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Allenes are cumulenes containing two consecutive carbon–carbon double bonds (C=C=C) whose terminal substituents lie in orthogonal planes. Allene synthesis is the set of transformations that install this 1,2-diene array. One established route is the propargyl Claisen rearrangement, in which a propargylic vinyl ether undergoes a 3,3-sigmatropic reorganization to an allenic carbonyl derivative ^[1]. Radical processes generate the cumulene by addition–elimination or hydrogen-atom transfer sequences that terminate in an allenic radical or closed-shell allene ^[2]. Fragmentative constructions create the two double bonds by scission of a C–C bond in a suitably disposed precursor, releasing the allene as the unsaturated fragment ^[3]. SN2' displacement of a propargylic leaving group by an organometallic nucleophile likewise produces a substituted allene. The defining structural criterion is the cumulative C=C=C unit, which distinguishes allenes from isolated dienes, conjugated 1,3-dienes, and alkynes. Axial chirality arises when the four termini are appropriately substituted, so stereochemical control is an intrinsic feature of many allene-forming bond reorganizations.