excluding redirect when logging in as admin

Forgive my newbiness but i have not been able to figure out how to exclude the wp-admin when logging on as admin. Is there a rule ordering scheme so that rules are examined in a certain order or ???

I have searched the forum and am not able to figure out the solution. What I am trying to achieve is to have an url rom site a.com redirected to the home page of site b.com but retain the capability of logging on to site a.com (a.com/wp-admin). Ny help would be appreciated.