A former Boston mob boss is appealing his 2013 conviction on the grounds that he was not able to completely present his defense.

James "Whitey" Bulger and his lawyers on Thursday called on to the federal appeals court to withdraw his lifetime sentence because they claim that he was promised immunity years ago.

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The 84-year-old Bulger was convicted of crimes tying him to 11 murders and other gang-related crimes.

The defense lawyers are claiming that Bulger was unfairly tried because the presiding judge did not let them tell jurors about the immunity promised by a late prosecutor.

In a letter written by Bulger's lawyers to the U.S. Court of Appeals, it said the defendants constitutional rights were violated and that he was "stripped of his right to testify" about how he was able to avoid prosecution for almost twenty-five years.

The letter, made by lawyers Hank Brennan and James Budreau, stated that Bulger didn't take the stand during the trial because he was forbidden to talk about the promised immunity.

The defense was banned from using the immunity during the trial and they said the judge also ruled that Bulger cannot make his own testimony.

Because of this "deprivation", the lawyers are pushing for the conviction to be overturned.

During his trial in Boston, the defendant told the presiding judge that the entire trial was a "sham" because now-dead prosecutor Jeremiah T. O'Sullivan promised him immunity, even for the murder charges.

In response, Judge Denise Casper said Sullivan, who died in 2009, did not have the authorization to grant the immunity.

The judge also noted that her ruling to bar the immunity defense was based on the defense's lack of evidence supporting the claim.

The prosecution has to file a response regarding the appeal within 30 days, though an extension can me requested.

Bulger is serving two life sentences at a federal penitentiary in Tucson.

TagsMob boss, Boston, whitey bulger