If the U.S. were to Warrantlessly use information about Americans obtained from the U.K., would that be legal? Assuming they were doing so simply as an end-run

If the U.S. were to Warrantlessly use information about Americans obtained from the U.K., would that be legal? Assuming they were doing so simply as an end-run around U.S. wiretap law, that's a pretty obvious violation of the spirit of the law, but the letter?