new, safe, and efficient immune-suppressive agent that prohibits B-cell proliferation
[28].
Dai et al. separated eighteen bergamotane sesquiterpenoids from the EtOAc extract of
Craterellus ordoratus: craterodoratins A–R (
7–
11,
29–
33,
53,
55,
56,
63–
65, and
71) and a new victoxinine derivative, craterodoratin S (
42), along with the previously isolated
5,
61,
77, and
88 by SiO
2/RP-18/Sephadex LH-20/preparative HPLC (
Figure 6)
[29].
Figure 6. Structures of tricyclic bergamotane sesquiterpenoids (56–63).
Their structures with absolute configurations were established by spectral, X-ray diffraction, and ECD analyses and NMR calculations. Compounds
29 and
71 possess a rare skeleton, where the C-14methyl in
71 showed a further 1,2-migration. On the other hand, compounds
7–
11,
53,
55,
56, and
63–
65 belong to β-pinene derivatives that produced
30–
33 through an alkyl migration (
Figure 7). Compounds
7–
10,
30,
42,
55,
61, and
88 demonstrated potent inhibitory potential versus LPS-caused B lymphocyte cell proliferation (IC
50s ranged from 0.67 to 22.68 μM) in BALB/c mice compared with cyclosporin A (IC
50 of 0.47 μM), where compound
61 (IC
50 0.67 μM) had the most potent effectiveness. Moreover, compounds
11 and
61 possessed inhibition (IC
50s of 31.50 and 0.98 μM, respectively) on T lymphocyte cells proliferation induced by ConA (concanavalin A) compared with cyclosporin A (IC
50 0.04 μM). Structurally, it was noted that the
α,
β-unsaturated-carboxylic acid unit could be the key functional group for the immunosuppressive potential of these metabolites. Furthermore, compounds
61 and
7–
10 with a
β-pinene main core had a wider range of bioactivities
[29].
Figure 7. Structures of tricyclic bergamotane sesquiterpenoids (64–71).
3.5. Antimicrobial Activity
From
Podospora decipiens, two new tetracyclic sesquiterpenoids, decipienolides A (
74) and B (
75), were separated from the EtOAc extract by SiO
2 CC and HPLC analyses. They were obtained as a mixture of inseparable epimers, having a 3-hydroxy-2,2-dimethylbutyric acid sidechain as elucidated by an NMR analysis (
Figure 8). The
74/
75 mixture had an antibacterial influence versus
B. subtilis (inhibition zone diameter of 9–10 mm, concentration of 200 µg/disk). Neither of them demonstrated capacity versus
Ascobolus furfuraceus NRRL6460,
Sordaria fimicola NRRL6459, and
C. albicans ATCC90029
[30]. Donacinolides A (
82) and B (
76) (concentration of 50 μg/mL) revealed weak inhibition versus
Salmonella enterica subsp.
enterica (inhibition rates of 24.3, 23.9, and 26.2%) in the microdilution assay
[31]. Furthermore, there were no observed antibacterial activity for purpurolides B (
83) and C (
84) (concentration of 50 μM) versus
E. coli ATCC25922,
M. smegmatis mc2155 ATCC70084,
S. aureus ATCC25923, and
S. epidermidis ATCC12228
[32].
Figure 8. Structures of tetracyclic bergamotane sesquiterpenoids (72–79).
3.6. Pancreatic Lipase Inhibition
Purpurolides B (83) and C (84) are new 6/4/5/5-tetracyclic sesquiterpenoids that were separated from Penicillium purpurogenum IMM003 cultures by SiO2/RP-18/preparative HPLC analysis. The structures and configurations of compounds 83 and 84 were established using spectral and X-ray analyses as well as ECD and GIAO NMR data calculations (Figure 9).
Figure 9. Structures of tetracyclic bergamotane sesquiterpenoids (80–93).
Compounds
83 and
84 demonstrated potent pancreatic lipase inhibition (IC
50s of 5.45 and 6.63 μM, respectively), compared with kaempferol (IC
50 of 1.50 μM)
[32]. These compounds were possibly biosynthesized via numerous the cyclization and enzyme-catalyzed oxidation of FPP (farnesyl pyrophosphate), leading to four- and six-membered rings and the formation of two five-membered heterocyclic rings (Scheme 3)
[32].
