pi-bond screening in benzonorbornadienes: The role of 7-substituents in governing the facial selectivity for the Diels-Alder reaction of benzonorbornadienes with 3,6-di(2-pyridyl)-s-tetrazine

Benzonorbornadiene 21, 7-spirocyclopropylbenzonorbomadiene 23, 7,7-dimethyl benzonorbomadiene 25, and 7-spirocyclopentylbenzonorbornadiene 27 have been reacted with 3,6-di(2-pyridyl)-s-tetrazine (rate: 21 > 23 > 25=27) to form symmetrical 4,5-dihydropyridazines which are stable towards fragmentation but rearrange with varying facility to their 1,4 isomers. The facial select ivity of attack on the pi -bond changes from exo-attack for 21 and 23 to en do-attack for 25 and 27. The 7-spirocyclopropyl benzonorbomadiene 23 typica lly forms a mixture of dihydropyridazines with exo-stereochemistry, which u ndergo further stereochemical isomerisation to an exo-fused product upon ac etylation (acetyl chloride in hot pyridine). Oxidation with DDQ of the dihy dropyridazines individually or as mixtures gives the corresponding fused 3, 6-di(2-pyridyl)pyridazines.