SYNTHESIS OF CARMINIC ACID, THE COLORANT PRINCIPLE OF COCHINEAL

The first synthesis of carminic acid (7 hyl-9,10-dioxo-9,10-dihydroant hracene-2-carboxylic acid) is described. Selective C-glycosylation at the 7-position of ethyl and benzyl ,10-pentamethoxy-1-methylanthracene -2-carboxylates with a-O-benzyl-1-trifluoroacetyl-alpha-D-glucopyranos e afforded intermediates which were oxidised to ethyl and benzyl ucopy ranosyl)-9,10-dihydroanthracene-2-carboxylate respectively. The benzyl compound was hydrogenolysed and the ethyl analogue hydrogenolysed and hydrolysed to give the same product, which was tetraacetylated and de methylated to afford 6-deoxycarminic acid tetraacetate, lucopyranosyl) -9,10-dihydroanthracene-2-carboxylic acid. The pentamethoxy intermedia tes were obtained from 2-chloronaphthazarin by Diels-Alder addition to ycarbonyl-2,4-bis(trimethylsiloxy)penta-2,4-dienes to give alkyl 6-de oxykermesates. Methylation afforded the corresponding trimethyl ethers , which by reductive methylation gave the required pentamethoxy compou nds. By known steps 6-deoxycarminic acid tetraacetate was converted in to the 5,8,9,10-bisquinone, acetoxylation of which gave carminic acid octaacetate. Acidic hydrolysis afforded carminic acid.