A319 Captain reported encountering wake turbulence 6 miles in trail of a B777 on approach to RDU.

Narrative

WAKE TURBULENCE ENCOUNTER. While descending on the TAQLE 2 RNAV STAR into RDU; Approach Control was concerned about spacing and speed between 777 ahead on the STAR and our flight. Approach Control issued speed reductions to our aircraft to ensure that there was adequate spacing; which appeared to be approximately six miles. While in IFR conditions at at 5;000 feet and between ESELL and DUWON on the RNAV (GPS) Y 23L approach; our aircraft began to encounter the wake from the preceding 777. FO was PF; and I instructed FO to disengage the autopilot to hand fly the jet to counter the effects of the wake. Bank angle did not exceed 20 degrees either side; but approximately 300 feet of altitude was lost as the FO attempted to keep wings level. I selected a 200 degree heading to remove our flight from the approach course and communicated to Approach Control that we needed a vector off the approach because of wake turbulence. As the plane turned to the 200 degree heading the wake encounter ended and ATC issued a clearance to descend to 3;000 (or 2;000). (Speed during the encounter appeared to remain relatively constant at the assigned 190KIAS.) ATC then issued a right turn to rejoin the approach when there was approximately ten miles separation with the 777. No further wake was encountered during the approach; which was in IMC; or landing. After parking at the gate; the number 1 flight attendant stated that he thought 'something scary' was happening during final approach and explained to him that we encountered the wake of the preceding heavy jet. None of the other flight attendants made remarks; and I was not made aware of any passenger comments. As the FO and I debriefed at the gate; the FO explained to me that full sidestick deflection was used to counteract the forces of the wake.Cause: Although I cannot assign a specific cause to this event; it appears that even with at least six miles of separation; there is still very significant wake that poses a risk to trailing airplanes. From my scan of the ND; there appeared to be that amount of separation; so; while a wake encounter is possible; the expectation is that the 777's vortices would dissipate within that distance. As for mitigation; I will ensure that anytime my flight is sequenced behind a heavy; I verbalize the threat of a wake turbulence encounter and briefly review the AOM maneuver if one is encountered. In reviewing the AOM; the manual does not discuss the option of making a heading adjustment; which seemed to be a very effective and quick means in this case to exit the wake encounter.