The Effect of p-Bond Screening on the Reaction of s-Tetrazines with 7-Substituted Benzonorbornadienes

Benzonorbornadiene 21, 7-spirocyclopropylbenzonorbornadiene 23, 7,7-dimethylbenzonorbornadiene 25 , and 7-spirocyclopentylbenzonorbornadiene 27 have been reacted with 3,6-di(2-pyridyl)-s-tetrazine (21>23>25=27) to form symmetrical 4,5-hydropyridazines which are stable towards fragmentation but rearrange with varying facility to their 1,4 isomers. The facial selectivity of attack on the p-bond changes from exo-attack for 21 and 23 to endo-attack for 25 and 27. The 7-spirocyclopropylbenzonorbornadiene 23 typically forms a mixture of dihydropyridazines with exo-stereochemistry, which undergo further stereochemical isomerisation to an exo-fused product upon acetylation (acetyl chloride in hot pyridine). Oxidation with DDQ of these dihydro compounds individually or as mixtures gives the corresponding fused 3,6-dipyridylpyridazines