Borane adducts of punicine and of its dehydroxy derivatives (pyridinium-1-yl)-2- and 3-phenolates

The natural product punicine (Punica granatum) exists in two tautomeric forms, the cross-conjugated mesomeric betaine 1-(pyridinium-1-yl)-2-hydroxy-phenyl-5-olate and the conjugated mesomeric betaine 1-(pyridinium-1-yl)-5-hydroxy-phenyl-2-olate. Punicine as well as its picoline derivatives reacted with tris(pentafluorophenyl)borane exclusively at the 2′-olate group to form zwitterionic borates. Correspondingly, the 5′-dehydroxy derivate of punicine, the conjugated heterocyclic mesomeric betaine 1-(pyridinium-1-yl)-phenyl-2-olate and its picoline derivatives also gave borates, whereas analogous reactions of the cross-conjugated isomer 2′-dehydroxypunicine [1-(pyridinium-1-yl)-phenyl-3-olate] did not result in the formation of stable adducts. [Display omitted] •Punicine exists as two tautomers, a conjugated and a cross-conjugated mesomeric betaine.•Reaction with tris(pentafluorophenyl)borane occurs exclusively at the conjugated 2"-olate group.•Dehydroxypunicines behave similarly.