Health Care REIT (HCN), the nation’s third-largest health care real estate investment trust, has agreed to acquire rival Sunrise Senior Living for nearly $845 million, yielding advisory roles for Arnold & Porter, Sidley Austin, and Wachtell, Lipton, Rosen & Katz.

The deal for Sunrise, which owns and operates more than 300 assisted living communities throughout North America and the United Kingdom, will help Toledo-based senior housing provider HCN expand its services as the baby boomer generation slides into retirement, Reuters reports. HCN’s portfolio already includes more than 900 properties it leases to senior living operators in Canada and 45 U.S. states.

According to a press release issued by the companies announcing the deal, the purchase price for Sunrise “reflects a real estate value of approximately $1.9 billion, of which approximately $950 million will be paid in cash and the balance through the assumption of debt.”

The transaction, which is expected to close in the first half of 2013, will see Health Care REIT pick up Sunrise’s 20 senior housing and retirement centers and its interest in joint ventures that own another 105 such facilities. SEC filings by HCN show that Toledo-based Shumaker, Loop & Kendrick, which has long served as primary outside general counsel to HCN through corporate partner Mary Ellen Pisanelli, has taken the lead on its client’s acquisition of Sunrise, along with Sidley Austin and Arnold & Porter.

Joseph Howe III, head of the tax practice at Arnold & Porter, and tax partner Cynthia Mann are leading a Washington, D.C.–based team from the firm serving as special REIT tax counsel to HCN on the structuring of the proposed transaction. Sidley corporate partners David Zampa and Matthew McQueen in Chicago are also working on the matter.

All three firms also advised publicly traded HCN last year on its $2.4 billion acquisition of 147 properties from Genesis HealthCare, according to our previous reports, as well as its $877 million stock-for-stock merger with Windrose Medical Properties Trust in 2006.

Jeffery Miller, a former partner at Shumaker, serves as general counsel for Health Care REIT, whose senior vice president, Daniel Loftus, has previously been of counsel to Bone McAllester Norton and Wyatt, Tarrant & Combs. Former Greenebaum Doll & McDonald of counsel William Ballard Jr., serves as an independent member of HCN’s board of directors, along with Daniel Decker, a former lawyer at Texas firm Winstead and founder of Dallas-based Munsch Hardt Kopf & Harr.

Wachtell M&A partner Adam Emmerich is leading a team from the firm serving as outside counsel to Sunrise on its sale to HCN. Also working on the matter for the firm are corporate partner David Lam, tax partner T. Eiko Stange, antitrust partner Joseph Larson, and restructuring and finance partner Scott Charles.

Wachtell previously advised Chicago-based Ventas—the largest owner of senior living centers in the United States—on its entry into the Canadian senior housing market in 2007 through its nearly $2 billion acquisition of properties owned by a Toronto-based predecessor of Sunrise. (Last year Emmerich’s Wachtell colleague, real estate and REIT M&A practice cohead Robin Panovka, was named an Am Law Daily Dealmaker of the Week for his role advising Ventas on its $7.4 billion all-stock merger with Nationwide Health Properties.)

Arnold & Porter and Canadian firm Stikeman Elliott took the lead advising Sunrise on the sale of its Canadian assets to Ventas five years ago. The deal also spawned a nasty litigation battle between Ventas and senior living and nursing home REIT rival HCP over the purchase price paid for control of select Sunrise assisted living facilities. (Jenner & Block prevailed for Ventas after HCP last year tried to appeal a $101 million verdict against it in that dispute, according to a report by The Am Law Litigation Daily.)

Earlier this year, McLean, Virginia–based Sunrise and certain affiliates sold 16 senior living communities to Ventas to raise $362 million in cash. David Haddock, a former lawyer at Baker Botts and Hogan & Hartson, has served as general counsel and corporate secretary for Sunrise since 2005.

Wachtell’s Emmerich says that his firm began advising Sunrise in 2010 after Gibson, Dunn & Crutcher guided the company through a settlement with the SEC over an investigation into its finances. The company, founded in Northern Virginia in 1981 by former CEO Paul Klaassen and his wife Terry, had been on the brink of bankruptcy only a year earlier. Emmerich says the proposed sale of Sunrise to HCN represents a remarkable turnaround for his client.