Ian Schrager and RFR Holding LLC, of which Aby Rosen and Michael Fuchs are partners, have entered an joint venture arrangement with SL Green Realty Corporation to convert the 700-foot-high office tower at One Madison Avenue to residential condominiums.

One of the world's greatest skyscrapers, it was the world's tallest building when it was erected in 1909. It retained that title for four years until the erection in 1913 of the Woolworth Building at 233 Broadway.

Designed by Pierre Le Brun of Napoleon Le Brun & Sons, it was modeled on the famous campanile in the Piazza San Marco in Venice, which is considerably shorter and has fewer windows.

An announcement by SL Green issued Friday indicated that SL Green will retain a 30 percent interest in the tower, and the arrangement provides Mr. Schrager and RFR, according to the announcement, with "the ability to increase their ownership interest if certain incentive return thresholds are achieved." "A timetable for the conversion and other project details will be announced at a later date," the announcement stated.

One Madison Avenue occupies the southeast corner at 24th Street and is part of a full-block property that SL Green acquired last year from MetLife for $918 million. The remainder of the block is a 14-story office building that SL Green, a real estate investment trust, is keeping. The announcement did not provide financial details of the arrangement with Mr. Schrager and RFR, nor whether it includes any unused air-rights from the low-rise portion of the block.

An article in The New York Post has indicated that Mr. Schrager and RFR may create about 100 condominium apartments in the tower.

The tower was originally clad in marble, which was replaced by limestone in 1964 in a renovation designed by Lloyd Morgan that stripped away much of the facade ornamentation. The tower, which is illuminated at night, has large clockfaces on each of its four sides.

"One Madison Avenue," Marc Holliday, president and chief executive officer of SL Green, said in the announcement, "is indicative of the type of opportunity that can be uncovered today in classic New York City office buildings. As soon as we were aware of its availability, we saw an opportunity to offer a world class address for both business and residential occupancy when we assumed ownership of The Clocktower as part of the acquisition. There is no one better than Ian Schrager at creating and defining cutting-edge lifestyle trends. He's proven that over and over again."

Mr. Schrager and Aby Rosen are developing 40 Bond Street, which will have a stunning green glass facade designed by Herzog & Meuron and Mr. Schrager is also involved in the development of 50 Gramercy Park North, and he developed the Paramount and Morgan's Hotels here and the Delano in Miami. He was a partner with Steve Rubell at Studio 54, the discotheque.

Mr. Rosen is developeing a mixed-use skyscraper at 610 Lexington Avenue at the rear of the Seagram Building on Park Avenue, of which he is an owner. He is also an owner of Lever House, also on Park Avenue.

The residential conversion of the clocktower building will be fourth major recent such conversion around Madison Square Park. The International Toy Center, comprising the buildings at 200 Fifth and 1107 Broadway, is underway as is the conversion of the large red-brick former Gift Building at 225 Fifth Avenue. The conversion and expansion of the small building on the northwest corner of Madison Avenue at 26th Street was completely recently.

While One Madison Avenue is the tallest and most prominent landmark on Madison Square Park, it is not the most famous. That honor is still held by the Flatiron Building on 23rd Street at the intersection of Fifth Avenue and Broadway.

Darcy Stacom of CB Richard Ellis was the exclusive agent for SL Green.