Photochemistry of 5-methylpyrimidin-4-ones in acetic acid solution: thermal rearrangements of Dewar pyrimidinones and 4-acetoxyazetidin-2-ones

Abstract

Irradiation of 3,6-dialkyl-5-methylpyrimidin-4-ones (1a–f) in acetic acid-acetonitrile (1 : 2, v/v) solution at 0 °C gave 4-acetoxy-3-(1-imino-2,2-dimethylpropyl)-1,3,4-trimethylazetidin-2-one (3a), 6-acetoxy-7-(1-imino-2,2-dimethylpropyl)-7-methyl-1-azabicyclo[4.2.0]octan-8-one (3b), 4-acetoxy-3-acetyl-1,3,4-trialkylazetidin-2-ones (4c–e), 4-acetoxy-3-acetyl-1,3-dimethylazetidin-2-one (4f), 3-acetyl-4-alkylidene-1,3-dimethylazetidin-2-ones (5d) and (5e) as the major products. Reaction of 1,4,6-trimethyl-3-t-butyl-2,6-diazabicyclo[2.2.0]hex-2-en-5-one (2a)(Dewar pyrimidinone) in acetic acid–acetonitrile (1 : 14, v/v) solution at 0 °C and thermolysis of (2a) without solvent at 80 ° C gave the imine azetidin-2-one (3a) and 2,3,5-trimethyl-6-t-butylpyrimidin-4(3H)-one (1a) respectively as the major products. Reactions of the imino azetidin-2-ones (3a) and (3b) and the 4-acetoxyazetidin-2-ones (4c–e) in the presence of acetic acid at 21–40 °C and without solvent at 100–110 °C were carried out to elucidate the rearrangements. The major products from (3a) and (3b) in acidic solutions at 40 °C were N-acetyl-3-acetamido-2,4,4,N-tetramethylpent-2-enamide (6a), N-methylacetamide (9a), 2,5-dimethyl-4-t-butyl-1,3-oxazin-6-one (10a), 2-piperidone (9b), and 1,3-oxazin-6-one (10b)=(10a), and from the thermolysis of (3a) and (3b) at 100–110 deg; C were the pyrimidin-4-ones (1a) and (1b), 1,3-dimethyl-3-pivaloyl-4-vinylazetidin-2-one (5a) and 7-methyl-7-pivaloyl-1-azabicyclo[4.2.0]octa-5-en-8-one (5b). Both reactions of (4d,e) at 23–24 °C in acidic solutions and of (4c,d) without solvent at 100–110 °C gave the 3-acetyl-4-alkylidene-1,3-dimethylazetidin-2-ones (5c–e) as the major products. The reaction mechanisms and intermediates of these reactions are discussed.