Highly diastereoselective Diels-Alder reactions with (R)-ethoxy p-tolyl vinyl sulfonium tetrafluoroborate
                                
                                    
                                        作者:B. Ronan、H.B. Kagan                                    
                                    
                                        DOI:10.1016/s0957-4166(00)82159-x
                                    
                                    
                                        日期:——
                                    
                                    p-Tolyl vinyl sulfoxide has been efficiently activated for Diels-Alder reactions by transformation into a sulfonium salt 3a or by addition of TMSOTf.  Very high diastereomeric excesses have been achieved (> 98%) at -20-degrees-C in many cases.  Best results have been obtained with cyclopentadiene, furan and 2,3-dimethyl-1,3-butadiene.  In this way enantiomerically pure oxanorbornenone 19a has been prepared.  The reaction mechanism is briefly discussed.