Synthesis of Bioactive Polyheterocycles: Regioselective Cyclisation of 5-Cyclohex-2-Enyl-6-Hydroxy[1]Benzopyran-2-One

Abstract

5-Cyclohex-2-enyl-6-hydroxycoumarin (4) was synthesised in 80% yield alongwith 4% of 7-cyclohex-2-enyl-6-hydroxycoumarin (5) by the thermal sigmatropic rearrangement of 6-cyclohex-2-enyloxycoumarin (3) in refluxing N, N-diethylaniline. Treatment of 4 with m-chloroperoxybenzoic acid in refluxing benzene for 4 h gave 8-hydroxy-7a,8,9,10,11,11a-hexahydrobenzofuro[3,2-f]benzopyran-3-one (6) in 90% yield which was then dehydrogenated to benzofuro[3,2-f]benzopyran-3-one (7) with excess of DDQ. Compound 4 was also cyclised with pyridine hydrotribromide or hexamine hydrotribromide to 2-bromo-1,3-propano-1,2,3-trihydropyrano[3,2-f]benzopyran-8-one (8) in 88% or 95% yield respectively. Compound 4 when treated with cold conc. H₂SO₄ at 0–5°C for 4 h furnished the cyclised 1,3-propano-1,2,3-trihydropyrano[3,2-f]benzopyran-8-one (10) in 90% yield.