Irreversible intramolecular electrocyclization followed by chain cleavage of cis-transoidal and cis-cisoidal stereoisomers of PPA is eliminated by dendronizing PPA and a reversible cis-cisoidal to cis-transoidal helix-helix transition allowing the design of molecular machines is generated.²⁶⁴ Reproduced with permission from ref. 264. Copyright 2005 and 2008, American Chemical Society.