The invention concerns pharmaceutically useful 1,3-dioxane alkenoic acid derivatives of formula VII and formula XI containing a pyridyl moiety at position 4 of the dioxane ring and in which the groups at positions 2, 4 and 5 have cis-relative stereochemistry, wherein X is hydrogen, alkoxy or hydroxy, Y is vinylene, n is 1 or 2, A.sup.1 is alkylene, R.sup.1 is a variety of substituents defined hereinafter, Ra and Rb are independently methyl or ethyl, R.sup.2 is hydroxy, a physiologically acceptable alcohol residue or alkanesulphonamido, and the pharmaceutically acceptable salts thereof.