摘要:
                                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.