Possibly the simplest approach to optimizing such a lead
is to modify the natural product by simple functional group
transformations. This can be achieved by chemical and/
or enzymatic methods. Large numbers of analogues can
be rapidly generated by such semisynthetic approaches;
however, many desired transformations cannot be accomplished
due to incompatibilities with pre-existing
functional groups or the lack of a feasible reaction. Thus,
the structural diversity of the analogues accessible by
derivatization is limited. There are numerous examples
of this approach including taxanes camptothecins
and combretastatins