After struggling for more than a year to lure retailers to the shiny new shopping center known as 6x6 on Mid-Market Street, the developer is now seeking to convert space on two floors of the $150 million building to offices.

Cypress Equities, the Dallas developer of the five-story project at 945 Market St., has filed a preliminary application with the San Francisco Planning Department seeking permission to lease the majority of the space on floors three and four to office users. That would reduce the amount of retail in the project by about 20 percent from 264,000 to 216,000 square feet. The basement and floors one, two and five would remain retail, and there would still be two retail spaces on the office levels. At present, the building is unused except for the underground parking garage.

The shift toward office space reflects the harsh realities facing owners of shopping centers, evidenced by many Macy’s stores and other brick-and-mortar outlets closing across the country.

“Things are changing so fast — faster than I have ever seen at any point in my career,” said Chris Maguire, chief executive officer of Cypress Equities. “The reality is that a lot of the set of retailers that we were doing business with four or five years ago, or even one year ago, are struggling or no longer in business.”

But even while it hedges its bets by proposing 47,522 square feet of office space at the Mid-Market building, Maguire said he continues to believe 6x6 will eventually attract a strong crop of stores, restaurants and entertainment venues.

He said Cypress has preliminary leases signed with several large tenants, although he would not name them because the deals are not finalized. The leases include an international fashion retailer, new to San Francisco, that would take 36,000 square feet on the basement level, and an entertainment tenant that would take 25,000 square feet on the top floor. Another half dozen restaurants and retailers have also signed letters of intent to move into the building.

“The fundamentals of 6x6 are sound,” he said.

The empty building between Fifth and Sixth streets, with its dramatic glass curtain facade, is an important piece of Mayor Ed Lee’s long-standing effort to pump economic life into Mid-Market, which has suffered from blight and high vacancies for decades.

The 6x6 building — on the site of the old St. Francis Theater, which closed in 2000 — was a decade in the making and went through several developers before Cypress Equities started construction in the fall of 2014.

While Cypress originally hoped to have the building fully leased by the completion of construction a year ago, that plan faded as the rise of online shopping decimated physical retail. This year alone, two dozen major retailers have filed for bankruptcy protection, including Payless, Sports Authority, Quicksilver, Wet Seal, Radio Shack, the Limited and American Apparel.

Macy’s has said it will close 100 stores, while JC Penney — which came close to signing a lease at 6x6 several years ago — is shuttering 138 stores. Two San Francisco Macy’s — the men’s store on Stockton Street and the store at Stonestown Galleria — will also close.

Maguire said that his group is fortunate to have a “financial structure that allows us to be patient.”

“It’s allowed us to be very picky and very cautious,” he said. “And frankly, we are fortunate we didn’t do some of the deals we could have done, because we would have a failing center today if we had.”

City Planning Director John Rahaim said he has not yet seen the 6x6 application, but is not surprised by the move.

“I can totally understand why they are doing it — there are real challenges in renting retail space,” Rahaim said. “I’d want to look at the proposal and how it functions and look at it against the original approvals. We also have to recognize that the nature of retail is changing pretty dramatically.”

The application comes at a time when several San Francisco landlords are seeking to convert upper-floor retail space to office, including the owner of the Macy’s men’s store on Stockton Street and the Saks Fifth Avenue men’s store on Post Street. Up to now, city planners have advocated limiting conversions to floors four and above, said Rahaim, although that could change. Unlike 6x6, the other buildings have long operated as multifloor retail.

“Should we be looking at allowing office above second floor rather than the third? I don’t know,” he said.

Class A office space in the area is in demand, leasing for an average of $72 a square foot, a rate near the top of the market. As a new building, 6x6 could fetch rents of more than $80 a square foot, said Colin Yasukochi, research director for brokerage CBRE. And having office space on the third and fourth levels makes sense, he added, as retailers want pedestrian traffic and tech workers are looking for natural light and views.

“It’s a logical opportunity,” he said. “Retail is still a difficult market right now, especially on upper floors.”

Maguire said his hope is to cater to companies that have both a tech and a retail component, as the Westfield San Francisco Centre has done with Bespoke, a co-working and events space on the top floor that is home to more than 75 “retail tech start-ups and brands.”

“We have had demand from the office users — a lot of the tech guys have come at us,” Maguire said.

Matt Holmes, a principal with the brokerage Retail West, said the change Cypress is seeking isn’t a reflection of 6x6 having shortcomings, but a symptom of a “diminishing retail sector and oversupply of available spaces.”

“To me, it is waving the white flag as far as the retail not working quite as well as expected,” he said.

J.K. Dineen is a San Francisco Chronicle staff writer. Email: jdineen@sfchronicle.com

Twitter: @sfjkdineen