Synthesis and characterization of d₅-barbarin for use in barbarin-related research

Based on structural similarities and equine administration experiments, Barbarin, 5-phenyl-2-oxazolidinethione from Brassicaceae plants, is a possible source of equine urinary identifications of aminorex, (R,S)-5-phenyl-4,5-dihydro-1,3-oxazol-2-amine, an amphetamine-related US Drug Enforcement Administration (DEA) controlled substance considered illegal in sport horses. We now report the synthesis and certification of d₅-barbarin to facilitate research on the relationship between plant barbarin and such aminorex identifications. D₅-barbarin synthesis commenced with production of d₅-2-oxo-2-phenylacetaldehyde oxime (d₅-oxime) from d₅-acetophenone via butylnitrite in an ethoxide/ethanol solution. This d₅-oxime was then reduced with lithium aluminum hydride (LiAlH₄) to produce the corresponding d₅-2-amino-1-phenylethan-1-ol (d₅-phenylethanolamine). Final ring closure of the d₅-phenylethanolamine was performed by the addition of carbon disulfide (CS₂) with pyridine. The reaction product was purified by recrystallization and presented as a stable white crystalline powder. Proton NMR spectroscopy revealed a triplet at 5.88 ppm for one proton, a double doublet at 3.71 ppm for one proton, and double doublet at 4.11 ppm for one proton, confirming d₅-barbarin as the product. Further characterization by high resolution mass spectrometry supports the successful synthesis of d₅-barbarin. Purity of the recrystallized product was ascertained by High Performance Liquid Chromatography (HPLC) to be greater than 98%. Together, we have developed the synthesis and full characterization of d₅-barbarin for use as an internal standard in barbarin-related and equine forensic research.