In what is being called the largest single-building life science transaction in the U.S. so far this year, the development team of Alexandria Real Estate Equities and National Development transferred a partial interest in the 12-story life science complex under construction at 15 Necco St. in Boston’s Seaport Innovation District set to house drugmaker Eli Lilly’s $700 million Lilly Institute for Genetic Medicine. The partial interest sale to Tokyo-based Mori Trust Co., Ltd.’s U.S. subsidiary, MORI America LLC., will recapitalize the development.