QUINOLINONE CYCLOADDITION AS A POTENTIAL SYNTHETIC ROUTE TO DIMERIC QUINOLINE ALKALOIDS

Acid-catalysed dehydration of the quinolinone allylic alcohol 24 and concomitant Diels-Alder cycloaddition of the resulting diene 25 under acid conditions, followed by further intramolecular cyclization, led to the isolation of isomeric tetracyclic compounds containing one quinolin-2-one and one quinolin-4-one ring (dimer A, 27 and dimer C, 30). Further intramolecular cyclization of dimer A 27 yielded the heptacyclic product (dimer B, 28) having a ring structure of similar type to the dimeric quinoline alkaloids (paraensidimerins). The structures of the cyclization products (dimer A, dimer B and dimer C) have been determined by spectroscopic and X-ray diffraction methods. Mechanistic pathways for the chemical synthesis of polycyclic quinolinone products and their relevance in the biosynthesis of dimeric quinoline alkaloids are discussed.