You will have to either:
a) include the case expression's switch field in the where clause, or
b) put the whole select in a derived table then build a where expression on the derived table's alias:
SELECT * FROM
(
Select
'ANormalColumn',
CASE
WHEN 1 = 1 THEN 'T'
ELSE 'F'
END AS ADynamicColumnName
) as dt
where dt.ADynamicColumnName = 'F'