a carbon to carbon double bond with water or a lower alkanol preferably methanol or ethanol this reaction is carried out in the presence of an acid preferably a mineral acid such as for example sulphuric acid a conventional inert organic solvent preferably tetrahydrofuran is expediently used as a solvent although temperature and pressure are not critical in this reaction the reaction is preferably carried out between about 0 u00b0 c and the reflux temperature of the reaction mixture especially between about 0 u00b0 c and 40 u00b0 c and at atmospheric pressure polyene compounds of formula i in which a is hydroxy or lower alkoxy are thus obtained an especially preferred procedure is that described in j am chem soc 91 5646 1969 this procedure involves reacting the polyene compound of formula i with water or an appropriate alcohol in the presence of a mercury salt the organomercury compound formed as an intermediate in this process can subsequently be reduced without isolating this intermediate by treating it with a reducing agent suitable mercury salts include mercury acetate and other acylates mercury nitrate mercury trifluoroacetate and mercury halides suitable reducing agents include the alkali metal borohydrides hydrazine and sodium amalgam treatment of the polyene compound of formula i as described above provides the polyene compound of formula i in.