Asymmetric [2,3]-Wittig Rearrangement of Oxygenated Allyl Benzyl Ethers in the Presence of a Chiral di-tBu-bis(oxazoline) Ligand: A Novel Synthetic Approach to THF Lignans

We have accomplished highly enantioselective [2,3]-Wittig rearrangements of functionalized allyl benzyl ethers in the presence of a chiral di-tBu-bis(oxazoline) ligand. In various oxygenated benzylic ethers, the reactions proceeded with excellent diastereo- and enantioselectivities, although the presence of a methoxy substituent at the ortho-position on the benzyl group drastically decreased the enantioselectivity. Conversely, o-ethyl and o-phenyl substituents had no significant effect on the selectivity. We found that the C2-substituent of the allylic moiety played an important role in producing high enantioselectivity. Highly enantioselective [2,3]-Wittig rearrangement in the presence of catalytic amounts of the chiral ligands is also described.