Sesquiterpenoids Specially Produced by Fungi: Structures, Biological Activities, Chemical and Biosynthesis (2015–2020)

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. This review examines the isolation, structural determination, bioactivities, and synthesis of sesquiterpenoids that were specially produced by fungi over the past five years (2015–2020).


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].
Our research group has been engaged in the study of the chemical composition of fungi for decades [14,15], while a large number of sesquiterpenoids have been reported [6]. It has been found that the vast majority of skeletons, such as alliacane, bergamotane, hirsutane, tremulane, etc., are specially produced by fungi. Many compounds displayed significant biological activities, and it is obvious that cytotoxic activity accounts for the  Our research group has been engaged in the study of the chemical composition of fungi for decades [14,15], while a large number of sesquiterpenoids have been reported [6]. It has been found that the vast majority of skeletons, such as alliacane, bergamotane, hirsutane, tremulane, etc., are specially produced by fungi. Many compounds displayed significant biological activities, and it is obvious that cytotoxic activity accounts for the largest proportion ( Figure 2). In addition, with the development of synthetic biology, the biosynthesis of many fungal sesquiterpenoids has been figured out. This review gives an overview about the structures, biological activities, chemical synthesis and biosynthesis of sesquiterpenoids specially produced by fungi presented from 2015 to 2020.    Our research group has been engaged in the study of the chemical composition of fungi for decades [14,15], while a large number of sesquiterpenoids have been reported [6]. It has been found that the vast majority of skeletons, such as alliacane, bergamotane, hirsutane, tremulane, etc., are specially produced by fungi. Many compounds displayed significant biological activities, and it is obvious that cytotoxic activity accounts for the largest proportion ( Figure 2). In addition, with the development of synthetic biology, the biosynthesis of many fungal sesquiterpenoids has been figured out. This review gives an overview about the structures, biological activities, chemical synthesis and biosynthesis of sesquiterpenoids specially produced by fungi presented from 2015 to 2020. Scheme 1. Plausible biogenetic pathways for 22−27 (Reference [26]).
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 [26,27]. The structures and absolute configurations of 22−27 were established by spectroscopic analysis, a single-crystal X-ray diffraction, and calculations of the 13  Expansolides C 28 and D 29 were two new bergamotane sesquiterpene lactones isolated from the plant pathogenic fungus Penicillium expansum [28]. 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 (IC50 0.50 mM) as compared with the positive control acarbose (IC50 1.90 mM) in an in vitro bioassay.
Expansolides C 28 and D 29 were two new bergamotane sesquiterpene lactones isolated from the plant pathogenic fungus Penicillium expansum [28]. 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.
Talaminoid A 53 was obtained from the fungus Talaromyces minioluteus [35]. 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 [36]. Tramspiroins A-D 55-58 have been isolated from the cultures of Basidiomycete Trametes versicolor [37].
Talaminoid A 53 was obtained from the fungus Talaromyces minioluteus [35]. Talaminoid A 53 showed a significant suppressive effect on the production of nitric oxide (NO) on lipopolysaccharide (LPS) induced BV-2 cell, with IC50 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 [36]. Tramspiroins A-D 55-58 have been isolated from the cultures of Basidiomycete Trametes versicolor [37].
Cyclonerotriol B 98 was isolated from the soil fungus Fusarium avenaceum [50]. Cyclonerodiol B 99 was isolated from the mangrove plant endophytic fungus Trichoderma sp. Xy24 [51]. 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.
Cyclonerotriol B 98 was isolated from the soil fungus Fusarium avenaceum [50]. Cyclonerodiol B 99 was isolated from the mangrove plant endophytic fungus Trichoderma sp. Xy24 [51]. 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.

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 1H-cyclopenta[α]pentalene [60]. Many compounds displayed a wide range of biological activities, such as cytotoxic, antimicrobial, and anti-inflammatory activities. A review gives an overview about 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 1H-cyclopenta[α]pentalene [60]. 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 [61]. the isolation, structure, biological activities, and chemical synthesis of linear triquinane sesquiterpenoids [61].
Ten new hirsutane-type sesquiterpenoids, sterhirsutins C-L 130-139, were isolated from the culture of Stereum hirsutum [67]. 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.
Ten new hirsutane-type sesquiterpenoids, sterhirsutins C-L 130-139, were isolated from the culture of Stereum hirsutum [67]. 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.
Antrodins A-E 145-149 were isolated from the fermentation of Antrodiella albocinnamomea [70]. Tremutin H 150 was isolated from cultures of the basidiomycetes Irpex lacteus [71]. 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.

