Bring a large pot of salted water to a boil. Add the spaghetti and cook for 8-10 minutes, until al dente or cooked to your liking. Drain and reserve a cup of the water.
Meanwhile, in a large pan, fry the garlic for 3 minutes over a low heat in olive oil.
Add the oregano and tomato purée and fry for 1 minute.
Add the cherry tomatoes and brown sugar and allow to simmer for 5 minutes.
Add the spaghetti and the reserved water, as well as the tuna, olives and capers. Mix everything together, making sure to let the tuna, capers and olives warm through.
Serve immediately with a generous sprinkling of freshly ground black pepper and basil leaves.