Heterocyclic studies. Part XXXVIII. Synthesis and disproportionation of pyrimido[4,5-b]quinolinium salts

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Abstract
Treatment of 4-chloro-6-N-ethylanilinopyrimidine-5-carbaldehyde (3) with boiling water gave 10-ethyl-3,4-dihydro-4-oxopyrimido[4,5-b]-quinolinium chloride (7) together with 10-ethyl-5,10-dihydropyrimido[4,5-b]-quinolin-4(3H)-one (8) and 1-ethyl-1,2-dihydro-2-oxoquinoline-3-carbonitrile (6). The salt (7) underwent disproportionation at pH values greater than 9 to the 5,10-dihydro-compound (8) and 10-ethylpyrimido[4,5-b]-quinoline-4,5(3H,10H)-dione (25). The reaction was shown by deuterum-labelling studies, ionisation studies, and crossing experiments with a closely related 3-methylpyrimidoquinolinium salt (29) to involve a hydride ion transfer from the 5-position of one molecule of the compound, as its pseudo-base anion, to the 5-position of another molecule in the cation form. Several other pyrimidoquinolinium salts behaved similarly. Reactions of the pyrimidoquinolines are described, including various oxidations, reductions, and disproportionations which enabled pyrimidoquinolines in the three possible oxidation states to be interconverted. Pyrimidoquinolinium salts were attacked at the 5-position by N,N-dialkylanilines to give 5-(4-dialkylaminophenyl)-5,10-dihydropyrimido[4,5-b]quinolines.

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