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.
J. Fungi 2021, 7, x FOR PEER REVIEW Antrodins A-E 145-149 were isolated from the fermentation of Antrodiella a namomea [70]. Tremutin H 150 was isolated from cultures of the basidiomycetes Irp teus [71]. The absolute configuration of 150 was determined by single-crystal X-r fraction analysis, and 150 shows a weak inhibitory effect on NO production with value of 22.7 µM.

Eudesmanolide, Gymnomitrane, and Humulane
Humulane-type sesquiterpenoids are found rarely in nature. They have been nized as being biogenetic precursors of many types of sesquiterpenoids [6]. The ma clic nature of members of the humulane group has proved to be troublesome for t termination of their absolute configurations.
Four new 12,8-eudesmanolides 151-154 were isolated from a mangrove rhizos derived fungus Eutypella sp. 1-15 [72]. Periconianone A 155, a polyoxygenated s erpenoid with a new 6/6/6 tricarbocyclic skeleton, was isolated from the endophyt gus Periconia sp., and the biosynthesis of the unusual six-membered carbonic ring was postulated to be formed through intramolecular aldol condensation (Scheme The first enantioselective total synthesis of the periconianone A 155 based on a post biogenesis has been reported (Scheme 4) [74].  An unusual type sesquiterpene 156 possessed an unusual 14(7-6)-cuparane sc (Scheme 5), and six rarely-encountered gymnomitrane-type sesquiterpenoids 15 were isolated from the medicinal mushroom Ganoderma lingzhi [75]. A new gym trane-type sesquiterpenoid 163 was isolated from the fruiting body of Ganoderma lu [76]. This compound 163 significantly inhibited the growth of epidermal growth receptor-tyrosine kinase inhibitor EGFR-TKI-resistant human lung cancer A549 a man prostate cancer PC3 cell lines. Antrodin F 164 was isolated from the fermenta Antrodiella albocinnamomea [70]. Antrodins A-E 145-149 were isolated from the fermentation of Antrodiella albocinnamomea [70]. Tremutin H 150 was isolated from cultures of the basidiomycetes Irpex lacteus [71]. 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 IC50 value of 22.7 µM.

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.
Nine new humulane-derived sesquiterpenoids, ochracenes A-I 165-173, were isolated from the Antarctic fungus Aspergillus ochraceopetaliformis [77]. 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. Nine new humulane-derived sesquiterpenoids, ochracenes A−I 165-173, were isolated from the Antarctic fungus Aspergillus ochraceopetaliformis [77]. 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 IC50 values of 14.6 and 18.3 µM, respectively.

Illudane, Illudalane, Protoilludane, Marasmane, and Norilludane
A review offers a comprehensive description of the investigations that started with the discovery of illudins in 1950, led to HMAF clinical trials against various tumors as a Nine new humulane-derived sesquiterpenoids, ochracenes A−I 165-173, were isolated from the Antarctic fungus Aspergillus ochraceopetaliformis [77]. 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 IC50 values of 14.6 and 18.3 µM, respectively.

Illudane, Illudalane, Protoilludane, Marasmane, and Norilludane
A review offers a comprehensive description of the investigations that started with the discovery of illudins in 1950, led to HMAF clinical trials against various tumors as a  Nine new humulane-derived sesquiterpenoids, ochracenes A−I 165-173, were isolated from the Antarctic fungus Aspergillus ochraceopetaliformis [77]. A biogenetic pathway for them was given in Scheme 6. The two unprecedented 8,9-

