Phototransposition Chemistry of 1-Methyl-4-phenylpyrazole. A New Intermediate on the P4 Pathway

The photochemistry of some pyranopyrazoles

“…The photochemistry of N-substituted pyrazoles has also been extensively studied and these compounds have been found to undergo photoisomerization to imidazoles [15][16][17][18][19][20][21] and photocleavage to yield enaminonitriles and enaminoisocyanides. [18][19][20][21] Although the photochemistry of the individual ring systems have been studied, we are unaware of any reports concerning the photochemistry of the pyranopyrazole ring system.…”

“…[18][19][20][21] Although the photochemistry of the individual ring systems have been studied, we are unaware of any reports concerning the photochemistry of the pyranopyrazole ring system. Because of our interest in these compounds, we have undertaken a study of the photochemistry of 1,3,6-trimethylpyrano [2,3-c] pyrazole-4(1H)-one (1) and of 3,6-dimethyl-1-phenylpyrano [2,3-c] pyrazole-4(1H)-one (2).…”

Photochemistry of Trifluoromethyl Substituted 1‐Methylpyrazoles.

“…(1) and (3) were synthesized by allowing methylhydrazine to react with 4-ethoxy-1,1,1-trifluoro-3-buten-2-one as previously described [8]. 1-Methyl-4-(trifluoromethyl)pyrazole (2) was prepared by condensation of 2-trifluoro-1,3bis(dimethylamino)trimethinium hexafluorophosphate and methylhydrazine in a modification of the published procedure [9].…”

“…Although the new absorption maximum at 200 nm suggests the formation of 1-methyl-4-(trifluoromethyl)imidazole (5) in this reaction, this latter compound does not absorb at 260 nm. Both enaminonitrile and enaminoisocyanide photocleavage products (Scheme 1) are known to absorb in the 260 nm region of the spectrum [2]. These two types of compounds have previously been distinguished on the basis of their stability in acidic medium [2].…”

“…Previous work in this [1][2][3] and other laboratories [4][5][6] indicates that the photochemistry of 1-methylpyrazoles involves competition between electrocyclic ring closure (Path A, Scheme 1) and cleavage of the N1-N2 bond (Path B, Scheme 1). Path A results in the formation of a 1,5diazabicyclo[2.1.0]pentene intermediate which undergoes one or two sigmatropic shifts of nitrogen followed by rearomatization of the two resulting isomeric 2,5-diazabicyclo[2.1.0]pentene species to yield 1-methylimidazoles with two different scrambling patterns identified as P 6 and P 7 respectively [7].…”