Synthesis of optically pure gomisin A and schizandrin: The first total synthesis of gomisin A and schizandrin having naturally occurring configurations
The totalsynthesis of gomisin A and schizandrin having natural configurations were accomplished for the first time. The key feature of these syntheses is a highly efficient intramolecular oxidative coupling of the intermediates 9 and 21, which can be obtained as both enantiomers in optically pure forms. The manipulation of the lactone moieties of 7 and 22 afforded natural enantiomers of schizandrin
The total syntheses of (+)-schizandrin (1), (+)-gomisin A (2), and (+)-isoschizandrin (3) having natural configurations were accomplished. Optically pure butyrolactones ((-)-9, (-)-31) were transformed to alpha-benzylidenebutyrolactones ((+)-10, (+)-32, (+)-35). By a highly efficient iron(III) perchlorate-mediated oxidative coupling reaction of 10, 32, and 35, the key intermediates with biphenyl skeletons ((-)-11, (-)-33) were constructed with high stereoselectivity. Several methods for the stereoselective introduction of the C6-hydroxyl group were examined. For the synthesis of schizandrin and gomisin A, the Mukaiyama hydration reaction of(-)-11 and (-)-33 provided the desired products with satisfactory selectivity. For the synthesis of isoschizandrin, the stereoselective epoxidation of allylic alcohol (+)-48 was successfully utilized taking advantage of its conformational features.