Walgreen said on Wednesday it would exercise an option to buy the 55 percent it does not already own of Alliance Boots for 3.13 billion pounds ($5.27 billion) in cash and 144.3 million Walgreen shares, after first taking a 45 percent stake in 2012. It added the combined company, with more than 11,000 stores in 10 countries, would keep its tax domicile in the United States, with headquarters in the Chicago area. Walgreen’s retreat is the third major possible tax “inversion” deal to collapse in recent months amid heightened political sensitivity in the United States to such transactions.