ammoxidation of lignin , 
 that is , the oxidative conversion of lignin 
 in aqueous ammonia , has been proposed as suitable approach to artificial 
 humic substances , with their large - scale availability being able to 
 greatly support the global efforts to combat soil erosion and desertification . the diversity of different nitrogenous moieties formed during ammoxidation 
 is a key feature of the process . as the various nitrogenous moieties 
 mineralize in soil at different rates , ammoxidized lignins can be 
 used as organo  mineral soil improving materials that supply 
 nitrogen to the plants over several vegetation periods . while older publications aimed at maximizing nitrogen incorporation 
 by application of high concentrations of ammonia , high oxygen pressure , 
 and temperature , it was later demonstrated 
 that ammoxidation in aqueous medium under rather mild conditions ( po2  0.2 mpa , t  
 100 c , cnh3  5% ) affords 
 products which resemble natural humic substances much better in terms 
 of nitrogen contents ( 36% ) , c / n ratio , chemical structure 
 and functional groups . the comparatively 
 low technical expenditure and energy demand of near ambient pressure 
 ammoxidation are strong pros with respect to economic viability and 
 render this technology a promising approach for large - scale conversion 
 of lignin to nitrogenous soil improving materials . next to ( purified ) 
 technical lignins whose price and multipurpose 
 utilization permanently increases , the huge quantities of globally 
 generated lignocellulosic harvest residues ( e.g. , rice straw , cotton 
 stalks , palm empty fruit bunches ) are increasingly considered as cheap 
 and suitable sources of organo  mineral soil improving materials . however , different from purified technical lignins lignocellulosic 
 sources are associated with varying amounts of mono- , oligo- , and 
 polysaccharides . the fate of saccharides under ammoxidative conditions , 
 and in particular the formation of nonheteroyclic nitrogenous compounds 
 has been therefore investigated in this study . recently we reported 
 about the constitution of product mixtures 
 obtained by ammoxidation of the monosaccharides glucose , xylose and 
 the corresponding polysaccharides cellulose and xylan under different 
 conditions . it was shown that the chemical 
 integrity of cellulose and xylan is largely maintained at moderate 
 temperature and oxygen pressure ( 70 c , 0.2 mpa ) which was evident 
 from the low amounts of low - molecular degradation products , the only 
 slightly reduced weight - average molecular weight ( mw 153 vs 139 kg / mol ) and the nearly constant amount of carbonyl groups , 
 as shown for cellulose . at elevated temperature and oxygen pressure 
 ( 140 c , 1.0 mpa ) cellulose was severely degraded ( mw = 43/kg mol ) and both alkaline peeling / stopping reactions and alkali - induced 
 chain scission start playing a major role , resulting in an increased 
 amount of monosaccharides and follow - up products . n - heterocyclic compounds such as 1h - imidazole- , pyridine- , 
 and pyrazine derivatives were demonstrated 
 to be formed from carbohydrates under ammoxidative conditions . while 
 the amounts of n - heterocyclic compounds formed at 
 70 c were extremely low for the polysaccharides ( 1630 
 g / g educt ) and low for glucose ( 15.4 mg / g educt ) , they increased 
 considerably at higher temperature ( polysaccharides at 140 c : 
 5.5216.03 mg / g educt ; monosaccharides at 100 c : 122.4160.5 
 mg / g educt ) . extracts obtained from the crude ammoxidation products 
 exhibited considerable ecotoxicity in terms of inhibiting the growth 
 of the fresh water algae pseudokirchneriella subcapitata ( oecd test 201 ) . 4-methyl-1h - imidazole , 4-(hydroxymethyl)-1h - imidazole and 3-hydroxypyridine were those carbohydrate - derived 
 ammoxidation products that had the highest ecotoxicity . the 
 reactions of carbohydrates under alkaline conditions in general , 
 in aqueous ammonium hydroxide in particular , or in maillard - type reactions 
 with amino acids have been the subject of numerous studies . lobry de bruyn - van ekenstein and amadori rearrangements are the 
 typical initial reactions that occur in alkaline medium and can convert 
 one single aldose into a set of isomeric sugars and aminodeoxysugars , 
 respectively . direct condensation of ammonia with the hemiacetal functionality 
 of the respective aldose leads to glycosylamines . under basic conditions , 
 reactive -dicarbonyl compounds ( osones ) form from sugars . aldonic 
 and deoxyaldonic acids are follow - up products formed by degradation 
 of sugars under alkaline conditions . hence it was assumed that a broad 
 spectrum of nitrogenous compounds besides the aforementioned n - heterocycles would also be present in the crude ammoxidation 
 mixtures of mono- and polysaccharides . the current study investigates 
 the temperature - dependent and time - dependent 
 formation of main classes of low - molecular , nitrogenous , water - soluble 
 reaction products , with an emphasis on non - n - heterocyclic 
 compounds ( aminosugars , glycosylamines , ammonium salts of aldonic , 
 deoxyaldonic and carbaminic acids , urea , -hydroxyamides , and 
 -amino acids ) under ammoxidative conditions , using d - glucose and d - xylose as model monosaccharides , which however 
