Titanium-Mediated Diastereoselective Formation of (E)- or (Z)-2-Substituted 1-Vinylcyclopropanols: Scope and Limitation, Applications

Abstract

Titanium-mediated cyclopropanation of α,β-unsaturated esters failed to provide 1-vinylcyclopropanol derivatives in useful yields, but (E)-2-substituted-1-vinylcyclopropanols were formed diastereoselectively from O-protected β-oxo- and β-halo esters, with the allylic double bond being created subsequently (Knoevenagel condensation or dehydrohalogenation). Titanium-mediated cyclopropanation of homoallyl alk-2-enoates, on the other hand, directly provided the corresponding Z diastereomers. Palladium(0)-catalysed azidation of their sulfonic esters (tosylate, mesylate), azide reduction, and subsequent double bond cleavage afforded (E)- or (Z)-2-alkyl-2,3-methanoamino acids, although improvements are required to perform the total asymmetric syntheses of molecules with three membered-rings by these methods. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)