Separation of Synephrine Enantiomers in Citrus Fruits by a Reversed Phase HPLC after Chiral Precolumn Derivatization

Racemic synephrine, which was transformed into diastereomers by derivatization with 2,3,4,6-tetra- O -acetyl-β-Dglucopyranosil isothiocyanate, was resolved by a reversed phase HPLC with UV detection at 254 nm. The total contents of synephrine enantiomers in citrus fruit samples were exocarp > mesocarp > endocarp > sarcocarp, suggesting that synephrine content of outer side of citrus fruits was higher than that of the inner side. ( R )-Synephrine was detected in exocarp of eleven fresh citrus fruits, except for lemon, lime, and grapefruit samples. ( S )-Synephrine was determined in the exocarp of four citrus fruits (mikan, orange, bitter orange, and ponkan samples) and the ratio of ( S )-synephrine to total synephrine was 0.5–0.9%. The racemization of ( R )-synephrine in aqueous solution during heating at 100°C was also examined. An increase in the heating time brought about an increase in the ( S )-synephrine content in a linear fashion. The racemization was found to be significantly reduced by the addition of D-fructose, D-maltose, D-glucose, D-mannose or D-galactose, but not D-sucrose or D-mannitol. It is suggested that the reducibility of sugars may result in the inhibition of racemization.