With a $39 billion offer in hand, Alexion Pharmaceuticals Inc. is the target of what could be the third largest takeover to date of a pure play biotech company. At a 45% premium, the proposed transaction, which is expected to close in the third quarter of 2021, is largely applauded by analysts who point to three years of languishing stock prices despite the promise of the Boston-based company’s C5 complement franchise and a sturdy pipeline.