A real estate investment company says in a $750 million lawsuit that it lost a chance to buy the General Motors Building in the city because of a bid-rigging conspiracy by a group controlled by billionaire investor George Soros.

Leslie Dick Worldwide Ltd. says in court papers the 50-story building at Fifth Avenue and 59th Street in Manhattan was sold to Macklowe Properties Inc.in a “sham”auction for $1.4 billion in September 2003 after the seller ignored Dick’s $1.5 billion offer.

The building was sold by Conseco Inc., which owned a majority interest, to raise cash after it declared bankruptcy in December 2002. Conseco, of Carmel, Ind., told the court that sale of the GM building “presents a unique opportunity to provide capital.”

After the auction, court papers say, Conseco accepted Macklowe’s $1.4 billion bid although “plaintiffs’ bid was superior to Macklowe’s bid in every material respect.”

Dick’s amended complaint, filed three weeks ago in Manhattan’s state Supreme Court, says Mr. Soros gave Macklowe $350 million, including the $50 million deposit Macklowe made, “essentially making Soros the real purchaser of the building.”

“The entire bidding and contract award process engaged in by the defendants was improper, unlawful and permeated with fraud,”Dick’s court papers say. “Plaintiffs have suffered financial losses and damages caused by defendants’ illegal acquisition and current illegal ownership of the GM building.”

Dick, whose original complaint was filed in April,asked the court to declare the bidding process “a fraud and a sham” and to void the sale, to impose a constructive trust to take control of the building until this case is over and to award Dick $750 million.

A Conseco spokesman, James Rosensteele, said yesterday, “We believe the suit is without merit, and we will defend it vigorously.”

A spokesman for Mr. Soros, Michael Vachon, said “the case is entirely without merit.”

Macklowe Properties spokesman Howard Rubenstein said his client called the lawsuit “absurd” and said it was “totally devoid of any merit.”

The building once was partly owned by developer Donald Trump, who bought it with Conseco in 1998 for $800 million. Trump’s name was spelled out across the front of the building in huge gold-colored letters.

Another losing bidder, Solow Building Corp., filed suit in Delaware in 2003 in an attempt to stop the building’s sale. Solow later withdrew the lawsuit.

A spokesman for Solow, Michael Gross, said lawyers for the company are “considering their options” in light of Dick’s amended complaint.