Photoreactions of diazines. Part 8. A di-π-methane photorearrangement of 4-substituted 1,4(or 3,4)-dihydropyrimidines leading to 5-substituted 1,2(or 2,3)-dihydropyrimidines

Abstract

Photolysis of 4-R-1,4(or 3,4)-dihydropyrimidines (1) caused rearrangement to 5-R-1,2(or 2,3)-dihydropyrimidines (2) provided that the substituent R contained a π-bond in a position α to the heterocyclic ring (R = phenyl, isobutenyl, and phenylethynyl). 4-Methyl-1,4(or 3,4)-dihydropyrimidine did not show this rearrangement. Chemical evidence is presented that the rearrangement of (1) occurs via the di-π-methane mechanism leading to 6-R-2,4-diazabicyclo[3,1,0]hex-2(or 3)-ene. This latter intermediate undergoes a thermal homo[1,5] hydrogen shift into 5-R-2,5-dihydropyrimidine which on tautomerization gives (2). The reaction could be sensitized by acetone. 5-Substituted 4-phenyl-1,4(or 3,4)-dihydropyrimidines (9a–c) did not rearrange under photochemical conditions.