János Harangi

Chemo-, stereo- and regioselective hydrogenolysis of carbohydrate benzylidene acetals. Synthesis of benzyl ethers of benzyl α-d-, methyl β-D-mannopyranosides and benzyl α-D-rhamnopyranoside by ring cleavage of benzylidene derivatives with the LiAlH4-AlCl3 reagent

Abstract Treatment of benzyl α-(1) and methyl β- d -mannopyranoside (2) with α,α-dimethoxytoluene gave the exo and endo isomers (3,5 and 4,6) of the dibenzylidene derivatives of 1 and 2. Hydrogenolysis of the exo isomers (3 and 5) with a molar equivalent of AlH2Cl gave the 3-0-benzyl-4,6-0-benzylidene derivatives (7 and 21), whereas the endo isomers (4 and 6) gave the 2-0-benzyl-4,6-0-benzylidene compounds (8 and 22). The 2-0-allyl ether 9 of 7, the 3-0-allyl derivative (10) of 8 and compounds 21 and 22 were treated with an additional molar equivalent of AlH2Cl at reflux and the products were the 4-0-benzyl-6-hydroxyl derivatives (11, 12, 23 and 24), whereas in the case of 22 the 6-0-benzyl-4-hydroxyl isomer (25) was also isolated. By deallylation of 11 and 12, 3,4-(13) and 2,4-di-0-benzyl (14) ethers of 1 were prepared. Tosylation of 11 and 12, and subsequent reduction of the products (15 and 16) made possible the preparation of the partially protected benzyl α- d -rhamnopyranoside derivatives (17–20). The structures of the compounds synthesized were characterized by 1H and 13C NMR spectroscopic investigation and by chemical methods.

Synthesis of chromogenic substrates of α-amylases on a cyclodextrin basis

Abstract One-pot acetylation and subsequent partial acetolysis of α-, β- and γ-cyclodextrins resulted in crystalline peracetylated malto-hexaose, -heptaose, and -octaose, respectively. Prolonged acetolysis of β-cyclodextrin gave a mixture of acetylated maltooligosaccharides, from which peracetylated malto-triose, -tetraose, and -pentaose were isolated. The acetylated oligosaccharides were converted into α-acetobromo derivatives, and then transformed into 4-nitrophenyl and 2-chloro-4-nitrophenyl β-glycosides. From the 4-nitrophenyl glycosides 4,6-O-benzylidene derivatives were prepared, which were used together with the free glycosides as substrates of porcine pancreatic α-amylase. One-pot acetylation and subsequent partial acetolysis of cyclodextrins resulted in the peracetylated maltooligomers (dp 3–8), which were transformed into 4-nitrophenyl and 2-chloro-4-nitrophenyl β-glycosides, the 4.6-O-benzylidene derivatives of NP-glycosides were also prepared

Hydrogenolysis of the isomeric 1,2:4,6-di-O-benzylidene-α-d-glucopyranose derivatives with the LiAlH4AlCl3 reagent

Abstract Both isomers of 1,2:4,6-di- O -benzylidene-α- d -glucopyranose (and their 3- O -acetyl and 3- O -benzyl derivatives) have been prepared and their 1 H- and 13 C-n.m.r. spectra assigned. The mode of hydrogenolysis of the dioxolane ring in these isomers by the LiAlH 4 AlCl 3 reagent is determined by the configuration at the acetal carbon and is independent of the electronic character of the two oxygen atoms.

Lewis-acid-catalysed isomerisation of benzylidene acetals of methyl α-l-rhamnopyranoside and methyl β-l-arabinopyranoside derivatives

Abstract The Lewis-acid-catalysed isomerisation of several derivatives of methyl 2,3- O -benzylidene-α- l -rhamnopyranoside and methyl 3,4- O -benzylidene-β- l -arabinopyranoside has been investigated. The presence of equatorial substituents vicinal to the benzylidene ring decreases the rate of isomerisation, and exo isomers isomerise more quickly than the corresponding endo isomers. The occurrence of isomerisation during the reductive cleavage reaction of acetal rings with LiAlH 4 -AlCl 3 has been demonstrated.

Synthesis of Methyl 1-Thio-L-Rhamnopyranoside-Derivatives

Abstract Both anomers of methyl 1-thio-L-rhamnopyranosides were prepared through methylation of the corresponding isothiouronium salt. 2,3-0-Isopropylidene-, benzylidene-and the until now unknown diphenyl-methylene acetals were synthesized. Phase-transfer catalysed benzylation and LiA1H4-AlCl3-type hydrogenolysis of benzylidene acetals were used to obtain partially benzylated derivatives. Comparing the C NMR data of the synthesized compounds with those of their 0-glycoside analogues revealed that the 0→S exchanges at the anomeric centres caused drastic upfield shifts (∼15 ppm) at C-1 and moderate downfield shifts at C-2 and C-5, as well. The chemical shift values of other carbons are not sensitive to the 0→S replacement.

