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.