Plaza Global Real Estate Partners, the JV, agreed to buy the stake for $72 million in a deal that is expected to close by the end of 2012.

521 Fifth Avenue.

SL Green plans to refinance the existing $150 million mortgage—it is from United Overseas Bank, according to data from Real Capital Analytics—with a new $170 million mortgage. All told, this will leave $86 million in net proceeds.

“The partial sale unlocks proceeds we intend to use to fuel our continued growth and allows us to benefit from the potential of this well located, top-quality asset,” SL Green President Andrew Mathias said in a prepared statement. “We look forward to continuing our successful track record of building value for joint venture partners with Quantum Global and LaSalle. This investment again highlights Manhattan’s extraordinary appeal to leading institutional investors.”

The sale marks LaSalle Investment Management’s first investment in New York City. Meanwhile, for Plaza, it marks a “continued commitment to buying core properties in leading locations,” according to Quantum managing director Antoine Castro.

“It also underlines our strategy to acquire value-preserving premium assets,” Mr. Castro said. “We are enthusiastic about partnering with New York’s premier office landlord in this iconic property.”

Eastdil’s Douglas Harmon and Adam Spies were assigned to sell the stake on behalf of SL Green.

cgaines@observer.com