Cook sugar in a dry 1 1/2-quart heavy saucepan over moderate heat, undisturbed, until it begins to melt. Continue to cook, stirring occasionally with a fork or flat whisk, until sugar is melted and turns a deep golden caramel. Tilt pan and carefully add cranberries and water (caramel will harden and vigorously steam). Simmer over moderately low heat, stirring, until caramel is completely dissolved, then pour syrup through a very fine sieve into a heatproof bowl, pressing hard on solids. Let cool.