The direct approach: The efficient use of substrate control has served as the basis for the enantioselective total synthesis of (R,R,R)‐α‐tocopherol. A single reagent directing group (ortho‐diphenylphosphanyl benzoate, o‐DPPB) served to control the stereoselectivity of a rhodium‐catalyzed hydroformylation reaction and the directed allylic substitution as the fragment‐coupling step (see picture).
The preparation of 26f was carried out on a 1.3-mmol scale from the metal acylate 8b23 according to the procedure described above for complex 26a with the reaction parameters indicated in Table l. A single organometallic product was observed by TLC and purified by chromatography on Florisil in the presence of air. The imino complex 26e (Rf 0.22) was obtained in 36% yield (212 mg, 0.47 mmol) as an orange/yellow solid: mp 87-88 °C. Spectral data for 26c:
Der direkte Weg: Eine effiziente Substratkontrolle wurde in der enantioselektiven Totalsynthese von (R,R,R)‐α‐Tocopherol genutzt. Hierbei steuert eine einzige ortho‐Diphenylphosphanylbenzoat‐Gruppe (o‐DPPB) die Stereoselektivität der rhodiumkatalysierten Hydroformylierung und der allylischen Substitution im Fragmentkupplungsschritt (siehe Strukturformel).
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