Fungi are widely distributed in the terrestrial environment, freshwater, and marine habitat. Only approximately 100,000 of these have been classified although there are about 5.1 million characteristic fungi all over the world. These eukaryotic microbes produce specialized metabolites and participate in a variety of ecological functions, such as quorum detection, chemical defense, allelopathy, and maintenance of symbiosis. Fungi therefore remain an important resource for the screening and discovery of biologically active natural products. Sesquiterpenoids are arguably the richest natural products from plants and micro-organisms. The rearrangement of the 15 high-ductility carbons gave rise to a large number of different skeletons. At the same time, abundant structural variations lead to a diversification of biological activity.
1. Introduction
Fungi are undoubtedly important resources for natural products discovery. With the advancement of natural product research, the importance of its biological resources has been infinitely enlarged. In the giant natural product system of fungi, sesquiterpenes, due to their carbon skeletons and amounts, are the largest of all types. The C-15-hydrocarbon skeletal system of various sesquiterpenoids isolated from fungi, bacteria, and plants are synthesized from farnesyl pyrophosphate (FPP) under the catalysis of sesquiterpene synthases
[1][2]. Sesquiterpene synthases catalyze different initial cyclization reactions to produce secondary or tertiary cyclic carbocation intermediates, which can then be further cyclized and reassembled until carbocation quenching at the active center, followed by the enzymatic release of the final sesquiterpenoid scaffold (
Figure 1)
[3]. A huge number of sesquiterpenoids were, consequently, produced
[4][5][6]. Among various other resources, fungal species have an enormous contribution owing to their potential to carry out the bio-transformations and drug synthesis under environmentally acceptable conditions. For instance, hydroxymethylacylfulvene (HMAF) is a semisynthetic antitumor agent based on the naturally occurring illudin S occurring in the mushroom
Omphalotus olearius [7]. It has been advanced into human clinical trials for the treatment of cancers
[8][9]. Trichothecenes, a class of tricyclic sesquiterpenes produced by a wide variety of fungi, are toxic to animals and humans and frequently present in cereal crops. They have attracted much attention in the areas such as agriculture, food contamination, and health care
[10][11][12][13].
Figure 1. Cyclization of FPP by characterized fungal sesquiterpene synthases (Reference
[3]).
2. Composition and Bioactivities
2.1. Alliacane, Cadinene, Azulene, and Zierane
Nine alliacane sesquiterpenoids inonoalliacanes A–I
1a/
1b–
6a/
6b–
7–
9 were isolated from the culture broth of the basidiomycete
Inonotus sp. BCC 22670
[14]. Inonoalliacane A
1 exhibited moderate antibacterial activity against
Bacillus cereus with a minimum inhibitory concentration (MIC) value of 25 µg/mL. Inonoalliacane B
2 showed antiviral activity against herpes simplex virus type 1 (HSV-1) with IC
50 of 17 μg/mL.
Clitocybulols G–O
10–
18, highly oxidized alliacane sesquiterpenoids, were isolated from the solid culture of the edible fungus
Pleurotus cystidiosus [15]. Clitocybulols G
10 and L
15 showed weak inhibitory activity against protein tyrosine phosphatase-1B (PTP1B) with IC
50 values of 49.5, 38.1 μM, respectively.
In the
1H NMR-guided fractionation of extracts from the edible mushroom
Lactarius deliciosus, two new azulene-type sesquiterpenoids
19 and
20 were characterized
[16]. Pestabacillin A
21 bearing a zierane-type sesquiterpene skeleton was isolated from the co-culture of the endophytic fungus
Pestalotiopsis sp. with
Bacillus subtilis [17]. Furthermore, the absolute configuration of
21 was confirmed by single-crystal X-ray diffraction analysis.
2.2. Bergamotane, Spiroaminal, and Spiroaxane
Bergamotane sesquiterpenes bearing a bridged 6/4 bicyclic ring incorporated with an isopentyl unit, are naturally occurring in plants and fungi
[18][19]. A new class of polyoxygenated bergamotanes with notable features inspired by a 6/4/5/5 tetracyclic ring system was very rare in nature and all examples of the polycyclic bergamotanes only derived from fungi
[20][21][22][23].
Purpurolide A
22, an unprecedented sesquiterpene lactone with a rarely encountered 5/5/5 spirocyclic skeleton, along with five new 6/4/5/5 tetracyclic sesquiterpene lactones (purpurolides B–F
23–
27), was isolated from the cultures of the endophytic fungus
Penicillium purpurogenum [24][25]. The structures and absolute configurations of
22–
27 were established by spectroscopic analysis, a single-crystal X-ray diffraction, and calculations of the
13C NMR and ECD data. The plausible biosynthetic pathway of
22–
27 is shown in
Scheme 1. Compounds
22–
27 showed significant inhibitory activity against pancreatic lipase with IC
50 values of 1.22–7.88 μM.
