The Wender Cedrene Synthesis Revisited: A Catalytic Enantioselective Entry to the Chiral Key Intermediate

Dedicated to Prof. Dr. Dieter Enders

Abstract

The seminal synthesis of the sesquiterpene (±)-α-cedrene reported by Wender in 1981 offers a uniquely short and elegant access to the bridged-tricyclic target compound by exploiting an intramolecular arene-olefin photocycloaddition. However, the synthesis was performed only in the racemic series so far. This synthesis was now re-investigated and the catalytic methods for the enantioselective preparation of the chiral key intermediate were evaluated. It was found that Cu-catalyzed allylic substitution of a cinnamyl chloride with MeMgBr in the presence of a Taddol-derived chiral phosphine-phosphite ligand affords the corresponding (1-methylallyl)arene with high enantioselectivity (94% ee). Hydroboration and subsequent Suzuki coupling gave (R)-curcuphenol methyl ether from which (–)-α-cedrene was prepared along the route paved by Wender.

Key words

terpenoids - total synthesis - enantioselectivity - asymmetric catalysis - allylation - photochemistry - copper - chiral P,P-ligands

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