Efficient synthesis and spectroscopic analysis of 8-nitro spiro C-3-annulated 2-benzazepines and their N-oxides
Nitroderivatives 9a-g have been prepared with great C-8 regioselectivity by allowing the corresponding 1,2,4,5-tetrahydrospiro[3H-2-benzazepine-3,1′-cycloalkanes] 8a-g to react with potassium nitrate and concentrated H2SO4. The oxidative reaction of a nitrogen-carbon bond in spirobenzazepines 8 and 9 was performed with H2O2 and catalytic amounts of sodium tungstate at room temperature affording nitrones 10a-d in moderate to high yields. Stereochemical assignments for all the 2-benzazepine derivatives obtained were derived from a full analysis of the 1H NMR spectroscopic data and an X-ray crystallographic analysis of 1,5dimethyl-8-nitro-1,2,4,5-tetrahydrospiro[3H-2-benzazepine-3, 1′-cyclohexane] 9c. These data indicate that the 2-benzazepines 8-11 in solution present the azepine ring preferably in the chair conformation, with the methyl substituent at C-5 disposed equatorially.