Preheat oven to 350°.
Drain pineapple, reserving juice. Add enough water to juice to equal 3/4 cup; set aside. Pat pineapple slices dry.
Coat a 9-inch round cake pan with melted butter; sprinkle with brown sugar. Arrange pineapple slices on top of brown sugar.
Combine pineapple juice mixture, cake mix, and egg whites; stir with a fork 1 minute. Pour batter into prepared cake pan.
Bake at 350° for 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 5 minutes on a wire rack. Loosen cake from sides of pan, using a narrow metal spatula. Invert cake onto a serving plate. Cut into wedges.