Abstract
The expected (E)-but-3-en-2-ones compounds I and II (half curcuminoids) were obtained by the Claisen–Schmidt reaction between aldehydes 3,4-dimethoxybenzaldehyde or 4-nitrobenzaldehyde with acetone. Concomitantly, 3-methylcyclohex-2-enones compounds III and IV arose from an unexpected reaction of but-3-en-2-ones in the cascade reaction of a Michael-type addition of a second molecule of acetone followed by Robinson annulation under strong basic conditions. Both enones exhibit the (E)-configuration, compound I displays s-trans conformation, whereas compound II exhibits conformational disorder as solid solution of s-cis and s-trans conformations. The related 3-methylcyclohex-2-enones exhibit envelope conformation. Compound III constitutes an example of the rarest case of racemic solid solution (pseudoracemate), where a lack of chiral discrimination with respect to the two enantiomers leads to an enantiomeric disorder of a racemic mixture with different occupancies at the reference site. Due to the lack of strong hydrogen-bond donors in all compounds, the crystal packing is mainly stabilized by weak intermolecular C-H···O interactions between the molecules. The present work provides a new perspective on the search for by-products normally overlooked in Claisen–Schmidt condensations.
Highlights
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.Licensee MDPI, Basel, Switzerland.Half curcuminoids [1,2,3] are typically prepared employing the classical Claisen–Schmidt condensation using strong base catalysis [4]
After completion of the reaction, the acetone was evaporated in vacuo and the reaction mixture was poured over ice and acidulated with diluted hydrochloric acid (HCl) to pH = 5
In compound IV, centrosymmetric dimers are built from interaction of the oxygen of the carbonyl group and the α-methylene of cyclohexenone (Figure 8, red molecules) and with an aromatic hydrogen (Figure 8, blue molecules) interconnected by the nitro group into a tridimensional network, reinforced by C–H···π and additional C–H···O1 interactions (Table 2)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The prop-2-en-1-one moiety provides a chemically reactive function for Michael-type additions [4,5,6,7,8], leading to the formation of cyclic structures through a domino reaction [9,10] (Scheme 1, Path II) in the presence of catalysts other than a strong base. Important in the synthesis of natural products and a wide range of biological activities have been associated with them, such as anti-inflammatory and anticancer effects [12,13]. They are used as food additives and in the synthesis of pheromones [14]. (E)-4-(4-nitrophenyl)but-3-en-2-one (II), yellow pale solid (126 mg, 40%), m.p 106
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