Sulfur tyranny: Thiiranes, instead of oxiranes, can be obtained in a highly stereoselective manner through the cycloaddition reaction of N‐acyl oxazolidine tethered diazo thione compounds with aldehydes catalyzed by RhII. Thus, this reaction provides versatile adducts S functionalized at both the α and β position, with concomitant generation of two contiguous stereocenters.