We report the nitrosylation of previously synthesized 3a-substituted 2,3,3a,4-tetrahydro-1H-benzo[d]- pyrrolo[1,2-a]imidazol-1-ones 1a,b in the reaction of 5R-3H-furan-2-ones with 1,2-diaminobenzene [1]. The nitrosylation reaction was carried out at 0–5 °C in aqueous acetonitrile. Nitrous acid was generated in situ and introduced directly to react with the substrate. The crystalline products have a color characteristic for nitroso compounds.

Since imidazolones 1a,b possess two reaction sites capable of reacting with electrophilic reagents, we could expect the formation of products of N-nitrosylation and/or C-nitrosylation.

The nitrosonium cation is a weak electrophile and, thus, attack at the secondary amino group in imidazolones 1a,b is more likely and would obey kinetic control. However, the NMR spectral data indicate the formation of compounds 2a,b, which were identified as 7-nitroso-3a-R-2,3,3a,4-tetrahydro-1H-benzo[d]pyr- rolo[1,2-a]imidazol-1-ones. The formation of these nitroso products is possible through a Fischer-Hepp rearrangement, which proceeds under acid catalysis conditions as an intramolecular migration of the NO+ cation, leading to the thermodynamically more stable C-nitrosylation products 2a,b.

The 1H NMR spectra were taken on a Varian-400 spectrometer at 400 MHz at 20–25 °C in CDCl3 with TMS as internal standard.

7-Nitroso-3a-phenyl-2,3,3a,4-tetrahydro-1H-benzo[ d ]pyrrolo[1,2- a ]imidazol-1-one (2a). Excess concentrated sulfuric acid was added to a solution of NaNO2 (0.07 g, 1 mmol) in cold water (3–5 ml), followed by a solution of compound 1a (0.25 g, 1 mmol) in 4:1 acetonitrile-water (10 ml) and then water (50 ml). The mixture was stirred for 2 h at room temperature. The precipitate was filtered off and washed with water to give 0.21 g (75%) pale-yellow crystalline nitroso derivative 2a; mp 96–98 °C. 1H NMR spectrum, δ, ppm (J, Hz): 1.62 (1H, br.s, NH); 3.61 (2H, t, J = 6.2, H-2); 3.88 (2H, t, J = 6.2, H-3); 7.47 (1H, d, J = 2.0, H-8); 7.49–7.65 (5H, m, C6H5); 8.12 (1H, d, J = 8.0, H-6); 8.25 (1H, d, J = 8.0, H-5). Found, %: C 69.00; H 5.15; N 15.23. C16H13N3O2 .Calculated, %: C 68.81; H 4.69; N 15.05.

3a-(4-Methylphenyl)-7-nitroso-2,3,3a,4-tetrahydro-1H-benzo[ d ]pyrrolo[1,2- a ]imidazol-1-one (2b) was obtained in 73% yield analogously from compound 1b (0.26 g, 1 mmol) and NaNO2 (0.07 g, 1 mmol); mp 120–122 °C. 1H NMR spectrum, δ, ppm (J, Hz): 1.68 (1H, br.s, NH); 2.26 (3H, s, CH3); 3.24 (2H, t, J = 6.0, H-2); 3.44 (2H, t, J = 6.2, H-3); 7.21, 7.39 (4H, d, J = 8.0, p-Tol); 7.47 (1H, d, J = 2.0, H-8); 8.12 (1H, d, J = 8.0, H-6); 8.25 (1H, d, J = 8.0, H-5). Found, %: C 69.49; H 5.06; N 14.69. C17H15N3O2. Calculated, %: C 69.61; H 5.15; N 14.33.

This work was carried out with the financial support of the Russian Basic Research Fund, Grant 10-03-00640-a.