Solvent and substituent effects on conjugated eliminations in propargylic systems

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 affords a mixture of (E) and (Z) aminocrotonates of which formation can be explained by a chemioselective removal of the acetalic proton leading to an intermediate allenyllithium that has equally been trapped by stannylation.