Chromophores from hexeneuronic acids: identification of HexA-derived chromophores

Hexeneuronic acids (HexA) have long been known as triggers for discoloration processes in glucuronoxylan-containing cellulosic pulps. They are formed under the conditions of pulping from 4-O-methylglucuronic acid residues, and are removed in an “A stage” along the bleaching sequences, which mainly comprises acidic washing treatments. The chemical structures of HexA-derived chromophoric compounds 4–8, which make up 90% of the HexA-derived chromophores, are reported here for the first time. The compounds are ladder-type, mixed quinoid-aromatic oligomers of the bis(furano)-[1,4]benzoquinone and bis(benzofurano)-[1,4]benzoquinone type. The same chromophoric compounds are generated independently of the starting material, which can be either a) HexA in pulp, b) the HexA model compound methyl 1-¹³C-4-deoxy-β-L-threo-hex-4-enopyranosiduronic acid (1) or c) a mixture of the primary degradation intermediates of 1, namely 5-formyl-furancarboxylic acid (2) and 2-furancarboxylic acid (3). Isotopic labeling (¹³C) in combination with NMR spectroscopy and mass spectrometry served for structure elucidation, and final confirmation was provided by X-ray structure analysis. ¹³C-Isotopic labeling was also used to establish the formation mechanisms, showing all the compounds to be composed of condensed, but otherwise largely intact, 2-carbonylfuran and 2-carbonylfuran-5-carboxylic acid moieties. These results disprove the frequent assumption that HexA-derived or furfural-derived chromophores are linear furanoid polymers, and might have a direct bearing on structure elucidation studies of “humins”, which are formed as dark-colored byproducts in depolymerization of pentosans and hexosans in different biorefinery scenarios.     

Chromophores from hexeneuronic acids: chemical behavior under peroxide bleaching conditions

Hexeneuronic acids (HexA) are a major cause of discoloration (yellowing/brightness reversion) in pulps from xylan-containing wood, being generated from the xylan’s 4-O-methylglucuronic acid residues. The HexA-derived chromophores, whose identification and structure confirmation have been described in the previous part of this series (Rosenau et al. in Cellulose, 2017), were subject to conditions of peroxide bleaching, i.e. treatment with hydrogen peroxide in alkaline medium. These chromophores, ladder-type oligomers of mixed aromatic-quinoid and mixed furanoid-benzoid character, are degraded relatively quickly to one major product, 2,5-dihydroxy-[1,4]-benzoquinone (DHBQ), and a minor component, 2,5-dihydroxyacetophenone (DHA). These two compounds, which have already been identified as two of the three key chromophores (besides 5,8-dihydroxy-[1,4]-naphthoquinone, DHNQ) in aged cellulosics, are potent chromophores themselves and are subsequently more slowly degraded to non-colored degradation products, according to pathways already described in previous parts of this series. The occurrence of DHBQ and DHA in the bleaching treatment of the HexA-derived chromophores establishes the link between HexA chemistry and the key chromophore classes of residual chromophores found in aged cellulosic materials.     

Arylidene pyruvic acids (APAs) in the synthesis of organic compounds

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Novel intermediates for the synthesis of carbocyclic spiro compounds

Cyclic dienaminones, synthesized from the α-carbon elongation reaction of cyclic ketones with vinamidinium salts, are useful synthetic intermediates to carbocyclic spiro compounds.     

Stereoselective synthesis of P-chiral phosphorus compounds from N-tert-butoxycarbonyl amino acids

Reaction of the N-t-Boc amino acids alanine and valine with PhPCl(2) gives the P-chiral trans-1,3,2-oxazaphospholidinones exclusively. Variable-temperature NMR and examination of the glycine derivative shows that the isomers observed are due to t-Boc rotation.     

The synthesis of novel macrocyclic multidentate compounds from dioxodioic acids

A series of novel macrocyclic multidentate crown compounds have been prepared from a series of dioxodioic acids. The dioxodioic acids were prepared by treating various oligoethylene glycols or amines with maleic, succinic, citraconic, and glutaric anhydrides. The crown compounds were then synthesized from the dioxodioic acid chlorides obtained from the diacids and various oligoethylene glycols or ethylene diamine. The following macrocyclic compounds have been prepared: 1,4,9,12-tetraoxacyclohexadecane-5,8,13,16-tetrone ( 1 ); 1,4,7,10,15,18-hexaoxacyclodocosane-11,14,19,22-tetrone ( 2 ); 1,4-dioxa-9,12-diazacyclohexadecane-5,8,13,16-tetrone ( 3 ); 1,4,7-trioxa-12,15-diazacyclonoadecane-8,11,16,19-tetrone ( 4 ); 1,4,7,10-tetraoxa-15,18-diazacyclodocosane-11,14,19,22-tetrone ( 5 ); 4-benzyl-1,7,12,15-tetraoxa-4-azacyclonona-decane-8,11,16,19-tetrone ( 6 ); 1,4,7,12,15,18-hexaoxacyclodocosa-9,20-diene-8,11,19,22-tetrone( 7 ); and 9,21-dimethyl-1,4,7,12,15,18-hexaoxacyclodocosa-9,20-diene-8,11,19,22-tetrone ( 8 ) (Figure 1).     

Stereo-controlled synthesis of intermediates of (±)-thienamycin

Stereo-controlled synthesis of (±)-thienamycin intermediates which have the correct relative configurations at three chiral centers is reported.     

