HNA now holds a 9.92% stake -- worth €3.4 billion ($3.7 billion) at Tuesday's closing price -- through C-Quadrat Investment, a European asset manager. It has built its holding from an initial stake of just over 3% in February.

The investment is one of a series of big money moves around the world by HNA, which has interests in a wide-range of industries, including airlines, hotels and technology.

Last year was its most acquisitive yet, including a $6.5 billion deal for a 25% stake in Hilton Worldwide.
It's also racked up a lot of debt in the process. HNA and its subsidiaries owed $13.3 billion as of April 12, according to Dealogic.

Beijing's recent moves to stem the flood of money pouring out of China has led to tougher policing of overseas deals by Chinese companies this year.
Despite this, HNA has remained a busy buyer of foreign businesses, signing more than 10 deals worth over $5 billion in the first three months of this year, most of them overseas.
It has done this partly by using its existing operations abroad to raise money for deals, a route some Chinese companies are taking in order to skirt the country's capital control rules.

Deutsche Bank isn't its first move into the finance sector
HNA already holds stakes in asset managers, banks, insurance companies and a hedge fund. Its financial-services arm, HNA Capital, claims to have more than 340 billion yuan ($49 billion) in assets.
Earlier this year, it reached a deal to buy Skybridge Capital, a U.S. hedge fund business, from Trump campaign fundraiser Anthony Scaramucci.

Chen Feng, a former provincial bureaucrat, set up Hainan Airlines with three other people in 1992 with a loan from the local government. That's the business out of which HNA grew to become a huge conglomerate.
Now HNA's chairman, Chen is also a Buddhist scholar and a big advocate of traditional Chinese culture. The 63-year-old wants to help the country become more respected globally.
His company became the subject of a Harvard Business School case study in 2015, which details his habit of visiting each business HNA buys and teaching its staff about the company's "Ten Commandments," which include compassion, integrity and modesty.