SQL - using an alias in a where clause

Question

I am having trouble using an alias in my SQL:

SELECT
    InvoiceDate,
    BillingAddress,
    BillingCity,
    Total,
    CASE
        WHEN Total < 2.00 THEN 'Baseline Purchase'
        WHEN Total BETWEEN 2.00 AND 6.99 THEN 'Low Purchase'
        WHEN Total BETWEEN 7.00 AND 15.00 THEN 'Target Purchase'
        ELSE 'Top Performer'
    END AS PurchaseType

FROM
    invoices

WHERE 
    PurchaseType = 'Top Performer'

ORDER BY
    BillingCity

I get the following error: Invalid column name 'PurchaseType'.