Illudane, Illudalane, Protoilludane, Marasmane, and Norilludane
A review offers a comprehensive description of the investigations that started with the discovery of illudins in 1950, led to HMAF clinical trials against various tumors as a 2.6. Illudane, Illudalane, Protoilludane, Marasmane, and Norilludane A review offers a comprehensive description of the investigations that started with the discovery of illudins in 1950, led to HMAF clinical trials against various tumors as a single agent and in combination therapy beginning in 2002, and culminated in the past decade of advances in chemical synthesis and mechanisms of toxicity of AFs, including biotransformation processes, DNA alkylation products, unique influences of DNA repair capacities, and enzyme inhibition properties [9]. The 4/6/5 ring-fused protoilludane-type sesquiterpenoids are the precursors of many other sesquiterpenoids, representing the largest group of sesquiterpene metabolites of fungal origin.
Phellinignin D 174 was isolated from the fungus Phellinus igniarius, which possessed a new carbon skeleton that might derive from an illudane framework by methyl shift and aromatization [78]. Phellinignin D 174 showed moderate cytotoxicities to three human cancer cell lines (HL-60, SMMC-7721, and SW480) with the IC 50 values of 21.1, 12.3, and 13.9 µM, respectively.
Illudadienes A 175 and B 176 were obtained from the wood-decomposing fungus Granulobasidium vellereum [79]. Phellinuin J 177 and sulphureuine A 178 were isolated from cultures of Phellinus tuberculosus and Laetiporus sulphureus [80]. Agrocybins H-K 179-184 were obtained from the edible mushroom Agrocybe salicacola [81]. Craterellins D 185 and E 186 were isolated from cultures of Craterellus cornucopioides [82]. Illudalane derivative, granulolactone 187, and a 15-norilludane, granulodione 188, were isolated from an agar plate culture of Granulobasidium vellereum [83]. single agent and in combination therapy beginning in 2002, and culminated in the past decade of advances in chemical synthesis and mechanisms of toxicity of AFs, including biotransformation processes, DNA alkylation products, unique influences of DNA repair capacities, and enzyme inhibition properties [9]. The 4/6/5 ring-fused protoilludane-type sesquiterpenoids are the precursors of many other sesquiterpenoids, representing the largest group of sesquiterpene metabolites of fungal origin. Phellinignin D 174 was isolated from the fungus Phellinus igniarius, which possessed a new carbon skeleton that might derive from an illudane framework by methyl shift and aromatization [78]. Phellinignin D 174 showed moderate cytotoxicities to three human cancer cell lines (HL-60, SMMC-7721, and SW480) with the IC50 values of 21.1, 12.3, and 13.9 µM, respectively.
Epicoterpenes A-E 205-209, and armilliphatic A 210 were isolated from Armillaria sp. by co-culture with the endophytic fungus Epicoccum sp. associated with Gastrodia elata [86]. Epicoterpene D 208 was the first example of an ent-protoilludane sesquiterpenoid scaffold bearing a five-membered lactone. Two new protoilludane sesquiterpene aryl esters 211 and 212 were isolated from the mycelium of Armillaria mellea [87]. Compound 212 showed cytotoxic activity for HepG2 cells with an IC 50 value of 18.03 µg/mL. Three new sesquiterpene aryl esters, melleolide N 213, Q 214, and R 215, were isolated from the EtOH extract of the mycelium of Armillaria mellea [88]. And 213-215 showed cytotoxicity to several human cancer cell lines. scaffold bearing a five-membered lactone. Two new protoilludane sesquiterpene aryl esters 211 and 212 were isolated from the mycelium of Armillaria mellea [87]. Compound 212 showed cytotoxic activity for HepG2 cells with an IC50 value of 18.03 µg/mL. Three new sesquiterpene aryl esters, melleolide N 213, Q 214, and R 215, were isolated from the EtOH extract of the mycelium of Armillaria mellea [88]. And 213-215 showed cytotoxicity to several human cancer cell lines.
Unified total syntheses of marasmane, mellolide, and protoilludane sesquiterpenoids have been achieved through a key organocatalytic enantioselective annulation (Scheme 7) [89]. The elaboration of key bicyclic lactone 216 was the molecular springboard from which the first enantioselective total syntheses of protoilludanes echinocidin B 220 and echinocidin D 221, and the mellolide armillaridin 219, as well as the synthesis of the marasmane isovelleral 222, were accomplished. The vanadium(II)/zinc(II) reductive coupling yielded the final ring of the densely functionalized cis-fused carbocyclic core. Finally, the unexpected semi-Pinacol-type ring contraction to establish cyclopropyl aldehyde 218 from cyclobutanediol 217 was potentially biomimetic in origin.
Unified total syntheses of marasmane, mellolide, and protoilludane sesquiterpenoids have been achieved through a key organocatalytic enantioselective annulation (Scheme 7) [89]. The elaboration of key bicyclic lactone 216 was the molecular springboard from which the first enantioselective total syntheses of protoilludanes echinocidin B 220 and echinocidin D 221, and the mellolide armillaridin 219, as well as the synthesis of the marasmane isovelleral 222, were accomplished. The vanadium(II)/zinc(II) reductive coupling yielded the final ring of the densely functionalized cis-fused carbocyclic core. Finally, the unexpected semi-Pinacol-type ring contraction to establish cyclopropyl aldehyde 218 from cyclobutanediol 217 was potentially biomimetic in origin.

