Photochemistry of pyrimidin-4-ones in aqueous solution and reactions of Dewar pyrimidinones with water and hydrogen sulphide: isolation of reversible hydrates and thiazines

Abstract

Irradiation of the 2,3,6-trimethylpyrimidin-4-one (1a) in aqueous solution gave an unstable product (3a) that was observed by ¹H n.m.r. spectra and reverted to the starting pyrimidin-4-one (1a) at room temperature. The reversible compound (3a) could not be separated from a large amount of the starting pyrimidin-4-one (1a) by column chromatography. Irradiation of 2,3-dialkylpyrimidin-4-ones (1e,f), 3,6-dimethylpyrimidin-4-one (1g), and 3-methylpyrimidin-4-one (1h) in aqueous acetonitrile gave (Z)-2-acetyl-3-amino-N-methylprop-2-enamide (4e), 2-(aminoethylene)-3-oxo-octane-8-lactam (4f), (Z)-N-methyl-2-acetyl-3-aminoprop-2-enamide (5g), and (Z)-3-amino-2-formyl-N-methylprop-2-enamide (5h) as the major crystalline products, respectively. In order to isolate the reversible compounds (3), 1,3,6-trialkyl-5-oxo-2,6-diazabicyclo[2.2.0]hex-2-enes (2a) and (2b) and 7-oxo-9-t-butyl-6,10-diazatricyclo[4.4.0.0^1,8]dec-9-ene (2c)(Dewar pyrimidinones) were prepared and treated in aqueous solutions at 15–20 °C. The isolated reversible products were (Z)-N-acetyl-2-amino-N-methylbut-2-enamide (3a), (Z)-N-acetyl-3-amino-4,4,N-trimethylpent-2-enamide (3b), and (Z)-N-(3-amino-4,4-dimethyl-1-oxopent-2-enyl)-2-piperidone (3c). The reactions of the Dewar isomers (2b) and (2c) and 6-methyl-5-oxo-1,3-di-t-butyl-2,6-diazabicyclo[2.2.0] hex-2-ene (2d) in acetonitrile solution containing hydrogen sulphide gave 2-amino2,3,6-trialkyl-3,4-dihydro-2H-1,3-thiazin-4-ones (8b) and (8d) and 9a-amino-5,6,7,8,9a-hexahydro-2,6-di-t-butyl-4H-pyrido[2,1-b][1,3]thiazin-4-one (8c). The hydrates (3a–c) and 1,3-thiazines (8b) and (8c) reverted quantitatively to the corresponding pyrimidin-4-ones (1a–c) at room temperature. The mechanisms and intermediates of the reactions are discussed and an X-ray crystallographic study of 3-amino-2-formyl-N-methylbut-2-enamide (4g) is reported.