The manufacture of optically active bifunctional compounds of the general formula ##STR1## wherein R represents protected hydroxymethyl, protected formyl or alkoxycarbonyl and R.sup.1 represents a group of the formula ##STR2## in which R.sup.2 and R.sup.3 represents lower alkyl or cycloalkyl or R.sup.2 and R.sup.3 together with the nitrogen atom represent a heterocyclic ring, by isomerizing compounds of the general formula ##STR3## wherein R, R.sup.2 and R.sup.3 have the above significance and R.sup.3 can additionally signify hydrogen, is described.The compounds of formula I are potential intermediates in the manufacture of, interalia, natural vitamin E and natural vitamin K.sub.1.