Stereoelectronic Model To Explain Highly Stereoselective Reactions of Seven-Membered-Ring Oxocarbenium-Ion Intermediates

Accumulation, Characterization and Reactivity of Chiral Ammonium-Carboxonium Dications in Superacid

The accumulation of chiral ammonium-oxocarbenium dications in superacid is evidenced by low-temperature NMR spectroscopy, X-ray diffraction analysis and confirmed by DFT calculations. Its potential for the diastereoselective remote hydrofunctionalization of non-activated alkene is also explored.

Involvement of an Oxonium Ion Intermediate in Controlling the Diastereoselectivity of Nucleophilic Substitution Reactions of Septanoses

The alkoxy substituents at C4 and C2 of septanoses control the stereochemical outcomes of O-glycosylation reactions of these seven-membered-ring intermediates. Isolation of a bicyclic acetal byproduct in some substitution reactions suggests that the C4 benzyloxy substituent engaged in long-range participation, stabilizing intermediates by the formation of an oxonium ion intermediate. Inductive destabilization of the carbocationic intermediate provided by the C2 substituent is crucial to the participation of the remote alkoxy group.

Diastereoselective Alkylations of Chiral Tetrazolo[1,5-a]azepines via Heterobenzylic Anion Intermediates

The alkylations of chiral seven-membered rings fused to tetrazoles are highly diastereoselective. The diastereoselectivity depended on the placement and the size of the substituent on the ring and on the electrophile. Subsequent alkylations occurred with high stereoselectivity, allowing for the construction of quaternary stereocenters. Computational studies revealed that torsional effects are responsible for the observed diastereoselectivities. Substituted products can be reduced to the corresponding secondary amines, thus providing an approach for synthesizing diastereomerically enriched azepanes.

Oxo-Rhenium-Mediated Allylation of Furanoside Derivatives: A Computational Study on the Mechanism and the Stereoselectivity

Properly substituted tetrahydrofuran (THF) rings are important building blocks in the synthesis of many natural metabolites. Having reliable procedures to control the stereoselectivity at the THF core while decorating it with different substituents is a fundamental requirement to achieve and fulfill the complexity of nature. We recently reported a new chemical approach to control the stereochemistry in the alkylation and arylation of furanoside derivatives by using a rhenium(V) complex to form an intermediate oxo-carbenium species able to react with proper soft nucleophiles. Here, we describe theoretical calculations, performed at the DFT B3LYP level, to disclose the important mechanistic features which regulate the entire catalytic cycle of the reaction of mono- and disubstituted furanosides with allyltrimethylsilane catalyzed by Re(O)Cl₃(OPPh₃)(Me₂S). Moreover, the key factors governing the allylation step were investigated, confirming that the stereoselectivity, which is independent of the anomeric configuration of starting acetal, mainly arises from the orientation of the substituent at C-4, with only marginal contribution of the substituent at C-5. Finally, puckering Cremer–Pople parameters were used to take trace of the structural modifications throughout the catalytic cycle.

Stereoselective Access to Iminosugar C,C-Glycosides from 6-Azidoketopyranoses

We report the synthesis of iminosugar C,C-glycosides starting from 6-azidoketopyranoses. Their Staudinger-azaWittig-mediated cyclization provided bicyclic N,O-acetals, which were stereoselectively opened with AllMgBr to afford β-hydroxyazepanes with a quaternary carbon α to the nitrogen. Their ring contraction via a β-aminoalcohol rearrangement produced the six-membered l-iminosugars with two functional handles at the pseudoanomeric position. Inversion of the free OH at the azepane level furnished the d-iminosugars.

