Allen & Overy (A&O) and Wachtell Lipton Rosen & Katz are among a line-up of firms advising on Walgreen's $6.7bn (£4.3bn) acquisition of a 45% stake in Alliance Boots, a tie-up that will create a global pharmacy giant with more than 11,000 stores in 12 countries.

Wachtell and A&O are both advising the US pharmacy chain, which plans to complete a full merger with Alliance Boots in three years for a further $9.5bn (£6bn), as well as the assumption of the retailer's then outstanding debt.

Amsterdam corporate partner Justin Steer led A&O's team, alongside competition partner Paul Glazener, while Watchell's team advising Walgreens – the largest healthcare retailer in the US – was headed up by partners Andy Brownstein and Ben Roth. Baer & Karrer acted as Swiss legal counsel to Walgreens.

Cleary Gottlieb Steen & Hamilton acted for Goldman Sachs, financial adviser to Walgreens, led by New York-based corporate partner Ethan Klingsberg, with support from senior lawyer Jennifer Bender.

Lazard, which is also serving as financial adviser to Walgreens, is being advised by Gibson Dunn & Crutcher partners Dennis Friedman and Eduardo Gallardo.

Meanwhile, Alliance Boots, which was bought by US private equity house Kohlberg Kravis Roberts & Co (KKR) for £9.7bn in 2007, is being advised by US firm Simpson Thacher & Bartlett, which is representing both KKR and Alliance Boots with a team led by corporate partner Mark Pflug.

France's Darrois Villey Maillot Brochier is also representing Alliance Boots, with a team including M&A partners Alain Maillot and Benjamin Burman and competition partners Didier Theophile and Igor Simic.

KKR's 2007 acquisition of Alliance Boots saw Clifford Chance instructed alongside the private equity house's regular adviser Simpson Thacher in what was seen as a coup for the magic circle firm.

Alliance Boots was established by a 2005 alliance between Boots Group and Alliance UniChem, forming a company with more than 3,330 retail stores in 11 countries.