Dioxolenium Ion Research Articles

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.

Stabilization of Glucosyl Dioxolenium Ions by “Dual Participation” of the 2,2-Dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA) Protection Group for 1,2-cis-Glucosylation

The stereoselective introduction of glycosidic bonds is of paramount importance to oligosaccharide synthesis. Among the various chemical strategies to steer stereoselectivity, participation by either neighboring or distal acyl groups is used particularly often. Recently, the use of the 2,2-dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA) protection group was shown to offer enhanced stereoselective steering compared to other acyl groups. Here, we investigate the origin of the stereoselectivity induced by the DMNPA group through systematic glycosylation reactions and infrared ion spectroscopy (IRIS) combined with techniques such as isotopic labeling of the anomeric center and isomer population analysis. Our study indicates that the origin of the DMNPA stereoselectivity does not lie in the direct participation of the nitro moiety but in the formation of a dioxolenium ion that is strongly stabilized by the nitro group.

Glycosyl Exchange of Unactivated Glycosidic Bonds: Suppressing or Embracing Side Reactivity in Catalytic Glycosylations.

While developing boron-catalyzed glycosylations using glycosyl fluoride donors and trialkylsilyl ether acceptors, competing pathways involving productive glycosylation or glycosyl exchange were observed. Experimental and computational mechanistic studies suggest a novel mode of reactivity where a dioxolenium ion is a key intermediate that promotes both pathways through addition to either a silyl ether or to the acetal of an existing glycosidic linkage. Modifications in catalyst structure enable either pathway to be favored, and with this understanding, improved multicomponent iterative couplings and glycosyl exchange processes were demonstrated.

Exploring the Neighbouring Group Participatory Mechanism in Glycosylation Reaction

Glycosyl donors have been experimentally shown to have a high tendency for acyl transfer to the alcohol nucleophile as a major side product during glycosylation reactions. Therefore, a neighbouring group participatory mechanism of glycosylation is explored using D-galactopyranose based donor having 2-O-acyl functionality by employing density functional theory. The reaction proceeds via galactopyranosyl dioxolenium ion as a stable intermediate, which leads to the formation of α-glycoside 4, orthoester (5 or 6) and acyl transfer 7 as side products. The mechanism of the stereoselective formation of β-glycoside is investigated. Moreover, all the possible intermediates and transition states have been explored.

Characterization of glycosyl dioxolenium ions and their role in glycosylation reactions

Controlling the chemical glycosylation reaction remains the major challenge in the synthesis of oligosaccharides. Though 1,2-trans glycosidic linkages can be installed using neighboring group participation, the construction of 1,2-cis linkages is difficult and has no general solution. Long-range participation (LRP) by distal acyl groups may steer the stereoselectivity, but contradictory results have been reported on the role and strength of this stereoelectronic effect. It has been exceedingly difficult to study the bridging dioxolenium ion intermediates because of their high reactivity and fleeting nature. Here we report an integrated approach, using infrared ion spectroscopy, DFT computations, and a systematic series of glycosylation reactions to probe these ions in detail. Our study reveals how distal acyl groups can play a decisive role in shaping the stereochemical outcome of a glycosylation reaction, and opens new avenues to exploit these species in the assembly of oligosaccharides and glycoconjugates to fuel biological research.

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.

Direct Experimental Characterization of Glycosyl Cations by Infrared Ion Spectroscopy.

Glycosyl cations are crucial intermediates formed during enzymatic and chemical glycosylation. The intrinsic high reactivity and short lifetime of these reaction intermediates make them very challenging to characterize using spectroscopic techniques. Herein, we report the use of collision induced dissociation tandem mass spectrometry to generate glycosyl cations in the gas phase followed by infrared ion spectroscopy using the FELIX infrared free electron laser. The experimentally observed IR spectra were compared to DFT calculated spectra enabling the detailed structural elucidation of elusive glycosyl oxocarbenium and dioxolenium ions.

A theoretical investigation of the reaction mechanism for hydrogenated furan formation under Prins reaction conditions in trifluoroacetic acid medium

The formation mechanism of hydrogenated furans by coupling between terminal alkenes and aldehydes in trifluoroacetic acid medium were investigated by ab initio calculations, the main attention was focused on the study of the rate controlling step. Two possible mechanisms for this reaction involving unsaturated alkoxycarbenium ions or dioxolenium ions as a key intermediate were considered. It was shown that the mechanism of formation of 3-alkylsubstituted hydrogenated furans in trifluoroacetic acid under Prins reaction conditions preferably includes recyclization of 3-(2-hydroxyethyl)-1-trifluoromethyl-2,5-dioxolenium ions.

