Synthese der enantiomeren (+)- und (−)-Rosmarinsäuremethylester

Summary.

 Starting from L-tyrosine (L-1), the phenyl lactic acid ester (−)-4 was prepared by reported procedures. (−)-4 could then be converted to the benzyl ether (−)-5. From (−)-5, the corresponding optical antipode (+)-5 is available via (+)-6 by Mitsunobu reaction. The en-antiomeric excess of 5 was determined by NMR spectroscopy from camphanic acid esters (+)- and (−)-8, respectively. The phenyl lactic acid esters (+)- and (−)-5 were acylated by caffeoyl chloride (9) to yield the O-protected enantiomeric rosmarinic acid esters (+)- and (−)-10. Deprotection of 10 by BCl₃ afforded the title compounds (+)- and (−)-11 in fair yields.