Cyclopenta[d]pyridazines and cyclopenta[d][1,2]oxazines
Abstract
Usually cyclopenta[d]pyridazines and cyclopenta[d][1,2]oxazines are protonated and react with other electrophiles in the five-membered ring at the 5- or the 7-position, although 2-phenyl-1,4-di-t-butylcyclopenta[d]pyridazxine can be brominated at the 6-position. The pyridazines are more reactive than the oxazines. Both pyridazines and oxazines can be brominated; the pyridazines, but not the oxazines, react with triethyl orthoformate to give aldehydes. The salts of both pyridazines and oxazines condense with diphenylcyclopropenone to form salts in which the positive charge is shared between the three-membered ring and the hetero-atoms; the pyridazinium salts also condense with p-dimethylaminobenzaldehyde to give intensely coloured products. N.m.r. spectra indicate more delocalisation of the double bonds in the pyridazines than in the oxazines.