Scheme 3. Biosynthetic pathway of purpurolides B and C (
83 and
84)
[32].
Xia et al. separated from
Penicillium purpurogenum IMM003 purpurolides D–F (
85–
87), which are new polyoxygenated 6/4/5/5-tetracyclic bergamotanes, using SiO
2/Sephadex-LH-20/RP-18 CC and preparative HPLC processing
[33]. Their elucidation was accomplished using spectral
13C NMR calculations coupled with DP
4+ probability and ECD analyses. Compound
87 had potent pancreatic lipase inhibition potential (IC
50 of 1.22 μM) compared with kaempferol (IC
50 of 1.50 μM) and orlistat (IC
50 of 0.75 μM), whereas compounds
85 and
86 (IC
50s of 6.50 and 7.88 μM, respectively) were five or six-fold less powerful than
87, revealing that the C-14 hydroxylated decanoic acid moiety increased the potency
[33]. Therefore, polyoxygenated bergamotanes could be viable candidates as pancreatic lipase inhibitors for further clinical development
[33].
3.7. Antidiabetic Activity
From the deep sea-derived
Paraconiothyrium brasiliense HDN15-135 EtOAc extract, new bergamotane sesquiterpenoids, brasilterpenes A-E (
66–
70), featuring an uncommon 6/4/5-tricyclic ring system, were separated by SiO
2/RP-18/Sephadex LH-20/HPLC and assigned by diverse NMR analyses and X-ray diffraction, ECD, and DFT-NMR (density functional theory calculations of nuclear magnetic resonance) data
[34]. Their hypoglycemic potential was estimated utilizing β-cell-ablated zebrafish larvae. Compounds
66 and
68 (concentration of 10 μM) remarkably lessened the glucose level down to 449.3 and 420.4 pmol/larva respectively, compared with the β-cell-ablated group (Teton+) (glucose level of 502.8 pmol/larva) and rosiglitazone (glucose level 395.6 pmol/larva) with no toxic influence on zebrafish larvae up to 200 μM. It was found that compounds
66 and
68 notably minimized free blood glucose in vivo in hyperglycemic zebrafish by suppressing gluconeogenesis and improving insulin sensitivity, which revealed that compound
68 had promising antidiabetic potential
[34]. The structure–activity study revealed that the activity may be linked to the C-14
S-configuration of compounds
66 and
68, which represent the main structural difference from
67 and
69. The existence of C-3-OH may weaken the influence in
68 versus
66; however, the △
2 endocyclic double bond may enhance the potential in
70 versus
69 [34]. Therefore, compound
68 may provide a scaffold for hypoglycemic drug development. Compounds
66–
70 are also biosynthesized by the FPP pathway (Scheme 4). The cyclization of FPP via NPP (nerolidyl diphosphate) followed by a bisabol intermediate yields the bergamotane skeleton. These compounds are created by further oxidation, 9-OH-nucleophilic attack, and methylation processes. Because of the nucleophilic attack direction flexibility during the furan ring formation, compounds
66–
69 appeared as C14-epimers in pairs
[34] (Scheme 4).
Scheme 4. Biosynthetic pathway of brasilterpenes A-E (
66–
70)
[34]. IPP: isopentenyl diphosphate; FS: farnesyl synthase; NPP: nerolidyl diphosphate; TC: terpenyl cyclase; DMAPP: dimethylallyl diphosphate; FPP: farnesyl diphosphate.
Ying et al. isolated two new derivatives, expansolides C (
73) and D (
81), in addition to
72 and
80 from the plant pathogen
Penicillium expansum ACCC37275
[35]. In an α-glucosidase inhibition assay; the
73/
81 epimeric mixture (ratio 2:1) possessed a more powerful effectiveness (IC
50 of 0.50 mM) compared with acarbose (IC
50 1.90 mM), while the
72/
80 epimeric mixture possessed no apparent potential. It was assumed that the acetyl group in compounds
72 and
80 impeded their binding with the α-glucosidase, resulting in loss of activity
[35].