Tremulane, Sterpurane, Phlebiane, Merulane, and Irlactane
Tremulane-type sesquiterpenoids are a class of sesquiterpenoids with a 5/7-ringfused perhydroazulene carbon skeleton. The first example was isolated from the wood-decaying fungus Phellinus tremulae in 1993 [97]. The biosynthesis pathway was elucidated through a 13 C-labeled feeding experiment revealed that tremulanes are derived from trans,transfarnesyl pyrophosphate via humulene and a key step of methyl migration [98].
A new irlactane-type, irlactin K 255, was isolated from the fermentation broth of the medicinal fungus Irpex lacteus [99]. The absolute configuration of 255 was established by single-crystal X-ray diffraction analysis. Irlactin K 255 could be derived from the tremulane type sesquiterpene irlactin E via a ring rearrangement [100]. Conosiligins A-D 256-259, four ring-rearranged sesquiterpenoids, were isolated from cultures of the basidiomycete Conocybe siliginea [101]. Conosiligins A 256 and B 257 possessed a 5/8-fused ring system, while conosiligin C 258 has a 5/6-fused backbone conjugated with a γ-lactone. Conosiligin D 259 was a 5,6-seco tremulane derivative with the loss of a skeletal carbon, featuring a tetracyclic system involving a pyranone moiety (Scheme 9). Conosiligins C 258 and D 259 inhibited Con A-induced T cell proliferation with IC 50 values of 12.3 and 6.6 µM, respectively. Antroalbocin A 260 possessing a bridged tricyclic system was isolated from cultures of the higher fungus Antrodiella albocinnamomea [102]. The structure with the absolute configuration was determined by extensive spectroscopic methods and single-crystal X-ray diffraction analysis and a plausible biosynthetic pathway for 260 was proposed (Scheme 10). Antroalbocin A 260 possessing a bridged tricyclic system was isolated from cultures of the higher fungus Antrodiella albocinnamomea [102]. The structure with the absolute configuration was determined by extensive spectroscopic methods and single-crystal X-ray diffraction analysis and a plausible biosynthetic pathway for 260 was proposed (Scheme 10).
Twenty-two tremulanes, irlactins F-J Antroalbocin A 260 possessing a bridged tricyclic system was isolated from cultures of the higher fungus Antrodiella albocinnamomea [102]. The structure with the absolute configuration was determined by extensive spectroscopic methods and single-crystal X-ray diffraction analysis and a plausible biosynthetic pathway for 260 was proposed (Scheme 10).  Nigrosirpexin A 293 was produced by Nigrospora oryzae co-cultured with Irpex lacteus [106]. Two new tremulanes 294 and 295 were obtained from different cocultures of Nigrospora oryzae and Irpex lacteus in a solid medium [107]. 5-Demethyl conocenol C 294 showed antifungal activities against Didymella glomerate and Colletotrichum gloeosporioides with MICs of 1 and 8 µg/mL, respectively.

Trichothecene, Merosesquiterpenoid, Norsesquiterpenoid, and Pyrone
Trichothecenes are a family of sesquiterpenoid mycotoxins produced by multiple genera of fungi, including plant and insect pathogens, and they are toxic to animals and humans and frequently detected in cereal crops [113]. Because of their diversity in structure and biological activity, trichothecenes are of concern in agriculture, food contamination, health care, and building protection.

