Synthesis, Structure, and <i>E</i>−<i>Z</i> Isomerization of β-(Hetero)aryl-α-nitro-α,β-enals
作者:Patricia Martínez-Bescos、Fernando Cagide-Fagín、Luis F. Roa、Juan Carlos Ortiz-Lara、Krzysztof Kierus、Lidia Ozores-Viturro、Marta Fernández-González、Ricardo Alonso
DOI:10.1021/jo702731b
日期:2008.5.1
The first general methodology for the gram-scale preparation of previously overlooked beta-(hetero)aryl-alpha-nitro-alpha,beta-enals (3) is reported. Condensation of (hetero) aromatic aldehydes with 2-nitroethanol gave the E-isomers of uncommon beta-(hetero)aryl-alpha-hydroxymethyl-alpha,beta-unsatured-nitroalkenes (2), as determined by NOE and X-ray studies. alpha-Nitro-alpha,beta-enals 3 were subsequently obtained by hypervalent iodine oxidation of 2 as E-Z-mixtures in solid form. They showed varied stability and solvent-dependent thermal-promoted and photopromoted E-Z interconversion. Starting with furfural, experimental conditions were developed to prepare the corresponding nitroenal 3a enriched in either the E or the Z isomer: E-3a/Z-3a approximate to 90/10 and 20/80, respectively. In contrast with other structurally related compounds, nitroenals 3 have their (hetero)aryl-vinyl unit and their formyl and nitro groups all in a planar arrangement, both in solid form and in solution; accordingly, they are colored compounds with predicted high dipole moments. As deduced from solution-NMR and X-ray data, the C=C and the C=O double bonds in 3 are exclusively s-cis-oriented; this disposition corresponds in fact to the DFT-computed most stable conformer.