. . . " is a radical of the formula --C(R.sub.a).dbd.C(R.sub.b)--, where R.sub.a and R.sub.b are each hydrogen or loweralkyl, the radical of the formula --C(R.sub.a).dbd.C(R.sub.b)-- is reduced by reduction of the double bond to the corresponding radical --CH(R.sub.a)--CH(R.sub.b)--, ord) for the preparation of compounds of the formula I in which X.sub.1 is a carbonyl or hydroxymethylene group, compounds" .