Reactivity of acyclic (pentadienyl)iron(1+) cations: Synthetic studies directed toward the frondosins

Abstract

A short, 4-step route to the scaffold of frondosin A and B is reported. The [1-methoxycarbonyl-5-(2′,5′-dimethoxyphenyl)pentadienyl]Fe(CO)₃⁺ cation was prepared in two steps from (methyl 6-oxo-2,4-hexadienoate)Fe(CO)₃. Reaction of this cation with isopropenyl Grignard or cyclohexenyllithium reagents affords (2-alkenyl-5-aryl-1-methoxycarbonyl-3-pentene-1,5-diyl)Fe(CO)₃ along with other addition products. Oxidative decomplexation of these (pentenediyl)iron complexes, utilizing CuCl₂, affords 6-aryl-3-methoxycarbonyl-1,4-cycloheptadienes via the presumed intermediacy of a cis-divinylcyclopropane.