Synthesis and evaluation of a new water‐soluble fluorescent red dye, xanthene bis‐ C ‐glycoside

Condensation of 2 eq. of C ‐ β ‐D‐glucosylphloroacetophenone with glyoxylic acid in an aqueous solution of Na 2 CO 3 , followed by air oxidation in MeOH in the presence of 11 eq. of pyridine to afford a 36% yield of a bright red dye, xanthene bis‐ C ‐glycoside. This dye is 10 times more fluorescent (Φ f [EtOH] 581 nm = 3.9 × 10 −2 ) and 7.5 times more water‐soluble (57 mg/mL H 2 O) than the natural red pigment, carthamin. Detailed NMR analysis of its methyl analogs was used to confirm the structure of the dye as methyl 4,5‐diacetyl‐1,3,8‐trihydroxy‐3‐oxo‐3 H ‐2,7‐di‐ C ‐ β ‐D‐glucopyranosylxanthene‐9‐carboxylate from among three possible ring‐closure isomers. Xanthene is safe and shows high light‐resistance; therefore, xanthene bis‐ C ‐glycoside could be used as a food colorant or an in vivo probe.