Fig. 4.

Graphical thought experiment considering putative interconversion of tryptorubin (1a) and its non-canonical atropisomer (1b). Theoretically, interconversion would require an unphysical inside-out flipping of the molecule, in which one macrocycle passed through the other (above). This is analogous to atropisomeric inversion of a rotaxane (center), which would require unphysical stretching of the ring (green) over the dumbbell. Such non-canonical atropisomers should be contrasted with prototypical atropisomers like binaphthol (below), which can interconvert through simple bond torsion.