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. (C) 2000 Elsevier Science Ltd.
Le Strat, Frédéric,Maddaluno, Jacques
p. 5367 - 5371
(2007/10/03)
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