Preheat oven to 350 degrees. Spray 9x9x2 inch metal baking pan with nonstick spray. Combine sugar, butter, pineapple juice concentrate, and molasses in heavy small saucepan. Bring to a boil over medium heat, whisking until sugar dissolves adn syrup is smooth; boil 1 minute. Pour evenly into prepared pan.
Cut off on 1/3-inch thick round from pineapple; cut out core. Place round in syrup in pan. Stand pineapple on end. Cut lengthwise into 1/3-inch thick slices. Using 3 round cutters of different sizes, cut out rounds from pineapple slices, avoiding core. Arrange rounds close together in single layer in free-form design in syrup.
Whisk first 6 ingredients for the cake together in a medium bowl. Beat butter in large bowl until fluffy. Add sugar and beat to blend. Beat in eggs, 1 at a time. Beat in molasses, then pumpkin. Beat in dry ingredients just until blended, occasionally scraping down sides of bowl. Beat in 1/2 cup boiling water. Pour batter evenly into pan.
Bake cake until tester inserted into center comes out clean, about 50 minutes. Cool cake in pan 45 minutes. Place platter over pan and invert. Let stand 5 minutes. Gently lift off pan. Serve cake warm or at room temperature with whipped cream.