Boston-based law firm Bingham McCutchen is expanding eastward. Last month, it announced the opening of a Hong Kong office, and today the Boston Business Journal reports that Bingham has acquired a 22-lawyer firm in Tokyo, Sakai & Mimura, known for financial restructuring and mergers and acquisitions. The BBJ says that Bingham already has a Japanese practice group of 30 lawyers, based mostly in New York but with one lawyer stationed in Japan.