Yes, you can do this
Well, first of all, the fireball spell description states:
The fire spreads around corners.
Rules designer Jeremy Crawford has unofficially clarified on Twitter what this means:
Your cover is foiled if an effect spreads around it and reaches you.
In addition, regarding spell targeting (and the targeting of Fireball in particular), Crawford has unofficially explained that this is possible:
Can a wiz target empty air with fireball or does it need to hit something solid to detonate? Are air bursts legal?
Fireball requires you to choose its point of origin. Its text doesn't require that point to be on an object. A point in space is fine.
How precisely can a wizard dictate that? Can he choose to cast it at a point exactly 23.5 feet away?
As precisely as the DM allows.
There are no official rules preventing spellcasters from precisely targeting their spells. The description of fireball simply says you choose a "point you choose within range" as the point of origin of the spell.
In general, you don't need a line of sight/effect from you to every point within the area of effect of a spell or ability unless it says otherwise. For area-of-effect spells, you simply need a clear path to the point of origin that you are targeting. After that point, the spell description describes how the effect is applied (and whether the spell spreads around cover).