Deripaska and the biggest firms in his empire, including Rusal and holding company En+, were included in April's U.S. Treasury Department sanctions blacklist.

This was designed to target allies of President Vladimir Putin and punish Moscow for alleged meddling in the 2016 U.S. presidential election and other so-called malign activities.

However after tumult in the aluminium markets and lobbying from stakeholders, the U.S. Treasury offered En+ and Rusal a way off the list if the Russian billionaire reduces his stake in London-listed En+ to below 50 percent, from 66 percent, and both companies appoint independent directors.

En+ and Rothschild both declined to comment on Monday.

The appointment of financial advisory firm Rothschild marks a step forward days after the Treasury extended the deadline for investors to divest holdings in companies covered by U.S. sanctions, including En+ and Rusal, buying En+ Chairman Greg Barker time to present concrete proposals.

Last week Reuters reported En+ is hoping to present a plan to the Treasury detailing how the stake will be reduced and who the new directors could be in the summer. The proposal will be contingent on sanctions being lifted.

In his quest to save En+ from sanctions, Barker has negotiated Derispaska's resignation from the board. The company's president Maxim Sokov also quit on Monday, hoping his resignation will help build the case for scrapping sanctions.

U.S. banks worked on the En+ initial public offering in November, but since the sanctions, sources have said some of them have taken a step back for fear of repercussions.

(Reporting by Dasha Afanasieva and Katya Golubkova; Editing by Alexander Smith)