Trichothecene, Merosesquiterpenoid, Norsesquiterpenoid, and Pyrone
Trichothecenes are a family of sesquiterpenoid mycotoxins produced by multiple genera of fungi, including plant and insect pathogens, and they are toxic to animals and humans and frequently detected in cereal crops [113]. Because of their diversity in structure and biological activity, trichothecenes are of concern in agriculture, food contamination, health care, and building protection.
Chartarenes A-D 324-327 were isolated from the sponge-derived fungus Stachybotrys chartarum [122].   [123,124]. The structure-activity relationship investigation of 328−330 with other known natural trichothecenes against a human colon cancer cell line (COLO201) and filamentous fungus Cochliobolus miyabeanus revealed that the 12-epoxide functionality is essential for the antifungal activity [124].
The semisynthesis of several trichodermin and trichodermol derivatives has been developed (Scheme 15) [127]. Some derivatives with a short chain at the C-4 position displayed selective antimicrobial activity against Candida albicans and they showed MIC values similar to those displayed by trichodermin. It was important to highlight the cytotoxic selectivity observed for compounds 350, 354, and 356, which presented average IC50 values of 2 µg/mL and were cytotoxic against tumorigenic cell line MCF-7 (breast carcinoma) and not against Fa2N4 (non-tumoral immortalized human hepatocytes).
Five new highly oxygenated α-pyrone merosesquiterpenoids, ochraceopones A−E 362−366, were isolated from an Antarctic soil-derived fungus Aspergillus ochraceopetaliformis [130]. The semisynthesis of several trichodermin and trichodermol derivatives has been developed (Scheme 15) [127]. Some derivatives with a short chain at the C-4 position displayed selective antimicrobial activity against Candida albicans and they showed MIC values similar to those displayed by trichodermin. It was important to highlight the cytotoxic selectivity observed for compounds 350, 354, and 356, which presented average IC50 values of 2 µg/mL and were cytotoxic against tumorigenic cell line MCF-7 (breast carcinoma) and not against Fa2N4 (non-tumoral immortalized human hepatocytes).
Five new highly oxygenated α-pyrone merosesquiterpenoids, ochraceopones A-E 362-366, were isolated from an Antarctic soil-derived fungus Aspergillus ochraceopetaliformis [130]. Ochraceopones A-D 363-366 were the first examples of α-pyrone merosesquiterpenoids possessing a linear tetracyclic carbon skeleton. Ochraceopone A 363 exhibited antiviral activities against the H3N2 influenza virus with IC 50 values of 12.2 µM. Yaminterritrem C 367 was isolated from a deep-sea-derived fungus Penicillium chryso-genum [131]. Verruculides A 368 and B 369 were isolated from a culture broth of the Indonesian ascidian-derived Penicillium verruculosum [132]. Verruculide A 368 inhibited the activity of PTP1B with an IC 50  ited antiviral activities against the H3N2 influenza virus with IC50 values of 12.2 µM. Yaminterritrem C 367 was isolated from a deep-sea-derived fungus Penicillium chrysogenum [131]. Verruculides A 368 and B 369 were isolated from a culture broth of the Indonesian ascidian-derived Penicillium verruculosum [132]. Verruculide A 368 inhibited the activity of PTP1B with an IC50 value of 8.4 µM.
Avertoxins B 392 and C 393 showed activity against human tumor HCT116 and HeLa cell lines with an IC50 value of 10 µM. And avertoxin B 392 was an active inhibitor against human acetylcholinesterase with the IC50 value of 14.9 µM.

