Elimination, Oxidation, and Transposition Reactions in Conformationally Biased Carbohydrates: Methyl 4,6-O-Benzylidene-α-D-Altropyranosides Containing the Phenylthio Group

The title compound containing a phenylthio group at C-3 and a methanesulfonate group at C-2 undergoes a quantitative transposition reaction in refluxing benzene to give the diequatorial isomer. In DMSO, a complex reaction takes place leading to elimination and oxidation at C-2. Oxidation of the title compound containing a phenylthio group at C-2, with acetic anhydride–DMSO gives the corresponding 3-uloside. With the isomeric 3-phenylthio derivative the enol acetate is formed. These results are rationalized based on mechanistic considerations.

Carbanions in Carbohydrate Chemistry: Approaches to Chemical Precursors of C-Nucleosides

The condensation of D-ribofuranosyl halides containing participating (benzoate) and non-participating (benzyl) substituents, with sodio dialkyl malonates and sodio triethyl 1,1,2-ethanetricarboxylate is described. In the presence of participating groups at C-2, the major and sometimes exclusive products were the 1,2-ketal derivatives. Both α- and β-anomeric D-ribofuranosyl malonates were formed in the non-participating series. The possibility of epimerization (anomerization) was experimentally demonstrated.Similar results were obtained with the more highly functionalized tricarbethoxy carbanion. For the participating series however, 20% of C-glycoside was obtained. Condensations with sodio diethyl malonate were also done in the D-arabino series (O-benzyl protecting group) and the anomeric C-glycosyl compounds were isolated and characterized. The mechanistic aspects of these condensations are discussed.

Carbanions in Carbohydrate Chemistry: Synthesis of C-Glycosyl Malonates

The condensation of 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide with sodio diethyl malonate led to crystalline diethyl 2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl) malonate. The corresponding dibenzyl ester proved to be a versatile intermediate for the preparation of crystalline β-D-glucopyranosyl malonic acid and β-D-glucopyranosyl acetic acid derivatives. The anomeric configuration in these C-glycosides was determined by a chemical correlation. With 2,3,4,6-tetra-O-acetyl- β-D-glucopyranosyl chloride and sodio diethyl malonate, the major product was a 1,2-O-ketal derivative resulting from an attack of the carbanion on the 1,2-acetoxonium ion. The condensation of 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl bromide with sodio diethyl malonate was conducted with, and without added bromide ion and the mechanistic implications of the results are discussed. C-Glycosides were also prepared in the D-mannofuranose series and their transformation into the D-lyxofuranose series (anomeric mixture) is described. The utility of n.m.r. shift reagents, and an apparent differential complexation by Eu(DPM)3 and Eu(FOD)3-d27 is demonstrated.

The Reaction of O-Benzylidene Sugars with N-Bromosuccinimide.: VII. Stereochemical and Mechanistic Aspects in the Pyrimidine Nucleoside Series

The reaction of 2′,3′-O-benzylideneuridine with N-bromosuccinimide (NBS) leads to 3'-O-benzoyl-2′,5-dibromouridine. The stereoselectivity of bromination in the sugar ring can be explained by invoking the sequential formation of benzoxonium ion and 2,2′-anhydronucleoside intermediates. Protonation of the appropriate 2,2′-anhydronucleoside, followed by treatment with NBS, recreates the conditions which prevail in the original reaction of the benzylidene acetal with NBS.

Chemistry and Synthetic Utility of α-(Dimethylamino)benzylidene and 1-(Dimethylamino)ethylidene Acetals

N,N-Dimethylbenzamide and N,N-dimethylacetamide dimethyl acetals react with vicinal cis-diols to give cyclic α-(dimethylamino)benzylidene and 1-(dimethylamino)ethylidene acetals respectively. In aqueous methanol, the acetals are hydrolyzed to the original diol. At pH 4–5, cleavage occurs to give benzoate and acetate esters, presumably by the sequential formation of acyloxonium and orthoacid intermediates. The acetals are thus useful as temporary protecting groups for vicinal diols and can be used for indirect, selective benzoylation and acetylation. They are compatible with the reaction conditions of acylation (acetylation, benzoylation, tosylation) and nucleophilic displacement [Formula: see text]

Carbanions in Carbohydrate Chemistry: Novel Methods for Chain Extension and Branching

Methods are described for the introduction of two and three carbon units in carbohydrates by the reaction of suitable epoxides with sodio diethyl malonate. The reactions represent a means of branching and chain extension in the carbohydrate series and lead to the incorporation of highly functionalized carbon units at predetermined sites in the sugar molecule.

The Reaction of O-Benzylidene Sugars with N-Bromosuccinimide.: VI. A New Synthesis of 2′-Deoxyuridine and some of its Selectively Substituted Derivatives

A new synthesis of 2′-deoxyuridine and its selectively substituted derivatives has been developed, based on the reaction of appropriately substituted 2′,3′-O-benzylideneuridines with N-bromosuccinimide (NBS). The preponderant products are uridines containing bromine and benzoate groups in the 2′- and 3′-positions respectively.

Methyl 2,3-O-Isopropylidene-β-D-ribo-pentodialdofuranoside: a Useful Intermediate in Chain Extension Reactions

Oxidation of methyl 2,3-O-isopropylidene-β-D-ribofuranoside with the "Pfitzner–Moffatt" reagent (methyl sulfoxide – dicyclohexylcarbodiimide) provides an efficient route to the corresponding 5-aldehydo derivative. Oxidation with methyl sulfoxide - acetic anhydride mixtures affords low yields of the aldehyde, the 5-acetate and 5-O-methylthiomethyl ether derivatives being the principal products.

Studies on the Ethylation of Nucleosides with Triethyloxonium Fluoroborate

Alkylation of inosine and adenosine triacetates with triethyloxonium fluoroborate leads to selective N-ethylation. Inosine triacetate affords 7-ethylinosine as a major product, while adenosine triacetate is ethylated at the N-6 and -1 positions. Treatment with base affords the 6-ethyl analog by rearrangement.

Utility of Shift Reagents in Nuclear Magnetic Resonance Studies of Polyfunctional Compounds: N- and O-Acetylated Carbohydrates and Nucleosides

The utility of n.m.r. shift reagents, Eu(DPM)3 and Pr(DPM)3, can be extended to polyfunctional molecules containing acetate and amide groups with significant spectral simplification and definitive peak assignments based on "signal crossover" phenomena.