Subselect AS and Where clause

Google being useless tonight - now that's new :-)

What I'm trying to do is the following and I'm certain there is a simple
solution which eludes me:

I have a query like this:

SELECT a,b,c, (select problem from other_table where id=a) as problem FROM
mytable WHERE a=1

So far so good. Actually "problem" always resolves to one record, so it's not
the "multiple records returned" problem.

What I try to do is this:

SELECT a,b,c, (select problem from other_table where id=a) as problem FROM
mytable WHERE a=1 and problem = 3

see the "problem=3" part in the where clause? The error I get is

 SQLError: (ProgrammingError) column "problem" does not exist

Do I miss something? Shouldn't the "as" assignment make this virtual column
available to the where clause? I think this should work and I know it works if
I'd make a view out of the query. However, the query is built dynamically, so
turning it into a view isn't really an option.

Any pointer will be greatly appreciated.

Uwe