Dithioacetals of sugars

Expeditious preparation of 1,6-anhydro-1-thio-β-d-hexopyranose derivatives

A convenient reaction condition has been developed for the preparation of 1,6-anhydro-1-thio-β-d-glycopyranose derivatives in excellent yield. The reaction condition is significantly fast, clean and can be scaled up for its use in the organic synthesis.

From a Carbohydrate Raw Material to an Important Building Block: Cost-Efficient Conversion of d-Fructose into 2-Deoxy-l-ribose

A straightforward method for the conversion of a low-cost carbohydrate (d-fructose) into an important carbohydrate building block (2-deoxy-l-ribose) is reported. This methodology involves a novel radical cyclization followed by a fragmentation reaction, selective enzymatic hydrolysis using a lipase, and oxidative cleavage of the vicinal diol. This method uses the cheapest starting material and employs the shortest synthetic route (7 steps) for converting a d-sugar into 2-deoxy-l-ribose.

Synthesis of l-Deoxyribonucleosides from d-Ribose

The preparation of 2-deoxy-l-ribose derivatives or mirror image deoxyribonucleosides (l-deoxyribonucleosides) from d-ribose is reported. Starting from inexpensive d-ribose, an acyclic d-form carbohydrate precursor was synthesized to study a unique carbonyl translocation process. In this novel radical reaction, not only was the configuration of the sugar transformed from the d-form to the l-form, but also deoxygenation at the C(2) position of the sugar was successfully achieved. This is one of the most practical methods for converting a d-sugar to a 2-deoxy-l-sugar in a one-step reaction. To further identify the reaction product, radical reactions followed by treatment with 1,3-propanedithiol and then benzoylation were performed to afford a dithioacetal derivative. The stereochemistry and configuration of the 2-deoxy-l-ribose dithioacetal derivative were confirmed by its X-ray crystal structure. To further apply this methodology, a diethyl thioacetal derivative was formed, followed by selective benzoyl protection, and an NIS-initiated cyclization reaction to give the desired ethyl S-l-2-deoxyriboside, which can be used as a 2-deoxy-l-ribosyl synthon in the formal total synthesis of various l-deoxyribonucleosides, such as l-dT.

Synthesis of the four 1-(1-deoxy-D-pentitol-1-yl)thymines and conformational properties of the acyclic sugar chain

Acetylated D-pentose diethyl dithioacetals were coupled by way of 1-bromo-1-ethylthio derivatives with 2,4-bis(trimethylsilyl)thymine to afford diastereomeric pairs of acyclic-sugar nucleoside analogues bearing a thymin-1-yl and an ethylthio group at C-1. Free-radical desulfurization by the action of tributylstannane removed the ethylthio group to afford the corresponding acetylated 1-(1-deoxy-d-pentitol-1-yl)thymines and subsequently the free title compounds in the arabino, lyxo, ribo, and xylo series. Conformations of the intermediates and products were studied in detail and the final products were evaluated for their potential as agents active against plant viruses and rice blast fungus.

A synthetic route to 9-(polyhydroxyalkyl)purines

Mercuric-ion promoted condensation of 6-chloropurine with acetylated dimethyl dithioacetals of D-ribose and D-arabinose in nitromethane afforded a separable mixture of 1'(S)-2,3,4,5-tetra-O-acetyl-1-(6-chloropurin-9-yl)-1-S-methyl-1-thio-D-ribitol (4) and its 1'(R) diastereomer, and the corresponding 1'(R)-arabinitol analogue (5); the structure of 4 was confirmed by X-ray crystallography. Desulfurization of 4 and 5 by tributylstannane in toluene gave 2,3,4,5-tetra-O-acetyl-1-(6-chloropurin-9-yl)-1-deoxy-D-ribitol (7) and the arabinitol analogue 8, convertible by the action of thiourea into the 1,6-dihydro-6-thioxopurin-9-yl analogues 9 and 10, which on deacetylation furnished the corresponding acyclic-sugar nucleosides 11 and 12.

Facile exchange of glycosyl S,S-acetals to their O,O-acetals and preparation of glycofuranosides from acyclic glycosyl S,S-acetals under metal-free reaction conditions in the presence of 1,3-dibromo-5,5-dimethylhydantoin

Exchange of acyclic glycosyl dithioacetals to their O,O-acetals has been achieved by a generalized reaction protocol mediated by 1,3-dibromo-5,5-dimethylhydantoin under mild, metal-free and neutral conditions. This methodology has been extended to the synthesis of alkyl glycofuranosides.

