66
H. Zhu et al.
and amiaspochalasins F (437) and G (438) [189]. This is the first report of actual
seco-aspochalasins, with ring C cleaved between C-17 and C-18 or between C-9 and
C-21.
2.4.4 Mero-aspochalasins and Aspochalasin Homodimers
Besides the merocytochalasans mentioned in Sects. 2.1 and 2.3, most merocytochalasans are generated by involving an aspochalasin unit (Table 19 and Fig. 19).
The discovery of structurally complex merocytochalasans within this group may
be considered to be a milestone in research on the cytochalasans, because of their
fascinating ring systems and scaffolds with the presence of multiple chiral centers.
The first type of mero-aspochalasins is formed by the connection of a peptide
or an amino acid moiety to an aspochalasin unit. Aspochalamins A–D (439–442)
[5, 6], four mero-aspochalasins constituted by the connection of a tripeptide-like
moiety to an aspochalasin unit at C-19, were isolated in 2004 from an endosymbiotic
fungus Aspergillus niveus, collected from the gut of a woodlouse species by Fiedler
and coworkers. These were the first examples of merocytochalasans, although the
term “merocytochalasan” had not yet been put forward at the time. However, to date,
insufficient attention to this group has been paid, and so their relative configurations at
C-17, C-18, and C-19 have still not determined. Recently, two new cysteine residuecontaining mero-aspochalasins, cyschalasins A (443) and B (444) [191] were isolated
from the endophytic fungus Aspergillus micronesiensis, which represent a new type
of merocytochalasan featuring a modified cysteine residue with an unusual sulfur
atom. A plausible biosynthesis pathway was proposed, and possibly this finding may
lead to additional attention from organic and biosynthesis chemists.
Recently, Liu and coworkers isolated a mero-aspochalasin named iizukine C (445)
[190] from the saline soil fungus Aspergillus iizukae, and it is the first aspochalasin
featuring a unique 1,2,4-triazole functionality at C-19. Its structure was determined
by extensive spectroscopic analysis inclusive of the
1 H15 N HMBC spectrum.
Another type of mero-aspochalasin is formed by the dimerization of one or more
aspochalasins with one or more epicoccine units. Spicarins A (446) and B (447)
[192], the first mero-aspochalasins with a epicoccine unit, were isolated from the
fungus Spicaria elegans KLA03 in 2016. The structure of spicarin A (446) was
determined by X-ray diffraction analysis. In the several years following, Zhang and
coworkers reported a series of mero-aspochalasins (448–468) belonging to this type.
The term “merocytochalasan” was put forward initially in 2017, and “heterodimer”,
“heterotrimer”, and “heterotetramers” have also gradually come into use for these
merocytochalasans.
Asperchalasine A (455) [193], the first cytochalasan heterotrimer featuring an
unusual decacyclic 5/6/11/5/5/6/5/11/6/5 ring system with as many as 20 chiral
centers, was isolated from the solid culture broth of Aspergillus flavipes, together
with three biogenetically related intermediates, asperchalasines B–D (448–450). The
structure of asperchalasine A (455), with multiple quaternary carbon atoms and chiral
H. Zhu et al.
and amiaspochalasins F (437) and G (438) [189]. This is the first report of actual
seco-aspochalasins, with ring C cleaved between C-17 and C-18 or between C-9 and
C-21.
2.4.4 Mero-aspochalasins and Aspochalasin Homodimers
Besides the merocytochalasans mentioned in Sects. 2.1 and 2.3, most merocytochalasans are generated by involving an aspochalasin unit (Table 19 and Fig. 19).
The discovery of structurally complex merocytochalasans within this group may
be considered to be a milestone in research on the cytochalasans, because of their
fascinating ring systems and scaffolds with the presence of multiple chiral centers.
The first type of mero-aspochalasins is formed by the connection of a peptide
or an amino acid moiety to an aspochalasin unit. Aspochalamins A–D (439–442)
[5, 6], four mero-aspochalasins constituted by the connection of a tripeptide-like
moiety to an aspochalasin unit at C-19, were isolated in 2004 from an endosymbiotic
fungus Aspergillus niveus, collected from the gut of a woodlouse species by Fiedler
and coworkers. These were the first examples of merocytochalasans, although the
term “merocytochalasan” had not yet been put forward at the time. However, to date,
insufficient attention to this group has been paid, and so their relative configurations at
C-17, C-18, and C-19 have still not determined. Recently, two new cysteine residuecontaining mero-aspochalasins, cyschalasins A (443) and B (444) [191] were isolated
from the endophytic fungus Aspergillus micronesiensis, which represent a new type
of merocytochalasan featuring a modified cysteine residue with an unusual sulfur
atom. A plausible biosynthesis pathway was proposed, and possibly this finding may
lead to additional attention from organic and biosynthesis chemists.
Recently, Liu and coworkers isolated a mero-aspochalasin named iizukine C (445)
[190] from the saline soil fungus Aspergillus iizukae, and it is the first aspochalasin
featuring a unique 1,2,4-triazole functionality at C-19. Its structure was determined
by extensive spectroscopic analysis inclusive of the
1 H15 N HMBC spectrum.
Another type of mero-aspochalasin is formed by the dimerization of one or more
aspochalasins with one or more epicoccine units. Spicarins A (446) and B (447)
[192], the first mero-aspochalasins with a epicoccine unit, were isolated from the
fungus Spicaria elegans KLA03 in 2016. The structure of spicarin A (446) was
determined by X-ray diffraction analysis. In the several years following, Zhang and
coworkers reported a series of mero-aspochalasins (448–468) belonging to this type.
The term “merocytochalasan” was put forward initially in 2017, and “heterodimer”,
“heterotrimer”, and “heterotetramers” have also gradually come into use for these
merocytochalasans.
Asperchalasine A (455) [193], the first cytochalasan heterotrimer featuring an
unusual decacyclic 5/6/11/5/5/6/5/11/6/5 ring system with as many as 20 chiral
centers, was isolated from the solid culture broth of Aspergillus flavipes, together
with three biogenetically related intermediates, asperchalasines B–D (448–450). The
structure of asperchalasine A (455), with multiple quaternary carbon atoms and chiral