Neighboring Group Participation in Glycosylation Reactions by 2,6-Disubstituted 2-O-Benzoyl groups: A Mechanistic Investigation

Variable yields and glycosylation stereoselectivity were obtained for NIS/TfOH-medi- ated reaction of 4-methoxyphenyl 2,4,6-tetra-O-acetyl-β-D-galactopyranoside and thiogalactosides bearing acetyl, benzoyl, 2,6-dimethoxylbenzoyl, 2,4,6-trimethylbenzoyl, or 2,6-dichlorobenzoyl groups at the 2-positions and acetyl at the remainder. X-ray structures of 4-methylphenyl 2,3,4,6-tetra-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyr anoside and 4-methylphenyl 3,4-O-isopropylidene-2,6-di-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside revealed slightly distorted 4 C 1 chair conformations. Variable temperature NMR revealed that activation of 4-methylphenyl 2,3,4,6-tetra-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside afforded only dioxolenium ion, whereas 4-methylphenyl 3,4,6-tri-O-acetyl-2-O-(2,4,6-trimethylbenzoyl)-1-thio-β-D-galactopyranoside gave a 1:1 mixture of dioxolenium ion and glycosyl triflate. However, the reaction intermediates formed from these deactivated donors do not influence the glycosylation stereoselectivity; instead, it is influenced by steric and electronic interactions at the transition states.

Cover Picture: Stereoselective Glycosylations Using (R)‐ or (S)‐(Ethoxycarbonyl)benzyl Chiral Auxiliaries at C‐2 of Glycopyranosyl Donors (Eur. J. Org. Chem. 22/2006)

AbstractThe cover picture shows the proposed mechanistic pathway for the stereoselective glycosylation based on Density Functional Theory calculations of the model compound. The protected sugar that has a chiral protecting group that is derived from mandelic acid and is attached to O‐2 after activation to an oxacarbenium ion species can form energetically favorable 6‐membered‐ring dioxolenium ions. The favored dioxolenium ion has the phenyl substituent in an equatorial position in both cases, allowing for the chirality of the protecting group to be transferred to the stereochemistry of the synthetically isolated glycoside. Details are discussed in the article by D. Whitfield, G.‐J. Boons et al. on p. 5007 ff.

Stereoselective Glycosylations Using (R)‐ or (S)‐(Ethoxycarbonyl)benzyl Chiral Auxiliaries at C‐2 of Glycopyranosyl Donors

AbstractThe stereoselective introduction of a glycosidic bond presents the greatest challenge to complex oligosaccharide synthesis. Important developments such as automated polymer‐supported oligosaccharide synthesis will not realize their full potential until this problem is addressed. In this paper, a novel approach for stereoselective glycosylations is described whereby a chiral auxiliary at C‐2 of a glycosyl donor controls the anomeric outcome of a glycosylation. It was found that participation of an (S)‐ethoxycarbonylbenzyl auxiliary led to the formation of 1,2‐cis glycosides, probably through a trans‐fused dioxolenium ion intermediate. On the other hand, the use of an auxiliary with (R) configuration gave 1,2‐trans glycosides, and this glycosylation proceeds through a cis‐fused dioxolenium ion intermediate. The auxiliary could conveniently be removed by Birch reduction. Computational studies support the formation of the proposed ethoxy‐carbonium ion intermediate with all pseudo‐equatorial substituents. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Is acyl migration to the aglycon avoidable in 2-acyl assisted glycosylation reactions?

This report unequivocally separates orthoester formation from acyl transfer for the first time and indicates possible routes to eliminate 2-O-acyl transfer during glycosylation reactions. Experimental evidence is shown that acyl transfer from 2-O-acyl-3,4,6-tri-O-benzyl-D-galactopyranose-derived glycosyl donors decreases in the order formyl > acetyl > pivaloyl. The 2-O-benzoyl derivatives are more variable, in some cases transferring easily, and in others not at all. Density functional theory (DFT) calculations of the structure and energetics of dioxolenium ion and related intermediates suggest that a proton transfer pathway from the nucleophile to O-2 provides an explanation for the observed trends. These DFT calculations of the proton transfer pathway support a mechanism in which a relay molecule is involved. Further DFT calculations used a constraint based on linear combinations of six bond lengths to establish the sequence of bond breaking and bond forming. The calculated anomeric carbon to former carbonyl oxygen bond that breaks during acyl transfer is the longest in the formyl case and shortest in those that exhibit little or no acyl transfer. Rotation about the aromatic to carbonyl Ph—C(=O) bond is different from the alkyl series. Analysis of this proposed TS led to the postulate that 2,6-substitution may hinder rotation even more. Thus, the 2,6-dimethylbenzoyl analogue was synthesized and it does not transfer directly or by rearrangement of its readily formed orthoester. DFT calculations suggested that 2,6-dimethoxybenzoyl should also not transfer easily. Experimentally, this proved to be the case and this new 2-O-acyl protecting group cleaves at 50 °C with a 1 mol/L solution of LiOH in methanol. Thus, a calculated transition state has led to a prototype of a protecting group that solves a major problem in oligosaccharide synthesis.Key words: glycosylation, carbohydrates, quantum chemistry, reaction mechanism, neighboring-group effects.

