IN order to build its own research facility, Mt. Sinai Hospital is selling a pre-war apartment building at 1212 Fifth Ave., along with the rights to construct a 315,000-foot building next to it between 102nd and 103rd streets that would overlook Fifth and Central Park.

Sources say around five potential bidders have emerged from an original group of 20 that expressed interest, including Related Cos. and Gary Barnett, with final bids amounting to more than $200 million expected on April 19.

Mt. Sinai’s investment advisers at Newmark Knight Frank refused to comment on the pending deal.

Sales around the city by universities, hospitals and museums to raise capital for expansion or to pay off debt have been stymied recently by a new tax abatement program that has encouraged developers to build in undesirable areas.

Mt. Sinai’s deal should squeak through, however, because along with the rental building, the new tower could market itself with a Fifth Ave. address and many units will have views.

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A number of new jewelry leases have been signed along Madison Avenue. According to retail queen Faith Hope Consolo, chairman of Prudential Douglas Elliman Retail, rents are therefore edging upward to $1,350 a foot and beyond.

“Soon they could reach Fifth Avenue levels of $1,500,” Consolo predicted.

Diamond specialists Graff just leased the entire six-story building at 710 Madison Ave., which is owned by a Boston-based company. The store on the northwest corner of 63rd Street is where Coach previously subleased to Givenchy before it closed a couple of weeks ago. Graff still has a small shop up the avenue at No. 721.

Another diamond dealer, Kwiat, leased its very first store at the Winter Organization’s 725 Madison Ave., where Chopard will close, as it is moving to the larger corner at 63rd Street, where Timberland used to be.

Meanwhile, Judith Ripka decided to renew her original store lease at 673 Madison Ave., even though she just opened a much larger store at No. 777.

And to top it off, Bulgari is expanding from two stories to four at its posh 783 Madison location.

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Taconic Partners, which coincidently owns 111 Eighth Ave., has teamed up with ING Clarion Partners to buy the 1,416 low-rise-apartment complex Eastchester Heights in the Bronx for $133 million.

Aaron Junreis of GFI Realty Services sold it to sellers Urban American and the City Investment Fund just two years ago for $97 million and has now helped them unload it in this off-market transaction.

Taconic recently listed its premier office tower at 450 Park Ave. through investment broker Douglas Harmon at Eastdil and hopes for sale of $500 million for the hedge fund-filled skyscraper at 57th Street.

Meanwhile, Morris Moinian just picked up the 85,000-foot, 12-story commercial loft building at 129 W. 29th St. for $26 million, or $302 a foot, through Shawn Sadaghati and Aaron Jungreis of GFI Realty Services.

Moinian says he will upgrade the building to a boutique office building but a decade down the road, may turn it into a boutique hotel.

“There are five hotels being built within a few blocks, including my Indigo,” said Moinian of his trendy new InterContinental franchise.

Nearby, Magnum Real Estate Group paid $50 million to acquire a long-term ground lease for the 100,000 foot parking lot at 140 West 28th St.

Partners Benjamin Shaoul and Marc Ravner plan to build their own 30-story boutique hotel of 200 and 300 rooms.

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Glam publisher Louise T. MacBain knows that in order to stay rich, you have to find a bargain.

Her LTB Media just inked a deal for 15,800 feet on part of the fourth floor at the Starrett-Lehigh Building at 601 W. 26th St.

Starrett-Lehigh, which is home to the offices of Martha Stewart and Tommy Hilfiger, has among the city’s most reasonable asking rents. CoStar Realty Corp. reports rents there are in the mid-$20s to mid-$30s a foot, depending on the size and configuration.

LTB, which publishes Art & Auction, Gallery Guide and Modern Painters, among others, is coming out of 111 Eighth Ave., which commanded double the rent of Starrett-Lehigh.

We also hear McBain hired architect Richard Meier to design the currently raw space.

The 81/2-year lease was made through Mark Lauzon of Cushman & Wakefield, who negotiated with owner’s rep Peter Thorsen.

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