| Abstract
| - The reactions of diazomethane and diazoethane with (S)-3-p-tolylsulfinylfuran-2(5H)-one (3) andits 4-methyl derivative (4) have been studied. The sulfinyl group was able to completely controlthe π-facial selectivity of all these reactions, which decreased when the polarity of the solventincreased and could be inverted in the presence of Lewis acids, Yb(OTf)3 being the most efficientcatalyst. This behavior made possible the stereodivergent synthesis of diastereoisomeric pyrazolinesin almost quantitative yields and de's higher than 98%. The endo/exo selectivity was also completein reactions of 3 with diazoethane, whereas 4 afforded an easily separable 1:1 mixture ofdiastereoisomers. Steric factors accounts for the endo/exo selectivity, whereas electrostaticinteractions must also be considered to explain the facial selectivity.
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