Carl Ichan, who owns more than 13 percent of SandRidge Energy Inc., called the Oklahoma energy company's planned purchase of Bonanza Creek Energy Inc. "insane" and "ridiculous."

Last month, SandRidge Energy (NYSE: SD) said it was buying Denver-based Bonanza Creek Energy (NYSE: BCEI) for $746 million.

Icahn opposed the deal, so SandRidge earlier this week enacted a "poison pill" plan that would stop Icahn from obtaining additional shares of SandRidge and prevent Icahn from scuttling the deal.

On Thursday, Icahn fired back at SandRidge in a letter that said the "poison pill (plan) is a complete travesty and represents a new low in corporate governance. ... Such actions would make a totalitarian dictator blush, yet you take them with a straight face and misrepresent the intentions of the pill by describing it as a 'shareholder rights plan.' We are not aware of a more insulting euphemism."

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