 are highly pertinent to carbohydrate residues in technical lignins . the kinetics of their formation and conversion to follow - up products 
 are discussed in the context of proposed reaction mechanisms found 
 in the literature . all chemicals including d - glucose and d - xylose 
 were purchased from sigma aldrich . ethyl acetate was dried over anhydrous 
 calcium chloride , distilled once with acetic anhydride and sulfuric 
 acid and subsequently over potassium carbonate , in order to remove 
 water , ethanol and acetic acid , respectively . pyridine and ethyl acetate 
 used for derivatization were stored over molecular sieves ( 4a ) , and 
 filtered through a 0.45 m syringe filter prior to use . was accomplished in a 4566 c series 100 
 ml laboratory - scale pressure vessel ( parr instruments , frankfurt , 
 germany ) equipped with a glass liner and a septum valve . ten ml of 10% aqueous ammonia 
 that contained 50 l of 2-methyl quinoxaline and 20 l 
 of phosphoric acid ( 85% ) as internal standards were placed into the 
 above - described reactor . after flushing three times with oxygen the 
 reactor was pressurized with o2 to 0.2 mpa and heated under 
 continuous stirring to the respective temperature ( 70 c , 100 
 or 140 c ) . it was left in connection with the gas supply via 
 a back - pressure valve in order to guarantee constant pressure throughout 
 the reaction . the effective pressure including the respective vapor 
 pressure of 5% aqueous ammonia was 0.27 mpa ( 70 c ) , 0.38 mpa 
 ( 100 c ) , 0.68 mpa ( 140 c ) . a solution of 1.0 g of the 
 respective monosaccharide in 10 ml of water was added through a septum 
 with a syringe , initiating the reaction ( t = 0 ) . samples of approximately 80 l were taken with a syringe after 
 1 , 3 , 5 , 10 , 15 , 20 , 30 , 45 min , and 1 , 1.5 , 2 , 3 , 4 , and 19 h. each 
 of those samples was split into three portions of 20.0 l , which 
 were immediately deep - frozen on dry ice , and subject to freeze - drying , 
 silylation , and gc / ms analysis ( n = 3 ) . freeze - dried aliquots ( 25 c , 
 1020 pa , 24 h ) of the crude reaction mixtures containing approximately 
 1 mg of dry matter were subject to gc / ms analysis after silylation 
 which was performed by adding 200 l of dry pyridine containing 
 1.5 g / l of the catalyst 4-(n , n - dimethylamino)-pyridine 
 ( dmap ) , and 10 l of a 10 g / l solution of the internal standard 
 phenyl--glucopyranoside in pyridine . the mixture was vortexed 
 for a minimum of 30 s , before 100 l of the silylation mixture 
 ( bstfa containing 10% trimethylchloro - silane ) was added . silylation 
 was performed at 70 c for 2 h. after cooling to room temperature , 
 900 l of dry ethyl acetate was added , the mixture was vortexed 
 again , and analyzed by gc / ms not later than 24 h after derivatization . one gram of d - glucose ( 5.55 mmol ) and 0.438 g 
 of ammonium hydrogen carbonate ( 5.55 mmol ) were dissolved in 27.75 
 ml of 26% aqueous ammonia , and stirred at 42 c for 36 h. the 
 solution was evaporated under reduced pressure to 10 ml and subsequently 
 freeze - dried . the resulting pale - brown solid of glucopyranosylamine 
 condenses at room temperature under emission of ammonia quantitatively 
 to di(glucopyranosyl)amine within two weeks . ei - ms ( 70 ev ) of 
 bis(2,3,4,6-tetra - o-(trimethylsilyl)-glucopyranosyl)amine : m / z 918 ( m ; 0.03% ) , 903 ( m ch3 ; 1.4% ) , 815 ( m ch2osime3 , 2.3% ) , 581 ( 24.0% ) , 568 ( 58.0% ) , 450 ( 5.0% ) , 
 378 ( 3.8% ) , 361 ( 49.8% ) , 332 ( 15.3% ) , 319 ( 6.7% ) , 305 ( 3.4% ) , 271 
 ( 8.0% ) , 243 ( 8.3% ) , 232 ( 8.0% ) , 217 ( 71.9% ) , 204 ( 100% ) , 191 ( 9.6% ) , 
 169 ( 7.4% ) , 147 ( 36.9% ) , 129 ( 17.3% ) , 117 ( 11.5% ) , 103 ( 16.9% ) , 73 
 ( 82.4% ) . gc - ms , data acquisition and processing , and tentative 
 peak assignment 
 were accomplished as described elsewhere . kinetic data following an exponential 
 decay were fitted with functions 1 or 2:12data that rise toward a 
 limit were fitted 
 with function 33for concentrations running through a maximum , 
 the giddings peak function , eq 4 , was used as implemented in the origin 7.1 software 
 package ( originlab corporation , northampton , ma)4 per - trimethylsilylation 
 and subsequent gc / ms analysis of the reaction 
 mixtures obtained by ammoxidation of glucose and xylose revealed a 
 far - reaching similarity of the low - molecular product pattern for the 
 two monosaccharides within the studied temperature range ( 70140 
 c , figure 1 , table 1 ) . differences between glucose and xylose were confirmed 
 to be mainly related to those compounds formed under far - reaching 
 preservation of the chain integrity and stereoconfiguration of the 
 respective monosaccharide , as shown for d - glucose ( figure 2 , 99 ) . this is the case for > c3 sugar acids ( e.g. , hexonic ( 100 ) , pentonic ( 101 ) , and tetronic ( 102 ) acid and their 2- and 
 3-deoxy derivatives ) , amino sugars ( e.g. , glucosamine / mannosamine 
 ( 103 ) , fructosamine ( 107 ) , di(glucopyranosyl)amine 
 ( 104 ) ) , imidazoles ( e.g. , 4-(d - arabino - tetrahydroxybutyl)-1h - imidazole ( 105 ) ) and pyrazines carrying oligohydroxyalkyl side chains that contain 
 more than one carbon atom , such as , for example , fructosazine and 
 deoxyfructosazine ( 106 ) . the preservation of stereochemical 
 information , however , is more pronounced with glucose than with xylose . 
