Where clause with windows function that will not work

WrongWhereWindow.sql

SELECT  DummyId,
        PartitionId,
        ROW_NUMBER() OVER (PARTITION BY PartitionId ORDER BY DummyId DESC) AS LastN
FROM    dbo.DummyTable
WHERE   ROW_NUMBER() OVER (PARTITION BY PartitionId ORDER BY DummyId DESC) <= 2;