Can the stereochemical outcome of glycosylation reactions be controlled by the conformational preferences of the glycosyl donor?

Previous static and dynamical density functional theory studies of the 2,6-di- O-acetyl-3,4- O-isopropylidene- d-galactopyranosyl cations and their methanol adducts has led to an hypothesis that these cations exist in two families of conformers characterized as 2 S O and B 2,5, respectively. These families differ by ring inversion, each with its own reactivity. New calculations on the 2,6-di- O-acetyl-3,4-di- O-methyl- d-galactopyranosyl cation confirmed these trends. Removing the isopropylidene group allows more flexibility, but two families of conformers can be discerned with the monocyclic oxocarbenium ions in the E 3 conformation and the bicyclic dioxolenium ions in the 4 H 5 conformation. Attack on the β-face of these monocyclic cations is favored by hydrogen bonding and the anomeric effect. The experimentally observed high β-stereoselectivity of mannopyranosyl donors and high α-stereoselectivity of glucopyranosyl donors with the 4,6- O-benzylidene protecting groups can be rationalized assuming that the trans-fused 1,3-dioxane ring allows population of only one family of conformers. The combination of hydrogen bonding and conformational changes of the pyranose ring in response to the C-5O-5C-1C-2 torsion angle changes are identified as key factors in stereoselectivity. Based on these observations a strategy to design face discriminated glycosyl donors that exist predominantly in only one family of conformers is proposed.

The conformational origin of the barrier to the formation of neighboring group assistance in glycosylation reactions: a dynamical density functional theory study.

Static and dynamical Density Functional Theory studies of 2,6-di-O-acetyl-3,4-O-isopropylidene-D-galactopyranosyl cation have shown that this cation can exist in two conformers characterized as (2)S(O) and B(2,5), respectively. The (2)S(O) conformer has the O-2 acyl group equatorial with the carbonyl syn to H-2 and is populated by monocyclic oxocarbenium ions. These conformational features are present in the structurally related glycosyl donor ethyl 2,6-di-O-benzoyl-3,4-O-isopropylidene-beta-D-galactothiopyranoside as determined by X-ray diffraction studies. The B(2,5) conformer has O-2 axial and allows the carbonyl to rotate and close the five-membered ring to form a bicyclic dioxolenium ion. Constraints based on natural internal coordinates were implemented to study this conformational transition. In this way the barrier to interconversion has been determined to be 34 kJ mol(-)(1) with a transition state characterized as (O)S(2) and a pathway involving pseudorotation. Thus, for the first time the structures and energetics of the key ions postulated to be involved in neighboring group assisted glycosylation reactions have been determined.

Zirconocene-catalyzed cationic Diels-Alder reactions

In situ prepared Cp 2ZrCl ⊕ catalyzes the formation of dioxolenium ions from α,β-unsaturated epoxy esters. As a consequence of this activation process, acrylate, methacrylate and crotonate derivatives undergo a rapid and stereoselective cationic [4+2] cycloaddition with a wide range of dienes. Ring-extended carboxylic acid derivatives are formed in 1–7 h at 0–21 °C and in 50–90% yield after saponification of the intermediate diol esters. Simple Lewis acid catalysis by Cp 2ZrCl ⊕ can be excluded on the basis of the experimental results.

Stabilities of some 2‐(para‐substituted‐phenyl)‐4,4,5,5‐tetramethyl‐1,3‐dioxolanes relative to their conjugate dioxolenium ions, radicals and carbanions as determined by thermodynamics for hydride and electron transfer in solution

AbstractA calorimetric method is described for the determination of the hydride affinities, ΔH(R+) of seven dioxolenium ions from the title compounds and also tropylium, trityl and 9‐phenyl xanthylium cations by hydride transfer to the carbocations from BH3CN−. Cyclic voltammetric methods yield free energies for reduction of the cations to the conjugate radicals and to the carbanions. The ΔH(R+) values correlate well with the first reduction potentials of the cations.

