[4 + 2]‐Cycloaddition of Singlet Oxygen and Phenyltriazolinedione with Bicyclo[6.1.0]nona‐2,4,6‐triene and Derivatives

Abstract

The dienic reactivity of the bicyclononatrienes l a ([4.3.0]-isomer) and 1 b ([6.1 .O]-isomer) and its oxa-derivative 6 (cyclooctatetraene epoxide) as well as 1,3,5-cyclooctatrienone (7), towards singlet oxygen ('0,) and phenyl-l,2,4-triazoline-3,5-dione (PTAD) as dienophiles has been investigated. Except for l a , which affords the endoperoxide 9a in 60% yield, the remaining substrates l b , 6 , and 7 do not react with '0,. With PTAD l a , l b , 6 , and 7 afford the urazoles 10a, 8b, 1212, and 11, respectively. Unusual is the fact that the bicyclo[6. products 2 a -c (Eq. 1). The valence isomer l b has been prepared some time ago'); its dienic reactivity has been investigated to some extent. For example, it was reported2a-c) that tetracyanoethylene gave the [2 + 21-cycloadduct 3 and the [4 + 21-cycloadduct 3c, while maleic anhydride afforded a mixture of the products 4a and 4c in 33% and 67%, respectively. Clearly, the [4 + 21-cycloadduct 4a must be derived from the valence isomer l a and the [4 + 21-cycloadducts 3c and 4c from l c in order to rationalize these results. Other dienophiles that have been employed include dimethyl acetyienedicarboxylatezd), chlorosulfonylisocyanateZe), and