3.8. Plant Growth Regulation
Kimura et al. purified the tricyclic amide sesquiterpenoid pinthunamide (
44) from the acetone extract of
Ampulliferina sp. at pH 2.0 utilizing SiO
2 and sephadex LH20 CC processing as well as crystallization from EtOAc extract, which gave positive NH
2OH-HCI-FeCl
3 and KMnO
4 reactions
[36]. The compound was assigned by X-ray diffraction and NMR methods. Its plant growth regulation effectiveness was evaluated using a lettuce seedling assay, where it (dose 300 mg/L) produced a 150% root growth acceleration over the control seedlings (100%) while scarcely influencing the hypocotyl elongation at the tested concentrations
[36]. Its structure combined a unique configuration of six-, five-, and four-membered rings that was proposed to be biosynthesized via the mevalonate/
trans-
cis-farnesol/bisabolene/bergamotane pathway (Scheme 5)
[36].
Scheme 5. Biosynthesis pathway of pinthunamide (
44)
[36].
Furthermore, in 1990, Kimura et al. purified another two new plant growth regulators, ampullicin (
91) and isoampullicin (
92) from
Ampulliferina sp. No. 27 associated with
Pinus thunbergii dead tree by SiO
2 CC utilizing benzene:acetone as an eluent
[37] (
Figure 10). They were stereoisomers that had γ-lactam rings. Additionally, they (doses of 300 and 30 mg/L) were shown to promote lettuce seedling root growth by 200% over the control lettuce seedlings (100%)
[37]. In 1993, the same group separated a minor metabolite, dihydroampullicin (
93), characterized by the absence of the C8-C9 double bond. The compound promoted a 160% growth rate in lettuce seedling roots (dose of 300 mg/L) compared with the control; however, it had no influence on the hypocotyl growth, indicating that the C8-C9-double bond (C8-C9) was substantial in lettuce seedlings` root growth
[38]. Bermejo et al. reported the synthesis of (
+)
−91 and
92 from (
R)-(-)-carvone with a 4.5% overall yield using a stereo-selective 18-step sequence application
[39]. The EtOAc extract of
Aspergillus fumigatus Fresenius separated from leaf litter yielded expansolides A (
72) and B (
80). They had 2
S/4
S/6
S/7
R/9
R/11
S and 2
S/4
R/6
S/7
R/9
R/11
S, respectively, based on modified Mosher’s method. The compounds noticeably prohibited etiolated wheat coleoptiles growth by 100% and 59% at 10
−3 M and 10
−4 M solution compared with LOGRAN (commercial herbicide) (%inhibition of 80 and 42%) at the same concentrations
[15].
Figure 10. Structures of tetracyclic bergamotane sesquiterpenoids (94–97).
3.9. Cytotoxic Activity
Compounds
3 and
4, which were new β-bergamotane sesquiterpenoids, were separated by SiO
2/RP-18/HPLC from the marine-associated
Aspergillus fumigatus-YK-7 EtOAc extract. Their antiproliferative effects on the U937 and PC-3 cell lines were measured in vitro in an MTT assay. Compound
4 revealed a weak growth inhibition capacity (IC
50 of 84.9 µM) versus the U937 cell line, while
3 had no activity (IC
50 > 100 µM) compared with doxorubicin hydrochloride (IC
50 of 0.021 µM). On the other sides, both had no effect versus PC-3 cells
[40]. Wu et al. reported the separation of two new derivatives, xylariterpenoids A and B (
16 and
17), from the EtOAc extract of
Xylariaceae fungus by Sephadex LH-20/ODS CC and reversed-phase HPLC processing
[41]. Their structures and stereo-configuration were proved utilizing NMR and CD methods. They are C-10 epimers having 2S/6S/7S/10R and 2S/6S/7S/10S configurations, respectively. Unfortunately, they (IC
50 > 40 μM) exhibited no cytotoxic potential versus HL-60, MCF-7, SMMC-7721, A-549, and SW480 in an MTT assay
[41].
