Efficient <i>trans</i>-Selectivity in the Cyclocondensation of (<i>S</i>)-2-[2-(<i>p</i>-Tolylsulfinyl)phenyl]acetaldehyde with Activated Dienes Catalyzed by Yb(OTf)<sub>3</sub>

Reactions of (<i>S</i>)-2-[2-(<i>p</i>-tolylsulfinyl)phenyl]acetaldehyde <b>1 </b>with Danishefsky's and related dienes took place in the presence of Yb(OTf)<sub>3</sub> in a completely stereoselective manner, mediated by a remote sulfinyl group (1,5-asymmetric induction), to afford the corresponding 2,3-dihydro-4<i>H</i>-pyran-4-ones. These reactions followed a stepwise mechanism, as was corroborated by isolation of the corresponding intermediates, with a high level of <i>trans</i>-selectivity for 4-methyl-substituted dienes. Treatment of the adducts with Raney Ni provided concomitant cleavage of the C−S bond and reduction of the conjugated carbonyl grouping.