52
H. Zhu et al.
and C-23, forming a ring system similar to that of pyrrole-chaetoglobosins described
later.
Certain oxidations at selected positions and functional groups are being presented
here. Armochaetoglasin G (261) [10] and oxichaetoglobosins A–F (264–269) [128]
are rare chaetoglobosins with a C-10 substituent that has been oxidized to a hydroxy
or methoxy group. More interestingly, the indole group of armochaetoglobin T (251)
[125] is oxidized and represents the first chaetoglobosin characterized by a 2
,3
-
epoxy-indole moiety. The same oxidation is also found in several other compounds
that include armochaetoglasin H (262) [10] and oxichaetoglobosins G (294) and
H (295) [128]. Additionally, it is notable that oxichaetoglobosin I (296) [128] is
the first example of a naturally occurring chaetoglobosin possessing a 2-nor-indole
group that is derived from chaetoglobosin W (292) [142] by oxidation.
• Pyrrole-chaetoglobosins
This is a group of chaetoglobosins with an unusual pyrrole ring fused to the macrocyclic scaffold of chaetoglobosins that are possibly formed via transamination,
cyclization, and dehydration reactions [144]. Penochalasins A–C (297–299) [143],
which represent the first pyrrole-chaetoglobosins, were isolated from a Penicillium sp. fungus separated from a marine alga Enteromorpha intestinalis in 1996
by Numafa and coworkers. Their structures were elucidated by NMR spectroscopic analysis and chemical transformations. A few years later, another pyrrolechaetoglobosin, penochalasin D (300) [120], was discovered from the same fungus,
in which the pyrrole ring is partially reduced. Then, for some time, there were no
reports of new pyrrole-chaetoglobosin derivatives. However, in 2015 Zhang and
coworkers isolated eight new pyrrole-chaetoglobosins named armochaetoglobins K–
R (301–308) [144], from Chaetomium globosum TW1-1, which inhabits the medicinal terrestrial arthropod Armadillidium vulgare. They also proposed a biosynthesis
pathway for pyrrole-chaetoglobosins from the precursor of chaetoglobosin A (210)
by reactions including transamination, cyclization, and dehydration. The structure
of armochaetoglobin K (301) was determined by X-ray diffraction analysis, and
this was the first time the skeleton of a pyrrole-chaetoglobosin was confirmed using
this technique. Isochaetoglobosin D b [148], separated from Chaetomium globosum
SNSHI-5, was assigned with an unusual structure having a pyrrole ring that differed
from other pyrrole-chaetoglobosins. However, its
1 H and
13 C NMR data are closely
related to those of penochalasin C (299) [143], and it appears that their structures are
identical. Since their NMR data were measured in different solvents and there was no
detailed structural elucidation description for isochaetoglobosin D b , this supposition
is not yet confirmed.
• [5.6.10.5]-Chaetoglobosins
Chaetoglobosins with a [5.6.10.5] ring system are a specific group derived from
normal [5.6.13]-chaetoglobosins by a nucleophilic reaction, leading to the connection
of C-17 and C-21. The first [5.6.10.5]-chaetoglobosin chaetoglobosin U (309) [145]
was isolated by Tan and coworkers from the solid culture of Chaetomium globosum
H. Zhu et al.
and C-23, forming a ring system similar to that of pyrrole-chaetoglobosins described
later.
Certain oxidations at selected positions and functional groups are being presented
here. Armochaetoglasin G (261) [10] and oxichaetoglobosins A–F (264–269) [128]
are rare chaetoglobosins with a C-10 substituent that has been oxidized to a hydroxy
or methoxy group. More interestingly, the indole group of armochaetoglobin T (251)
[125] is oxidized and represents the first chaetoglobosin characterized by a 2
,3
-
epoxy-indole moiety. The same oxidation is also found in several other compounds
that include armochaetoglasin H (262) [10] and oxichaetoglobosins G (294) and
H (295) [128]. Additionally, it is notable that oxichaetoglobosin I (296) [128] is
the first example of a naturally occurring chaetoglobosin possessing a 2-nor-indole
group that is derived from chaetoglobosin W (292) [142] by oxidation.
• Pyrrole-chaetoglobosins
This is a group of chaetoglobosins with an unusual pyrrole ring fused to the macrocyclic scaffold of chaetoglobosins that are possibly formed via transamination,
cyclization, and dehydration reactions [144]. Penochalasins A–C (297–299) [143],
which represent the first pyrrole-chaetoglobosins, were isolated from a Penicillium sp. fungus separated from a marine alga Enteromorpha intestinalis in 1996
by Numafa and coworkers. Their structures were elucidated by NMR spectroscopic analysis and chemical transformations. A few years later, another pyrrolechaetoglobosin, penochalasin D (300) [120], was discovered from the same fungus,
in which the pyrrole ring is partially reduced. Then, for some time, there were no
reports of new pyrrole-chaetoglobosin derivatives. However, in 2015 Zhang and
coworkers isolated eight new pyrrole-chaetoglobosins named armochaetoglobins K–
R (301–308) [144], from Chaetomium globosum TW1-1, which inhabits the medicinal terrestrial arthropod Armadillidium vulgare. They also proposed a biosynthesis
pathway for pyrrole-chaetoglobosins from the precursor of chaetoglobosin A (210)
by reactions including transamination, cyclization, and dehydration. The structure
of armochaetoglobin K (301) was determined by X-ray diffraction analysis, and
this was the first time the skeleton of a pyrrole-chaetoglobosin was confirmed using
this technique. Isochaetoglobosin D b [148], separated from Chaetomium globosum
SNSHI-5, was assigned with an unusual structure having a pyrrole ring that differed
from other pyrrole-chaetoglobosins. However, its
1 H and
13 C NMR data are closely
related to those of penochalasin C (299) [143], and it appears that their structures are
identical. Since their NMR data were measured in different solvents and there was no
detailed structural elucidation description for isochaetoglobosin D b , this supposition
is not yet confirmed.
• [5.6.10.5]-Chaetoglobosins
Chaetoglobosins with a [5.6.10.5] ring system are a specific group derived from
normal [5.6.13]-chaetoglobosins by a nucleophilic reaction, leading to the connection
of C-17 and C-21. The first [5.6.10.5]-chaetoglobosin chaetoglobosin U (309) [145]
was isolated by Tan and coworkers from the solid culture of Chaetomium globosum