1-O-Alkylation of D-Glucopyranose

Neighboring-Group Participation by C-2 Acyloxy Groups: Influence of the Nucleophile and Acyl Group on the Stereochemical Outcome of Acetal Substitution Reactions

A single acyloxy group at C-2 can control the outcome of nucleophilic substitution reactions of pyran-derived acetals, but the extent of the neighboring-group participation depends on a number of factors. We show here that neighboring-group participation does not necessarily control the stereochemical outcome of acetal substitution reactions with weak nucleophiles. The 1,2-trans selectivity increased with increasing reactivity of the incoming nucleophile. This trend suggests the intermediacy of both cis-fused dioxolenium ions and oxocarbenium ions in the stereochemistry-determining step. In addition, as the electron-donating ability of the neighboring group decreased, the preference for the 1,2-trans products increased. Computational studies show how the barriers for the ring-opening reaction on the dioxolenium ions and the transition states to provide the oxocarbenium ions change with the electron-donating capacity of the C-2-acyloxy group and the reactivity of the nucleophile.

Diastereoselective Substitution Reactions of Acyclic β-Alkoxy Acetals via Electrostatically Stabilized Oxocarbenium Ion Intermediates

Substitution reactions of acyclic β-alkoxy acetals proceeded with generally high diastereoselectivities (>90:10) to form the anti product. Mechanistic experiments supplemented with computational studies suggest that, upon activation of the acetal, the resulting oxocarbenium ion is electrostatically stabilized by the β-alkoxy group. This stabilization defines the conformation of the reactive intermediate, which can be attacked preferentially from the more exposed face, leading to the observed products.

Using Neighboring-Group Participation for Acyclic Stereocontrol in Diastereoselective Substitution Reactions of Acetals

Neighboring-group participation of an ester enabled stereocontrol in substitution reactions of acyclic acetals. The ester group formed a trans-fused dioxolenium ion intermediate, which underwent a substitution reaction at the acetal carbon atom to afford the product with high diastereoselectivity. Neighboring-group participation was confirmed by isolating dioxolane products resulting from nucleophilic addition at C-2 of a 1,3-dioxolenium ion intermediate. Using a pivaloate ester as the participating group in combination with strong nucleophiles produced substitution products with diastereoselectivities of ≥90:10.

Synthesis of Simplified Azasordarin Analogs as Potential Antifungal Agents

A new series of simplified azasordarin analogs was synthesized using as key steps a Diels-Alder reaction to generate a highly substituted bicyclo[2.2.1]heptane core, followed by a subsequent nitrile alkylation. Several additional strategies were investigated for the generation of the key tertiary nitrile or aldehyde thought to be required for inhibition at the fungal protein eukaryotic elongation factor 2. This new series also features a morpholino glycone previously reported in semisynthetic sordarin derivatives with broad spectrum antifungal activity. Despite a lack of activity against Candida albicans for these early de novo analogs, the synthetic route reported here permits more comprehensive modifications of the bicyclic core and structure-activity relationship studies that were not heretofore possible.

Ready Access to Fluorinated Phosphonate Mimics of Secondary Phosphates. Synthesis of the (alpha,alpha-Difluoroalkyl)phosphonate Analogues of L-Phosphoserine, L-Phosphoallothreonine, and L-Phosphothreonine

In addition to the previously recorded reactions of diethyl lithio(difluoromethyl)phosphonate (8) with primary triflates and aldehydes, we report here that 8 reacts with functionalized, but unactivated, methyl esters to give efficient acyl substitution. Thus, 8 reacts cleanly (-78 degrees C, THF) with the following methyl esters (product, yield): methyl (S)-isopropylideneglycerate (14, 99%), methyl (S)-3-O-(tert-butyldimethylsilyl)-2 -O-tetrahydropyranylglycerate (16, 85%), and the Garner ester derived from D-serine (15, 77%). Expeditious treatment of the resultant alpha,alpha-difluoro-beta-keto phosphonates with hydride or Grignard reagents followed by alcohol deoxygenation provides a general method for the synthesis of (alpha,alpha-difluoroalkyl)phosphonate analogues of secondary phosphates. For tertiary alcohols, Dolan-MacMillan deoxygenation conditions are employed. The requisite methyl oxalate esters are obtained by an improved procedure wherein the lithium alkoxide of the hindered tertiary alcohol is irreversibly generated at low temperature and then condensed with methyl oxalyl chloride. Relative stereochemistry is assigned via conversion of the Garner ester derived Boc-amino alcohols to the corresponding cyclic, six-membered phosphonate esters and examination of their (1)H NMR spectra. The relevant vicinal coupling constants are extracted from these spectra by performing double quantum-filtered phase-sensitive COSY experiments. This new (difluoromethylene)phosphonate anion-methyl ester condensation, Grignard (hydride) addition, deoxygenation sequence has been applied to the synthesis of (alpha,alpha-difluoroalkyl)phosphonate analogues of L-phosphoserine (>/=96% ee) and L-phosphoallothreonine (93% ee) from D-serine and of L-phosphothreonine (91% ee) from L-glycerate, respectively.

4,6-O-benzylidene-D-glucopyranose and its sodium salt: new data on their preparation and properties

An improved method for the preparation of 4,6-O-benzylidene-D-glucopyranose (BG), and new or correlated data on its 1H and 13C NMR spectra, specific rotations, and tautomeric equilibria, and on those of its anomeric sodium salt (BGNa), are reported. Evidence is presented in favour of the hypothesis that crystalline BGNa exists entirely in its beta-anomeric form and that it can be useful in the access to beta-glucosides in reactions with strong electrophiles under strictly heterogeneous conditions.

