Bake cake according to directions on box in 9 x 13" pan. While cake is cooking, mix 20 oz. can crushed pineapple, sugar, and cornstarch and bring to a boil. Cook low until it is clear in color and a little thick. Poke holes in cake and pour warm mixture over it so it will soak in. When completely cool, sprinkle with coconut. This is very moist and delish!