Highly enantioselective [2,3]-Wittig rearrangement of functionalized allyl benzyl ethers: a novel approach to lignan synthesis

Highly enantioselective [2,3]-Wittig rearrangement of functionalized allyl benzyl ethers was accomplished using a chiral di-tert-butyl bis(oxazoline) ligand. The reaction proceeded with excellent diastereo- and enantioselectivity when no methoxy substituent was present at the ortho-position on the benzyl group. On the other hand, the enantioselectivity was drastically decreased in the presence of an ortho-methoxy group.

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(1R,2S)-2-Methyl-1-phenyl-3-[(triisopropylsilyloxy)methyl]-3-buten-1-olC21H36O2SiEe = 98%[α]D23=-12.2 (c 1.24, CHCl3)Source of chirality: chemical reactionAbsolute configuration:(1R,2S)

(1R,2S)-1-(4-Methoxyphenyl)-2-methyl-3-[(triisopropylsilyloxy)methyl]-3-buten-1-olC22H38O3SiEe = 98%[α]D25=-11.5 (c 0.92, CHCl3)Source of chirality: chemical reactionAbsolute configuration: (1R,2S)

(1R,2S)-1-(3-Methoxyphenyl)-2-methyl-3-[(triisopropylsilyloxy)methyl]-3-buten-1-olC22H38O3SiEe = 95%[α]D22=-12.2 (c 0.73, CHCl3)Source of chirality: chemical reactionAbsolute configuration: (1R,2S)

(1R,2S)-1-(3,4-Dimethoxyphenyl)-2-methyl-3-[(triisopropylsilyloxy)methyl]-3-buten-1-olC23H40O4SiEe = 87%[α]D24=-12.2 (c 0.85, CHCl3)Source of chirality: chemical reactionAbsolute configuration: (1R,2S)

(1R,2S)-1-(3,5-Dimethoxyphenyl)-2-methyl-3-[(triisopropylsilyloxy)methyl]-3-buten-1-olC23H40O4SiEe = 99%[α]D25=-10.3 (c 1.74,CHCl3)Source of chirality: chemical reactionAbsolute configuration: (1R,2S)

(1R,2S)-2-Methyl-3-[(triisopropylsilyloxy)methyl]-1-(3,4,5-trimethoxyphenyl)-3-buten-1-olC24H42O5SiEe = 85%[α]D25=-12.7 (c 0.72, CHCl3)Source of chirality: chemical reactionAbsolute configuration: (1R,2S)

(1R,2S)-1-(2-Ethylphenyl)-2-methyl-3-[(triisopropylsilyloxy)methyl]-3-buten-1-olC23H40O2SiEe = >99%[α]D20=+2.6 (c 1.40, CHCl3)Source of chirality: chemical reactionAbsolute configuration: (1R,2S)