NEW YORK -- Two significant financial fraud trials that tell separate, unsavory tales about the ways and means of Manhattan's wealthy business elite are set to open in federal court here this week.

In both cases, the principal criminal defendant is a young New York professional accused by the government of betraying the trust of his well-to-do clients. One trial involves allegations of a massive, allegedly fraudulent tax shelter scheme, while the other concerns an alleged embezzlement and insider stock trading conspiracy.

The main defendant in the tax shelter case is familiar to some Washingtonians. He is Charles A. Atkins, a one-time associate of Edward A. Markowitz, the former tax shelter promoter and part owner of the Washington Capitals hockey team who pleaded guilty to fraud charges in 1985.

About a year before entering his guilty plea, Markowitz pledged to cooperate with federal investigators, and he is expected to be a prosecution witness at the Atkins trial here, although sources familiar with the case say Markowitz is not expected to be one of the government's most important witnesses against Atkins.

Markowitz still lives in the Washington area, according to his attorney, Austin Doyle. Markowitz's sentencing on criminal fraud charges has been indefinitely postponed, pending completion of his cooperation with the government. Doyle said he could not predict when Markowitz would be sentenced. "That's up to the court," he said.

Atkins, 32, is charged with promoting a fraudulent shelter scheme that generated more than $350 million in improper tax deductions for wealthy investors, who included such Manhattan financiers as CBS Inc. chief executive Laurence Tisch and Lazard Freres & Co. Chairman Michael David-Weill, as well as such Hollywood celebrities as Michael Landon, Sidney Poitier and Norman Lear.

None of the investors has been charged with any wrongdoing, but they have been required to pay back taxes, interest and, in some cases, penalties.

After arriving in New York at the age of 24, Atkins, the son of former Ashland Oil Co. Chairman Orin Atkins, rapidly accumulated a substantive personal fortune by selling tax shelters. Documents on file in related civil lawsuits brought by investors in his shelters show that at the height of his success, Atkins earned more than $5.5 million annually. His business holdings included an interest in a Los Angeles savings and loan association.

Atkins spent lavishly, according to former investors and others. His tax shelter business, The Securities Groups, leased offices in a luxury Park Avenue building in which Atkins had a partial ownership interest. And his Manhattan apartment was filled with expensive works of art.

Atkins' sizable empire began to unravel when federal investigators questioned the legitimacy of the tax shelters he sold. The shelters generated tax losses through trading in government securities. The government charges that many of the trades made by Atkins' firm were fictitious, designed solely to generate tax losses and interest expenses, which is illegal.

Atkins and his two codefendants, William S. Hack, 62, and Ernest M. Grunebaum, 52, deny the charges, saying they carried on a legitimate economic enterprise. To prevail at trial, the defendants will have to overcome expected testimony by four former employes of The Securities Groups who pleaded guilty to fraud charges last April, sources familiar with the case said.

David W.C. Clark, the defendant in the other financial fraud trial scheduled to open in Manhattan's federal courthouse on Monday, also will have to overcome expected prosecution testimony by former clients and business associates.

Clark's case is an offshoot of the R. Foster Winans scandal, in which the former Wall Street Journal reporter was convicted of leaking confidential information about his stock market columns to a stockbroker and others.

Winans and his coconspirators profited by anticipating the effect of his newspaper columns on the price of individual stocks. But not all of Winans' alleged confederates were charged when the former reporter's scheme was first exposed in 1984.

Earlier this year, Clark, a Manhattan attorney, was indicted by a federal grand jury for allegedly profiting from tips passed to him by Peter Brant, a Kidder, Peabody & Co. stockbroker who has admitted receiving improper leaks from Winans. Clark was also charged with embezzling money from clients in order to fund his illegal stock trading, evading taxes on his illegal income, and committing perjury.

Last July, a federal judge postponed Clark's trial indefinitely pending the outcome of an appeal to the U.S. Supreme Court by Winans. Attorneys for Winans have urged the high court to strike down the prevailing legal theory that underlies the crime of insider trading. (Oral arguments are scheduled before the Supreme Court this week, but a decision on the appeal is not expected until early next year.)

The judge in Clark's case ruled that since it was possible Winans would prevail in the U.S. Supreme Court, it would be "unfair to the defendant to make him undergo the ordeal of a trial under such circumstances."

Earlier this month, however, the Manhattan U.S. attorney moved to dismiss the insider trading charges against Clark, choosing to move quickly to trial on 17 remaining embezzlement, perjury and related counts. The same federal judge who had earlier granted the postponement promptly scheduled Clark's case for trial.

Clark, partner in a small law firm known for its social connections in fashionable Manhattan neighborhoods and in the elite summer beach communities of the Hamptons on Long Island, is accused of embezzling funds from the trust account of an actor, Roger Wilson.

"Wilson alleges," according to a federal prosecutor's affidavit filed in a related civil case, "that Clark and Brant looted Wilson's brokerage accounts by means of Clark's power of attorney over Wilson's financial affairs. Documents produced ... show that approximately $2,280,000 was removed from Wilson's account."

Clark is described by acquaintances in the Manhattan legal world as a charming, socially ambitious attorney who belonged to some of New York's most exclusive mens' clubs and used the personal connections established by his older partners to build his legal practice. Clark will be defended at his trial by the renowned criminal attorney, F. Lee Bailey.