* Buechner's takeover defense alienated some investors
* Vanlancker has to deliver on performance promises
* As CEO, analysts say Buechner transformed the business (Updates with details on Buechner, analyst comment)
AMSTERDAM, July 19 (Reuters) - The chief executive of Dutch paintmaker Akzo Nobel, Ton Buechner, has stepped down for health reasons two months after fending off a takeover bid, and will be replaced by chemicals division chief Thierry Vanlancker, the firm said.
Buechner, 52, had repelled a 26.3 billion euro ($30.35 billion) takeover attempt by U.S. rival PPG Industries, alienating a chunk of the Dutch firm's shareholders who felt combining the companies made strategic sense.
Vanlancker, a Belgian who joined the firm in 2016, will now have to deliver on the promises made by Buechner and Chairman Anthony Burgmans in their struggle to avoid a merger, including stronger sales growth and margin improvements.
Analysts have said some promises will be tough to achieve.
In his resignation statement, Buechner said: "For me this was an extraordinarily difficult decision to make but my focus must now be on my health."
Burgmans declined to elaborate on details of Buechner's illness, saying he regretted "only that he has decided to step down."
Buechner "felt that if he continued to subject himself to the pressures of his office, that would endanger his health," Burgmans told reporters on a conference call.
"What caused it, I cannot tell, I'm not a doctor."
Buechner told Akzo's board about his decision on Tuesday, Burgmans said.
Vanlancker was previously identified as the person who would take Buechner's job as part of emergency contingency planning.
In his current post, Vanlancker was expected to oversee the sale or initial public offering (IPO) of Akzo Nobel's specialty chemicals division. Vanlancker previously worked for Chemours, which was spun off from DuPont in 2015.
Buechner's surprise announcement means his tenure at Akzo Nobel has ended almost as it began.
In September 2012, a half year after he took the top job, Buechner stepped down on his doctor's advice, suffering from what the company described as "over-tiredness." After several delays, he resumed work in December the same year.
Buechner's tenure was mostly regarded as successful, as he made a series of strategic and operational changes to improve the company's financial health.
"Buechner has been responsible for overseeing the successful transformation of the business, including portfolio reshaping ..., de-risking of the pension funds, cutting costs and driving efficiency gains that have raised both margins and returns," Morgan Stanley analyst wrote in a note.
But the firm faced a minor shareholder rebellion after Buechner and Burgmans rejected talks with PPG, leading to a shareholder lawsuit still in progress seeking to have Burgmans ousted.
Faced with opposition from Akzo's board and Dutch politicians, PPG walked away from its bid for Akzo on June 1. Under Dutch market rules, PPG cannot approach Akzo again during a six month cooldown period.
Akzo reports second quarter earnings July 25.
($1 = 0.8666 euros) (Reporting by Toby Sterling; Editing by Himani Sarkar and Amrutha Gayathri)