The intramolecular [4+2]-cycloadditions of di- and trisubstituted nitroalkenes with unactivated olefins are described. The cycloadditions proceed readily at low temperatures in the presence of SnCl4. The reactions are shown to be stereospecific in the preservation of dienophile configuration in the product. The configuration of the heterodiene controls the stereochemistry of the ring fusion but the selectivity is high only with trisubstituted nitroalkenes. The rate of cycloaddition of a cis dienophile is ca. 20-fold faster than a trans dienophile. The implications for mechanism and transition structure are discussed.