| Research Abstract |
In view of the recent progress in the chiral coordinating agent-based asymmetric synthesis, we have been exploring the various radical reactions induced by the chiral coordinating agent-based protocol. Disclosed herein are the enantioselective[1,2]-Witting rearrangement via the asymmetric lithiation with RLi/chiral bisoxazoline and Stevens rearrangement with chiral alkoxides.
Enantioselective[1,2]-Witting Rearrangement
The carbanion-radical rearrangement of dibenzyl ether(1), when induced with t-BuLi in the presence of chiral bisoxazoline[(S,S)-Box-i-Pr(2)], was found to afford the [1,2]-rearrangement product, benzyl alcohol 3 in 63% ee. This represents the first example of an enantioselective[1,2]-Witting rearrangement. Next, we also examined the asymmetric catalytic version of the present rearrangement. Thus, the rearrangement of 1 was carried out using 10 mol% of(S,S)-2, and 2 equiv of t-BuLi under the same conditions. As anticipated , alcohol(S)-3 was obtained in comparably high %ee and chemical yield(60% ee, 86%). Furthermore, the synthetic potential of the enantioselective[1,2]-Witting rearrangement has been demonstrated by the enantioselective synthesis of tertiary alcohols(up to 65% ee)from racemic ethers.
Enantioselective Stevens Rearrangement
Benzyldimethylamine derived ammonium salt(racemic)was treated with suger derived chiral alkoxide to afford the enantio-enriched rearrangement product in 38% ee. This is the first example of the enantioselective version of Stevens rearrangement.
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