Characterization of the low energy conformations and differential reactivities of D-glucose and D-mannose based oxepines

Carbohydrate-based oxepines are valuable intermediates for the synthesis of septanose carbohydrates. Here we report the characterization of the preferred conformations of D-glucose and D-mannose based oxepines 1 and 2 using computational chemistry and NMR spectroscopy. Monte Carlo conformational searches on 1 and 2 were performed, followed by DFT optimization and single-point energy calculations on the low energy conformations of each oxepine. Coupling constants were computed for all unique conformations at a B3LYP/6-31G(d,p)u+1s level of theory and weighted based on a Boltzmann distribution. These values were then compared to the experimental values collected using ³J_H,H values collected from ¹H NMR spectra. Information from the MC/DFT approach was then used in a least squares method that correlated DFT calculated and observed ³J_H,H coupling constants. The conformations of 1 and 2 are largely governed by a combination of the rigidifying enol ether element in combination with the reduction of unfavorable interactions. The vinylogous anomeric effect (VAE) emerged as a consequence, rather than a driver of conformations. Oxepine 1 showed greater reactivity in Ferrier rearrangement reactions relative to oxepine 2, in line with its greater %VAE.

Highly Stereoselective Glycosylation Reactions of Furanoside Derivatives via Rhenium (V) Catalysis

A novel approach for the formation of anomeric carbon-functionalized furanoside systems was accomplished through the employment of an oxo-rhenium catalyst. The transformation boasts a broad range of nucleophiles including allylsilanes, enol ethers, and aromatics in addition to sulfur, nitrogen, and hydride donors, able to react with an oxocarbenium ion intermediate derived from furanosidic structures. The excellent stereoselectivities observed followed the Woerpel model, ultimately providing 1,3-cis-1,4-trans systems. In the case of electron-rich aromatic nucleophiles, an equilibration occurs at the anomeric center with the selective formation of 1,3-trans-1,4-cis systems. This anomalous result was rationalized through density functional theory calculations. Different oxocarbenium ions such as those derived from dihydroisobenzofuran, pyrrolidine, and oxazolidine heterocycles can also be used as a substrate for the oxo-Re-mediated allylation reaction.

CF3-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

"The extraordinary instability of such an "ion" accounts for many of the peculiarities of organic reactions" - Franck C. Whitmore (1932). This statement from Whitmore came in a period where carbocations began to be considered as intermediates in reactions. Ninety years later, pointing at the strong knowledge acquired from the contributions of famous organic chemists, carbocations are very well known reaction intermediates. Among them, destabilized carbocations - carbocations substituted with electron-withdrawing groups - are, however, still predestined to be transient species and sometimes considered as exotic ones. Among them, the CF3-substituted carbocations, frequently suggested to be involved in synthetic transformations but rarely considered as affordable intermediates for synthetic purposes, have long been investigated. This review highlights recent and past reports focusing on their study and potential in modern synthetic transformations.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Impact of Aromatic Stacking on Glycoside Reactivity: Balancing CH/π and Cation/π Interactions for the Stabilization of Glycosyl-Oxocarbenium Ions

Carbohydrate/aromatic stacking represents a recurring key motif for the molecular recognition of glycosides, either by protein binding domains, enzymes, or synthetic receptors. Interestingly, it has been proposed that aromatic residues might also assist in the formation/cleavage of glycosidic bonds by stabilizing positively charged oxocarbenium-like intermediates/transition states through cation/π interactions. While the significance of aromatic stacking on glycoside recognition is well stablished, its impact on the reactivity of glycosyl donors is yet to be explored. Herein, we report the first experimental study on this relevant topic. Our strategy is based on the design, synthesis, and reactivity evaluation of a large number of model systems, comprising a wide range of glycosidic donor/aromatic complexes. Different stacking geometries and dynamic features, anomeric leaving groups, sugar configurations, and reaction conditions have been explicitly considered. The obtained results underline the opposing influence exerted by van der Waals and Coulombic forces on the reactivity of the carbohydrate/aromatic complex: depending on the outcome of this balance, aromatic platforms can indeed exert a variety of effects, stretching from reaction inhibition all the way to rate enhancements. Although aromatic/glycosyl cation contacts are highly dynamic, the conclusions of our study suggest that aromatic assistance to glycosylation processes must indeed be feasible, with far reaching implications for enzyme engineering and organocatalysis.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.