Photochemical generation of a vinylketen from 5-acetyl-3,3-dimethyl-3H-pyrazole and the identification of the end products as 7-acetyl-3,5,5,9,9-pentamethyl-1,6-diazabicyclo[4,3,0]nona-3,7-dien-2-one by X-ray crystallographic analysis

Abstract

The direct photolysis (λ > 290 nm) of 5-acetyl-3,3-dimethyl-3H-pyrazole (1) in ether gives 2-acetyl-1,1-dimethylcyclopropane (4; 5%) and 7-acetyl-3,5,5,9,9-pentamethyl-1,6-diazabicyclo[4,3,0]nona-3,7-dien-2-one (7; 46%), the structure of which has been determined by X-ray crystallography; the latter compound is formed via a vinylketen (6), which can be trapped with dimethylamine as the amide (5) and with azobenzene as the [2+2] adduct (8).