create table sintax

Hello,

If I run

create table newtable (like oldtable including constraints);

in the SQL window with works just file.
But if i execute

execute 'create table '||newtable||' (LIKE '||oldtable||' including
constraints)';

inside a function, in a LOOP, the constraints aren't created.
What is the problem?
Tanks,

julio almeida