Starting from racemic naringenin ((±)-1), a mixture of dracocephin A stereoisomers 6-(2"-pyrrolidinone-5"-yl)naringenin (±)-2a-d and its regioisomer, dracocephin B 8-(2"-pyrrolidinone-5"-yl)naringenin (±)-3a-d originally isolated from Dracocephalum rupestre, have been synthesized in a one-pot reaction. The separation of 2a-d and 3a-d was achieved by preparative HPLC. The four stereoisomers of each natural product were separated by analytical chiral HPLC and their absolute configuration was studied by the combination of HPLC-ECD measurements and TDDFT-ECD calculations. The synthesized flavonoid alkaloids were further characterized by physicochemical and in vitro pharmacological studies.
Keywords: Dracocephalum rupestre; ECD calculation; HPLC–ECD; absolute configuration; dracocephins A–B; flavonoid alkaloids.