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The company expanded to national status in 1965, when Union Oil merged with the Pure Oil Company, headquartered in what was then Palatine, Illinois, and now Schaumburg, Illinois. Over the next two decades, Union became the major oil producer in southern Alaska and a major natural gas producer in the Gulf of Mexico. The company was reorganized in 1983, and Union Oil Company of California became an operating subsidiary of a new Delaware-based holding company, Unocal Corporation. In 1985, Mesa Petroleum, controlled by billionaire T. Boone Pickens, attempted a takeover of Unocal Corp. that resulted in the Delaware Supreme Court landmark decision Unocal v. Mesa Petroleum, which upheld Unocal's takeover defense. Virtually all operations are conducted by Union Oil Company of California (Union Oil).