A shortcut hydroformylation route to 7-formyl-5,6-dihydroindolizine from 1-allyl-2-formylpyrrole

When 1-allyl-2-formylpyrrole (1) was subject to hydroformylation conditions with Rh4(CO)12 as catalyst precursor, at 100 atm total pressure and 100°C, 7-formyl-5,6-dihydroindolizine (2′) was produced together with the expected branched aldehyde (3), the linear isomer (2) being obtained in traces only. An intramolecular aldol condensation between the carbon atom adjacent to the formyl group in the chain and the carbonyl group directly bonded to pyrrole ring most likely generates the indolizine structure.