New York developer Brickman Associates is suing International Paper over a plan to sell a Connecticut estate to conservationists, saying the pact breached their own $15 million purchase agreement.

The lawsuit was filed on October 23 in Connecticut Superior Court in Stamford by Fifth Avenue-based Brickman Associates.

A Brickman executive signed a letter of intent at the end of August to purchase the Connecticut conference center on 115 acres of land that straddles Greenwich and Stamford and abuts the Mianus River Park.

Known as “Treetops,” the 33-room mansion and estate was the former home of actress and torch singer Libby Holman. Other well-known figures such as Elizabeth Taylor and Tennessee Williams were among those entertained at the home. But Holman became known as the “Angel of Death'' after her first two husbands died young. In 1971, her third husband found her dead of carbon monoxide poisoning in the front seat of a Rolls Royce in the estate garage.

Brickman intended to develop luxury homes on the grounds. Although an International Paper executive countersigned the 30-day letter of intent on September 1, the lawsuit claims he and other International Paper officials intentionally sloughed off attempts by Brickman to wire its $250,000 deposit, and did not complete negotiations in good faith.

Under terms of the letter of intent, International Paper was to notify Brickman of any “bona fide” offers made by conservationists so that Brickman could match these offers.

Instead, on October 17, International Paper announced a sales agreement with state and local officials.

The agreement enabled International Paper to keep the conference center and 15 acres, and sell 95 acres to the State of Connecticut through the Trust for Public Land for $11.5 million. Ten acres surrounding the mansion would also be restricted from development by a conservation easement.

The Brickman lawsuit alleges that despite their agreement, International Paper continued to negotiate with the non-profit and government officials, and ultimately decided not to sell the property to Brickman. It charges the paper company “manipulated the timing” of the offer until after October 1, 2000 to “deprive Brickman of its rights.”

A spokesperson for the Trust said they had been negotiating to buy Treetops for more than a year, first with Champion International, and then with International Paper, after that company purchased Champion.

She told The Post that negotiations continued during the summer, including the months of August and September. “There were other potential buyers interested in the property so it was a difficult negotiation,” she said.

The State of Connecticut has already committed $3.5 million for the purchase while the Trust and municipal officials have until January 31, 2001 to fund the rest.

A spokesperson for International Paper declined to comment on the pending legal action but said they have not yet decided if they will sell the conference center itself.