Synthesis and hydrogenolysis of the methylene, ethylidene, isopropylidene, and diastereoisomeric 1-phenylethylidene acetals of β-l-arabino- and α-l-rhamnopyranoside derivatives

Abstract Both diastereoisomers of 1-phenylethylidene acetals (acetophenone acetals) of methyl and benzyl β- l -arabinopyranoside and α- l -rhamnopyranoside were prepared. Acetal-exchange reactions gave only the endo -phenyl isomers; their 2- O - and 4- O -acetyl derivatives were isomerised into the exo -phenyl compounds. 1 H-N.m.r. data were used to determine the absolute configuration at the acetal carbon atom in these compounds. The protons of the methyl group of the exo -phenyl isomers resonate at lower field than those of the endo -phenyl isomers. Hydrogenolysis of various methylene, ethylidene, and isopropylidene derivatives gave axial ethers. The endo -phenyl isomers of the acetophenone derivatives also gave axial 1-phenylethyl ethers in two diastereoisomeric forms. The exo -phenyl isomers of the arabinosides were stable towards the reagent (LiAlH 4 AlCl 3 ), whereas the corresponding rhamnopyranosides gave the 2-(1-phenylethyl) ethers, but cleavage required prolonged reaction time and higher temperature.

Synthesis of everninose, a non-reducing disaccharide component of the orthosomycin-type oligosaccharide antibiotics

Abstract Glycosylation of 3,4-di- O -benzyl-2- O -methyl- l -lyxopyranose ( 1 ) with 3,4-di- O -benzyl-2,6-di- O -methyl-α- d -mannopyranosyl chloride afforded a mixture of the α,α- ( 12 ) and α,β-disaccharide derivative ( 10 ). Reaction of 1 with 3,4-di- O -benzyl-2,6-di- O -methyl-α- d -mannopyranosyl trichloroacetimidate gave 10 exclusively. O -Debenzylation of 12 and 10 gave everninose and isoeverninose, respectively. The 1 H- and 13 C-n.m.r. spectra were assigned on the basis of 2D homo- and heteronuclear correlation experiments.

A conformational study of dioxolane-type acetals of some carbohydrate derivatives by N.M.R. spectroscopy

Abstract The conformation of the methylene, isopropylidene, benzylidene, and 1-phenylethylidene acetals of some rhamnopyranoside and arabinopyranoside derivatives has been investigated by n.m.r. spectroscopy. In each series, the pyranoside ring adopts a distorted chair conformation. For the benzylidene acetals, the pyranoside ring is more distorted in the endo -phenyl isomers, and these isomers are compared to the methylene and isopropylidene acetals. For the 1,3-dioxolane ring, only the coupling constant of the acetal carbon atom with the axial bridgehead proton is measurable; for the benzylidene acetals, it is larger in the exo -phenyl isomer. For the other types of acetals, the coupling constant depends on the nature of the endo substituent at the acetal carbon atom. If it is bulky, the coupling constant and the conformation of the dioxolane ring are similar to those of the corresponding endo -phenyl benzylidene derivative.

Synthesis and 13C-n.m.r. spectroscopic investigation of three methyl rhamnotriosides

Abstract Methyl 2,3-O-isopropylidene-4-O-(2,3-O-isopropylidene-α- l -rhamnopyranosyl)- α- l -rhamnopyranoside, methyl 2,4-di-O-benzyl-3-O-(2,3-O-isopropylidene- α- l -rhamnopyranosyl)-α- l -rhamnopyranoside, and methyl 3,4-di-O-benzyl-2-O- (2,3-O-isopropylidene-α- l -rhamnopyranosyl)-α- l -rhamnopyranoside, obtained by isopropylidenation of the respective methyl rhamnobiosides, were glycosylated with 2,3,4-tri-O-acetyl-α- l -rhamnopyranosyl bromide. Removal of the protecting groups from the products gave the methyl glycosides of α- l -Rhap-(1→4)-α- l - Rhap-(1→4)-α- l -Rhap, α- l -Rhap-(1→4)-α- l -Rhap-(1→3)-α- l -Rhap, and α- l - Rhap-(1→4)-α- l -Rhap-(1→2)-α- l -Rhap. These trisaccharide glycosides and their hepta-acetates have been studied by 13C-n.m.r. spectroscopy.

Mosaic increments for predicting the gas chromatographic retention data of the chlorobenzenes

The chlorinated organic compounds are very important from the point of view of the chemical industry and environmental protection, and therefore the gas chromatographic analysis of these compounds is very interesting for analytical chemists. In this paper we studied the relationship between the molecular structure and gas chromatographic retention on several stationary phases having different polarity and at several temperatures of benzene and 12 chlorobenzene compounds as model compounds. A coding system involving primary (mosaic increments) and secondary (bond increments)calculation methods was developed. The retention indices of benzene and the chlorobenzenes calculated on HP-5 at 120 degrees C shows a better performance of the mosaic increments (average absolute deviation delta of 1.7 retention index units) compared with the bond increments (delta = 11.7 retention index units). Retention factors, k, calculated with mosaic increments for chlorobenzenes on SPB-1 and WAX-10, at 140 degrees C, yield average relative errors of epsilon = 0.9 and 3.5%, respectively. Therefore, the presented paper provides a new possibility for precalculation of the retention data.