Scheme 1. Plausible biogenetic pathways for
22–
27 (Reference
[24]).
Expansolides C
28 and D
29 were two new bergamotane sesquiterpene lactones isolated from the plant pathogenic fungus
Penicillium expansum [26]. The epimeric mixture of expansolides C
28 and D
29 (in a ratio of 2:1 at the temperature of the bioassay) exhibited more potent
α-glucosidase inhibitory activity (IC
50 0.50 mM) as compared with the positive control acarbose (IC
50 1.90 mM) in an in vitro bioassay.
Donacinolides A
30 and B
31 and donacinoic acids A
32 and B
33, four new rare tetracyclic bergamotane-type sesquiterpenoids, were isolated from the mushroom-associated fungus
Montagnula donacina [27]. Two new β-bergamotane sesquiterpenoids
34 and
35 were isolated from the marine-derived fungus
Aspergillus fumigatus [28]. Brasilamides K–N
36–
39 were isolated from the plant endophytic fungus
Paraconiothynium Brasiliense [29].
Sporulaminals A
40 and B
41, a pair of unusual epimeric spiroaminal derivatives bearing a 6/4/5/5 tetracyclic ring system derived from bergamotane sesquiterpenoid (
Scheme 2), were isolated from a marine-derived fungus
Paraconiothyrium sporulosum [30]. Pleurospiroketal F
42, a new perhydrobenzannulated 5,5-spiroketal sesquiterpene was isolated from solid-state fermentation of
Pleurotus citrinopileatus, and the absolute configuration of
42 was determined by single-crystal X-ray diffraction analysis
[31].
Scheme 2. Plausible biosynthetic pathways of sporulaminals A
40 and B
41 (Reference
[30]).
Flammuspirones A–J
43–
52, ten spiroaxane sesquiterpenoids, were obtained from the edible mushroom
Flammulina velutipes [32]. Flammuspirones A
43 and C
45 showed inhibition on HMG-CoA reductase with IC
50 of 114.7 and 77.6 μM, respectively. Flammuspirones C–E
45–
47 and H
50 showed inhibitory activity on DPP-4 with IC
50 values in the range from 70.9 to 83.7 μM.
Talaminoid A
53 was obtained from the fungus
Talaromyces minioluteus [33]. Talaminoid A
53 showed a significant suppressive effect on the production of nitric oxide (NO) on lipopolysaccharide (LPS) induced BV-2 cell, with IC
50 of 5.79 μM. In addition, talaminoid A
53 exhibited significant anti-inflammatory activities against the production of TNF-
α and IL-6. Further immunofluorescence experiments revealed the mechanism of action to be inhibitory the NF-κB-activated pathway. A new sesquiterpenoid
54 was isolated from the fungus
Pholiota nameko [34]. Tramspiroins A–D
55–
58 have been isolated from the cultures of Basidiomycete
Trametes versicolor [35].
2.3. Carotane, Cyclonerane, Cyclofarnesane, and Longifolene
A new dimeric sesquiterpene divirensol H
59 and two exceptionally novel trimeric sesquiterpenes trivirensols A
60 and B
61 were purified from an endophytic fungus
Trichoderma virens [36]. Divirensol H
59 showed significant activities against fungi
Penicillium italicum,
Fusarium oxysporum,
Fusarium graminearum,
Colletotrichum musae, and
Colletotrictum gloeosporioides with MIC values of 6.25 to 25 μg/mL. Rhinomilisin A
62 and four new heptelidic acid derivatives, rhinomilisin B–E
63–
66, were isolated from the endophytic fungus
Rhinocladiella similis [37]. Rhinomilisins A
62 showed moderate cytotoxicity activity against the mouse lymphoma cell line L5178Y with an IC
50 value of 5.0 μM.
Peniterester
67, a new tricyclic sesquiterpene was isolated from the secondary metabolites of an artificial mutant
Penicillium sp. T2-M20
[38]. Peniterester
67 showed significant activities against
Bacillus subtilis,
Escherichia coli, and
Staphylococcus aureus in vitro with MICs of 8.0, 8.0, and 4.0 μg/mL, respectively.
Piltunines A–F
68–
73 and penigrisacids A–D
74–
77, ten new carotane sesquiterpenoids, were isolated from the marine-derived fungus
Penicillium griseofulvum and
Penicillium piltunense, respectively
[39][40]. Penigrisacid D
75 showed a weak effect on ECA-109 tumor cells with an IC
50 value of 28.7 µM
[39]. Trichocarotins A–H
78–
85, eight new carotane sesquiterpenes, were isolated from the culture of the fungus
Trichoderma virens [41]. Trichocarotins C–E
80–
82 and H
85 displayed potent inhibition against the four marine phytoplankton species (
Chattonella marina,
Heterosigma akashiwo,
Karlodinium veneficum, and
Prorocentrum donghaiense) tested, especially against
C. marina with IC
50 values ranging from 0.24 to 1.2 μg/mL.
