A major share of the National Place property is being sold to a syndicate of private investors for $60 million, as the financing partner of the group that developed the mixed-use project between 13th and 14th streets on Pennsylvania Avenue NW moves to liquidate its interest in the complex.

Aetna Casualty and Surety Co., which owned two-thirds of the recently opened hotel, retail and office project, is selling its interest to Twelve AMH Associates Limited Partnership of Boston. Twelve AMH is raising the cash by selling 600 limited partnership shares for $96,250 each.

The Marriott Corp. and NP Limited Partnership, the other two members of thegroup -- known as Square 254 Limited Partnership -- that developed the complex and will continue to operate it, will retain their ownership interests of one-sixth each. Quadrangle Development Corp. is the primary partner of NP Limited Partnership.

The syndication was offered privately by Winthrop Securities Co. Inc. of Boston to "sophisticated investors," those with the experience to understand the project and enough income and net worth to be able to support possible future calls for additional capital. The general partners of Twelve AMH Associates are a limited partnership made up of several principal officers of First Winthrop Corp., a subsidiary of Winthrop Securities, and Two Winthrop Properties Inc., a wholly owned subsidiary of First Winthrop.

Winthrop officials refused to comment on the sale, saying they didn't want to jeopardize the project before closing, scheduled to take place yesterday. Despite the current glut of office space east of 15th Street, sources aware of the sale said that the project sold out in one week. Winthrop will make more than $9 million on the sale.

The property at National Place includes a 99-year lease from the Pennsylvania Avenue Development Corp. for use of roughly two-thirds of the land of the 1300 block north of Pennsylvania Avenue, a 99-year lease from the PADC for the National Theatre, the 71,000-square-foot retail mall known as The Shops at National Place, 418,000 square feet of office space, the 774-room J. W. Marriott Hotel, and a 400-space underground parking garage.

The Rouse Co., famous for retail developments in Boston, Baltimore and elsewhere, holds a 30-year contract to operate The Shops, and Quadrangle Development has contracted to operate the office space. The nonprofit New National Theatre Corp. has a 35-year contract to lease and operate the theater. Square 254 Limited Partnership collects rents from Rouse and Quadrangle as well as 80 percent of the proceeds from the Marriott. Aetna will continue to hold a $130 million mortgage on the property.

The prospectus for the offering forecast that for an investor in the 50 percent tax bracket, ownership of one share in the partnership would yield $132,528 in tax benefits and $115,585 in cash over the 15-year life of the partnership, assuming an annual growth rate of 6 percent. If the growth rate is 10 percent, one share would yield $85,677 in tax benefits and $214,852 in cash.

The prospectus also forecast that, under three different sets of assumptions, the future sale of the project could net investors $91,695 per unit, $164,324 or $266,588. However, a financial analyst familiar with the Washington market said he believed the highest projection was based on assumptions that, for land along Pennsylvania Avenue, were "too optimistic."

Also, the prospectus was drawn up June 11, and its calculations are based on tax law as of that date. Since then, passage of the deficit reduction package has altered some provisions of the tax code to make them less favorable to ventures such as this one. The prospectus mentions this possibility but makes no detailed calculations of the changes' impact.

Income for the partnership is projected at increasing from roughly $16 million estimated for 1985, when the project comes up to speed, to roughly $40 million by 1998. Those projections are based on an estimated 6 percent annual growth in rental rates and revenue, although the prospectus also says the project may suffer from competition from other office and retail developments in the area. The average room price at the Marriott, which is $94 a night today, is expected to swell to $233 over the next 15 years.

The project's office space is currently 38 percent leased, according to the prospectus, at an average annual rent of about $23 a square foot. The prospectus' financial forecasts assume that the space will be 60 percent leased by the end of this year and 90 percent leased by the end of 1985. It also notes that it assumes "all leases signed in 1984 will include six months' free rent."