How to use command line proxy authentication with AUTHENICATION REQUIRED clause

Greetings,
I am exploring the use of the command line proxy authentication feature of our 10.2 databases with a third-party client application. The initial results are promising, and seem to allow for the increased auditing capabilities we are seeking.

I am having difficulty, however, using command line proxy authentication with the ALTER USER ... GRANT CONNECT THROUGH ... AUTHENTICATION REQUIRED statement. For example, I am able to connect using the following:

  SQL> ALTER USER user1 GRANT CONNECT THROUGH user2;
  User altered.

  SQL> CONNECT user2[user1]/user2
  Connected.

I cannot find any syntax that will allow me use the AUTHENTICATION REQUIRED clause, though:

  SQL> ALTER USER user1 GRANT CONNECT THROUGH user2 AUTHENTICATION REQUIRED;