Other Types
Three new sesquiterpenoids, chermesiterpenoids A-C 395−397, were isolated and identified from the marine red algal-derived fungus Penicillium chermesinum [139].  Fomitopins A-L 398−409 were isolated via bioassay-guided purification from the bracket fungus Fomitopsis pinicola [140]. Fomitopin K 408 exhibited the most potent antiinflammatory activity with IC50 of 0.81 µM for inhibition of superoxide anion generation and IC50 of 0.74 µM for inhibition of elastase release. Fomitopins J 407 and L 409 also exhibited moderate inhibition of superoxide anion generation with IC50 of 1.66 and 1.72 µM, respectively.
Emericellins A 417 and B 418, representing a new type of sesquiterpenoid with an unprecedented tricyclo [1,2,4,4]hendecane scaffold (Scheme 18), were isolated from the liquid cultures of an endophytic fungus Emericella sp. associated with the leaves of Panax notoginseng [144]. Emericellins A 417 [140]. Fomitopin K 408 exhibited the most potent antiinflammatory activity with IC50 of 0.81 µM for inhibition of superoxide anion generation and IC50 of 0.74 µM for inhibition of elastase release. Fomitopins J 407 and L 409 also exhibited moderate inhibition of superoxide anion generation with IC50 of 1.66 and 1.72 µM, respectively.
Stereumenes A-C 419-421 were isolated and identified from the fungus Stereum sp. [145]. Stereumene B 420 showed weak nematicidal activity against Caenorhabditis elegans, which killed 41.1% of C. elegans at 200 mg/L in 24 h. Sesteralterin 422 was obtained from the culture extract of an Alternaria alternata strain isolated from the surface of the marine red alga Lomentaria hakodatensis [146]. Colletotrichine A 423 was obtained from the endophyte fungus Colletotrichum gloeosporioides [147].
Four novel mixed terpenes, stereumamides A-D 424-427, which were sesquiterpenes combined with α-amino acids to form quaternary ammonium hybrids, were isolated from the mycelium of mushroom Stereum hirsutum [148]. Stereumamides A 424 and D 427 showed antibacterial activity against Escherichia coli, Staphylococcus aureus, and Salmonella typhimurium, with MIC values of 12.5-25.0 µg/mL. Stereumenes A-C 419−421 were isolated and identified from the fungus Stereum sp. [145]. Stereumene B 420 showed weak nematicidal activity against Caenorhabditis elegans, which killed 41.1% of C. elegans at 200 mg/L in 24 h. Sesteralterin 422 was obtained from the culture extract of an Alternaria alternata strain isolated from the surface of the marine red alga Lomentaria hakodatensis [146]. Colletotrichine A 423 was obtained from the endophyte fungus Colletotrichum gloeosporioides [147].
Hypocoprins A-C 429-431 have a distinctive ring system consisting of fused cyclopropane and cyclodecene units were isolated from the Coprophilous fungus Hypocopra rostrate [150]. Pestaloporonins A-C 432-434, three new sesquiterpenoids related to the caryophyllene-derived punctaporonins, were isolated from cultures of a fungicolous isolate of Pestalotiopsis sp. MYC-709 [151]. Among them, pestaloporonins A 432 and B 433 contained new bicyclic and tricyclic ring systems, respectively, and the absolute configuration of 432 was confirmed by single-crystal X-ray crystallographic analysis.
Phomanoxide 435 was isolated from the solid substrate fermentation cultures of the fungus Phoma sp. [152]. Colletotrichine B 436 was produced by the fungal Colletotrichum gloeosporioides [153]. Three new chlorinated sesquiterpenes, lepistatins A-C 437-439, were isolated from the culture broth of Basidiomycete Lepista sordida [154]. The structures of lepistatins A-C 437-439 feature the indanone core structure but differ from other indanonecontaining sesquiterpenes of fungal origin by the alkyl substitution pattern. This indicates that lepistatins A-C 437-439 probably possessed a new sesquiterpene scaffold derived from the common precursor, trans-humulyl cation, by an alternative cyclization (Scheme 20). Hypocoprins A−C 429−431 have a distinctive ring system consisting of fused cyclopropane and cyclodecene units were isolated from the Coprophilous fungus Hypocopra rostrate [150]. Pestaloporonins A−C 432−434, three new sesquiterpenoids related to the caryophyllene-derived punctaporonins, were isolated from cultures of a fungicolous isolate of Pestalotiopsis sp. MYC-709 [151]. Among them, pestaloporonins A 432 and B 433 contained new bicyclic and tricyclic ring systems, respectively, and the absolute configuration of 432 was confirmed by single-crystal X-ray crystallographic analysis.
Phomanoxide 435 was isolated from the solid substrate fermentation cultures of the fungus Phoma sp. [152]. Colletotrichine B 436 was produced by the fungal Colletotrichum gloeosporioides [153]. Three new chlorinated sesquiterpenes, lepistatins A-C 437-439, were isolated from the culture broth of Basidiomycete Lepista sordida [154]. The structures of lepistatins A-C 437-439 feature the indanone core structure but differ from other indanone-containing sesquiterpenes of fungal origin by the alkyl substitution pattern. This indicates that lepistatins A-C 437-439 probably possessed a new sesquiterpene scaffold derived from the common precursor, trans-humulyl cation, by an alternative cyclization (Scheme 20).

Scheme 20. Plausible biosynthetic pathway for lepistatin A 437 and pterosin A (Reference [154]).
A novel sesquiterpene methylcyclopentenedione, penicilliumin B 440, was obtained from a deep sea-derived fungus Penicillium sp. F00120 [155]. Penicilliumin B 440, presenting the first example with the sesquiterpene cyclopentenedione skeleton as natural products (Scheme 21), was structurally determined by analysis of the NMR and MS spectroscopic data, while the absolute configurations were assigned by single-crystal X-ray experiments. Penicilliumin B 440 with low toxicity showed significant potential to inhibit the kidney fibrogenic action in vitro by a mechanism dependent on disruption of oxidative stress. Seiricardine D 441 was a new bicyclic sesquiterpene obtained from the endophytic fungus Cytospora sp. [156]. Twenty new sesquiterpenes (442−461) were isolated from the endophytic fungus Pseudolagarobasidium acaciico [157]. Among them, compounds 459 and 460 displayed cytotoxicity against several cancer and normal cell lines. A novel sesquiterpene methylcyclopentenedione, penicilliumin B 440, was obtained from a deep sea-derived fungus Penicillium sp. F00120 [155]. Penicilliumin B 440, presenting the first example with the sesquiterpene cyclopentenedione skeleton as natural products (Scheme 21), was structurally determined by analysis of the NMR and MS spectroscopic data, while the absolute configurations were assigned by single-crystal X-ray experiments. Penicilliumin B 440 with low toxicity showed significant potential to inhibit the kidney fibrogenic action in vitro by a mechanism dependent on disruption of oxidative stress. Seiricardine D 441 was a new bicyclic sesquiterpene obtained from the endophytic fungus Cytospora sp. [156]. Twenty new sesquiterpenes (442-461) were isolated from the endophytic fungus Pseudolagarobasidium acaciico [157]. Among them, compounds 459 and 460 displayed cytotoxicity against several cancer and normal cell lines.

