NEW YORK CITY-Boston Properties said late Friday afternoon it had closed on its acquisition of 510 Madison Ave. from Macklowe Properties, a $280.5-million deal first reported last month. Separately, SL Green Realty Corp. said in a statement that the closing, and the retirement of debt SL Green held in conjunction with the boutique officer tower, was the main factor in raising its full-year earnings guidance from about $4 per share to $4.75.

At the start of 2010, SL Green had bought the first mortgage loan and senior mezzanine loan on 510 Madison, with a then-outstanding aggregate balance of $249.3 million, for $180.5 million. The sale to BPX, which in 2008 paid $3.95 billion for the General Motors Building and other Macklowe assets, meant SL Green received full repayment of the loans. SL Green says it has realized an unleveraged IRR of 55% on its investment.

“The repayment of our loans in connection with the sale of 510 Madison marks the conclusion of one of SL Green’s most successful debt investments to date,” SL Green CEO Marc Holliday says in a statement. “It illustrates the overall success of our structured finance investment program as an activity that complements our core business and has the potential to provide significant value to our shareholders.” SL Green also cites better-than-expected leasing results in revising its earnings guidance upward.

Boston-based BPX says it expects to spend about $15 million to complete construction at 510 Madison. The 350,000-square-foot, 30-story tower is currently vacant; it’s geared toward boutique financial tenants such as hedge funds.