Nitroalkene (4+2) cycloadditions of achiral and chiral vinyl ethers. The stereochemical consequence of the Lewis acid promoter and studies toward the total synthesis of (+)-pretazettine
Chiral enol ether dienophiles derived from (1R,2S)-($-$)-2-phenylcyclohexanol and (R)-($-$)-2,2-diphenylcyclopentanol (($-$)-6 have been found to provide high levels of asymmetric induction in tandem inter (4+2) /intra (3+2) nitroalkene cycloadditions. Either enantiomeric series of tandem cycloaddition/hydrogenolysis product is available from the chiral auxiliary of a single absolute configuration by judicious selection of the Lewis acid promoter, Ti(Oi-Pr)$\sb{2}$Cl$\sb{2}$ (98% ee, ($-$)) or methylaluminum bis-(2,6-diphenylphenoxide) (MAPh) (93% ee, (+)). Propenyl ethers derived from ($-$)-6 undergo endo-selective (4+2) cycloadditions (2.3/1-8.2/1) in the presence Ti(Oi-Pr)$\sb2$Cl$\sb2$, however, exoselective (4+2) cycloadditions (10.2/l-54.8/l) are observed in the presence of MAPh.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign