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[摘要]：The title compound (short version: BTE) occurs in (E)- and (Z)-isomers (both with b.p. of ca. 100 degrees) which equilibrate with nucleophilic catalysts. Both undergo (2+2) cycloadditions with methyl vinyl ether at 25 degrees. Three stereogenic centers in the cyclobutanes led to four rac-diastereoisomers, which were obtained in pure and crystalline state. The structures were elucidated by 19F-NMR spectroscopy and confirmed by two X-ray analyses. The cycloadditions were not stereospecific: e.g., (E)-BTE furnished 73% trans-adducts (with respect to the CF3 groups) and 27% cis-adducts. The loss of stereochemical integrity occurs in the intermediate gauche-zwitterions which can cyclize or rotate, but not dissociate. Under extreme conditions (2M LiClO4 in Et2O, 70 degrees, 3 months), the thermodynamic equilibrium of the four cyclobutanes was achieved. Considerations of Coulombic attraction and conformational strain in the zwitterionic intermediates allow us to rationalize the observed proportions of diastereoisomeric cyclobutanes. Ethyl vinyl ether and butyl vinyl ether furnished cyclobutanes in similar diastereoisomer ratios. |
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