Investment groups from Chicago and New York said Monday they have acquired the landmark building at 360 N. Michigan and plan to convert its office space into a hotel. Chicago-based Oxford Capital Group LLC teamed with Angelo, Gordon & Co. to buy the 21-story building. Formerly known as the London Accident & Guarantee Building, it opened in 1923 at Michigan and Wacker and is part of one of the city’s most impressive architectural corners. Terms of the sale were not disclosed, but a source said the property went for $57 million. The sale includes a tiny adjacent parking lot at 77 E. Wacker that could be used for an addition to the building.