New York (CNN) The fiancée of a beloved New Jersey high school principal who died earlier this year while attempting to donate bone marrow to a child has filed a wrongful death lawsuit alleging that doctors treating him were negligent.

Derrick Nelson was described as a man of dignity and courage by his fiancée Sheronda Braker, who is the mother of his 5-year-old daughter.

Braker filed a lawsuit in Union County Superior Court on Monday for malpractice involved in the treatment of Nelson.

The defendants are Jerry Baratta, Hackensack University Medical Center, Hackensack Anesthesiology Associates of New Jersey Healthcare Specialists, The John Theurer Cancer Center and unnamed doctors and nurses overseeing Nelson's care.

Braker is demanding a jury trial and unspecified compensation.

"I'm seeking justice for the untimely death of Derrick," Braker told CNN.

Nelson underwent the procedure to donate bone marrow at Hackensack University Medical Center in New Jersey on February 18 to help an anonymous 14-year-old boy suffering from cancer in France.

For months, Nelson had been in touch with Be the Match -- a foundation that manages a global marrow registry to help those who suffer from blood cancers, said Braker. They informed him that he was a match for the boy.

Nelson, who was the principal of Westfield High School, told the school newspaper before the surgery, "If it's just a little bit of pain for a little bit of time that can give someone years of joy, it's all worth it."

The medical team kept Nelson under anesthesia despite his low oxygen level, according to Braker's complaint.

The complaint alleges the medical team then failed to supply him with additional oxygen, at which point Nelson's heart rate slowed down. This was a late sign of hypoxemia, meaning he had low levels of oxygen in his blood, the complaint says.

Finally realizing Nelson's heart rate had slowed, the medical team attempted to provide ventilation through a mask airway -- but once again failed to provide sufficient care, according to the complaint.

This led Nelson to suffer permanent brain injury, the complaint says. He fell into a comatose state and remained on life support until the 44-year-old man died in April, said Braker's attorney David Mazie.

"This is not a situation where we want people to be worried about and not to proceed with donating bone marrow," Mazie told CNN. "The case is about an anesthesiologist who didn't do what he was supposed to do. This doctor didn't do his job."

Nelson was overweight and had sleep apnea, two factors that put him at a higher risk for undergoing anesthesia, according to the complaint.

One of the defendants, anesthesiologist Jerry Baratta, did not return telephone call and e-mail requests for comment. Baratta is board certified and has been in practice for nearly 30 years, according to the Osteopathic Board of New Jersey.

Hackensack Anesthesiology Associates did not respond to a request for comment.

Mary Jo Layton, a spokesperson for Hackensack Meridian Health, which owns Hackensack University Medical Center, said, "We were saddened by the tragic death of Dr. Derrick Nelson and have shared our deepest sympathies with his family, his students, the community, his friends and colleagues he touched."

"He leaves a remarkable legacy as an educator and veteran. It is important to note that the safety of our patients remains our primary focus."

A military veteran, Nelson's "kindness, compassion, integrity and endlessly positive attitude" touched those he knew, wrote superintendent of Westfield Public Schools, Margaret Dolan, in a letter to parents following Nelson's death.

"We want to prevent this from happening to anyone else," Braker told CNN. "Our family has been forever changed."

The pretrial judge assigned to the case is the James Hely.