1,3-Dipolar Cycloadditions of Diazoalkanes to Activated Sulfoxides:Influence of Lewis Acids

摘要

The reactions of diazomethane and diazoethane with(S)-3-p-tolylsulfinylfuran-2(5H)-one(3)and its 4-methyl derivative(4)have been studied.The sulfinyl group was able to completely control the pi-facial selectivity of all these reactions,which decreased when the polarity of the solvent increased and could be inverted in the presence of Lewis acids,Yb(OTf)_3 being the most efficient catalyst.This behavior made possible the stereodivergent synthesis of diastereoisomeric pyrazolines in almost quantitative yields and de's higher than 98%.The endolexo selectivity was also complete in reactions of 3 with diazoethane,whereas 4 afforded an easily separable 1:1 mixture of diastereoisomers.Steric factors accounts for the endolexo selectivity,whereas electrostatic interactions must also be considered to explain the facial selectivity.