A deal to take Walgreens Boots Alliance Inc. private would be a “stretch” at a time when investors have become more discriminating, said Stephen Schwarzman, head of one of the world’s biggest buyout firms. “It might be possible. It’s a huge stretch doing things over $50 billion,” the Blackstone Group Inc.’s chief executive officer said Wednesday at a Reuters Newsmaker event in New York. His firm has led some of the largest leveraged buyouts ever, including the Hilton hotel chain and Equity Office Properties Trust.