Trichocaranes E
86 and F
87 were isolated from cultures of the insect pathogenic fungus
Isaria fumosorosea [42]. Trichocaranes E
86 and F
87 showed potent cytotoxic activities against six tumor cell lines MDA, MCF-7, SKOV-3, Hela, A549, and HepG2 with IC
50 values in a concentration range of 0.13–4.57 μg/mL. Two new carotane-type biogenetically related sesquiterpenes, aspterrics A
88 and B
89, were isolated from the deep-sea-derived fungus
Aspergillus terreus [43].
Two new cycloneranes
90 and
91 were isolated from the marine alga endophytic fungus
Trichoderma citrinoviride [44]. The compound
90 had an inhibition to the marine phytoplankton species
Karlodinium veneficum with an IC
50 value of 8.1 μg/mL. Six new cycloneranes
92–
97 were isolated from the fungus
Trichoderma harzianum [45][46][47]. The three new ones
95–
97 all exhibited growth inhibition of the four phytoplankton species (
Chattonella marina,
Heterosigma akashiwo,
Karlodinium veneficum, and
Prorocentrum donghaiense) with IC
50 values ranging from 0.66 to 75 μg/mL
[47].
Cyclonerotriol B
98 was isolated from the soil fungus
Fusarium avenaceum [48]. Cyclonerodiol B
99 was isolated from the mangrove plant endophytic fungus
Trichoderma sp. Xy24
[49]. Cyclonerodiol B
99 exhibited significant neural anti-inflammatory activity by inhibiting LPS-induced NO production in BV2 cells with the inhibitory rates of 75.0% at 0.1 μM, which are more potent than curcumin, positive control with the inhibitory rate of 21.1% at 0.1 μM.
Ten new cycloneranes
100–
109 were isolated from the algicolous endophytic fungus
Trichoderma asperellum [50][51]. The seven new ones,
100–
104,
108, and
109, all exhibited growth inhibition of the four phytoplankton species (
Chattonella marina,
Heterosigma akashiwo,
Karlodinium veneficum, and
Prorocentrum donghaiense) with IC
50 values ranging from 2.4 to 76 μg/mL
[50].
A new sesquiterpenoid
110 was isolated and identified from an endophytic fungus
Umbelopsis dimorpha grown on host-plant
Kadsura angustifolia and wheat bran
[52]. Inonofarnesane
111, a new cyclofarnesane sesquiterpenoid, was isolated from cultures of the wood-rotting basidiomycete
Inonotus sp. BCC 23706
[53].
One new norbisabolane sesquiterpenoid degradation, isopolisin B
112, was isolated from the fungus
Pestalotiopsis heterocornis [54]. Koninginol D
113 as a new farnesane sesquiterpenoid was isolated from the endophytic fungus
Trichoderma koningiopsis [55].
Bipolenin F
114, a new
seco-longifolene sesquiterpenoid, and two new
seco-sativene sesquiterpenoids, bipolenins D
115 and E
116, and two novel sesquiterpenoid-xanthone adducts, bipolenins I
117 and J
118, were obtained from cultures of potato endophytic fungus
Bipolaris eleusines [56][57]. Bipolenins I
117 and J
118 exhibited potent inhibitory activity against the plant pathogens
Alternaria solani with MIC values of 8 and 16 μg/mL, respectively
[57].
2.4. Cerapicane, Cucumane, Cuparene, Hirsutane, Isohirsutane, and Triquinane
Cuparane-type sesquiterpenoids of fungal origin possess a skeleton with a six-membered ring connected to a five-membered ring, of which the six-membered ring is always aromatic. Linear triquinane sesquiterpenoids have a basic skeleton 1
H-cyclopenta[α]pentalene
[58]. Many compounds displayed a wide range of biological activities, such as cytotoxic, antimicrobial, and anti-inflammatory activities. A review gives an overview about the isolation, structure, biological activities, and chemical synthesis of linear triquinane sesquiterpenoids
[59].
Enokipodins A–D
119–
122, highly oxygenated cuparene-type sesquiterpenes were obtained from the fungi
Flammulina rossica and
Flammulina velutipes. In addition, enokipodins B
121 and D
122 are oxidized compounds of enokipodins A
119 and C
120, respectively
[60].
