Current Organic Chemistry - Volume 18, Issue 9, 2014

Cycloaddition reactions of azomethine ylides are the most direct way to synthesise pyrrolidine derivatives. They have been studied for several decades and have become an indispensable tool in synthesis of pyrrolidines and pyrrolidine derived natural products. Amongst many methods for generating azomethine ylides, various processes involving imines derived from amino acid esters have been the most frequently studied. The use of Lewis acids to promote imine-ylide-cycloaddition sequence under mild conditions, in recent years, has led to the development of highly stereoselective metal catalysed methodologies for the preparation of pyrrolidine derivatives. In the last few years, the concept of organocatalysis has been incorporated in cycloaddition reactions of azomethine ylides providing an alternative access to chiral pyrrolidines. Several classes of typical organocatalysts such as prolines, phosphoric acids, thioureas, guanidines and sulphuric acid derivatives have been used for these purposes. Various mechanistic pathways have been proposed, based on either the activation of only one reacting partner, 1,3-dipole (imine) or dipolarophile (alkene), or both of them simultaneously. While the first three classes of organocatalysts appear to afford pyrrolidines, generally, in good yields and with high levels of stereoselectivity, guanidines and sulphuric acid derivatives are less efficient, but also the least studied catalyst group. A whole range of electron deficient dipolarophiles (alkenes) have been used in these cycloaddition processes, while, regarding the dipole precursor imine, aromatic aldimines seem to be more efficient than their aliphatic equivalents. There is no doubt that the recent progress in organocatalytic cycloadditions of azomethine ylides created new possibilities for synthesis of pyrrolidine derivatives and enriched this useful synthetic methodology.

Recent applications of activation parameters variation approach to the elucidation of acyl-transfer mechanisms have led to further clarifications of structures of intermediates and transition states involved in the concerted and stepwise reaction pathways. Acyltransfer reactions in solution are reviewed with special emphasis of activation parameter variation ΔX‡ (X = H, S, G) with substituents in the nucleophile, acyl and leaving groups applying linear free energy relationships in order to evaluate the resultant δΔX‡ reaction constants. The use of internal enthalpy reaction constant δΔH‡ int as a mechanistic tool is stressed when one of the steps of the acyl-transfer reaction is the single rate-determining step.

4-Oxothiazolidine core, owing to the wide range of pharmacological activities exhibited by its derivatives, has been recognized as an important structural motif in biologically active compounds. A subclass constitutes 4-oxothiazolidines with an exocyclic C=C double bond at the C(2)-position. Some of these derivatives are registered as active substances for the treatment of various diseases, such as Ralitoline, an antiepileptic, Etozolin, a diuretic, and Piprozolin, a choleretic. The exocyclic C=C double bond in these compounds contains one or two electron-accepting groups at its other end, so that they also belong to the class of the so-called push-pull alkenes. In the case of the nitrogen-unsubstituted molecules, the enamino tautomeric form with the exocyclic double bond is stabilized by the extended electron delocalization arising from the push-pull effect. In the absence of push-pull effect, the imino form would dominate. Therefore, these compounds exhibit properties characteristic for both 4-oxothiazolidine ring and push-pull alkenes. They are also proved to be useful synthetic interamediates for the formation of various mono- and polyheterocycles. The importance of 4-oxothiazolidine derivatives is witnessed by several review articles, the latest published in 2012, mainly dealing with the chemistry and biological activity of various 2-imino, 2-oxo, 2-thioxo and 2-alkyl(aryl)-substituted compounds. The lack of a comprehensive review on 2-alkylidene-4- oxothiazolidines has prompted us to collect the literature covering their synthesis, structure, reactivity and biological activity. Derivatives with two exocyclic C=C double bonds are included, too.

The concept of sustainable chemistry includes three main parameters; the use of renewable biofeedstock for syntheses of chemical compounds, nontoxic solvents, and eco-friendly processing systems. Until now, many attempts have been made to meet these parameters in chemical synthesis. In hydroxymethylfurfural (HMF) synthesis, most studies have focused on the conversion of cellulosic/ lignocellulosic biomass to HMF. In addition to these biomasses, starch has also been intensively studied for the sustainable synthesis of HMF in ionic liquids. These studies demonstrated that starch-based biomaterials have high potential as renewable feedstocks for HMF synthesis by affording substantial yields of HMF. Starch is a more favorable biofeedstock than other carbohydrate materials because it is available at a relatively low cost and is likely applicable to the cost-effective mass production of HMF. Our studies confirmed the high yields of HMF from starch-based biomaterials in ionic liquid. Based on our results and related data, this review describes the current knowledge on the concept of chemical sustainability, importance of starch-based biomaterials as a biomass source for sustainable production of HMF, nature and application of HMF and ionic liquids, and bioengineering technologies for improving starch quantity and quality.

This review provides a comprehensive compilation of available literature concerning the reactions of 1,2- & 1,4- dihydropyridines, which have opened new frontiers in the area of synthetic and medicinal chemistry. These reactions mainly include cycloaddition, 1,3-Dipolar cycloaddition, alkoxy halogenation, diamination, hydroxyamination, oxidation, hydroxylation, photooxygenation reaction, nucleophilic addition, cyclization, electrophilic substitution, multicomponent reactions (MCRs) and rearrangement reactions. 1,4-Dihydropyridine has also been used in hydrogen transfer reactions as a synthetic NADH model for the reduction of several important molecules in excellent yields.

Alkenyl succinic anhydride (ASA) is one of the most prominent paper sizing agents worldwide. Commercial products start from isomerized C16-18 olefins with certain distribution of the double bond along the alkyl chain. This distribution is then found again in the product ASA which renders mechanistic studies on ASA binding to the cellulose surface difficult. For studies on ASA reactivity, olefins with defined double bond location (2-, 4-, 6-octadecene and 6-icosene) were synthesized according to Wittig protocols, and further reacted with 13C-labeled maleic anhydride to give ASA model compounds with defined double bond location and 13C-labeled carbonyl group. The olefinic products of the Wittig synthesis, rich in cis-isomers, afforded exclusively trans-configured compounds upon the enereaction with maleic anhydride. The reactions afforded four products, two regioisomeric compounds with different position of the double bond, each of which as threo- and erythro-isomer in different ratios. The products were comprehensively analytically characterized including a full NMR resonance assignment in the 1H and 13C domain. The model compounds are now used in combination with gel and solid-state NMR for studies on ASA reactivity towards cellulosic surfaces with a focus on the questions whether - and to what extent - covalent binding of ASA to cellulose can occur under conditions relevant to paper production and paper sizing.

We had synthesized a series of β-hydroxy sulfonamides D-glucosamine and applied them as ligands in the titanium tetraisopropoxide-promoted enantioselective addition of phenylacetylene and hexyne-1 to benzaldehyde and selected aromatic and aliphatic aldehydes. The N-3,5-bis(ditrifluoromethyl)benzenesulfonamido-D-glucosamine derivative performed best and we choose it as the most efficient ligand for the addition of phenylacetylene and hexyne-1 to aldehydes. High enantiomeric excesses, up to 96%, were obtained for some aromatic aldehydes.