Perfluoroalkanesulfonylimides and their lithium salts: synthesis and characterisation of intermediates and target compounds

ECF processes have been extensively experienced and developed since early 1970s at the Fluorine Chemistry Laboratory of Padua University: several classes of perfluorinated inert and functional compounds have been obtained, in particular perfluoro heterocyclics and perfluorinated acid fluorides.Recently the demand for electrolyte salts, applied in lithium batteries, drove our interest to investigate on the perfluoroalkanesulfonylimides.A series of the perfluoroalkanesulfonylfluoride precursors has been obtained by ECF and their metathesis to the related imides and lithium salts has been investigated.A number of representative products has been obtained and characterized in their structure and ionic conductivity.     

Studies on the syntheses of heterocyclic compounds—DXCI : Total synthesis of (±)-cherylline and corgoine through quinonoid intermediates

Acid catalysed cyclisation of N - (4′ - benzyloxy - β - methoxyphenethyl) - 3 - benzyloxy - 4 - methoxy - N-methylbenzylamine (18) gave (±)-cherylline (1), one of the Amaryllidaceae alkaloids. Fusion of 4-hydroxybenzyl alcohol (22) with 1,2,3,4-tetrahydroisoquinoline (24) also gave corgoine (5).     

Alkali metal polyphosphides as intermediates in the synthesis of organophosphorus compounds from elemental phosphorus

Russian Chemical Bulletin - Methods for the preparation of alkali metal polyphosphides, as well as their reactivity with respect to organic substrates, namely, electrophilic reagents and...     

Studies on synthesis and kinetics of thermodecomposition of the nickel(Ⅱ) compounds with Schiff base derived from amino acids

Monoaqua salicylaldehyde-o-aminobenzoic acid Ni(Ⅱ) monohydrate (cp1) and monoaqua o-vanillin-o-aminobenzoic acid Ni(Ⅱ) monohydrate (cp2) were synthesized. The composition and structures of these two compounds were analyzed. Their thermal stability and non-isothermal kinetics were also investigated by use of TG and DTG curves. The possible reaction mechanisms in their first steps of thermal decomposition reactions were deduced by means of integral and differential methods. Thermodecomposition kinetic equations of the compounds are as follows:Cp1: da/At = A . exp(-E/RT) . 3/2(1 - a)4/3 . [1/(1 - a)1/3 -1]-1 Cp2 : da/At = A . exp(-E/RT) . (1 - a)     

Platinum-mercury compounds as intermediates to mono- and di-arylplatinum(II) complexes

The reaction of HgR₂ (R = 2,5-C₆H₃Cl₂; 2,3,4- and 2,4,6-C₆H₂Cl₃; 2,3,4,5-,2,3,4,6- and 2,3,5,6-C₆HCl₄ and C₆Cl₅) with Pt(PPh₃)₃ gives the new stable compounds [(PPh₃)₂RPt(HgR)] containing PtHg bonds. When R contains an ortho chlorine atom (R = 2,5-C₆H₃Cl₂; 2,3,4-C₆H₂Cl₃ and 2,3,4,5-C₆HCl₄) refluxing xylene solutions of these compounds gives the complexes [PtR₂-(PPh₃)₂], with simultaneous precipitation of mercury. In the other cases the initial compounds are recovered unaltered. All the compounds containing the PtHg bond react readily with CF₃COOH to give a new series of compounds of formula [Pt(O₂CCF₃)R(PPh₃)₂].     

Enantiopure N-acyldihydropyridones as synthetic intermediates: asymmetric synthesis of (-)-slaframine

[formula: see text] An asymmetric synthesis of (-)-slaframine and N-acetylslaframine has been accomplished starting from an enantiopure dihydropyridone building block. The oxygen-carbon bond at C-1 was incorporated with complete stereoselectivity by using an efficient phenylselenocyclocarbamation reaction.     

N-ethoxycarbonylamidines as starting materials and intermediates in the synthesis of heterocyclic compounds

Treatment of N-ethoxycarbonylthioamides ( 1 ) with primary aromatic amines yields N-aryl-N′-ethoxy-carbonylamidines ( 2 ), which thermally cyclize to 2-aryl-4(3H)-quinazolinones ( 6 ). Analogous reactions of 1 with ethyl 3-aminocrotonate and with 2-amino-2-thiazoline lead respectively to ethyl 2-aryl-3,4-dihydro-6-methyl-4-oxo-5-pyrimidinecarboxylates ( 10 ) and to 2-aryl-6,7-dihydro-4H-thiazolo[3,2-a]-1,3,5-triazin-4-ones ( 14 ), presumably through the corresponding N-ethoxycarbonylamidines.     

Enantiospecific syntheses of intermediates in the total synthesis of pseudomonic acids

Enantiospecific syntheses of 1S,6S-3,7-dioxabicyclo[4.3.0]non-4-en-8-one (1) and of the enantiomer (2) from D- and L-arabinose respectively have been achieved by two different routes. The conversion of (1) to 6S-(3R-acetanilido)-3,6-dihydro-2H-pyranyl-N,N-dimethylacetamide (3), a key intermediate in the synthesis of pseudomonic acids, is described.     

Preparative synthesis of palladium(II) acetate: Reactions, intermediates, and by-products

Comparison of various laboratory procedures for the synthesis of palladium acetate demonstrated that the purest product containing no nitrite or nitrate impurities is formed in up to 90% yields upon the reaction of palladium nitrate with alkali metal acetates in aqueous acetic acid. Other laboratory syntheses are more labor-consuming and do not ensure high purity of the product. The synthesis by-products are described and possible reaction schemes are proposed.     

Tris(allyl) indium compounds: synthesis and structural characterization

Allyl and 2-methylallyl indium compounds were prepared by salt metathesis starting from indium trichloride and a Grignard reagent. They are highly fluxional in solution and reveal coordination numbers of the indium atoms of four and five in the solid state.