Place raspberries and 1 cup water in a blender; process until smooth. Strain raspberry mixture through a fine sieve; discard solids. Combine raspberry puree, remaining 1 1/2 cups water, and sugar in a small saucepan. Combine juice and cornstarch in a small bowl, stirring with a whisk until blended. Add cornstarch mixture to raspberry mixture; bring to a boil over medium heat. Cook 5 minutes or until thickened, stirring constantly. Strain through a fine sieve; discard solids. Cool. Cover and chill.