Effect of metal cationization on the tandem mass spectra of glycosyl dithioacetals

The effect of metal cationization on the tandem mass spectra of glycosyl dithioacetals of glucose, mannose, galactose, rhamnose, arabinose and xylose was studied by electrospray ionization mass spectrometry under ammonium and metal (Li, Na, Ag and Cu) ion cationization conditions. The ammonium-cationized glycosyl dithioacetals fragment by loss of ammonia followed by either two molecules of EtSH or one molecule of EtSH and one molecule of H2O. Lithium cationization leads to additional eliminations such as EtSEt and EtSSEt and C-C cleavages. Elimination of EtSH is not observed under sodium cationization. Silver cationization, on the other hand, leads to additional fragmentations involving the elimination of silver as AgOH and AgSEt. Copper cationization results in adducts where copper has undergone a change of oxidation state from II to I. Li+, Ag+ and Cu+ cationization seem to favour cyclization resulting in elimination of EtSH. However, the mechanisms seem to be differently affected by different metal ions. Li+ and Ag+ cationization appear to be non-specific and favour cyclization involving C2-, C4- and C5-hydroxyl hydrogens, whereas Cu+ cationization seems to favour cyclization involving C4-hydroxyl hydrogen.

Tandem mass spectra of transition-metal ion adducts of glycosyl dithioacetals; distinction among stereoisomers

Electrospray ionization mass spectra of some glycosyl dithioacetals recorded in the presence of transition-metal chlorides, XCl2 (where X = Co, Mn and Zn), give abundant adduct ions such as [M+XCl]+ and [2M-H+X]+ and minor ions such as [M-H+X]+ and [2M+XCl]+. The tandem mass spectra of these adducts show characteristic elimination of neutral molecules such as H2O, HCl, EtSH, CH2O, C2H4O2/C2H4O. [M+XCl]+ ions fragment readily and the fragmentation appears to be stereochemically controlled as the relative abundances of the fragments are different for three stereoisomers. The added metal is lost as neutral molecules in the form of XCl(OH) and XCl(SEt). This is a predominant pathway in the ZnCl+ adducts. [2M+XCl]+ ions fragment preferentially by elimination of HCl, indicating strong metal interactions in the resulting dimeric [2M-H+X]+ ion. As there are several electron-rich centers in the molecule, the dimeric complex [2M-H+X]+ can have several structures and the observed fragmentations may reflect the sum of those of all these structures. The dimeric complexes fragment by elimination of neutral molecules leaving the dimeric interactions intact. The extent of fragmentation varies for the stereoisomers, leading to stereochemical differentiation.

Determination of the absolute configuration of sugar residues using gas chromatography. Method with potential for elimination of references

The absolute configuration of a sugar can be determined by gas-liquid chromatography of the acetylated or trimethylsilylated dithioacetals from 1-phenylethanethiol. The isolation of both enantiomers of 1-phenylethanethiol is also described. Using the acetates and both thiol reagents the absolute configuration of C-2 can be determined, provided it is a hydroxyl group, with great certainty. A new way of determining the absolute configuration of sugars, without references, is thereby provided. The sugars analysed include aldoses, deoxyaldoses, 2-acetamido-2-deoxyaldoses and uronic acids. The analysis is made using columns with non-chiral stationary phase and the electron impact mass spectra of the acetylated and trimethylsilylated bis(1-phenylethyl)dithioacetals are described.

Synthesis of L-gulose, L-galactose, and their acetylated aldehydo forms from 6-S-phenyl-6-thio-D-hexoses

Methyl 6-S-phenyl-6-thio-a-D-glucopyranoside, prepared in high yield from methyl a-D-glucopyranoside by the action of diphenyl disulfide and tributylphosphine in pyridine, was converted into 6-S-phenyl-6-thio-D-glucitol pentaacetate (7) by sequential hydrolysis, borohydride reduction, and acetylation. Oxidation of 7 with 3-chloroperoxybenzoic acid gave the corresponding S-epimeric sulfoxides, which underwent Pummerer rearrangement to 1-epimeric L-gulose S-phenyl monothiohemiacetal hexaacetates. Boron trifluoride-catalyzed reaction of the latter with thiophenol gave the analogous diphenyl dithioacetal, whereas base-catalyzed methanolysis led to free L-gulose. Treatment of 7 with N-chlorosuccinimide afforded 1-epimeric 1-chloro-1-S-phenyl-1-thio-L-gulitol pentaacetates, which were hydrolyzed to provide aldehydo-L-gulose pentaacetate. The same reaction sequences were performed with 6-S-phenyl-6-thio-D-galactose, synthesized in two steps from 1,2:3,4-di-O-isopropylidene-a-D-galactopyranose, furnishing ultimately L-galactose, its diphenyl dithioacetal pentaacetate, and aldehydo-L-galactose pentaacetate. Similar reaction sequences for the chain-terminal interchange of oxidation state in other omega-S-phenyl-omega-thioaldoses may prove useful for the preparation of less-common sugar derivatives.

Resolution of D- and L-galactose peracetates as their bis(ethyl L-lactate) acetals by gas-liquid chromatography

Reaction between ethyl L-lactate and each of a pair of sugar enantiomers, the peracetylated D-galactose and L-galactose diethyldithioacetals, produced two acyclic diasterioisomers. They could be separated by conventional gas-liquid chromatography. The corresponding fucose diastereomers were also separated. This process should make it possible to develop a general analytical method by which small amounts of enantiomeric sugars can be identified and their quantities measured.