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 hippeastrinesynthesis

The heterocycloaddition reaction between nitrosobenzene and 2-methylcyclohe xa-1,3-diene preferentially affords the regioisomer with the N-Ph group nex t to the methyl substituent, and this product has been characterised by X-r ay crystallography. As a model study for a tricarbonyliron mediated route t o the alkaloid hippeastrine, a 2-methyl-6-phenylcyclohexadiene analogue has been prepared from tricarbonyl(eta(5)-3-methylcyclohexadienyl) iron(1+) he xafluorophosphate(1-) and has been examined in the heterocycloaddition reac tion. A novel in situ heterocycloaddition trapping procedure has been devel oped based on this chemistry. Tricarbonyl(eta(4)-2-methyl-6-phenylcyclohexa diene)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.