For cake: Position rack in center of oven and preheat to 350°F Butter 3, 8-inch in diameter cake pans. Cut 3 parchment rounds to fit bottoms of pans and butter these too.
Mix sour cream and soda in bowl and let stand 10 minutes.
Sift flour, baking powder and salt in a small bowl.
Cream butter with sugar in a large bowl for 8 minutes, until very light and fluffy. Add yolks, one at a time, beating well after each addition. Beat in juices, zest and sour cream mixture. Mix in dry ingredients in 3 additions.
Beat whites in a medium bowl to soft peaks.
Fold 1/4 of the whites into the cake batter to lighten and then gently fold in remaining whites.
Divide batter among 3 prepared pans. Bake about 30 minutes, or until a tester comes out clean. Cool in pans 5 minutes. Run small, sharp knife around sides to loosen. Turn out and cool completely.
For syrup: Stir icing sugar and juice in small saucepan over low heat until sugar dissolves. Increase heat and bring to boil. Brush syrup over cakes (can be prepared 1 day ahead).
For filling: Mix pineapple and cornstarch in medium saucepan. Add butter, sugar and salt. Stir over medium heat until mixture boils and thickens, about 10 minutes. Cool, cover and refrigerate until well chilled (can be prepared 1 day ahead).
For Lemon Whipped Cream Frosting (about 4 cups): Mince lemon zest with sugar in processor or mini-blender. Whip cream in large bowl to soft peaks. Gradually beat in sugar.
To assemble: Place 1 cake layer, syrup side up, on plate.
Gently spread 1/2 of pineapple filling on top.
Ice with 1/2 cup of frosting.
Top with second layer and repeat.
Top with third layer.
Ice top and sides of cake with remaining frosting.
Press coconut onto sides of cake (can be prepared 8 hours ahead. Cover and refrigerate. Let stand at room temperature 1 hour before serving).