Asperterpenoid A
A putative three-gene cluster for asperterpenoid A was identified [158]. Stepwise reconstitution of this gene cluster in Aspergillus oryzae reveals that astC encodes a sesterterpene cyclase to synthesize preasperterpenoid A 462, which was dually oxidized by a P450 enzyme AstB to give asperterpenoid A 463 along with a minor product asperterpenoid B 464, and asperterpenoid A 463 was further oxidized by another P450 enzyme AstA to afford a new sesterterpenoid asperterpenoid C 465 (Scheme 22). Asperterpenoids A 463 and B 464 exhibit potent inhibitory activity against Mycobacterium tuberculosis protein tyrosine phosphatase B with IC 50 values of 3-6 µM.

Asperterpenoid A
A putative three-gene cluster for asperterpenoid A was identified [158]. Stepwise reconstitution of this gene cluster in Aspergillus oryzae reveals that astC encodes a sesterterpene cyclase to synthesize preasperterpenoid A 462, which was dually oxidized by a P450 enzyme AstB to give asperterpenoid A 463 along with a minor product asperterpenoid B 464, and asperterpenoid A 463 was further oxidized by another P450 enzyme AstA to afford a new sesterterpenoid asperterpenoid C 465 (Scheme 22). Asperterpenoids A 463 and B 464 exhibit potent inhibitory activity against Mycobacterium tuberculosis protein tyrosine phosphatase B with IC50 values of 3-6 µM.

Cuparene
Use of the ku70-deficient strain of Coprinopsis cinerea enabled confirmation within the native context of the central role the sesquiterpene synthase Cop6 plays in lagopodin biosynthesis [159]. Furthermore, yeast in vivo bioconversion and in vitro assays of two cytochrome P450 monooxygenases Cox1 and Cox2 allowed elucidation of the network of oxidation steps that build structural complexity onto the α-cuprenene framework during the biosynthesis of lagopodins (Scheme 23). Three new compounds 466-468 were identified as intermediates formed by the redox enzymes.

Cuparene
Use of the ku70-deficient strain of Coprinopsis cinerea enabled confirmation within the native context of the central role the sesquiterpene synthase Cop6 plays in lagopodin biosynthesis [159]. Furthermore, yeast in vivo bioconversion and in vitro assays of two cytochrome P450 monooxygenases Cox1 and Cox2 allowed elucidation of the network of oxidation steps that build structural complexity onto the α-cuprenene framework during the biosynthesis of lagopodins (Scheme 23). Three new compounds 466-468 were identified as intermediates formed by the redox enzymes.

Hirsutenoid
The identification and successful cloning of the previously elusive hirsutene synthase from the wood-rotting mushroom Stereum hirsutum provide the biosynthetic pathways of hirsutane-type sesquiterpenoids (Scheme 25) [161]. The hirsutene synthase, as an unexpected fusion protein of a sesquiterpene synthase (STS) with a C-terminal 3-hydroxy-3methylglutaryl-coenzyme A (3-hydroxy-3-methylglutaryl-CoA) synthase (HMGS) domain, was part of a biosynthetic gene cluster that includes P450s and oxidases that were expressed and could be cloned from cDNA.

Hirsutenoid
The identification and successful cloning of the previously elusive hirsutene synthase from the wood-rotting mushroom Stereum hirsutum provide the biosynthetic pathways of hirsutane-type sesquiterpenoids (Scheme 25) [161]. The hirsutene synthase, as an unexpected fusion protein of a sesquiterpene synthase (STS) with a C-terminal 3-hydroxy-3methylglutaryl-coenzyme A (3-hydroxy-3-methylglutaryl-CoA) synthase (HMGS) domain, was part of a biosynthetic gene cluster that includes P450s and oxidases that were expressed and could be cloned from cDNA.

Protoilludenes
Sixteen sesquiterpene synthases genes as full-length cDNAs have been isolated by using RT-PCR, and heterologous expression revealed that the sesquiterpene synthases could produce a series of sesquiterpene scaffolds with distinct metabolic profiles (Scheme 27) [163].

