In large, shallow, non-metallic dish, combine reserved juice, lemon juice, oil, oregano, and garlic powder . Reserve 1/4 cup. Add chicken to remaining marinade, turning to coat all sides. Cover and marinate 15 minutes in refrigerator. Discard marinade.
Grill or broil chicken and pineapple slices, brushing occasionally with reserved marinade, 5 to 10 minutes on one side turn over. Add pineapple slices to grill with chicken and brush with marinade 10 minutes or until chicken is no longer pin in center. Discard any remaining marinade.