MILD OXIDATIVE CYCLISATION OF AN ISOPENTENYL SIDE-CHAIN

Colupulone, on mild aerial oxidation undergoes an oxidative cyclisation to give a dihydrofuran the suggested structure of which is supported by physical and chemical evidence. The reaction provides a model for the biosynthesis of dihydrofuran rings from isopentenyl side-chains. The dihydrofuran is hydrogenolysed to 4,6-dihydroxy-2-(1-hydroxy-1-methylethyl)-7-isobutyryl-5- isopentyl-2,3-dihydrobenzo[b]furan which is then dehydrated and rearranged with acid to 7-isobutyryl-4,6-dihydroxy-5-isopentyl-2-isopropylbenzo[b]furan. The last-named compound is synthesised from 2,4,6-trihydroxy-3- isopentylisobutyrophenone and isoprene dibromide.