From
Paraconiothynium brasiliense Verkley, new bergamotane sesquiterpenoids brasilamides K-N (
49–
52), featuring 4-oxatricyclo-(3.3.1.0
2,7)-nonane (as in
49) and 9-oxatricyclo-(4.3.0.0
4,7)-nonane (as in
50–
52) skeletons in addition to the formerly reported brasilamides A and C (
45 and
46), were purified from the fungus scale-up fermentation cultures using SiO
2/Sephadex LH-20/HPLC processing. They were elucidated via NMR analyses and compound
52’s configuration was assured using modified Mosher’s method. Compound
49 is a
45-hydrogenated analog that has a tetrahydro-2H-pyrone unit linked at C-2 and C-5 to the bicyclo(3.1.1)heptane framework, forming a 4-oxatricyclo-(3.3.1.0
2,7)-nonane skeleton, whereas compounds
50–
52 displayed unusual 9-oxatricyclo-(4.3.0.0
4,7)-nonane skeletons. Compounds
50 and
51 are hydrogenated and oxygenated derivatives of
46, respectively, while
52 differed from
46 by having a C-8-carbonyl, C-1-methyl, and C-12 hydroxyl group instead of methylene, oxy-methylene, and ketone carbonyl, respectively. These metabolites (concentration of 50 µM) possessed no potential versus A549, A375, MCF-7, CNE1-LMP1, EC109, MGC, PANC-1, and Hep3B-2 in the MTS assay
[42].
Montagnula donacina (edible mushroom) biosynthesized rare tetracyclic bergamotane sesquiterpenoids, donacinolides A (
82) and B (
76) and donacinoic acids A (
88) and B (
6), which were separated using SiO
2 CC/Sephadex LH-20 CC/HPLC processing and were characterized using spectroscopic data, X-ray diffraction analysis, and computational methods. Compounds
76 and
82 are C9 epimers with a spiroketal moiety having 1S/5S/6S/9R and 1S/5S/6S/9S configurations, respectively, whereas
88 and
6 exhibited α,β-unsaturated carboxylic acid moiety and had 1R/2R/5S/6S/9S/14S and 1R/3S/5R/6R/9S configurations, respectively. These metabolites lacked a marked cytotoxic potential (IC
50 > 40 μM) versus HL-60, SW480, A549, SMMC-7721, and MCF-7
[31].
In addition, purpurolides B (
83) and C (
84) had no cytotoxicity versus M14, HCT-116, U87, A2780, BGC-823, Bel-7402, and A549
[32], whereas compounds
85–
87 (concentration of 50 μM) were inactive versus HCT-116, BGC-823, and Bel-7402 cell lines
[33].
The chemical investigation of Arctic fungus
Eutypella sp. D-1′s EtOAc extract yielded new derivatives, eutypellacytosporins A–D (
94–
97), which were established by spectroscopic analysis and modified Mosher’s method. Structurally, these metabolites are related to decipienolides and cytosporins. They exhibited (IC
50s ranging from 4.9 to 17.1 μM) weak-to-moderate cytotoxic influence versus DU145, SW1990, Huh7, and PANC-1 in the CCK-8 assay, whereas Huh7 and SW1990 cell lines had more sensitivity to
94–
97 (IC
50s ranging from 4.9 to 8.4 μM). On the other hand, compounds
95 and
97 possessed noticeable potential versus PANC-1 (IC
50s of 7.9 and 7.5 μM, respectively) compared with cisplatin (IC
50 4.5 μM). The results revealed that the decipienolide moiety was substantial for activity; however, the C-33 configuration did not affect the activity
[43]. It was proposed that compounds
94–
97 are created from gentisaldehyde precursor with subsequent isoprenyl unit addition, double bond epoxidation, keto group hydrogenation, and an aliphatic chain addition (Scheme 6). The other precursor, the 14-OH of decipienolide A
74 or B
75, is produced from hydroxylation, allylic oxidation, and cyclization of farnesyl diphosphate to give
I with a bicycle[3.1.1]heptane. Additionally, (14S)-14-OH-expansolide C, (14R)-14-OH-expansolide C, (14S)-14-OH-expansolide D, and (14R)-14-OH-expansolide D are formed via two steps of reface- and si-face attacks of the OH groups on the ketone and aldehyde groups, respectively. After these steps, compounds
94–
97 were produced from the two groups of 14-OH-expansolides C and D through condensation reactions with (S)-3-hydroxy-2,2-dimethylbutanoic acid and cytosporin D, respectively
[43].
Scheme 6. Biosynthetic pathway of eutypellacytosporins A–D (
94–
97)
[43].