 the significantly larger number of very small peaks between 30 and 
 40 min retention time in the case of ammoxidized xylose is predominantly 
 caused by isomeric oligohydroxyalkyl pyrazines . while for glucose , 
 isomerization reactions in early stages of ammoxidation ( lobry de 
 bruijn - van ekenstein reaction ) are mostly restricted to the c1 and 
 c2 stereocenters , they involve also other stereocenters ( c3 , possibly 
 others ) to a greater extent with xylose , which caused a larger variety 
 of diastereomeric oligohydroxyalkyl pyrazines . however , apart from 
 these differences , comparison of the product spectra of glucose and 
 xylose upon ammoxidation show large similarities , and thus suggest 
 that both cases share the same underlying reaction pathways . therefore , d - glucose was further used as a representative monosaccharide 
 to study the impact of the ammoxidation temperature on the product 
 pattern . gc / ms spectra ( after per - trimethylsilylation ) of the low - molecular 
 product fractions obtained from glucose and xylose , respectively , 
 after ammoxidation ( 100 c , 0.2 mpa o2 , 3 h ) . ammoxidation products of glucose with preserved 
 glucose stereoconfiguration . identified by comparison with 
 authentic samples . the gas chromatograms 
 of the per - trimethylsilylated crude products 
 obtained from glucose at 70 c , 100 c , and 140 c 
 ammoxidation temperature ( 0.2 mpa o2 , 3 h ) are shown in 
 figure 3 . the pie diagrams give an overview 
 of principal changes in the product pattern in terms of both overall 
 concentration ( size of pie chart ) and the main classes of relevant 
 compounds ( parts of pie chart ) , that is , sugars , amino sugars ( containing 
 both amadori products and glycosyl amines ) , organic acids including 
 amides , and n - heterocyclic compounds . the latter 
 were grouped into 1h - imidazole and pyrazine derivatives 
 to better visualize the temperature - dependent contribution of the 
 main reaction pathways to n - heterocyclic compounds . pyridines , as another group of azabenzene derivatives formed upon 
 ammoxidation , were integrated into the group  others  
 due to both their small amounts ( highest value 0.7% at 140 c ) 
 and the mechanism of their formation which substantially differs from 
 that of the 1h - imidazole and pyrazine derivatives , 
 respectively . chromatograms of the 
 crude reaction mixtures obtained by ammoxidation 
 of d - glucose at different temperature ( 70 c , 100 c , 
 140 c ; 0.2 mpa o2 , 3 h ) after freeze - drying and per - trimethylsilylation . inserts : pie diagrams show the semiquantitative constitution of the 
 crude products in terms of main classes of organic compounds . the 
 area of the pie diagrams corresponds to the total peak area of gc / ms - detectable 
 compounds in the respective chromatogram . sugars ( 18% of the total peak area ) , amino sugars ( 24% ) 
 and organic acids / amides ( 38% ) constituted the largest product fraction 
 at 70 c , whereas only a very low quantity of n - heterocyclic compounds is formed at this temperature . at 100 c , 
 the overall yield of low molecular compounds decreased . short - chain 
 acids ( c2 to c6 ) , pyrazines and 1h - imidazoles accounted here for more than three - fourths of the total 
 peak area , while sugars and amino sugars were already largely consumed 
 by follow - up reactions , their fraction being reduced to about 10% . the amount of gc / ms - detectable compounds decreased further when raising 
 the ammoxidation temperature to 140 c . organic acids / amides 
 ( 47% ) and pyrazines ( 42% ) were the dominating products , while sugars 
 and amino sugars were reduced to low levels ( about 0.2% and 0.1% , 
 respectively ) . the significant reduction in the absolute amount of low - molecular 
 compounds within the studied temperature range of 70140 c 
 can be caused by both increasing oxidative degradation to gaseous 
 compounds , such as co2 , and polymerization reactions of 
 low - molecular intermediates . the occurrence of the latter was evident 
 from the observed temperature - dependent discoloration of the reaction 
 mixtures which was inversely correlated with the total gc / ms peak 
 area . while an only faintly colored solution was obtained after 18 
 h of ammoxidation at 70 c , the reaction mixture at 140 c 
 turned dark brown within the first five minutes . this observation 
 is in accordance with literature , which stated that the color of maillard - type 
 reaction products originates almost exclusively from polymeric substances 
 which can comprise up to 90% of the total product mixture . the time - dependent and temperature - dependent consumption of 
 glucose 
 and isomeric sugars , the formation of main classes of nitrogenous 
 reaction products , and their involvement in onward reactions are summarized 
 in figure 4 . the kinetics of the conversion 
 of glucose and isomeric sugars confirm that the concentration of the 
 monosaccharides in the reaction mixtures decreases exponentially with 
 time , with the highest conversion rate at 140 c . the shape of 
 the respective curves , following the function f(x ) = a1(e+ e ) , furthermore 
 indicates that glucose is consumed by at least two reaction pathways . 