Synthesis of 2,9,10-trioxatricyclo[4.3.1.0]decane analogues of resiniferatoxin

Structurally simplified analogues of the daphnane diterpene resiniferatoxin (RTX)1, possessing the unusual 2,9,10-trioxatricyclo[4.3.1.0]decane system have been synthesised stereoselectively from cyclohexa-1,3-diene: functionalisation of the diene afforded the anti-epoxide, 1,4-di-O-benzyl-t-2,t-3-epoxycyclohexane-r-1,c-4-diol 4, the ring-opening of which was examined using various organo-metallic reagents; organoaluminium species were found to be the most efficient to effect this reaction. When trimethylsilyl (in place of benzyl) ethers were used to protect the diol, selective deprotection of 1,4-di-O-trimethylsilyl-2-O-(p-tolylsulfonyl)-c-3-[3-(tert-butyldiphenylsilyloxy)-prop-1-ynyl]cyclohexane-r-1,t-2,c-4-triol 16 was achieved using citric acid in methanol—the equatorially disposed trimethylsilyl ether was found to be more easily cleaved than the axially orientated one. Formation of the tricyclic orthoester was achieved by the generation of a dioxolenium ion from 1-O-phenylacetyl-2-O-(p-tolylsulfpnyl)-c-3-[3-(tert-butyldiphenylsilyloxy)-prop-1-ynyl]cyclohexane-r-1,t-2,c-4-triol 19, by heating in 2,4,6-trimethylpyridine, with in situ intramolecular trapping by the suitably orientated hydroxy group to give 1-benzyl-7-(3-tert-butyldiphenylsilyloxyprop-1-ynyl)-2,9,10-trioxatricyclo[4.3.1.0] decane 20.

Action du N-bromosuccinimide sur les dioxolannes en série bicyclo [2.2.1 ]heptanique. Limitation des transpositions de wagner-meerwein

Vicinal benzoyloxy- trans-bicyclo [2.2.1] heptanic bromhydrins, otherwise difficult to obtain are formed from N-bromosuccinimide and norbornanes bearing a 2-aryl-1,3 dioxolane skeleton. The greater stability of the intermediate dioxolenium ion reduces the tendency towards both Wagner-Meerwein rearrangement and neighbouring group participation by chlorine, processes usually observed when a less stable intermediate is involved. Nevertheless, methoxycarbonyl neighbouring group participation and Wagner Meerwein rearrangment occur in methoxycarbonylbicyclo [2.2.1]heptane and in a bornanic compound respectively.

Carbonyl oxygen exchange of glycol monoesters. Rate and equilibrium constants for the formation of a tetrahedral intermediate

Kinetic investigations of the hydrolysis of the 2-phenyl-4,4,5,5-tetramethyl-1,3-dioxolenium ion and 2-phenyl-2-methoxy-4,4,5,5-tetramethyl-1,3-dioxolane furnish rate constants for all three reaction stages of the ortho ester hydrolysis: (1) generation of the dioxolenium ion, (2) hydration of this ion to form hydrogen ortho ester, and (3) breakdown of this species to pinacol monobenzoate. The equilibrium constant for stage (2) can also be obtained. This study complements a previous investigation of 2-phenyl-2-alkoxy-1,3-dioxolanes where similar information was obtained.The rate constants for carbonyl oxygen exchange of the ester products of these reactions, pinacol monobenzoate and ethylene glycol monobenzoate, have been measured. This reaction is shown to proceed by a different mechanism to that normally associated with exchange of carboxylic acid derivatives: cyclization of the glycol monoester to form hydrogen ortho ester, followed by loss of the labelled exocyclic OH group to give 1,3-dioxolenium ion. Reversal of these steps, initiated by an unlabelled water molecule, results in exchange. The relationship of this mechanism with that of the ortho ester hydrolysis is obvious; it is shown that the exchange provides rate constants for the reverse of stage (3). This means that both the forward and reverse rates of this process have been obtained, and this provides the equilibrium constant.

Nucleophilic substitution in glycerol derivatives, part II: unsymmetrical dioxolenium ions as intermediates in the reactions of diacylglycerosulphonates with carboxylate ions

Nucleophilic substitution in glycerol derivatives, part II: unsymmetrical dioxolenium ions as intermediates in the reactions of diacylglycerosulphonates with carboxylate ions