Résumé
2-(2-Tetrahydrofurylidene)propionates, usually obtained only in the more stable (E) configuration 1, were efficiently isomerised into their (Z) isomers 2 by use of Lewis acids or metallated bases. The (E)/(Z) isomerisation was governed by different factors (type of chelating agent, nature and steric demand of the α,β-unsaturated group). We demonstrated that judicious selection of the α,β-unsaturated group, in combination with the chelating agent, afforded yields of up to 95% with either LDA [R = N(Me)2, tBu] or MgBr2 and ZnI2 (R = OBn), and that the reaction was also influenced by 1,3-allylic strains.