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Enolate Anions and Condensation Reactions
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
The term simply means that the enolate of one ketone or aldehyde reacts with another molecule of the same ketone or aldehyde. Under equilibrium conditions both the enolate anion and the starting aldehyde or ketone are present and can react with one another in an aldol reaction, a “self-condensation.” What is a “mixed” aldol condensation?
Biocatalysts: The Different Classes and Applications for Synthesis of APIs
Published in Peter Grunwald, Pharmaceutical Biocatalysis, 2019
These enzymes belonging to the class of lyase enzymes catalyze the formation of C–C bonds known from Organic Chemistry as the aldol-reaction. The aldolase-catalyzed reaction represents a reversible stereoselective addition of a nucleophile to an electrophile. In living cells, aldolases play a key role in degradation as well as in synthesis of carbohydrates and keto acids. Natural donors (see scheme below) of aldolases are dihydroxyacetone phosphate (DHAP), pyruvate and phosphoenol pyruvate (PEP), acetaldehyde and glycine. Several aldolases, among them KDPG and KDPGal aldolase, or N-acetyl neuraminic acid lyase (Walters and Toone, 2017; Stockwell et al., 2016) accept fluoropyruvate as an alternative donor substrate to pyruvate, a property used by Windle et al. (2017), to investigate the stereoselective synthesis of fluorinated compounds such as α-fluoro β-hydroxy carboxyl derivatives.
Structural Design for Molecular Catalysts
Published in Qingmin Ji, Harald Fuchs, Soft Matters for Catalysts, 2019
Qingmin Ji, Qin Tang, Jonathan P. Hill, Katsuhiko Ariga
The role of solvents or additives is important for the selectivity of the proline-catalyzed reactions. In the case of aldol reactions, polar aprotic organic solvents such as dimethyl sulfoxide and dimethylformamide may lead to high enantioselectivity, while polar protic solvents are better solvents for Michael reactions. Pihko et al. have observed significantly higher yields for the proline-catalyzed intermolecular aldol reaction by the addition of water [85]. The use of additives or combination of two solvents may also be a viable strategy to refine the yield and selectivity of proline-catalyzed reactions. When unmodified aldehydes were used as donors, the cross-Mannich reaction was shown to proceed faster than the competing cross-aldol reaction [86–88]. The three-component cross-Mannich reaction exhibited higher stereo- and chemoselectivity (kMannich > Kaldol) at temperatures below 0°C (Fig. 2.10). With electron-rich aromatic acceptor aldehydes, the stereodetermining proton transfer from the carboxylic acid in proline to the alkoxide is facilitated. Therefore, the aldehyde substituent occupies a pseudo-equatorial position in the aldol reaction, while in Mannich reaction the substituent is forced into a pseudoaxial orientation. These results bring insights on the importance of reaction media for the enamine formation and the stereoselectivities of the reactions.
Asymmetric cross-aldol reaction of isatin and ketones catalyzed by crude earthworm extract as efficient biocatalyst
Published in Green Chemistry Letters and Reviews, 2020
Fatemeh Shams, Javad Mokhtari Aliabad, Morteza Rouhani
Cross-aldol reaction has been considered one of the most powerful and efficient measures for the formation of carbon–carbon bond at C-3 position of isatin derivatives (10–15). In this context, the cross-aldol reaction between isatin and carbonyl compounds has attracted much attention. In the recent years, different research groups employed different catalyst for this reaction. Tomasini demonstrated the cross-aldol reaction of isatin with acetone by a dipeptide as an organocatalyst (16), Toru et al. used sulfonamides for the enantioselective cross-aldol reaction of acetaldehyde with isatin (17), Zhao et al. described the use of quinidine thiourea for the enantioselective synthesis of 3-hydroxyindolin-2-ones (18), Lin et al. disclosed the enantioselective cross-aldol reaction of isatin derivatives with cyclic ketones in by enzymes (19) and Chen et al. developed amino acid salts catalyzing cross-aldol reactions of isatin with ketones (20). Despite these reports, it is still important and desirable to develop new environmentally safe, accessible and cheap catalysts for the cross-aldol reactions of isatin and ketones.
Nanostructured assemblies of liquid-crystalline supermolecules: from display to medicine
Published in Liquid Crystals, 2019
The helical nanofilaments form a network which acts as a porous nanoconfinement medium of large internal area, with the guest material confined to nanoscale interstitial volumes between the filaments [58,71,72]. The network can produce nanocomposites for applications, such as semiconductors and photovoltaics [73]. However, to the best of our knowledge, no catalytic activity has been reported for the layered chiral conglomerates so far. We studied the reaction of acetone with benzaldehyde in the presence of a catalytic amount of trimer 4-(9,9) in the DC phase. According to the precedent shown with proline [74], we attempted to perform the reaction. Reacting trimer 4-(9,9) (0.15 mmol) in DMSO (15 ml)/acetone (15 ml) with benzaldehyde (1.5 mmol) at room temperature for 264 h gave the aldol product in 40% yield [75]. The product did not exhibit optical activity. To determine if the observed catalytic activity was a unique property of trimer 4-(9,9), we conducted several experiments. Interestingly, neither trimer 4-(9,9) in the crystalline state nor a bent-core molecule, P-8-O-PIMB, in the HNF phase proceeded the aldol reaction at all. We surmise that the porous network in the DC phase offers an enzyme-mimetic reaction field (Figure 26).
Amino acids: Building blocks for the synthesis of greener amphiphiles
Published in Journal of Dispersion Science and Technology, 2018
Nausheen Joondan, Sabina Jhaumeer Laulloo, Prakashanand Caumul
The presence of the secondary amine architecture in proline based surfactants has been found to be useful for the catalysis of Aldol reaction. The proline based surfactants 58a and 58b have been reported to exhibit better catalytic activities compared to the parent proline for Aldol reactions in water, giving rise to high yields, good enantioselectivity and anti-diastereoselectivities.[69] Yang et al. reported the catalytic activities of proline based surfactants containing aryl sulfonamide scaffold on asymmetric Aldol reactions.[133] The presence of water in the reaction medium was found enhance the product yield and selectivity.