Studies of chromenes. Part 4. Oxidation of hexahydro-5H-chromeno[3,4-c]-pyridazines and conformational isomerism in tetra-and hexa-hydro-N,N′-dialkoxycarbonyl-5H-chromeno[3,4-c]pyridazines

Abstract

3,4-Bis(t-butoxycarbonyl)- and 3,4-bis(methoxycarbpnyl)-8-methoxy-5,5-dimethyl-2,3,4,4a-tetra-hydro-5H-chromeno[3,4-c]pyridazines and their 2,2-dimethyl analogues were hydrogenated to give hexahydrochromenopyridazines possessing only a trans-ring junction. Removal of the t-butoxy-carbonyl groups from the trans-products afforded the corresponding 1,2,3,4,4a,10b-hexahydro-5H-chromeno[3,4-c]pyridazines which were oxidised first to 1,2,3,10b-tetrahydro- and then 1,2-dihydro-5H-chromenopyridazines. The last-named dihydro compounds possess an unusual [3,4]pyridazine hydrogenation pattern. The dialkoxycarbonyl derivatives of both the tetra- and hexahydrochromenopyridazines exist as pairs of conformational isomers whose interconversion is acid catalysed. The isomerism is considered to be due to restricted rotation about the less hindered N(3)–CO bond, the N(4)–CO bond being locked in one conformation by 1,3 interactions.