The total synthesis of (-)-rothrockene, trans-1-(1-methylethenyl)-2-(2-methyl-1-propenyl)cyclopropane, and its enantiomer is described. Utilization of (-)-(R)-N,S-dimethyl-S-phenylsulfoximine in the synthesis allowed for resolution of a precursor, regio- and diastereoface selectivity in a Simmons-Smith cyclopropanation, determination of absolute configurations based on X-ray crystallography, and introduction of a side-chain double bond. The absolute configuration of the natural substance [from Artemisia tridentata rothrockii] is 1R,2S.