On naming attributes in a WITH clause

Hello,

Is it possible to rename columns in a WITH clause which includes VALUES?

None of the following parse:

WITH map_table AS
(
    VALUES ('1','r'),('2','w'),('3','a'),('4','d')
) AS (a,b)
SELECT * FROM map_table;

WITH map_table AS
(
    VALUES ('1','r'),('2','w'),('3','a'),('4','d') AS t (a,b)
)
SELECT * FROM map_table;

I would like the map_table columns to have names other than column1 and column2.

WITH map_table AS
(
    VALUES ('1','r'),('2','w'),('3','a'),('4','d')
)
SELECT column1 AS a, column2 AS b FROM map_table;

is close but I would prefer to rename the columns as part of the WITH clause before the (poorly) named column1 and
column2hit the main query. Is it possible? 

Cheers,
M