20
H. Zhu et al.
pathways [2]. Several years later, chamiside A (97) [61] was reported by Lou and
coworkers from the endophytic fungus Chaetomium nigricolor F5, which shares the
same ring A as that of both chaetoconvosins A (93) and B (94), while differing in the
cleavage of ring C. In their paper, the authors proposed a biosynthesis pathway, and
the most intriguing step was the oxidation and rearrangement in the five-membered
pyrrolinone ring to generate the six-membered ring A, which also applies to chaetoconvosins A (93) and B (94). In any event, the structures of these three compounds
are extraordinary and they are rare examples of cytochalasans with a six-membered
ring A.
Curtachalasins A (95) and B (96) were reported in 2018 by Liu and coworkers [60]
from the endophytic fungus Xylaria curta E10, which are rare cytochalasans with an
even-numbered (10-) carbocyclic ring C, while most other examples contain an oddnumbered (9-, 11-, and 13-) ring C. Condensation and rearrangement are proposed
to be the key steps for the construction of their carbon skeleton. One year later, from
the same fungus, curtachalasins C–E (98–100) [62] were identified, which share a
similar ring C, but have an unprecedented bicyclo[3.3.1]lactam ring as compared
with curtachalasins A (95) and B (97). From the fermented medium under optimized
conditions, xylarichalasin A (101) [63], an unexpected halogenated cytochalasin, was
isolated, and it has an unprecedented 6/7/5/6/6/6-fused polycyclic structure. The ring
C part of xylarichalasin A (101) is similar to those of the curtachalasins A (95) and B
(96), but the connection of Me-11 and the benzene ring (C-25) via a radical reaction
generates a new cycloheptane ring. Moreover, the presence of two chlorine atoms
located at C-10 and C-13 is rare. These findings show that the reconstruction of
the isoindolone core, whether it is a bicyclo[3.3.1]lactam in 98–100 or an additional
cycloheptane fused to the core structure in 101, adds a new dimension to the structural
diversity of the cytochalasin family.
Interestingly, some compounds of this subclass were obtained by bioconversion
procedures. Chen and coworkers reported the first microbial transformation of L696,474 (26) [35], a cytochalasin isolated from Hypoxylon fragiforme ATCC 20995,
using Actinoplanes sp. ATCC 53771, which led to the two new cytochalasins, 22hydroxy-L-696,474 (29) and 16β,22-dihydroxy-L-696,474 (30), and several new
pyrichalasins (165–168). This work suggested that microbial biotransformation is a
powerful tool to generate novel cytocalasin derivatives.
[5.6.13]-Cytochalasins and Related Derivatives
[5.6.13]-Cytochalasins (Table 3 and Fig. 3) differ from [5.6.11]-cytochalasins by the
occurrance of two more carbons in the macrocycle fused to the isoindolone core.
In addition, [5.6.13]-cytochalasins normally exhibit only one methyl group on the
macrocycle at C-16 (102–104 and 106–108), with the one exception of cytochalasin
K Fex (105) [64]. The macrocycle of cytochalasin K Fex (105) is similar to those of
the chaetoglobosins (Sect. 2.3), with methyl groups both at C-16 and C-18.
H. Zhu et al.
pathways [2]. Several years later, chamiside A (97) [61] was reported by Lou and
coworkers from the endophytic fungus Chaetomium nigricolor F5, which shares the
same ring A as that of both chaetoconvosins A (93) and B (94), while differing in the
cleavage of ring C. In their paper, the authors proposed a biosynthesis pathway, and
the most intriguing step was the oxidation and rearrangement in the five-membered
pyrrolinone ring to generate the six-membered ring A, which also applies to chaetoconvosins A (93) and B (94). In any event, the structures of these three compounds
are extraordinary and they are rare examples of cytochalasans with a six-membered
ring A.
Curtachalasins A (95) and B (96) were reported in 2018 by Liu and coworkers [60]
from the endophytic fungus Xylaria curta E10, which are rare cytochalasans with an
even-numbered (10-) carbocyclic ring C, while most other examples contain an oddnumbered (9-, 11-, and 13-) ring C. Condensation and rearrangement are proposed
to be the key steps for the construction of their carbon skeleton. One year later, from
the same fungus, curtachalasins C–E (98–100) [62] were identified, which share a
similar ring C, but have an unprecedented bicyclo[3.3.1]lactam ring as compared
with curtachalasins A (95) and B (97). From the fermented medium under optimized
conditions, xylarichalasin A (101) [63], an unexpected halogenated cytochalasin, was
isolated, and it has an unprecedented 6/7/5/6/6/6-fused polycyclic structure. The ring
C part of xylarichalasin A (101) is similar to those of the curtachalasins A (95) and B
(96), but the connection of Me-11 and the benzene ring (C-25) via a radical reaction
generates a new cycloheptane ring. Moreover, the presence of two chlorine atoms
located at C-10 and C-13 is rare. These findings show that the reconstruction of
the isoindolone core, whether it is a bicyclo[3.3.1]lactam in 98–100 or an additional
cycloheptane fused to the core structure in 101, adds a new dimension to the structural
diversity of the cytochalasin family.
Interestingly, some compounds of this subclass were obtained by bioconversion
procedures. Chen and coworkers reported the first microbial transformation of L696,474 (26) [35], a cytochalasin isolated from Hypoxylon fragiforme ATCC 20995,
using Actinoplanes sp. ATCC 53771, which led to the two new cytochalasins, 22hydroxy-L-696,474 (29) and 16β,22-dihydroxy-L-696,474 (30), and several new
pyrichalasins (165–168). This work suggested that microbial biotransformation is a
powerful tool to generate novel cytocalasin derivatives.
[5.6.13]-Cytochalasins and Related Derivatives
[5.6.13]-Cytochalasins (Table 3 and Fig. 3) differ from [5.6.11]-cytochalasins by the
occurrance of two more carbons in the macrocycle fused to the isoindolone core.
In addition, [5.6.13]-cytochalasins normally exhibit only one methyl group on the
macrocycle at C-16 (102–104 and 106–108), with the one exception of cytochalasin
K Fex (105) [64]. The macrocycle of cytochalasin K Fex (105) is similar to those of
the chaetoglobosins (Sect. 2.3), with methyl groups both at C-16 and C-18.