C-Alkylation of Methyl leuco-6-Deoxykermesate by Aldol Reactions and its Application to Synthesis of Carminic Acid

In a non-aqueous medium in the presence of piperidinium acetate, methyl leuco-6-deoxykermesate reacts in aldol fashion with aldehydes regioselectively to give 6-alkyl products while under aqueous alkaline conditions over a prolonged time, 7-alkyl compounds are selectively formed; the structures of the 6-alkyl series was confirmed by an X-ray crystal structure determination of the 6-methyl member, namely methyl 3,5,8-trihydroxy-1,6-dimethylanthra-9,10-quinone-2-carboxylate; in aqueous alkaline conditions during a short mild reaction period, intermediate 7-α-hydroxyalkyl compounds can be isolated, and in an application to a synthesis of 6-deoxycarminic acid, the aldol reaction of 2,3,4,5,6-penta-O-benzyl-D-glucose with methyl leuco-6-deoxykermesate was examined.