One new cerapicane cerrenin A
123, and two new isohirsutane sesquiterpenoids cerrenins B
124 and C
125, were isolated from the broth extract of
Cerrena sp. which was isolated from
Pogostemon cablin [61]. Trefoliol C
126, one new cucumane sesquiterpenoid, was isolated from cultures of the basidiomycetes
Tremella foliacea [62]. A new sesquiterpenoid
127 was isolated from the crude extract of
Antrodiella albocinnamomea [63]. Two new hirsutane-type sesquiterpenoids, chondrosterins N
128 and O
129, were isolated from the marine fungus
Chondrostereum sp.
[64].
Ten new hirsutane-type sesquiterpenoids, sterhirsutins C–L
130–
139, were isolated from the culture of
Stereum hirsutum [65]. Sterhirsutins C
130 and D
131 possessed an unprecedented chemical skeleton with a 5/5/5/6/9/4 fused ring system, and the absolute configuration of sterhirsutin C
130 was assigned by single-crystal X-ray diffraction experiment. Sterhirsutin L
139 was the first sesquiterpene coupled with a xanthine moiety. Sterhirsutins C–L
130–
139 showed cytotoxicity against K562 and HCT116 cell lines, and sterhirsutin K
138 induced autophagy in HeLa cells. Sterhirsutin G
133 inhibited the activation of the IFNβ promoter in Sendai virus-infected cells.
Cerrenins D
140 and E
141, two new triquinane-type sesquiterpenoids, were obtained from the endophytic fungus
Cerrena sp. A593
[66]. Chondrosterins K–M
142–
144 were isolated from the marine fungus
Chondrostereum sp.
[67]. Chondrosterins K–M
142–
144 showed different degrees of cytotoxicities against various cancer cell lines (CNE1, CNE2, HONE1, SUNE1, A549, GLC82, and HL7702) in vitro, with IC
50 values ranging from 12.03 to 58.83 µM.
Antrodins A–E
145–
149 were isolated from the fermentation of
Antrodiella albocinnamomea [68]. Tremutin H
150 was isolated from cultures of the basidiomycetes
Irpex lacteus [69]. The absolute configuration of
150 was determined by single-crystal X-ray diffraction analysis, and
150 shows a weak inhibitory effect on NO production with an IC
50 value of 22.7 μM.
2.5. Eudesmanolide, Gymnomitrane, and Humulane
Humulane-type sesquiterpenoids are found rarely in nature. They have been recognized as being biogenetic precursors of many types of sesquiterpenoids
[6]. The macrocyclic nature of members of the humulane group has proved to be troublesome for the determination of their absolute configurations.
Four new 12,8-eudesmanolides
151–
154 were isolated from a mangrove rhizosphere-derived fungus
Eutypella sp. 1–15
[70]. Periconianone A
155, a polyoxygenated sesquiterpenoid with a new 6/6/6 tricarbocyclic skeleton, was isolated from the endophytic fungus
Periconia sp., and the biosynthesis of the unusual six-membered carbonic ring of
155 was postulated to be formed through intramolecular aldol condensation (
Scheme 3)
[71]. The first enantioselective total synthesis of the periconianone A
155 based on a postulated biogenesis has been reported (
Scheme 4)
[72].
Scheme 3. Hypothetical biosynthetic pathway of periconianone A
155 (Reference
[71]).
Scheme 4. Total synthesis of periconianone A
155 (Reference
[72]).
An unusual type sesquiterpene
156 possessed an unusual 14(7-6)-cuparane scaffold (
Scheme 5), and six rarely-encountered gymnomitrane-type sesquiterpenoids
157–
162, were isolated from the medicinal mushroom
Ganoderma lingzhi [73]. A new gymnomitrane-type sesquiterpenoid
163 was isolated from the fruiting body of
Ganoderma lucidum [74]. This compound
163 significantly inhibited the growth of epidermal growth factor receptor-tyrosine kinase inhibitor EGFR-TKI-resistant human lung cancer A549 and human prostate cancer PC3 cell lines. Antrodin F
164 was isolated from the fermentation of
Antrodiella albocinnamomea [68].
Scheme 5. Proposed biosynthetic pathway of
156 and
162 (Reference
[73]).
Nine new humulane-derived sesquiterpenoids, ochracenes A–I
165–
173, were isolated from the Antarctic fungus
Aspergillus ochraceopetaliformis [75]. A biogenetic pathway for them was given in
Scheme 6. The two unprecedented 8,9-secocyclic sesquiterpenoids, ochracenes B
166 and C
167, exhibited inhibitory effects on LPS-induced NO release in RAW 264.7 mouse macrophage cell with IC
50 values of 14.6 and 18.3 μM, respectively.
Scheme 6. Postulated biogenetic pathway for ochracenes A–I
165–173 (Reference
[75]).