HMPA is a highly polar aprotic solvent which coordinates very well to the lithium ion. As expected, silylated telluronium allylide, which was generated in situ from the corresponding telluronium salt and LDA, reacted with methyl cinnamate to afford trans-2-phenyl-trans-3-((trimethylsilyl)vinyl)cyclopropyl ester in high yield with high selectivity in the absence of HMPA. The same reaction provided trans-2-phenyl-cis-3-((trimethylsilyl)vinyl)cyclopropyl ester in the presence of HMPA. The ability of HMPT, TMEDA, PMDETA and 12-crown-4 to affect the stereochemical course were also studied. Further studies showed that the amount of HMPA used in this reaction also influenced the stereoselectivity and the yield. The generality of this tuning reaction was established by investigating a variety of α,β-unsaturated carbonyl compounds as substrates. Either β-aryl-substituted or β-alkyl substituted α,β-unsaturated esters or amides were used, these reactions proceeded smoothly. HMPA, like a switch of stereochemistry, could play a crucial role to tune the stereoselectivity of these cyclopropanation reactions.
The switch-like method for the stereocontrol of ylide cyclopropanation provides a facile means for the synthesis of the two geometrical isomers of multifunctionlized 3-vinylcyclopropane derivatives.
2. Highly Diastereoselective synthesis of vinylcyclopropane derivatives with (-)-8-phenylmenthol as chiral auxiliary
It was reported that (-)-8-phenylmenthol is powerful chiral auxiliary in Michael addition reaction. The first step of ylide cyclopropanation is usually considered to be a Michael addition step, so we carried out the asymmetrical ylide cylcopropanation with (-)-8-phenylmenthol as a chiral auxiliary. It was found that silylated telluronium allylide could react with (-)-8-phenylmenthyl α,β-unsaturated esters to afford vinylcyclopropanes with high diastereoselectivity in high yields. The scope the substances was also investigated. It was found a variety of α,β-unsaturated esters can react smoothly to afford the corresponding products.
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