(E) Enol ethers from the stereoselective reduction of α-alkoxy-β-ketophosphonates and Wittig type reaction

When alpha-alkoxy-beta-ketophosphonates, prepared by the Rh(II) mediated insertion reaction of alpha-diazo-beta-ketophosphonates into the OH bond of primary alcohols, were reduced either by NaBH4 in the presence of CaCl2 or by DIBAL, they respectively gave the corresponding anti or syn stereomeric hydroxyphosphonates with pronounced to complete stereoselectivity. Submitted to the action of potassium tert-butoxyde, syn isomers led to the corresponding pure (E) enol ethers in moderate to good yields. Under the same conditions anti isomers led to a mixture of (Z) and (E) enol ethers in rather poor yields. The sequence was applied to the preparation of some allyl-vinyl ethers with a (E) configuration for the vinylic double bond.