An unusual boron tribromide-mediated, one-pot bromination/cyclization reaction. Application to the synthesis of a highly strained cyclopenta[1,3]cyclopropa[1,2-b]pyrrolizin-8-one

Performing Jefford’s cyclization of ethyl 2-pyrrol-1-ylcyclohex-2-ene-1-carboxylate (5) using boron tribromide in refluxing dichloromethane led to a trans–cis bromopyrrolohydrindolone 7 whose debromination in alkaline medium afforded a highly strained cyclopenta[1,3]cyclopropa[1,2-b]pyrrolizin-8-one 6. Compound 7 and two of its diastereoisomers were synthesized in order to better understand this unusual reaction and more generally the reactivity of these systems.