She will now pursue opportunities as a producer in the US and UK.
Tartan says the $6m convertible loan has been agreed with a private investor group.
As part of the restructuring, the company's separate PR and marketing departments will be merged. Mylene Bradfield will head Group Marketing, and Sarah Bemand and John Dunning will head Group Publicity.
The home entertainment division, headed by Alan Partington and Sam Dunn, will move from a separate office into Tartan's Dean Street headquarters in London. Partington will be part of the management team alongside founder Hamish McAlpine and Carole Siller.
Tartan has brought in Richard Brooman and Lisa Halkett from myFD as Group Financial Director and Group Financial Controller, respectively.
Tartan chairman Hamish McAlpine said of the changes: 'As we are now moving into a new digital age where films will become a virtual rather than a physical reality, it is important that Tartan also adapts and changes with these new exciting times. Many new opportunities are going to present themselves and we need to be able to fully take advantage of them.
A new COO will be announced after AFM.
De Casto, who has been with Tartan since 1999, said: 'I have thoroughly enjoyed my time at Tartan and am delighted to have made such successes of a wide range of titles including Secretary, Oldboy, Super Size Me, and Black Book, having worked with so many extraordinary, visionary filmmakers. Having fulfilled my ambitions in distribution, I now look forward to exploring other opportunities in production.'
McAlpine said of De Casto: 'We are hoping to continue our close friendship and relationship and hope that Tartan will be the first port of call for her projects.'
Tartan's forthcoming releases include Paranoid Park, Silent Light, Chaotic Ana, Funny Games, and P2.