Abstract: Compounds of the formula ##STR1## where one of Z and Y is CO and the other is C--W--R.sup.2 and the dotted line indicates a double bond is present where necessary to meet valency requirements, W is O, S(O).sub.n, N(R.sup.3), N(R.sup.3)(R.sup.4), N(R.sup.3)O or ON(R.sup.3); R.sup.1 is hydrogen, or an optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, phenyl or heterocyclyl group; R.sup.2, R.sup.3 and R.sup.4, which may be the same or different, are as defined above for R.sup.1, or are acyl, or R.sup.2 and R.sup.3 or R.sup.2 and R.sup.4 or R.sup.3 and R.sup.4 together with the nitrogen or oxygen to which they are attached form an optionally substituted ring which may contain other hetero atoms; each X, which may be the same as or different from any other X, is halogen, CN, NO.sub.2, SF.sub.5, B(OH).sub.2, triakylsilyl or a group E, OE or S(O).sub.n E where E is a group as defined hereinbefore for R.sup.