Preheat the oven to 350°F (176°C). Butter an ovenproof gratin or casserole dish.
Arrange the sweet potato slices in the prepared baking dish and season generously with salt and pepper.
In a large saucepan, combine the sugar, butter, bourbon, and syrup, and bring the mixture to a boil over medium-high heat. As soon as the sauce begins to boil, pour it over the sweet potatoes.
Bake the casserole, turning the sweet potatoes and spooning the liquid over the top occasionally, until the sweet potatoes are tender, 45 to 60 minutes. Taste and adjust for seasoning with salt and pepper.
