i built rpms for this the other day, and i found that java-1.4.2-bea (or whichever version that binary expects) was the key to making ant run. it is possible that the necessary files are on your system, but in non-standard locations.
i would install java-1.4.2-bea (rhn current version is java-1.4.2-bea-1.4.2.05-1jpp_13rh) and see if that helps. that is the extent to which i can offer advise on this since i just build the package, i don't know what it does.