Abstract: The present invention discloses a new class of trans-cyclooctenes (TCOs), “a-TCOs,” that are prepared in high yield via stereocontrolled 1,2-additions of nucleophiles to trans-cyclooct-4-enone, which itself was prepared on large scale in two steps from 1,5-cyclooctadiene. Computational transition state models rationalize the diastereoselectivity of 1,2-additions to deliver a-TCO products, which were also shown to be more reactive than standard TCOs and less hydrophobic than even a trans-oxocene analog. Illustrating the favorable physicochemical properties of a-TCOs, a fluorescent TAMRA derivative in live HeLa cells was shown to be cell-permeable through intracellular Diels-Alder chemistry and to washout more rapidly than other TCOs.