Synthesis and reactivity of a new glycosyl donor: O-(1-phenyltetrazol-5-yl) glucoside

A new glycosyl donor possessing an anomeric O-(1-phenyltetrazol-5-yl) group is prepared from 2,3,4,6-tetra-O-benzyl-D-glucose (2) and commercially available 5-chloro-1-phenyl-1H-tetrazole (1). The synthesis of glycosides derived from the donor and a few primary and secondary alcohols is reported.

Synthesis of Kdo-alpha-glycosides of lipid A derivatives

The synthesis of the lipopolysaccharide fragment O-(4,5,7,8-tetra-O-acetyl-3-deoxy-N-methyl-alpha-D-manno-2- octulopyranosylonamide)-(2-->6)-O-(2-deoxy-2-[(3R)-3- dodecanoyloxytetradecanamido]-4-O- phosphono-3-O-tetradecanoyl-beta-D-glucopyranosyl)-(1-->6)-1-O-acetyl-2- deoxy-2 - [(3R)-3-dodecanoyloxytetradecanamido]-3-O-tetradecanoyl-alpha-D- glucopyranose (35 alpha) is performed via anomeric O-alkylation. With this objective, the 2-azido-3-O-benzyl-2-deoxy-6-O-trifluoromethanesulfonyl-beta-D-glu copyranosides 5, 7, and 19 alpha, beta were synthesized from D-glucal and employed as alkylating agents. Reaction of 5 with the O-cyclohexylidene-protected Kdo-derivative 10 afforded the desired alpha-linked disaccharide, tert-butyldimethylsilyl 4-O-allyl-2-azido-3-O-benzyl-2-deoxy-6- O-(4,5:7,8-di-O-cyclohexylidene-3-deoxy-N-methyl-alpha-D-manno-2- octulopyranosylonamide)-beta-D-glucopyranoside (11); even better yields of the structurally related disaccharide 12 were obtained with the 4-O-unprotected 7 as alkylating agent. 1-O-Desilylation of 12 furnished the lactol 20, which could be alkylated at the anomeric position with 1-O-allyl protected alkylating agents 19 alpha and 19 beta, both of which furnished exclusively the desired beta-(1-->6)-linked trisaccharides allyl O-(4,5:7,8-di-O-cyclohexylidene-3- deoxy-N-methyl-alpha-D-manno-2-octulopyranosylonamide)-(2-->6)-O-( 2-azido-3- O-benzyl-2-deoxy-beta-D-glucopyranosyl)-(1-->6)-2-azido-3, 4-di-O-benzyl-2-deoxy-alpha- (21 alpha) and -beta-D-glucopyranoside (21 beta), respectively. Phosphorylation with diphenyl phosphorochloridate, replacement of the O-cyclohexylidene protective group by O-triethylsilyl (TES) protective groups, removal of the 1-O-allyl group, azido group reduction, subsequent N-acylation, and then O-acetylation provided the key 1-O-acetyl protected intermediate 30 alpha. Removal of the O-TES groups, subsequent O-acetylation, and hydrogenolytic O-debenzylation furnished O-[4,5:7,8-tetra-O-acetyl-3-deoxy-N-methyl-alpha-D-manno-2- octulopyranosylonamide]-(2-->6)-O-(2-deoxy-4-O-diphenoxyphospho ryl-2-[(3R)- 3-dodecanoyloxytetradecanamido]-beta-D-glucopyranosyl)-(1-->6)-1-O -acetyl-2- deoxy-2[(3R)-dodecanoyloxytetradecanamido]-alpha-D-glucopyranose (33 alpha), which underwent the required selective O-tetradecanoylation at the 3-O- and 3'-O-position, thus furnishing, after hydrogenolytic O-dephenylation of the diphenoxyphosphoryl group, the target molecule 35 alpha.

O-alkylation at the anomeric centre for the stereoselective synthesis of Kdo-alpha-glycosides

O-Alkylation at the anomeric centre of the dianion of 4,5:7,8-di-O-cyclohexylidene-3-deoxy-N-methyl-alpha-D-manno- octulopyranosonamide (1) with several triflates led diastereoselectively to the alpha-glycosides. In this way, lipopolysaccharide building-blocks containing alpha-Kdo-(2----6)-GlcN and alpha-Kdo-(2----6)-beta-GlcN-(1----6)-GlcN moieties were obtained and deployed in the synthesis of decyl 4,5,7,8-tetra-O-acetyl-3-deoxy-N-methyl-alpha-D-manno-2-octulopyranos idonamide (18), 2,3-di-O-tetradecanoyl-D-glycer-1-yl 4,5,7,8-tetra-O-acetyl-3-deoxy-N-methyl- alpha-D-manno-2-octulopyranosid-onamide (22), methyl 2,3,4-tri-O-acetyl-6-O-(methyl 4,5,7,8-tetra-O-acetyl-3-deoxy-alpha-D- manno-2-octulopyranosylonate)-alpha-D-glucopyranoside (28), 1-O-acetyl-2-deoxy-6-O-(methyl 4,5,7,8-tetra-O-acetyl-3-deoxy-alpha-D-manno-2- octulopyranosylonate)-2-tetradecanoylamino-alpha-D-glucopyra nose (33), tert-butyldimethylsilyl O-(methyl 4,5:7,8-di-O-cyclohexylidene-3-deoxy-alpha-D- manno-2-octulopyranosylonate)-(2----6)-O-(3,4-di-O-benzyl-2-deoxy- 2- tetradecanoylamino-beta-D-glucopyranosyl)-(1----6)-3,4-di-O-benzyl -2-deoxy-2- tetradecanoylamino-alpha-D-glucopyranoside (36).