LONDON — Alliance Boots GmbH, the pharmaceutical retailer and wholesaler partly owned by Walgreen Co., has taken its first step into Latin America with a deal to acquire 1,400 drug stores in Mexico and Chile.

Boots agreed to purchase the retail pharmacy business of Grupo Casa Saba S.A.B., one of Mexico’s largest pharmaceutical distributors, in a deal valuing the retail unit at $638 million.

Grupo Casa Saba owns a controlling stake in Farmacias Ahumada S.A., a Chilean company also known as FASA, that owns the third-largest drug store chain in Mexico (Farmacias Benavides), with 1,000 outlets. It also operates 400 drug stores in Chile under its own banner.

Boots is Europe’s biggest drug wholesaler, serving more than 170,000 pharmacies, hospitals and clinics in 20 countries in Europe, Asia and the Middle East. It also has 3,300 retail pharmacies, mostly in Europe but also in Asia.

“We are confident in the high potential for sustainable growth and value generation that will be unlocked by this acquisition, including the opportunity for consumers in Mexico and Chile to access, for the first time, leading Boots product brands,” says Stefano Pessina, executive chairman of Boots.

Pessina merged his pharmaceutical and medical products distributor, Alliance UniChem, with British pharmacy operator Boots Group in 2006. The following year he teamed with New York private equity firm Kohlberg Kravis Roberts & Co. to take Boots private in what was Europe’s largest private equity deal to date.

Walgreens last summer paid $6.7 billion for a 45% interest in Boots, with an option to take over the whole company for another $9.5 billion by 2016.

Pessina signaled his commitment to the plan by opting to be paid in shares, acquiring an 8% stake in Walgreens and taking a seat on its board. His stake would increase to 16% if Walgreens buys the rest of Boots.

Boots’ move into Latin America was endorsed by a Walgreens spokesman. “[The] announcement is another step toward the vision Walgreens and Alliance Boots outlined together in June 2012, when we launched our strategic partnership to establish the first global, pharmacy-led health and well-being enterprise,” says Michael Polzin, Walgreens vice president of corporate ­communications.

The two pharmacy giants operate more than 11,000 drug stores in the United States, Europe and Asia under the Walgreens and Boots banners.

Walgreens gets most of its revenue from filling prescriptions through its 8,600 pharmacies in all 50 states, the District of Columbia and Puerto Rico.