Specifying Roles in web.config of an asp.net MVC application

I am creating an MVC application with forms auth. I am authenticating against active directory and so have created a custom RoleProvider. My application is only concerned with a small set of roles which up until now I have been defining in the appSettings section of my web.config:

<appSettings>
  <add key="DirectorRole" value="Domain\Directors" />
  <add key="ManagementRole" value="Domain\Managers" />
  ...
</appSettings>

However I have run into a couple of problems with this approach:

1. I cannot reference these setting in my contoller data annotations: [Authorize(Roles = ConfigurationManager.AppSettings["DirectorRole"])] as it wont compile so I have to specify the name of the group again: [Authorize(Roles = "Domain\\Directors")].
2. In my web.config, I would like to specify the groupsToUse for my role provider and just reference a pre-existing list, rather than maintain two seperate lists of the same set of roles.

It seems that there must be a better/reusable way to define the roles in the web.config, can someone point me in the right direction please?