Daiichi Sankyo Co. plans to purchase a controlling stake in Ranbaxy Laboratories Ltd. for approximately $4.6 billion, acquiring 50.1% of the company, according to a company statement. Ranbaxy’s chief executive officer, Malvinder Singh, will retain his current position.     The purchase would position Daiichi Sankyo third in the generic drug market behind Teva Pharmaceutical Industries Ltd. and Novartis AG’s Sandoz unit. The acquisition also gives the company more reach in emerging...