Based on the different reaction results with (-)-menthyl, (-)-8-(2-naphthyl)menthyl or (-)-8-phenylmenthyl as the chiral auxiliary, it is was proposed that the possible p-stacking effect between the aryl of chiral auxiliary and dienyl group of α,β-unsaturated esters is involved in the diastereoselectivity of these reactions.
The easy recovery of chiral auxiliary, good selectivity and high yield of this reaction make it a potential method in the synthesis of multifunctional optically active cyclopropane derivatives.
3. A novel chiral sulfonium ylide: Highly enantioselective synthesis of vinylcyclopropanes
In our previous research, we found lithium ion played a crucial role on the stereoselective tuning of the cyclopropanation reaction of telluronium allylides with α,β-unsaturated esters or amides. On the basis of its mechanistic insight, we proposed that it is possible to enhance its selectivity by formation of a rigid transition state if a coordination group (such as hydroxy or amino) is introduced into a chiral ylide molecule in the presence of a metal ion. Thus, the corresponding chiral sulfonium ylide was designed and its precursor was found to be readily available from the coresponing sulfide that was easily prepared from D-camphor in two steps.
Two special structural features of the sulfonium salt were proved to stabilize its corresponding ylide and depress the 2,3-sigma rearrangement. One is that there is a trimethylsilyl in the end of the allylic double band. This silicon atom can stabilize a α-carbon anion and the corresponding ylide. Another is that there is hydroxy group in the beta-position of sulfonium atom. The oxygen of this hydroxy group may stabilize the sulfonium cation.
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