The Harry and Jeanette Weinberg Foundation announced yesterday the largest single grant in its history -- $15 million toward the redevelopment of Baltimore's Sinai Hospital. The gift, believed to be one of the largest to an area hospital, will be given in five separate "challenge grants" aimed at leveraging an additional $45 million in donations from other sources. "Harry Weinberg took great pride in some of the institutions he watched grow up from very small beginnings," Bernard Siegel, president of the Weinberg Foundation, said at a media briefing ++ yesterday. "Sinai was one of those institutions he took an interest in." Mr. Weinberg, a real estate billionaire, started the foundation in 1959, but it existed in relative obscurity until his death in 1990. Enriched by cash infusions in the years before his death, the foundation became one of the top 25 nationally with assets of nearly $800 million. The money will be dispersed in five installments of $3 million each over a 15-year period. Each installment will require an additional $9 million in private giving to trigger the next payment. "It's a magnificent gift that is being bestowed upon Sinai by a foundation that is in the forefront of doing the right thing for the community," said Warren A. Green, Sinai president. Mr. Green said the unrestricted grant will support the hospital's ambitious redevelopment program, which will include a new emergency center, an inpatient facility with 160 to 215 beds and an outpatient wing. Last spring, construction began on the first phase of the project: an employee and physician resource center, which will house a 450-seat cafeteria, kosher restaurant, physicians' dining room and office suites. The project will modernize and transform the look of the hospital with a 1,000-foot-long addition that will rise six stories and curve around the existing hospital. Sinai sits between Belvedere Avenue and Northern Parkway in Northwest Baltimore. Construction is expected to take about seven years. All told, the redevelopment is expected to cost $120 million, with $60 million coming from private sources and the balance from bond issues, bank loans and cash spun off from operating profits.