The Spanish and European champions have been linked with the 22-year-old winger for some time and Bartomeu is hopeful a deal will be reached.
"We are optimistic, but this has just begun. We are negotiating for Sanchez," he told La Tercera.
"We know he is a valuable player; that is why there are so many teams after him. We have to continue talking and we will see what is going to happen in the next few days."
Bartomeu insists the player's club hold the key to his arrival in Barcelona.
"We will first have to come to an agreement with Udinese," he said.