 in alkaline medium the lobry - de bruijn  van ekenstein reaction 
 leads to equilibria of glucose with isomeric sugars such as fructose 
 and mannose . simultaneously , the presence of ammonia leads to the 
 formation of an equilibrium with glycosyl amine and diglycosyl amine . 
 until the respective equilibria are reached , glucose consumption is 
 very fast . then , the different compounds involved react in slower 
 follow - up reactions , which is evident from the declining slope of 
 the monosaccharide concentration curve . change in concentration 
 of different substance classes during ammoxidation 
 ( isobar 0.2 mpa o2 ) of glucose at 70 c ( a ) , 100 c 
 ( b ) , and 140 c ( c ) . legend : solid black squares , sugars ; open 
 white squares , aminosugars ; solid black circles , acids and amides ; 
 open white circles , n - heterocyclic compounds ; solid 
 gray triangles , nonidentified compounds . the concentration profiles of the amino sugars ( amadori products , 
 glycosyl amines ) were found to differ largely with ammoxidation temperature . 
 while at 70 c the concentration profile of amino sugars followed 
 the typical kinetics of intermediate products , and the concentration 
 proceeded through a maximum , their initial formation was so fast at 
 100 c and 140 c that the concentration increase could 
 not be detected with the used analytical setup . after consumption 
 of sugars and amino sugars , acids and amides formed the largest product 
 fraction , followed by heterocyclic compounds . heterocycles , 
 acids and amides can be considered end products of 
 the reaction at 70 c , as the respective maximum concentrations , 
 once reached , did not further change within the studied time period . 
 at 100 c , the kinetics of the formation of acids and amides 
 was quite similar to that observed at 70 c , however , the decline 
 in concentration at long reaction times ( > 240 min ) suggests that 
 some 
 of the formed n - heterocyclic compounds were consumed 
 in follow - up reactions . at 140 c , not only the concentration 
 of n - heterocycles started to decline even earlier 
 ( maximum at 30 min ) , but also the concentration of acids / amides was 
 declining significantly at > 120 min reaction time . the recurring 
 increase 
 of the acid / amide concentration as observed at 140 c after about 
 four hours of ammoxidation was owing to the formation of glycolic 
 and oxalic acids . as the above considerations on the kinetics 
 of whole compound classes 
 do not properly reflect the great differences in the product distribution 
 within one class , a closer look into the reaction mechanisms leading 
 to the formation of major representatives of the different compound 
 classes seemed appropriate . glycosyl amines , aminodeoxy sugars , and isomers 
 of the nonaminated sugars are the very first reaction products of 
 reducing sugars and ammonia . while glycosyl amines are simply formed 
 by condensation of ammonia with the hemiacetal functionality of the 
 respective aldose , isomeric sugars and aminodeoxysugars ( i.e. , 1-deoxy-1-amino 
 ketoses and 2-deoxy-2-amino aldoses ) are formed by lobry de bruyn - van 
 ekenstein and amadori rearrangements , respectively . the 1,2-enediols 
 and 1,2-enolamines , which are intermediates of these rearrangements , 
 can be either oxidized or dehydrated to form osones , that is , -dicarbonyl 
 compounds , which in turn are key intermediates in many follow - up reactions . the multitude of these sugars , amino sugars , and dicarbonyl compounds 
 forms a pool of compounds from which smaller fragments are formed 
 by base - catalyzed retro - aldol and retro - claisen scissions , leading to shortened fragments , and 
 small acids and aldehydes . especially the latter are formed independently 
 from the starting monosaccharide , and their generation is the main 
 reason for the largely similar product spectra despite different starting 
 carbohydrates ( e.g. , glucose versus xylose ) . gc / ms analysis 
 of the crude products after trimethylsilylation , however , did not 
 show any carbohydrates of lower chain length than that of the educt , 
 -dicarbonyl compounds or small aldehydes . this is due to the 
 high reactivity of such compounds which are usually immediately consumed 
 by follow - up reactions affording products of higher stability . -dicarbonyl 
 compounds under the respective ammoxidation conditions , for example , 
 are so reactive that they can be only detected when trapped with a 
 suitable reagent . therefore , only the 
 presence of fructose , mannose and glucosamine could be verified by 
 comparison with commercially available compounds . based on the fragmentation 
