Stereoselective aldol condensations via alkenyloxy dialkoxyboranes : mechanistic and stereochemical details
1984; Elsevier BV; Volume: 40; Issue: 20 Linguagem: Inglês
10.1016/0040-4020(84)85085-1
ISSN1464-5416
AutoresCesare Gennari, Lino Colombo, Carlo Scolastico, Roberto Todeschini,
Tópico(s)Organometallic Compounds Synthesis and Characterization
ResumoA detailed investigation of the enolization of ketones with ethylenechloroboronate ( ECB ) in the presence of a tertiary amine and the subsequent aldol condensations of these boron enolates was conducted. The enolization with ECB- DPEA system was found to be regioselective except for the case of butanone. The stereochemistry of the enolates derived from ethyl ketones was defined as Z on the basis of 1H-NMR comparison to the Z enolates obtained by a stereodefined route. A mechanistic model for the enolization is proposed to explain the enolization selectivity. E enolates were found to be more reactive than the Z enolates. The product aldol stereochemistry ( syn ) was correlated to the enolate geometry via a chairlike transition state ( Z enolates ) or via a boatlike transition state ( E enolates ).
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