AbstractAdducts formed from benzoyl isothiocyanate and l‐morpholinocyclopentene or β‐(N,N‐diethylamino)styrene and that prepared from acetyl isothiocyanate and l‐pyrrolidinoeyclopen‐tene gave 4‐pyrimidinethiones when treated with primary amines or ammonia. In some cases intermediates, the products of amine exchange, were isolated. These intermediates were readily cyclized to 4‐pyrimidinethiones with