 pattern of the prepared persilylated di(glucopyranosyl ) amine ( 73 ) , other smaller peaks in the reaction mixture of glucose 
 at 70 c were assigned , such as that of hexopyranosyl - pentosyl 
 amine , or the major aminoglycoside of xylose ammoxidized at 70 c , 
 di(pentopyranosyl ) amine . the silylated crude ammoxidation product 
 of glucose obtained at 
 70 c consisted of 18 different monoglycosyl amines ( retention 
 time 3640 min ; figure 1 ) and diglycosylamines 
 ( retention time 4149 min ) , respectively , with largely similar 
 mass spectra . additional peaks in the same retention time ranges did 
 show typical hexopyranosyl fragments ( i.e. , m / z 451 , 361 , 217 , 204 , 190 ) , but could not be unambiguously 
 assigned . the ratio of diglucosyl amines to monoglycosyl amines increased 
 among otherwise identical samples with longer storage time , higher 
 storage temperature and longer lyophilization duration . just as for 
 the sugars , also the glycosyl amines contributed a major fraction 
 to the reaction mixture at 70 c , they were found only in traces 
 at 100 c , and they were absent at 140 c . a considerable fraction of the reaction 
 products , in particular of those obtained at 70 c , consisted 
 of a homologous series of aldonic acids , i.e. aldoses with their terminal 
 aldehyde / hemiacetal group having been converted into a carboxyl group 
 ( table 2 ) . values were calculated as ratio 
 of the relative peak areas of aldonic acids and the internal standard 
 phenyl -glucoside ( 200 g ) . the concentration of all detected aldonic acids covering 
 the range 
 from c2 ( glycolic acid ) to c6 acids ( mannonic and gluconic acid ) rose 
 quickly as long as monosaccharides were present in the reaction mixture 
 in significant amounts , with the reaction rate being proportional 
 to the monosaccharide concentration ( figure 5 ) . the highest total amounts of aldonic acids were obtained at 70 
 c consisting mainly of c5/c6 acids . at 100 c and above , 
 the concentration of the longer acids ( c5 and c6 ) declined in favor 
 of those with shorter chains after consumption of monosaccharides . glycolic and glyceric acid concentrations rose throughout the observed 
 reaction time , while the tetronic acids had their maxima between 6 
 and 18 h reaction time . upon ammoxidation at 140 c , hexonic 
 acids reached their maximum after 30 min and were completely consumed 
 after 4 h , while glycolic acid concentration rose until 6.5 h reaction 
 time , and only showed the beginning of a concentration decline at 
 the last sampling time of 18 h. this is in contradiction to other 
 authors , who claim that aldonic acids 
 are stable end products of alkaline sugar degradation , albeit in their 
 studies the effect of oxygen and ammonia beyond that of mere alkalinity 
 was not considered . change in concentration of different aldonic acids during 
 ammoxidation 
 ( isobar 0.2 mpa o2 ) of glucose at 70 c ( a ) , 100 c 
 ( b ) , and 140 c ( c ) . legend : solid black squares , glycolic acid ; 
 open white squares , glyceric acid ; solid black circles , tetronic acid ; 
 open white circles , pentonic acid ; solid black triangles , hexonic 
 acid . the tetronic , pentonic , and hexonic 
 acids were shown to afford 
 two peaks , each of very similar mass fragment patterns due to the 
 formation of diastereomers . the different diasteromers were identified 
 by comparison with the retention times and mass spectra of silylated 
 authentic samples . of all the possible diastereomeric aldonic acids 
 of respective chain lengths , only the corresponding c-2 epimers were 
 found , which implies that the hexonic acids had the same configuration 
 at c-3c-5 as glucose ( gluconic and mannonic acids , but not , 
 for example , allonic acid ) . similarly , in pentonic acids the configuration 
 of c-4 and c-5 of glucose was preserved : arabinoic and ribonic acids 
 were found , but not , for example , xylonic acid . this observation 
 allows for some important conclusions with regard 
 to the mechanisms leading to the formation of aldonic acids . base - catalyzed 
 migration of the carbonyl group through the whole sugar backbone , 
 which would lead to complete epimerization of the carbon centers , 
 does not occur under the chosen conditions . such carbonyl migrations , 
 however , were observed for reactions in near - neutral , oxygen - free 
 media ( ph 7.4 phosphate buffer ) and lower concentrations of amino 
 acids and also for monosaccharides in n - methylmorpholine - n - oxide . under the present , harsher conditions they are 
 probably outrun by faster oxidation , rearrangement , condensation , 
 and chain scission reactions . also , build - up of pentoses and hexoses 
