[2,3] Sigmatropic Wittig Rearrangement of Chiral Allyloxyacetaldehyde Hydrazones in the Diastereo‐ and Enantioselective Synthesis of Protected, γ,δ‐Unsaturated α‐Hydroxyaldehydes and Cyanohydrins

The polyfunctional chiral building blocks 1 (X = CHO, CN) are accessible by [2,3] sigmatropic Wittig rearrangements. They were synthesized in three or four steps (with purification of an intermediate by chromatography), some of which with syn/anti selectivities of >98% syn and enantiomeric excesses of 94–97%. R1 = Me, Et, nPr, iPr; R2 = H, Me.