Preheat oven to 350.
Grease a 9 X 13 dish, and set aside.
In a large bowl, add the flour, sugar, eggs, vanilla, baking soda, salt, and crushed pineapple.
Stir well, and pour cake batter into the greased 9 X 13 dish.
Place the dish in the oven, and bake at 350 degrees for 35 - 40 minutes, or until a toothpick inserted in the center comes out clean.
Remove from the oven to a wire rack to cool.
While the cake is cooling, place a heavy saucepan on the stove, and add the butter, and sugar.
Melt the butter and sugar until smooth and clear.
Add the sweetened condensed milk, shredded coconut, and crushed pretzels, and stir until the mixture boils.
Turn heat to low, and continue to cook for 1 minute.
Remove from heat, and cool partially.
When the Frosting is cooled a bit, but still warm, spoon the frosting on the top of the cake, and smooth evenly across cake.
Allow cake and frosting to cool completely. Frosting will become thick as it cools.