(Adds Braskem request, total amount of debt owed by the group, context)
SAO PAULO, June 17 (Reuters) - Brazilian conglomerate Odebrecht SA filed on Monday for bankruptcy protection, aiming to restructure 51 billion reais ($13 billion) of debt in what would be one of Latin America's largest-ever in-court debt restructurings.
The bankruptcy filing comes after years of struggles for Odebrecht, the biggest of the Brazilian engineering groups caught in a sweeping political corruption investigation that has rippled across Latin America.
In the filing, the company asks the judge to bar the group's seven largest creditors - six banks and an investment fund - from taking possession or selling shares in the group's crown jewel, its controlling stake in petrochemical company Braskem SA .
Shares in Braskem are pledged as collateral to the creditors. But Odebrecht says the Braskem stake is essential to its restructuring, as the petrochemical company was responsible for nearly 80% of the conglomerate's revenues in 2018.
Odebrecht said the bankruptcy protection was the best way to conclude its debt restructuring as creditors have sought to seize assets pledged as collateral for unpaid loans.
The debt restructuring relates to the parent company Odebrecht SA and a network of holding companies.
The group's main operating businesses are excluded, including Braskem, construction unit OEC, oil company Ocyan, shipbuilder Enseada, Odebrecht Transport and homebuilder Incorporadora OR. Sugar and ethanol subsidiary Atvos Agroindustrial Participacoes SA, which already filed for a separate bankruptcy protection, is also excluded.
Odebrecht said its total debt reaches 98.5 billion reais, including intercompany loans and debt that is not subjected to in-court restructuring.
FALL FROM GRACE
Odebrecht expanded from its construction roots into one of Brazil's biggest conglomerates but began its fall from grace in 2014, when it became a principal target of the country's largest-ever corruption probe.
Former chief executive Marcelo Odebrecht, grandson of the founder, was arrested in 2015 and later sentenced to 19 years in jail for corruption. He has been under house arrest since 2017, barred from having any say in the company's running.
In 2016, Odebrecht agreed to the world's largest-ever corruption leniency fine with prosecutors in Brazil, the United States, and Switzerland, paying at least $3.5 billion. The scandal over bribes for public works contracts spread to other countries where Odebrecht did business, including Peru, Mexico, Argentina and Colombia.
Since then, the group has been selling assets to raise cash as borrowing costs climbed. It sold Brazilian sanitation company Odebrecht Ambiental to Canada's Brookfield Asset Management for $800 million and Peruvian hydroelectric plant Chaglla to China Three Gorges Corporation for $1.4 billion.
Still, some of the group's businesses have been forced to restructure debts as their revenues dwindled.
Odebrecht's construction unit OEC is in talks to restructure $3 billion of debt with bondholders. The company proposed a 70% haircut, which was rejected by its creditors.
After a failed attempt to negotiate an out-of-court solution with its creditors, ethanol unit Atvos filed for bankruptcy protection in May.
Odebrecht had been negotiating a sale of Braskem to LyondellBasell Industries NV for a year and a half, but talks ended with no deal earlier this month.
The conglomerate's largest creditors are Brazilian state-owned lenders Banco do Brasil SA, Caixa Economica Federal and BNDES, as well as private-sector lenders Banco Bradesco SA, Itaú Unibanco Holding SA , Banco Santander Brasil SA and an investment fund, totaling 33 billion reais in debt.
The group is being advised by financial restructuring firm RK Partners and law firm E. Munhoz Advogados.
($1 = 3.8913 reais) (Reporting by Aluisio Alves, Carolina Mandl and Tatiana Bautzer Editing by Brad Haynes and Rosalba O'Brien)