When you are medium, have a reach weapon, and are adjacent to a large creature, are you allowed to make an attack on the parts of the creature that are further away?
Strict RAW would say no:
Reach: You use a reach weapon to strike opponents 10 feet away, but you can't use it against an adjacent foe. (source; emphasis mine)
Since the large creature is adjacent, a reach weapon can't be used against it.
If parts of the creature is 10' away, it would make sense that You could attack that area if there is nothing blocking your weapon. So I would say yes