 from smaller sugars by aldol - type or formose - type condensation can 
 be ruled out as a major reaction pathway , as it would cause epimerized 
 c3 or c4 centers of the hypothetic products , which however were not 
 observed . other higher carbohydrates formed by aldol - type condensation 
 were also not detected in the ammoxidation products , such as various 
 > c6 sugars and acids which were reported by de bruijn et al . who treated glucose with 10 mm koh at 78 c 
 for 7h . several possible formation mechanisms are discussed 
 in literature 
 for the formation of aldonic acids ( figure 6 ) , most of which start from -dicarbonyl compounds ( 109 ) . suggested pathways are oxidative cleavage ( d ) ; benzilic acid rearrangement ( f ) ; 
 hydrolytic -dicarbonyl cleavage via oh addition 
 and subsequent elimination of a formyl anion ( e ) ; isomerization to a -dicarbonyl compound and subsequent retro - claisen 
 scission ( g and h ) lead to aldonic acids 
 shortened by two carbon atoms . oddly , direct oxidation of the aldehyde 
 group without involvement of adjacent groups , for example , by canizzarro 
 reaction ( i ) or direct action of oxygen or reactive oxygen 
 species ( j ) , is usually not considered in the context 
 of alkaline sugar degradation . reaction mechanisms of the formation of 
 aldonic acids and 2- and 
 3-deoxyaldonic acids . besides aldonic acids , two series 
 of deoxyaldonic acids were detected in the gas chromatograms : 2-deoxyaldonic 
 acids and 3-deoxyaldonic acids . the homologous series of 3-deoxyaldonic 
 acids comprised the chain lengths c3 (= lactic acid ) to 
 c6 . similar to the case of aldonic acids , we observed peak 
 pairs of quite similar mass spectra for 3-deoxyaldonic acids of c5 and c6 chain lengths due to epimer formation , 
 while only one peak was observed for the 2-deoxyaldonic acids , regardless 
 of their chain lengths ( table 3 ) . values were calculated as ratio 
 of the relative peak areas of aldonic acids and the internal standard 
 phenyl -glucoside ( 200 g ) . 3-deoxyaldonic acids are well - known products of alkaline 
 sugar 
 degradation (  metasaccharinic acids  ) . , aldoses form 3-deoxy-1,2-dicarbonyl 
 compounds ( deoxyosones , figure 6 , pathway a ) . these undergo similar reactions as their nondeoxy counterparts : 
 benzilic acid rearrangement ( pathway f ) affords 3-deoxyaldonic 
 acids ( 110 ) , while oxidative cleavage leads to 2-deoxyaldonic 
 acids shortened by one carbon . an alternative literature 
 pathway proceeding through a ketene intermediate ( c ) was deemed unlikely due to the high expected 
 activation energy of such a process . for the 3-deoxyhexonic 
 acids , a preferential formation of one of 
 the 2-epimers was observed , which has also been reported and explained in literature for similar reaction systems . as compared to the 3-deoxyhexonic acid epimers , this 
 is likely due to the higher equilibrium concentration of the furanose 
 form of the respective precursor molecule 3-deoxy - pentosone in comparison 
 to 3-deoxyhexosone which leads to less stereochemical 
 induction . the amount of deoxyaldonic acids and of shorter - chain 
 acids were 
 shown to increase with ammoxidation temperature at the expense of 
 longer - chain and nondeoxy acids . at 70 c , shorter - chain ( < c6 ) deoxyaldonic acids were found in relatively small amounts , 
 and only the two products that can be directly formed from 3-deoxyglucosone , 
 that is , 3-deoxygluconic acid and 2-deoxypentonic acid , were present 
 in larger amounts . c 
 bond cleavage by retro - aldol / retro - claisen reactions , while 2-deoxypentonic 
 acid is formed by oxidation reactions ( enediol oxidation followed 
 by oxidative cleavage , figure 6 ) , suggesting 
 that the latter require less activation energy than c  c bond 
 cleaving reactions . all of the deoxyaldonic acids found originate 
 from 3-deoxyosones 
 as reactive intermediate , while no follow - up products of the frequently 
 described 1-deoxyosones were found , such as parasaccharinic acids 
 ( i.e. , 2-c - methylpentonic acids ) or pyrazines with 
 a 2-oligohydroxyalkyl-3-methyl substitution pattern , at least in glucose 
 ammoxidation mixtures . ( 5 ) formation of the 
 1-deoxyosone requires tautomeric migration of the aldose s 
 carbonyl group to form the respective 3-ketoaldose , followed by -hydroxy 
 carbonyl elimination of the 1-hydroxy group . this again indicates 
 that , at least for glucose , tautomeric migration of the carbonyl group 
 along the chain over more than one carbon does not occur to a measurable 
 extent . however , parasaccharinic acids ( c4c5 ) were detected among the ammoxidation products of xylan ( 100 
 c - 140 c , 0.2 mpa o2 , 3 h , data not shown ) . 
