Cationic 31-pyridylation of a chlorophyll-a derivative using Barluenga reagents

Abstract

The 3-vinyl group in methyl pyropheophorbide-a was transformed into the cationic 1-pyridinioethenyl group by the action of a commercially available Barluenga reagent, bis(pyridine)iodonium(I) tetrafluoroborate, via the dehydroiodation of the 1-pyridinio-2-iodoethenyl adduct. Similar addition–elimination reactions with synthetic Barluenga reagent analogs possessing p-methyl, methoxy, and amino groups on the pyridine ligand afforded the 31-(p-substituted pyridinio) counterparts. Using another Barluenga reagent analog comprised of sterically demanded 2,4,6-collidine, the above chlorophyll-a derivative was reacted with p-(un)substituted pyridines to give the corresponding 31-pyridinio substitutes.