Levoglucosenone, formation

The goal of the present study was to investigate the effect of structural peculiarities of different types of cellulose on levoglucosenone formation under catalytic fast pyrolysis conditions. Moreover, amount of phosphoric acid and pretreatment temperature should be determined to provide maximum possible yields of levoglucosenone. 

All the research to date supports the preferential formation of levoglucosenone despite the possibility of double dehydration with alternative formation of isolevoglucosenone. This formation has never been detected in the pyrolysate and is only available through a total synthesis. 

Addition of iodine to levoglucosenone has been conveniently performed by the treatment of this enone with a solution of iodine in anhydrous pyridine (17), resulting in the formation of 3-iodolevoglucosenone in moderate (55%) yield. 

The stereochemistry of compounds 8, 9, and 10 was determined by NOE spectroscopy. It has been reported 20a that the methyl and pentyl adducts of levoglucosenone (2) when oxidized by peracetic acid give y-lactones that correspond to lactone 10, but no intermediate formates were reported. 

The formation of levoglucosan and levoglucosenone is found to be strongly dependent on the presence of catalytic amounts of various cations during pyrolysis. However, the literature is inconsistent regarding the effect of these cations. By varying the amount of acid catalyst used, Faix et al. were able to control the relative amount of the two products.441,442... 

Dehydration Reactions. Detailed analysis of the pyrolysis tar as discussed previously (Figure 12 and Scheme 3) shows the presence of levoglucosan, its furanose isomer (1,6-anhydro-p-D-glucofuranose) and their transglycosylation products as the main components. In addition to these compounds, the pyrolyzate contains minor amounts of a variety of products formed from dehydration of the glucose units. The dehydration products detected include 3-deoxy-o-erythrohexo-sulose, 5-hydroxymethyl-2-furaldehyde, 2-furaldehyde (furfural), other furan derivatives, levoglucosenone (l,6-anhydro-3,4-dideoxy-P-D-glycerohex-3-enopyranos-2-ulose), l,5-anhydro-4-deoxy-D-hex-l-ene-3-ulose, and other pyran derivatives. The dehydration products are important as intermediate compounds in char formation. 

The pyrolytic process of cellobiosan can also explain the formation of levoglucosenone and hydroxymethylfurfural ... 

Scheme 2. Stereoselective Michael addition of 1 thio-p-D-glucose to levoglucosenone with the formation of (1-4)-S-thiodisaccharides Ref. 1-2...

Scheme 4. Stereoselective Sn2 displacement of iodine in 3-iodo-levoglucosenone by 1-thio-p-D-glucose with stereoselective formation of (1-3)-S-thiodisaccharides. Ref. 4...

This stereoselectivity as observed previously in levoglucosenone conjugate addition proceeds by the attack of an incoming nucleophile (thiol) at the alkene face opposite the 1,6-anhydro ring. The sterically hindered 1,6-anhydro bridge in isolevoglucosenone is, therefore assumed to effectively prevent formation of the opposite stereoisomer. 

The Michael addition of 1-thio-P-D-glucose to this enone proceeded smoothly (6) with the formation of P-(l-5)-4-deoxy-5-C-thiodisaccharide in 94% yield. The proton-proton coupling constants in the H NMR spectrum of the addition product confirmed that only the 5-axial adduct was obtained as a single product. This stereoselectivity, as observed previously in levoglucosenone conjugate additions proceeds by the attack of incoming nucleophile at the top of the alkene face of the enone ring as depicted in Scheme 6. 

Reaction of levoglucosenone with amides of a-nitrocarboxylic acids resulted in the formation of the tetrahydropyridones 77 and with acetoacetic acid amides, mixtures of the keto and enol tautomers were obtained [137]. 

Reaction of levoglucosenone with urea, thiourea, or J -cyano- or W-ni-troguanidine resulted in the formation of the pyrimidine systems 99 in a stereospecific manner [153,154], Its reaction with a-aminoazoles yielded azolo[l,5-a]pyrimidine systems fused with a carbohydrate fragment. The reaction occurs much more smoothly than in the case of other a,jS-unsaturated ketones. The reactions of levoglucosenone with /3-dicarbonyl compounds (dimedone, barbituric acid) in the presence of a base resulted in pyran ring closure [153,154]. 

An alternative 1,2-hydride shift leading to a hydroxycarbenium ion at C-4 does not occur since the corresponding levoglucosenone isomer known as isolevoglucosenone was not found in the pyrolysate. The hypothethical mechanism of the formation of levoglucosenone via three alternative routes is depicted in Scheme 5 (21). 

Paton and coworkers (45) reported a highly regio- and stereospecific cycloaddition reaction of benzonitrile oxide to levoglucosenone leading to the formation of two exo-isomers and the endo- adduct in 70 % yield. A similar approach to functionalized levoglucosenone was based on the stereoselective construction of fused heterocyclic systems (45-49). 

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