Mr Loeb demanded the change in leadership in an open letter to Mr Ruprecht in which he criticises him for not taking the business forward and failing to compete properly with arch-rival Christie's and failing in his policy to focus on the top end of the market.
Praising Mr Ruprecht for his handling of Sotheby's during the financial crisis of the past few years, Mr Loeb is reported as saying that the company now faced a "crisis of leadership".
In response, an official company statement said that results in recent times had been "superior", including "a share price increase exceeding the Standard & Poor's Midcap index over the one, five and ten-year periods" and declined to comment further at this stage on what it called the "incendiary and baseless comments".
Mr Loeb's letter comes a week after Sotheby's announced that they would be replacing their chief financial officer of 17 years, William S. Sheridan, and a matter of weeks after he and other "investor activists" acquired a 5.7% stake in the company (now reportedly raised to 9.3%) with the expressed intention of mounting talks with the board about improving shareholder value.
Mr Loeb, who is also reported to be seeking a seat on Sotheby's board, is known for sending similar letters to chief executives of firms in which he has invested - there is even a website dedicated to this correspondence http://danloebletters.blogspot.co.uk/