Synthesis of carthamin acetate, the red pigment in safflower petals

Carthamin acetate, (3S,3′S)-1-[5-(p-acetoxycinnamoyl)-3-C-(β-D-2″,3″,4″,6″-tetra-O-acetylglucopyranosyl)-3,4-dihydroxy-2,6-diketo]cyclohex-4-enylidene-1′-[5′-(p-acetoxycinnamoyl)-3′-C-(β-D-2″′,3″′,4″′, 6″′-tetra-O-acetylglucopyranosyl)-2′,3′,4′-trihydroxy-6′-keto]cyclohexa-1′,4′-dienylmethane 7a and its (3R, 3′R)-epimer 7b were synthesized via the C-glycosylation of 2-acetyl-1,3,4,5-benzenetetrol using acetobromoglucose in the presence of sodium hyride, followed by aldol condensation with p-hydroxybenzaldehyde after methyl-protection of the enolic hydroxy group, and subsequent dimerization with triethyl orthoformate after demethylation, in a total yield of 0.6%. Their absolute configurations were determined by X-ray analysis of the key intermediate 4b.