Two isomers of diethyl 7-methyl-9-oxo-2,4-diphenyl-3-oxa-7-azabicyclo[3.3.1]nonane-1,5-dicarboxylate I (R = Me) were obtained by condensation of diethyl diphenyl-1-oxa-4-oxo-cyclohexane-3,5-dicarboxylate with methylamine and formaldehyde. Their crystal structures were detd. by x-ray diffraction. One isomer has the boat-chair conformation, and the other one has the chair-chair conformation. The formation of I (R = Me) causes a configurational change of the Ph substituents. The two species are characterized by NMR spectroscopy. Another comparable bicyclononanone I (R = CMe3) (III), with a bulky substituent at the N atom was prepd. and was investigated spectroscopically. II should have chair-boat conformation (with the boat conformation in the N-heterocyclic ring) whereas I (R = Me) has the boat conformation in the O-heterocyclic ring.