Particular examples of X1 are the groups --NH2 ; --CO2 H; --N=C=S; N=C=O; or an acyl group having the formula --R2 --CO--G wherein G may be a group such as OH, halogen, ORb, or a ##STR1## group; Ra and Rb being, independently, a C1 -C12 alkyl, alkenyl, alkynyl or a phenyl containing chain, which is optionally substituted, e.g. by halogen.