 in xylan ammoxidation mixtures , low concentrations of free sugars 
 were formed constantly by hydrolysis of the polysaccharide chain ( data 
 not shown ) . this demonstrates that the product spectrum and the prevalence 
 of the different reaction pathways is also dependent on the sugar 
 concentration : higher sugar concentrations apparently facilitate condensation 
 reactions leading to n - heterocycles and prevent extensive 
 isomerization , while low sugar concentrations preferably promote base - catalyzed 
 isomerizations and rearrangement reactions , thus affording a greater 
 variety of carbonic acids , but less n - heterocyclic 
 compounds . the overall picture of the reaction kinetics of deoxyaldonic 
 acids 
 was similar to that of the aldonic acids : at 70 c , all acids 
 were following pseudo - first - order formation kinetics , while at 100 
 c significant degradation of the longer - chain acids occurred 
 as competitive consumption process , and at 140 c , after a very 
 short build - up period , most of the acids were progressively consumed 
 ( figure 7 ) . however , there are some differences : 
 the stability of 3-deoxy acids was much higher as compared to the 
 corresponding aldonic acids . while gluconic acid , for example , was 
 consumed within 30 min at 140 c ( figure 5 ) , 3-deoxygluconic acid stayed the most abundant acid even after 
 3 h reaction time . this suggests that degradation and further consumption 
 reactions of the onic acids involve the hydroxy groups in - 
 and -position to the carboxylic acid , for example , by a facilitated 
 elimination of the -hydroxy group and formation of an enol 
 in conjugation to the carboxyl group as the key step . at 140 c 
 however , even the deoxyaldonic acids are relatively rapidly degraded 
 over time , with dehydration and fragmentation reactions in the polyhydroxyalkyl 
 tail of the acids likely being responsible . change in concentration 
 of different deoxyaldonic acids during 
 ammoxidation ( isobar 0.2 mpa o2 ) of glucose at 70 c 
 ( a ) , 100 c ( b ) and 140 c ( c ) . legend : solid black squares , 
 lactic acid ; open white squares , 3-hydroxypropanoic acid ; solid black 
 circles , 2,4-dihydroxybutyric acid ; solid black triangles , 3,4-dihydroxybutyric 
 acid ; open white circles , 2-deoxypentonic acid ; open white triangles , 
 3-deoxypentonic acid . surprisingly , 
 minor amounts of the -amino acids glycine , alanine and serine 
 were found in all crude products , together with larger amounts of 
 the -hydroxy amides glycol amide and lactamide . the total 
 amounts of both amino acids and hydroxy amides were shown to be a 
 function of the ammoxidation temperature ( table 4 ) . while the highest level of amino acids was reached at 100 c , 
 the total amount of -hydroxy amides decreased from 70 to 140 
 c . among the amino acids , glycine contributed most and serine 
 fewest at all temperature levels . glycol amide as the major -hydroxy 
 amide was preferably formed at lower temperature ( 70 c ) . values were calculated as ratio 
 of the relative peak areas of aldonic acids and the internal standard 
 phenyl -glucoside ( 200 g ) . the kinetics of -hydroxy 
 amide and -amino acid formation 
 and consumption at 70 c , 100 c , and 140 c ammoxidation 
 temperature is shown in figure 8 . as in the 
 case of aldonic acids , -amino acids and -hydroxy amides 
 followed pseudo - first - order kinetics at 70 c . at 100 c , the amino acids reached a plateau value , while both hydroxy amides 
 start to undergo follow - up reactions within the studied time period , 
 probably due to alkaline hydrolysis . this corroborates their formation 
 via a rearrangement reaction rather than by direct conversion of the 
 acid with hot aqueous ammonia . at 140 c , both -amino 
 acids and -hydroxy amides undergo degradation after a steep 
 concentration increase in the first few minutes of the reaction . while 
 for the amides , also a base - catalyzed hydrolysis can be assumed , the 
 amino acids could take part in maillard reaction cascades with their 
 amino group . in that sense , amino acids may be regarded as important 
 intermediary products in the reaction of ammonia with sugars . change in concentration 
 of the amino acids glycine , alanine and 
 serine and of the -hydroxy amides glycol amide and lactamide 
 during ammoxidation ( isobar 0.2 mpa o2 ) of glucose at 70 
 c ( a ) , 100 c ( b ) , and 140 c ( c ) . legend : solid black 
 squares , glycine ; open white squares , glycol amide ; solid black circles , 
 alanine ; open white circles , lactamide ; solid black triangles , serine . to our best knowledge , the formation 
 of -amino acids and 
 -hydroxy amides from saccharides and we suggest that the formation of -amino acids and -hydroxy 
 amides follows a common mechanism , which is similar to the formation 
 mechanism of aldonic acids from -dicarbonyl compounds ( see discussion above ) . this can occur 
 either by oxidation of amino sugars ( pathway g ) , by condensation 
 of a dicarbonyl compound with ammonia 
 ( pathway h ) or by classic maillard reaction from already 
 formed amino acids and saccharides via amadori rearrangement , strecker 
 degradation , oxidation , and dehydration ( not depicted ) . the latter pathway allows the new formation and 
 interconversion of amino acids in the maillard reaction . the -carbonyl 
 imines are then attacked by a hydroxide ion ( figure 9 , pathway k ) either at the carbonyl group or 
 at the imino group . subsequent benzilic acid rearrangement affords 
 either -amino acids or -hydroxy amides , depending on 
 whether the rearrangement starts from the imino or the carbonyl group . this mechanism could apply to both -keto aldimines and -imino 
 aldehydes ( not depicted ) . overview of main reactions of carbohydrates 
 under ammoxidative 
 conditions : pathway a : lobry de bruyn  van ekenstein 
 transformation . direct conversion of the respective acids into amides in 
