A group of real estate investors is selling a portfolio of six buildings in Boston’s Seaport District, hoping to cash in on rapidly rising rents in the neighborhood.

National Development and Angelo Gordon & Co. are selling 407,000 square feet of office space in turn-of-the-century buildings on Farnsworth, Congress, and Summer streets. The space is about 96 percent occupied by a range of companies, from the software company Bullhorn to Isaacson Miller, an executive search firm.