The deprotonation of 4-methoxy-but-2-ynal diethyl acetal by n-butyllithium induces an acetylenic–allenic isomerization (in diethylether) or a conjugated elimination reaction (in THF), providing the corresponding 1, 4-dialkoxycumulene. An allenyllithium, that has been trapped as an allenylstannane, is proposed to be a common intermediate to both pathways. Also, the deprotonation of 4-dialkylamino-but-2-ynal diethyl acetals in the same conditions ...