Albofungin and chloroalbofungin: antibiotic crystals with 2D but not 3D isostructurality

The potent antibiotics albofungin [systematic name: (1 S ,4 R ,8a R )‐13‐amino‐1,15,16‐trihydroxy‐4‐methoxy‐12‐methyl‐3,4,8a,13‐tetrahydro‐1 H ‐xantheno[4′,3′,2′:4,5][1,3]benzodioxino[7,6‐ g ]isoquinoline‐14,17(2 H ,9 H )‐dione, C 27 H 24 N 2 O 9 , 1 ] and its chlorinated analogue chloroalbofungin (the 11‐chloro analogue, C 27 H 23 ClN 2 O 9 , 2 ) have been crystallized following their isolation from the bacterial strain Streptomyces chrestomyceticus and their structures determined by single‐crystal X‐ray diffraction. The novel N ‐aminoquinolone molecular arrangement shows N—N bond lengths of 1.4202 (16) and 1.424 (2) Å in 1 and 2 , respectively. The regiochemistry of chloro substitution in the A ‐ring is para to the quinolone O atom, with a C—Cl bond length of 1.741 (2) Å. The absolute stereochemistry at three chiral centres of the xanthone rings ( i.e. 10 S , 13 R and 19 R ) is confirmed. Both compounds crystallize in chiral Sohncke space groups consistent with enantiopurity, but are not fully isostructural. A preserved supramolecular construct (SC) confers two‐dimensional (2D) isostructurality, but the SC self‐associates via either a twofold screw operation in 1 , giving a monoclinic P 2 1 structure, or a twofold rotation in 2 , affording a monoclinic C 2 structure with a doubled unit‐cell axis.