Wash and dry sweet potatoes and bake for 1 hour or until potatoes are soft to the touch. Remove from the oven and let cool slightly then remove skin from potatoes. Reduce oven temperature to 350 degrees F. While the potatoes are baking, combine remaining ingredients in a saucepan. Bring to a boil, then reduce the heat and simmer for 30 to 45 minutes until sauce is slightly thickened. Slice the potatoes 1/2-inch thick into a medium casserole dish. Pour syrup over them and return to the oven and bake for approximately 30 minutes.