In studies on N aminomethylation (the Mannich re action 1,2 ) of 4,4 bis(nitroxymethyl)oxazolidin 2 one (1) with piperidine (2) and dimethylamine (3) as the amino components, we found that it was not the expected Mannich bases 4 and 5 that formed as the final stable reaction products but rather quaternary ammonium salts 6 and 7 (Scheme 1).We assume that the nitroxy group in the transient com pounds 4 and 5 undergoes intramolecular nucleophilic substitution 3-5 by the nitrogen atom of a tertiary amine, with concominant elimination of the nitrate anion and the nitrogen atom quaternisation.The structures of compounds 6 and 7 were established based on data from 1 H, 13 C and 14 N NMR spectroscopy. The signal assignment is based on a comparison of chemi cal shifts, multiplet forms, and signal integrated intensi ties for products 6 and 7 and compounds 1 and 8, as well as literature data 6-8 for related compounds, and data from DEPT 135 and COSY 13 C-1 H experiments.In the 1 H NMR spectra of com pounds 1 and 8, CH 2 protons of the oxazolidine ring and NCH 2 N protons of compound 8 resonated as singlets. In the CH 2 ONO 2 group (compound 1) and in the CH 2 OH group (com pound 8), the observed magnetic nonequivalence (0.05 and 0.12 ppm, respectively) is typical of such compounds. 6 Cyclisation with formation of rifid structures 6 and 7 makes all gemi nal positions nonequivalent. Hence a complex overlap ping AB type pattern is observed at δ 3.5-5.1, as well as poorly resolved (even on a 500 MHz instrument) piperi dine ring multiplets at δ 1.5-3.8 in compound 6 and two methyl group singlets in compound 7 that became non equivalent are observed. 13 C-1 H correlation spectroscopy was used for the assignment. In the 13 C NMR spectrum of compound 6, two broad signals at δ 66.6 and 76.0 were observed (broadening occurs probably due to spin spin coupling with the quaternary nitrogen atom), which were Scheme 1 † Deseased.