The Synthesis of Sugars from Non-Carbohydrate Substrates

De novo asymmetric Achmatowicz approach to oligosaccharide natural products

The development and application of the asymmetric synthesis of oligosaccharides from achiral starting materials is reviewed. This de novo asymmetric approach centers around the use of asymmetric catalysis for the synthesis of optically pure furan alcohols in conjunction with Achmatowicz oxidative rearrangement for the synthesis of various pyranones. In addition, the use of a diastereoselective palladium-catalyzed glycosylation and subsequent diastereoselective post-glycosylation transformation was used for the synthesis of oligosaccharides. The application of this approach to oligosaccharide synthesis is discussed.

De novo asymmetric synthesis of the pyranoses: from monosaccharides to oligosaccharides

The various methods for the de novo asymmetric synthesis of the pyranose sugars are surveyed. The presentation begins with the work of Masamune and Sharpless with the use of the Sharpless asymmetric epoxidation for the synthesis of all eight l-hexoses. The development of other asymmetric reactions and their application for the synthesis of specific hexopyranoses are further discussed. The broad application of the Achmatowicz rearrangement with asymmetric catalysis, for the synthesis of various pyranones and imino sugars, is also presented. Finally, the use of a diastereoselective palladium-catalyzed glycosylation with the Achmatowicz approach for the synthesis of oligosaccharides and applications to medicinal chemistry are discussed.

Catalyst-directed diastereoselectivity in hydrogenative couplings of acetylene to alpha-chiral aldehydes: formal synthesis of all eight L-hexoses

Hydrogenative coupling of acetylene to alpha-chiral aldehydes 1a-4a using enantiomeric rhodium catalysts ligated by (S)-MeO-BIPHEP and (R)-MeO-BIPHEP delivers the diastereomeric products of carbonyl-(Z)-butadienylation 1b-4b and 1c-4c, respectively, with good to excellent levels of catalyst directed diastereofacial selectivity. Diastereomeric L-glyceraldehyde acetonide adducts 1b and 1c were converted to the four isomeric enoates 6b, 8b, 6c, and 8c, representing a formal synthesis of all eight L-hexoses.

De novo asymmetric syntheses of C-4-substituted sugars via an iterative dihydroxylation strategy

A short and highly efficient route to various C-4 substituted sugar lactones has been developed. The key to the overall transformation is the sequential osmium-catalyzed dihydroxylation reaction of substituted 2,4-dienoates and an allylic substitution at the C-4 position. When the Sharpless AD-mix procedure is used in a matched sense for the second dihydroxylation reaction, it results in an exceedingly diastereo- and enantioselective synthesis of several C-4-substituted sugars.

A Palladium-Catalyzed Glycosylation Reaction: The de Novo Synthesis of Natural and Unnatural Glycosides

A highly stereoselective and sterospecific palladium-catalyzed glycosylation reaction of a variety of alcohols is reported. The reaction selectively converts alpha-2-substituted 6-carboxy-2H-pyran-3(6H)-ones into alpha-2-substituted 6-alkoxy-2H-pyran-3(6H)-ones with complete retention of configuration and similarly converts the pyranones with beta-carboxy groups into pyranones with beta-alkoxy groups. The reaction works equally well with both amino acid- and carbohydrate-based alcohols. To demonstrate the utility of this process for carbohydrate chemistry several of the products were selectively converted into alpha-manno-pyranosides in two additional steps. Because the 2-substituted 6-carboxy-2H-pyran-3(6H)-ones are prepared by asymmetric synthesis, this reaction can be used for the preparation of either d- or l-pyranones.

Syntheses of four D- and L-hexoses via diastereoselective and enantioselective dihydroxylation reactions

An expeditious approach to various protected hexoses has been developed by the use of the Sharpless catalytic asymmetric dihydroxylation reaction. Applying the Sharpless catalytic asymmetric dihydroxylation reaction on vinylfuran, diols with high enantioexcess are produced. The resulting diols can be stereoselectively transformed into either protected D- or L-mannose in five steps and approximately 39% yield from furfural. Similarly, both D- and L-talose and gulose have been synthesized in 19% overall yields, respectively. Using a modified strategy, both protected D- and L-gulo- and allo-sugar-delta-lactones were synthesized in eight steps and approximately 20%, overall yield from furfural.

Synthesis of mycarose and epi-axenose from non-carbohydrate precursors

A six-step synthesis of racemic mycarose from allylacetylene is described. Key transformations include the threo-selective epoxidation of (E)-4-methyl-1,4-heptadien-6-ol and the alpha-opening of xylo-4,5-epoxy-4-methylhept-1-en-6-ol (7), which was accomplished via a neighboring group-assisted reaction of xylo-4,5-epoxy-4-methyl-6-(N-phenylcarbamoyloxy)hept-1-ene (12). The latter conversion proceeded wih lower selectivity (3:1) than observed with disubstituted epoxyurethanes because of the greater tendency of trisubstituted epoxides to undergo substitutions with SN1 character at the tertiary center. Methanolysis of ribo-4-methylhept-1-ene-4,5,6-triol 5,6-carbonate, obtained from 12 in up to 61% yield, afforded ribo-4-methylhept-1-ene-4,5,6-triol, which was converted into mycarose by ozonolysis. Similarly, ozonolysis of lyxo-4-methylhept-1-ene-4,5,6-triol, which was prepared (64%) by hydrolysis of 7, afforded racemic 3-epi-axenose.