Synthesis and intramolecular transformations of 8-substituted 11-nitro-2-phenyl-5,6-dihydro-2H-2,6-methanobenzo[g][1,3,5]oxadiazocin-4(3H)-one diastereomers

The Biginelli reaction between 5-R-salicylic aldehyde (R = H, Me, and Br), α-nitroacetophenone, and urea affords 8-R-11-nitro-2-phenyl-5,6-dihydro-2H-2,6-methanobenzo-g]1,3,5]oxadiazocin-4(3Н)-ones as mixtures of two diastereomers. The ratio of diastereomers depends on the catalyst (HCl) concentration in the reaction medium. In DMSO and DMF, the resulting compounds undergo oxadiazocine ring opening with establishment of a three-component equilibrium between 4-(2-hydroxy-5-R-phenyl)-5-nitro-6-phenyl-3,4-dihydropyrimidin-2(1Н)-one as the major component and diastereomeric methanobenzoxadiazocines as two minor components. Dilution of these solutions with water favors the oxa-Michael reaction resulting in the reverse transformation of dihydropyrimidinones into the corresponding starting diastereomers.