Phosphonic acids and esters–XVIII : Preparation of β,γ-unsaturated phosphonates by the stobbe condensation with diethyl β-carbethoxyethylphosphonate; stereochemical dependencies of homoallylic 31P 1H spin-spin coupling constants

Abstract

The base catalyzed condensation of the title compound ( 4) with benzaldehyde and a variety of ketones follows a normal Stobbe course yielding β,γ-unsaturated phosphonates, i.e. ethyl hydrogen β-carbethoxy-γ-substituted allylphosphonates ( 5). Despite the unsymmetrical nature of 4, condensation is specific at the position adjacent to the carbethoxy function; neither the isomeric α,β-unsaturated phosphonates nor the dephosphonated (Wittig) products, β,γ-unsaturated carboxylates, are observed. The specificity presumably reflects the greater stabilization afforded an α-anion by a carbonyl function than by a dialkoxyphosphono function. Condensations with unsymmetrical carbonyl components (acetophenone, benzaldehyde) yield mixtures of cis- and trans- 5 in a nonstereoselective manner. Gross structures and stereochemical relationships were assigned on the basis of PMR spectroscopy. In 5 derived from acetophenone and acetone, a long-range (five bond) 31P 1H coupling is observed. Based upon studies of these and model compounds (dimethyl γ,γ-dimethylallylphosphonate), it was shown that the transoid 5J PH is of larger magnitude than the corresponding coupling involving cisoid geometry.