The crystal structure and absolute configuration of the antibiotic actinobolin

The crystal and molecular structure of N-acetylactinobolin: the α-helix in a small peptide

“…The isocoumarin ring system is in the double chair conformation as predicted by Antosz, Nelson, Herald & Munk (1970) based upon a careful NMR study in water and dimethyl sulfoxide (Antosz, 1969) and also found in actinobolin. HI (Wetherington & Moncrief, 1975). In particular, the apparent coupling constants (9-10 Hz) between the protons in trans-axial positions at C(5)-C(6), C(5)-C(10) and C(6)-C(7) are in excellent agreement with the torsion angles (Table 4) for these linkages.…”

“…The other bonds in the isocoumarin ring system are normal C-C and C-O single bonds and are similar to those found for the four independent molecules in actinobolin. HI (Wetherington & Moncrief, 1975) despite the completely different molecular packing and hydrogen-bonding scheme found in that structure.…”

“…The six chiral centers are (R)-C(3), (R)-C(4), (R)-C(5), (R)-C(6), (R)-C(10) and (S)-C(14). Since the absolute configuration of actinobolin has been determined (Antosz, Nelson, Herald & Munk, 1970;Wetherington & Moncrief, 1975) the molecule is shown in the correct enantiomorphous form.…”

“…Its structure was established from studies of its acid hydrolysis and the base hydrolysis of N-acetylactinobolin (II) (Munk, Nelson, Antosz, Herald & Haskell, 1968;Antosz, Nelson, Herald & Munk, 1970). More recently the structure of the hydroiodide salt of actinobolin was reported (Wetherington & Moncrief, 1975). This extremely complex structure with four molecules in each asymmetric unit confirmed the chemical studies and displayed an extensive hydrogen-bonding scheme.…”

“…This hydrogen-bonding scheme is quite different from that found in actinobolin. HI (Wetherington & Moncrief, 1975) where the amide hydrogen atoms on all four molecules bond to the carbonyl on the isocoumarin ring of an adjacent molecule. In addition, there is no interaction between the C(13)O(13) carbonyl and the O(5)H hydroxyl or any other hydroxyl in contrast to what had been found for one of the molecules in actinobolin.…”