 the presence 
 of ammonia can be excluded , as this would require temperatures clearly 
 above 100 c and nonaqueous conditions . there is no satisfying explanation yet why only the above - mentioned 
 three amino acids were formed , and not all products that are possible 
 following pathway k ( figure 9 ) , 
 that are , all c2 to c6 2-deoxy-2-amino aldonic 
 acids , 2,3-dideoxy-2-amino aldonic acids and the respective -hydroxy 
 amides . one explanation could be that the proposed mechanism occurs 
 only in the more reactive open - chain c2 and c3 compounds , whereas longer - chain compounds are protected by hemiacetal 
 formation . however , for most of these additional compounds no references 
 are available so that the compounds may remain hidden in the crowded 
 regions of the chromatograms and thus elude identification . besides the 
 already discussed compound classes , a few compounds not fitting into 
 the other compound classes were also found . depending on the reaction 
 temperature , either the carbamate ( 70 c ) or urea ( higher temperatures ) 
 is formed . oxalic acid is formed continuously throughout the 
 reaction at all temperatures , suggesting that it is the final oxidation 
 product of c2 units that were split off in the various reaction pathways . acetamide could form in a similar manner as acetic acid from respective aza - analogous intermediates ; i.e. either by retro - claisen 
 scission of -iminocarbonyl compounds or oxidative cleavage 
 of -iminocarbonyl compounds . the main pathways involved in the formation of 
 the different classes of nitrogenous compounds from monosaccharides 
 under ammoxidation conditions have been summarized in a simplified 
 overall reaction scheme ( figure 9 ) . first , 
 sugars undergo base - catalyzed isomerizations and fragmentation ( retro - aldol 
 and retro - claisen scissions ) reactions , yielding a broad spectrum 
 of sugars ( and small aldehydes ) of all chain lengths between c1 and c6 ( in case of a hexose as starting material ) . aldol condensations of shorter fragments back to larger carbohydrate 
 derivatives were not observed under ammoxidation conditions . these 
 different sugars undergo either condensation with ammonia , followed 
 by amadori rearrangement to afford amino sugars ( figure 9 , pathway c ) ; oxidation by oxygen to osones ( pathway e ) , or base - catalyzed dehydration to deoxyosones ( d ) . oxidative cleavage ( i ) or benzilic acid rearrangement 
 ( j ) of these compounds afford aldonic acids and their 
 2- and 3-deoxy derivatives . oxidation ( g ) or dehydration 
 ( f ) of amino sugars or condensation of osones with ammonia 
 ( h ) affords -carbonyl imines . these compounds , 
 which also form through strecker degradation in the reaction of -amino 
 acids and sugars , either undergo a benzilic 
 acid rearrangement ( k ) to form an -amino acid 
 or an -hydroxy amide or condense to form heterocyclic structures . condensation with a reactive aldehyde and ammonia affords imidazoles 
 ( l ) , condensation with an -amino carbonyl compound 
 ( i.e. , amino sugar ) affords pyrazines ( m ) , and condensation 
 with ammonia and an -dicarbonyl compound affords pyrazinols 
 ( n ) . cyclization and dehydration 
 of amino sugars affords pyridinols ( o ) . for the 
 sake of clarity , for instance , the condensation of two amino - sugars 
 forms dihydropyrazines , which are subsequently oxidized to pyrazines . while higher reaction temperatures favor fragmentation , dehydration , 
 polymerization , and formation of pyrazines , lower temperatures promote 
 product mixtures dominated by long - chain amino sugars , sugar acids , 
 and imidazoles . the results of the current study that investigated 
 the fate of 
 saccharides upon ammoxidation of lignocellulosic materials revealed 
 that a large variety of partially phytotoxic nitrogenous compounds can be formed from this fraction , depending 
 on the reaction conditions and amount of saccharides present in the 
 parent material . however , ammoxidation using mild reaction conditions 
 as realized by the near ambient pressure technology ( po2  0.2 mpa , t  70 c , cnh3  5% in water ) was confirmed to afford only low 
 amounts of ecotoxic side - products . their quantity can be further reduced 
 by implementing a washing step prior to ammoxidation that is capable 
 of reducing the monosaccharide content of the respective lignocellulosic 
 source .
ammoxidized 
 technical lignins are valuable soil - improving materials 
 that share many similarities with native terrestrial humic substances . 
 in contrast to lignins , the chemical fate of carbohydrates as typical 
 minor constituents of technical lignins during the ammoxidation processes 
 has not been thoroughly investigated . 
 recently , we reported the formation 
 of n - heterocyclic , ecotoxic compounds ( oecd test 
 201 ) from both monosaccharides ( d - glucose , d - xylose ) 
 and polysaccharides ( cellulose , xylan ) under ammoxidation conditions 
 and showed that monosaccharides are a source more critical than polysaccharides 
 in this respect . 
 gc / ms - derivatization analysis of the crude product 
 mixtures revealed that ammoxidation of carbohydrates which resembles 
 the conditions encountered in nonenzymatical browning of foodstuff 
 affords also a multitude of nonheterocyclic nitrogenous compounds 
 such as aminosugars , glycosylamines , ammonium salts of aldonic , deoxyaldonic , 
 oxalic and carbaminic acids , urea , acetamide , -hydroxyamides , 
 and even minor amounts of -amino acids . 
 d - glucose 
 and d - xylose afforded largely similar product patterns which 
 differed from each other only for those products that were formed 
 under preservation of the chain integrity and stereoconfiguration 
 of the respective monosaccharide . the kinetics and reaction pathways 
 involved in the formation of the different classes of nitrogenous 
 compounds under ammoxidation conditions are discussed .