Topping our latest roundup of deals worth $1 billion or more is UnitedHealth Group's acquisition of Schaumburg, Illinois-based Catamaran for $12.8 billion. The acquired pharmacy benefits manager will be combined with UnitedHealth's OptumRx.

Advising UnitedHealth are Sullivan & Cromwell and Canadian firm Blake, Cassels & Graydon, which recently placed second on Acritas' 2015 Canadian law firm brand index. Squire Patton Boggs also advised UnitedHealth on antitrust matters. Leading the charge for Catamaran are Sidley Austin and Baker & McKenzie.

The deal follows Rite Aid's announcement in February that it would purchase pharmacy benefits manager Envision Pharmaceutical Services for $2 billion in an effort to more closely compete with CVS Health Corp. following its $21 billion acquisition of Caremark Rx Inc. in 2006.