July 10 (Reuters) - UBS cannot comply with a U.S. request to disclose the identity of 52,000 U.S. secret account holders, the bank's chief executive Oswald Gruebel said in an internal memorandum, according to a source familiar with the situation. [ID:nLA315492]

Following is a timeline of recent major events at UBS:

April 1, 2008 -- Doubles its writedowns, dumps its chairman, Marcel Ospel, and seeks more emergency capital. It proposes its lawyer, Peter Kurer, as Ospel's successor.

May 6 -- Says it will axe 5,500 jobs and sell billions of dollars of ailing assets to weather the subprime crisis.

June 19 -- A former UBS banker who once smuggled a client's diamonds into the U.S. in a toothpaste tube pleads guilty to helping a billionaire hide $200 million in assets from U.S. tax authorities, part of a broader tax evasion probe of UBS.

Oct. 16 -- Announces it is to get 6 billion francs from the Swiss government for a 9.3 percent stake and is to unload $60 billion of toxic assets into a new central bank fund.

Nov. 12 -- Raoul Weil, head of UBS AG's wealth management business, is charged with conspiring to help thousands of wealthy Americans hide $20 billion of assets from U.S. tax authorities in Swiss bank accounts.

Feb. 10, 2009 - Posts a 2008 loss of 19.7 billion francs, the biggest ever loss for a Swiss company. Cuts 2,000 more jobs.