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Barton esters have been introduced into the side chain of carbohydrates with high yields in only a few steps from easily available glycals. Their radical reactions afford 2-C-methyl and 2-C-bromomethyl hexoses, pentoses and disaccharides in good yields in analytically pure form. Since the Barton esters have been synthesized by an oxidative radical addition and their transformations by reductive radical processes, our results demonstrate the power of such reactions in carbohydrate chemistry.
C-branched carbohydrates are of current interest for glycochemistry, are widely found in nature and serve as important subunits in many antibiotics, bacterial polysaccharides and macrolides. Among C-functionalized saccharides, 2-C-branched carbohydrates represent challenging structures for synthetic chemists, since in contrast to C-glycosides they are not easily accessible from glycosyl bromides or other simple precursors. In this perspective we want to summarize recent approaches to 2-C-branched carbohydrates over the past fifteen years. The two main strategies are based on ring-opening of 1,2-cyclopropanated carbohydrates by various reagents, as well as radical additions to glycals and further transformations, developed in our group. Both methods are characterized by high stereoselectivities and good yields and give access to a broad variety of functionalized carbohydrate 2-C-analogs.
Regioselective synthesis of alkylarenes by two-step ipso-substitution of aromatic dicarboxylic acids
(2012)
A strategy for the regioselective alkylation of arenes was developed, starting from commercially available and inexpensive terephthalic acid or naphthalene-1,4-dicarboxylic acid. The method entails a formal ipso-substitution of the carboxylate groups by a sequence of reductive alkylation under Birch conditions and subsequent acid-mediated rearomatization with loss of carbon monoxide and carbon dioxide. More than 20 different arenes with various side-chains were synthesized. With naphthalene-1,4-dicarboxylic acid as starting material, we were able to control the degree of alkylation by choosing the appropriate electrophile in the Birch reduction. Thus, bisalkylated naphthalenes and naphthoic acids became available chemoselectively. All reactions afforded a single regioisomer exclusively in high yields. Overall, aromatic dicarboxylic acids are suitable substrates for a two-step ipso-substitution that allows the selective synthesis of alkylated benzenes and naphthalenes.
The esters of 4-hydroxy-cyclohexanone and a series of carboxylic acids R-COOH with R of different electronic and steric influence (R=Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, sec-Bu, t-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, and CBr3) were synthesized and the conformational equilibria studied by 1H and 13C NMR spectroscopy at 103 K and at 295 K, respectively. The geometry of optimized structures of the axial/equatorial chair conformers was computed at the ab initio MO and DFT levels of theory. Only one preferred conformation was obtained for the axial and the equatorial conformer as well. When comparing the conformational equilibria of the cyclohexanone esters with those of the corresponding cyclohexyl esters a certain polarity contribution of the cyclohexanone framework was revealed, which is independent of the substituent effects and increases the stability of the axial conformers by a constant amount.
The esters of 4-hydroxy-cyclohexanone and a series of carboxylic acids R-COOH with R of different electronic and steric influence (R=Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, sec-Bu, t-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, and CBr3) were synthesized and the conformational equilibria studied by H-1 and C-13 NMR spectroscopy at 103 K and at 295 K, respectively. The geometry of optimized structures of the axial 'equatorial chair conformers was computed at the ab initio MO and DFT levels of theory. Only one preferred conformation was obtained for the axial and the equatorial conformer as well. When comparing the conformational equilibria of the cyclohexanone esters with those of the corresponding cyclohexyl esters a certain polarity contribution of the cyclohexanone framework was revealed, which is independent of the substituent effects and increases the stability of the axial conformers by a constant amount.
An experimental and computational study on the impact of functional groups on the oxidation stability of higher acenes is presented. We synthesized anthracenes, tetracenes, and pentacenes with various substituents at the periphery, identified their photooxygenation products, and measured the kinetics. Furthermore, the products obtained from thermolysis and the kinetics of the thermolysis are investigated. Density functional theory is applied in order to predict reaction energies, frontier molecular orbital interactions, and radical stabilization energies. The combined results allow us to describe the mechanisms of the oxidations and the subsequent thermolysis. We found that the alkynyl group not only enhances the oxidation stability of acenes but also protects the resulting endoperoxides from thermal decomposition. Additionally, such substituents increase the regioselectivity of the photooxygenation of tetracenes and pentacenes. For the first time, we oxidized alkynylpentacenes by using chemically generated singlet oxygen (O-1(2)) without irradiation and identified a 6,13-endoperoxide as the sole regioisomer. The bimolecular rate constant of this oxidation amounts to only 1 X 10(5) s(-1) M-1. This unexpectedly slow reaction is a result of a physical deactivation of O-1(2). In contrast to unsubstituted or aryl-substituted acenes, photooxygenation of alkynyl-substituted acenes proceeds most likely by a concerted mechanism, while the thermolysis is well explained by the formation of radical intermediates. Our results should be important for the future design of oxidation stable acene-based semiconductors.