HARTFORD, Conn. — A huge nationwide spike in hospitalizations last month caused by a class of drugs often called “synthetic marijuana” illustrates the potency and dangers of the chemicals used to make them, and the shifty tactics authorities believe manufacturers are using to evade regulation. Poison control centers nationwide reported 359 cases in January of illnesses from synthetic cannabinoids, which mimic the effects of the main psychoactive ingredient in marijuana but can be far more potent. There were 273 in February and 269 in March.