See
http://www.designers-guide.org/Forum/YaBB.pl?num=1165836451/2#2.
Whilst various synthesis tools have been created for specific classes of circuits, there aren't really any good general solutions. Years ago I wrote (along with some colleagues) a switch capacitor filter synthesis tool, which worked pretty well - but it was targeted at one specific type of circuit.
So normally it needs a designer to "synthesize" the design from a high level spec (or model) to transistor level (of course, you may be able to build the design from existing blocks such as opamps, data converters, comparators, etc).
The closest approach that has been reasonably successful is using optimization to size the design once the topology has been developed. This is mainly beneficial in terms of design centering and yield improvement though (and relies on you capturing the appropriate measurements and specifications to ensure that the optimizer is sufficiently well constrained).
Regards,
Andrew.