Bill Ackman’s Pershing Square Capital Management LP joined with Winthrop Realty Trust to buy $300 million in defaulted mezzanine loans on Stuyvesant Town-Peter Cooper Village, Manhattan’s largest apartment complex. The joint venture paid $45 million for the senior-level mezzanine debt and began foreclosure proceedings on the property, the companies said today in a statement. Pershing Square, based in New York, owns 77.5 percent of the venture.