You do not have to be in controlled airspace to fly IFR and you do not have to fly IFR in controlled airspace. The problem you may have, will be if you fly a route where you drop below the base of CAS and then intend to re enter CAS as its base lowers, you MUST obtain a clearance to do so. Do not assume that filing a flight plan automatically gives you clearance to enter controlled airspace. Normally itis not a problem BUt it would be prudent to have an alternative routing in case the controller says those unwelcome words 'remain clear of controlled airspace' and you are IMC at lower levels!