A fire in the kitchen of the Burger King on Boylston Street spread into the ductwork of the downtown Boston building before Boston firefighters put out the blaze, officials said. According to a Fire Department tweet, the fire was discovered around 7:27 a.m. inside the restaurant at 551 Boylston St. The roof of the four-story building across from Copley Square Park was broken open around the duct, and the fire was out by 8:15 a.m. The two-alarm blaze was caused by grease in the ductwork, and fire officials estimated damages at $100,000. No injuries were reported, officials said.Catalina Gaitan can be reached at email@example.com. Follow her on Twitter at @catalina_gaitan.