X-ray crystallographic proof of regioselectivity in a nitrosobenzene cycloaddition, and development of a novel in situ heterocycloaddition trap for use during decomplexation of tricarbonyliron diene complexes in hippeastrine synthesis

Abstract

The heterocycloaddition reaction between nitrosobenzene and 2-methylcyclohexa-1,3-diene preferentially affords the regioisomer with the N–Ph group next to the methyl substituent, and this product has been characterised by X-ray crystallography. As a model study for a tricarbonyliron mediated route to the alkaloid hippeastrine, a 2-methyl-6-phenylcyclohexadiene analogue has been prepared from tricarbonyl(η 5-3-methylcyclohexadienyl)iron(1+) hexafluorophosphate(1−) and has been examined in the heterocycloaddition reaction. A novel in situ heterocycloaddition trapping procedure has been developed based on this chemistry. Tricarbonyl(η 4-2-methyl-6-phenylcyclohexadiene)iron(0) can be converted directly into the cycloadduct in this way by reaction with trimethylamine N-oxide in the presence of nitrosobenzene.