The US Federal Reserve is sitting on significant paper losses on the real estate assets it acquired in the Bear Stearns rescue, with much of the red ink coming from debt used to back some of the most highprofile buy-out deals of the bubble years. Among the debts weighing on the central bank’s portfolio are those used in financing the acquisitions of Hilton Hotels, which is being restructured, and hotel operator Extended Stay, which is in bankruptcy, people familiar with the matter say.