Protoilludenes
Sixteen sesquiterpene synthases genes as full-length cDNAs have been isolated by using RT-PCR, and heterologous expression revealed that the sesquiterpene synthases could produce a series of sesquiterpene scaffolds with distinct metabolic profiles (Scheme 27) [163]. Scheme 27. Reaction pathways of protoilludene metabolism by PpSTS-08 and PpCYPs (Reference [163]).
An artificial metabolic route to an unnatural trichothecene was designed by taking advantage of the broad substrate specificities of the T-2 toxin biosynthetic enzymes of Fusarium sporotrichioides [167]. By feeding 7-hydroxyisotrichodermin, a shunt pathway Scheme 29. Biosynthetic approach to generate 7-hydroxy T-2 toxin 483. Red arrows indicate the metabolic pathway of F. sporotrichioides, whereas blue arrows indicate that of F. graminearum. Solid arrows indicate the main pathway, and dotted arrows indicate the shunt pathway (Reference [167]). Scheme 29. Biosynthetic approach to generate 7-hydroxy T-2 toxin 483. Red arrows indicate the metabolic pathway of F. sporotrichioides, whereas blue arrows indicate that of F. graminearum. Solid arrows indicate the main pathway, and dotted arrows indicate the shunt pathway (Reference [167]).
The candidate gene, Clm2, a second structural gene required for culmorin biosynthesis in the plant pathogen Fusarium graminearum, encodes a regio-and stereoselective cytochrome P450 monooxygenase for C-11 of longiborneol 484 (Scheme 30) [168]. Clm2 gene disruptants were grown in liquid culture and assessed for culmorin production via HPLC-evaporative light scattering detection. The analysis indicated a complete loss of culmorin 485 from the liquid culture of the ∆Clm2 mutants. Culmorin production resumed in a ∆Clm2 complementation experiment. A detailed analysis of the secondary metabolites extracted from the largescale liquid culture of disruptant ∆Clm2D20 revealed five new natural products: 486-490. The structures of the new compounds were elucidated by a combination of HRMS, 1D and 2D NMR, and single-crystal X-ray crystallography analysis.

Conclusions and Future Prospects
Natural products, in particular bioactive molecules as precursor pharmaceutical compounds, have attracted particular attention in the field of health promotion and drug discovery and development. Compared with other sources, fungal species play a decisive role in bio-transformations and drug synthesis owing to their wide varieties, easy cultivation, diverse chemical compositions, and distinct biological activities. This process has been accelerated by considerable advances in microbial genome research and in understanding the structure of genes and their corresponding products. Genome mining-based natural products discovery programs mainly use the most identifiable terpene synthases and prenyltransferases to locate and quickly identify new terpenoids. In the last five years, nearly 500 new sesquiterpenes, including about 20 new skeletons were identified from fungi. These sesquiterpenoids exhibit various biological activities, such as anti-tumor, anti-viral, anti-microbial, anti-inflammatory, etc. These efforts have clearly led to a global promotion of discovery and characterization of fungal terpenoids and offer optimism for the future of fungal terpenoid discovery.
This review summarized the isolation, chemical structures, plausible biosynthetic pathways, bioactivity, chemical synthesis, and biosynthesis of 490 recent sesquiterpenoids. This could be a useful reference for modern researchers studying this category of compounds.

Conclusions and Future Prospects
Natural products, in particular bioactive molecules as precursor pharmaceutical compounds, have attracted particular attention in the field of health promotion and drug discovery and development. Compared with other sources, fungal species play a decisive role in bio-transformations and drug synthesis owing to their wide varieties, easy cultivation, diverse chemical compositions, and distinct biological activities. This process has been accelerated by considerable advances in microbial genome research and in understanding the structure of genes and their corresponding products. Genome mining-based natural products discovery programs mainly use the most identifiable terpene synthases and prenyltransferases to locate and quickly identify new terpenoids. In the last five years, nearly 500 new sesquiterpenes, including about 20 new skeletons were identified from fungi. These sesquiterpenoids exhibit various biological activities, such as anti-tumor, anti-viral, anti-microbial, anti-inflammatory, etc. These efforts have clearly led to a global promotion of discovery and characterization of fungal terpenoids and offer optimism for the future of fungal terpenoid discovery.
This review summarized the isolation, chemical structures, plausible biosynthetic pathways, bioactivity, chemical synthesis, and biosynthesis of 490 recent sesquiterpenoids. This could be a useful reference for modern researchers studying this category of compounds.