Abstract The reaction of the lithium derivatives of N, N, N′, N′-tetramethyldiamides of arylmethanephosphonic acids (1-Li) with ortho-and para-substituted benzaldehydes 2 is studied. Reaction conditions are found for erythro-stereoselective addition of 1 to 2 with high diastereomeric purity (95–99%) of the erythro adducts 3, 4 (yields 35–72%). By thermolysis of the adducts in neutral medium the corresponding (Z)-ortho and para-substituted ...