SEATTLE — Negotiations to end a 45-day machinists union strike against the Boeing Co. will resume Thursday with a federal mediator in Washington, D.C. Arthur Rosenfeld, director of the U.S. Federal Mediation and Conciliation Service, told both sides “that he wants the parties to reconvene negotiations under the auspices of the FMCS,” according to an agency statement Monday. The announcement came one week after two days of bargaining, the first since the strike began Sept. 6, ended in failure.