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

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(eta(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(eta(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. (C) 2000 Elsevier Science Ltd All rights reserved.