Pyridine-catalysed condensation of citral with phloroglucinols, a novel reaction leading to tetracyclic bis-ethers and chromenes. Two-step synthesis of (±)-deoxybruceol
Citral condenses with phloroglucinol under pyridine catalysis (1 mol.) to give the tetracyclic bis-ether (II). Treatment of the latter with acids can give carbonium ions (XVII) and (XV) and the major products, resulting from solvent trapping, elimination, or further reaction, are from the ion (XV) having ring-B ruptured.
Preparation of 5,7-dihydroxycoumarin by condensation of phloroglucinol with propiolic ester, followed by condensation with citral in the presence of pyridine, gives a simple two step synthesis of deoxybruceol (VIII) which occurs in Eriostemon brucei.
When citral is condensed with phloroglucinol in the presence of 5 × 10–4N hydrochloric acid the major products are, in contrast to those from the pyridine-catalysed reaction, the cis- and trans-forms of the B-ring ethers (XXIII) and (XXIV). Small amounts of the bridge-ring A ethers (XVI) and (V) are found in the reaction.