Synthesis and intramolecular conversion of substituted 2-methyl-11-nitro-5,6-dihydro-2H-2,6-methanobenzo[g][1,3,5] oxadiazocin-4(3H)-ones in different solvents

A Biginelli reaction of 5-R-salicylic aldehydes (R = H, Me, Br) with nitroacetone and urea in each case leads to a predominant formation of 2 R *,6 S *,11 S * diastereomers of 8-R-2-methyl-11-nitro-5,6-dihydro-2 H -2,6-methanobenzo[ g ][1,3,5]oxadiazocin-4(3 H )-one. In solutions in DMF and DMSO, these diastereomers undergo the oxadiazocine ring opening with setting a three-component equilibrium between predominant 4-(2-hydroxy-5-R-phenyl)-6-methyl5-nitro-3,4-dihydropyrimidin-2(1 H )-ones and 2 R *,6 S *,11 S * and 2 R *,6 S *,11 R * diastereomers of methanobenzoxadiazocine as two minor components.