Organocatalytic synthesis of higher-carbon sugars : efficient protocol for the synthesis of natural sedoheptulose and D-glycero-L-galacto-oct-2-ulose

Herein we report a short and efficient protocol for the synthesis of naturally occurring higher‐carbon sugars—sedoheptulose (d‐altro‐hept‐2‐ulose) and d‐glycero‐l‐galacto‐oct‐2‐ulose—from readily available sugar aldehydes and dihydroxyacetone (DHA). The key step includes a diastereoselective organocatalytic syn‐selective aldol reaction of DHA with d‐erythrose and d‐xylose, respectively. The methodology presented can be expanded to the synthesis of various higher sugars by means of syn‐selective carbon–carbon‐bond‐forming aldol reactions promoted by primary‐based organocatalysts. For example, this methodology provided useful access to d‐glycero‐d‐galacto‐oct‐2‐ulose and 1‐deoxy‐d‐glycero‐d‐galacto‐oct‐2‐ulose from d‐arabinose in high yield (85 and 74 %, respectively) and high stereoselectivity (99:1)

Low-coordinate erbium(III) single-molecule magnets with photochromic behavior

[Image: see text] The structures and magnetic properties of photoresponsive magnets can be controlled or fine-tuned by visible light irradiation, which makes them appealing as candidates for ternary memory devices: photochromic and photomagnetic at the same time. One of the strategies for photoresponsive magnetic systems is the use of photochromic/photoswitchable molecules coordinated to paramagnetic metal centers to indirectly influence their magnetic properties. Herein, we present two erbium(III)-based coordination systems: a trinuclear molecule {[Er(III)(BHT)(3)](3)(dtepy)(2)}(.)4C(5)H(12) (1) and a 1D coordination chain {[Er(III)(BHT)(3)(azopy)}(n)·2C(5)H(12) (2), where the bridging photochromic ligands belong to the class of diarylethenes: 1,2-bis((2-methyl-5-pyridyl)thie-3-yl)perfluorocyclopentene (dtepy) and 4,4′-azopyridine (azopy), respectively (BHT = 2,6-di-tert-butyl-4-methylphenolate). Both compounds show slow dynamics of magnetization, typical for single-molecule magnets (SMMs) as revealed by alternating current (AC) magnetic susceptibility measurements. The trinuclear compound 1 also shows an immediate color change from yellow to dark blue in response to near-UV irradiation. Such behavior is typical for the photoisomerization of the open form of the ligand into its closed form. The color change can be reversed by exposing the closed form to visible light. The chain-like compound 2, on the other hand, does not show significant signs of the expected trans–cis photoisomerization of the azopyridine in response to UV irradiation and does not appear to show photoswitching behavior

Chemistry of pyruvate enolates : anti-selective direct aldol reactions of pyruvate ester with sugar aldehydes promoted by a dinuclear zinc catalyst

A chiral dinuclear zinc complex can effectively catalyse the direct aldol reactions of pyruvic acid ester with various chiral sugar aldehydes, thus functionally mimicking the pyruvate-dependent type II aldolases. Application of sterically hindered aryl esters allows for the elusive aldol reaction of the pyruvate donor with controlled anti-selectivity en route to the short and efficient synthesis of 3-deoxy-2-ulosonic acids. Pyruvic acid ester is here used as a chemical equivalent of phosphoenol pyruvate (PEP) in imitation of the synthetic principle used in nature. The presented biomimetic methodologies use enol formation for the highly efficient and flexible formation of various C6–C9 ulosonic acids. Particularly, efficient and concise syntheses of 3-deoxy-D-erythro-hex-2-ulosonic acid (KDG, overall 50% yield), 3-deoxy-D-ribo-hept-2-ulosonic acid (DRH, overall 53% yield) and 3-deoxy-D-glycero-D-talo-non-2-ulosonic acid (4-epi-KDN, overall 78% yield) are described. This direct efficient application of pyruvic esters does not require additional demasking steps and thus surpassess previously methodologies utilising masked pyruvic synthons such 2-acetylthiazole and pyruvic aldehyde dimethyl acetal

Rapidly renewable silver amalgam annular band electrode for voltammetry and polarography

This work describes a novel type of working electrode for use in voltammetry and polarography — the renewable silver liquid amalgam film–modified silver solid amalgam annular band electrode (AgLAF–AgSAE). The electrode is produced by mechanically refreshing the silver liquid amalgam film (AgLAF) before each measurement. The main constituents of the electrode are: a specially constructed silver solid amalgam annular band electrode (AgSAE), two silicon O-rings, silver liquid amalgam and a polypropylene electrode body. Contaminants from the analyzed solution are removed and the AgSAE surface is covered with a thick layer of fresh amalgam while pulling the AgSAE into the sensor body. During movement in the reverse direction AgLAF is formed and homogenized. The time needed to refresh the film is less than 1 s. The electrode is characterized by excellent surface repeatability (∼1%) and long-term stability (over ten thousand measurement cycles). Keywords: Mercury film electrodes, Silver amalgams, Voltammetry, Polarography, Tensammetr

Synthesis of 2-keto-D- and L-gluconic acid via stereoselective direct aldol reactions

Stereoselective direct aldol reaction between optically pure d- or l-glyceraldehyde and hydroxyacetylfuran is demonstrated as an efficient and straightforward methodology for the synthesis of six-carbon atom d- and l-<i>arabino</i>-hex-2-ulosonic acids. <i>syn</i>-Selective aldol reactions realized by using either tertiary amines or a dizinc aldol catalyst constitute two parallel routes to the <i>de novo</i> synthesis of orthogonally protected biologically relevant 2-keto-d- and l-gluconic acids