1Preheat the oven to 350F. Butter and flour a 10-inch tube or Bundt pan. Put the flour, baking soda and salt in a medium bowl and stir to blend; reserve.
2Put the zest, 2 1/2 cups of the granulated sugar, and the butter in a large bowl. Using a hand mixer, mix the sugar, butter and zest on low speed for 30 seconds to combine. Mix on medium speed until creamed and light- about 3 1/2 to 4 minutes. Scrape the bowl.
3Add the eggs, yogurt, and extracts; mix on low speed until well blended, about 1 minute. Scrape the bowl.
4Add the dry ingredients and mix on low speed for 30 seconds. Scrape the bowl and mix until completely blended, about 30 to 40 seconds. Transfer the batter to the prepared pan and spread evenly. Bake in the preheated 350F oven until a tester inserted in the center comes out clean, 65 to 75 minutes.
5While the cake is in the oven, prepare the syrup by combining 1/2 cup of the grapefruit juice with the remaining 1/2 cup of granulated sugar in a small saucepan. Cook over medium heat until the sugar is dissolved, 3 to 5 minutes; keep warm and reserve.
6Cool cake in pan on a rack for 5 minutes, then turn out onto the rack; wash and dry the pan. Prick the top and sides of the cake with a cake tester and return it to the pan. Prick the bottom of the cake with the cake tester. (I copied this directly as it was written, but I don't know why we are taking the cake out, pricking it, and putting it back in the tube pan. The point is to get the syrup into the cake, and if you know a better way, I think you can figure it out.)
7Pour the warm grapefruit syrup slowly over the cake, allowing the cake to absorb the syrup. When the syrup has been absorbed, turn the cake out onto the rack to cool completely.
8Combine the powdered sugar with the remaining grapefruit juice and stir until blended and smooth. Drizzle the pink grapefruit glaze over the cooled cake. Let rest 30 minutes before cutting.