Three syntheses of 3,8-dimethyl-1H-pyrano[4,3-b][1]benzopyran-10-one, a model for the synthesis of the fungal metabolite, fulvic acid

Abstract

Compouds with the 1H-pyrano[4,3-b][1]benzopyran-10-one nucleus characteristic of fulvic acid have been prepared by three methods from chromone precursors; the characteristic steps are (i) the acetylation of the 2-methyl group of 3-methoxymethyl-2,6-dimethylchromone by acetyl chloride and lithium di-isopropylamide at –70 °C, (ii) the acetylation of the 2-methyl group of 3-chloromethyl-2,6-dimethylchromone by N,N-dimethylacetamide in phosphoryl chloride followed by acid hydrolysis, and (iii)(the best route) the reverse Diels–Alder addition of 2-methoxypropene to 3-formyl-6-methylchromone and the rhodium-catalysed isomerisation of the product to regain the chromone nucleus.