Stereoselective synthesis of nucleosides from 1-thio and 1-seleno glycosides through consecutive 1,2-migration and glycosylation under Mitsunobu conditions

Viso, Antonio; Poopeiko, Nicolai E.; Castillón, Sergio

doi:10.1016/s0040-4039(99)02071-7

Cited by 18 publications

(14 citation statements)

References 18 publications

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“…[13,16] With optimization, the sulfur system could be rendered diastereoselective, but only low selectivity was observed for the selenium series. From these results, the authors concluded that these reactions do not involve episulfonium or episelenonium ions.…”

Section: 2-sulfur Migrations and Episulfonium Ionsmentioning

confidence: 99%

“…From these results, the authors concluded that these reactions do not involve episulfonium or episelenonium ions. [13,16] Scheme 10. Unselective 1,2-sulfur migration/glycosylation reaction.…”

Section: 2-sulfur Migrations and Episulfonium Ionsmentioning

confidence: 99%

“…[12] Although mechanisms involving bridged intermediates such as the episulfonium ions 4 and 8 provide concise and satisfying explanations for observations such as those shown in Schemes 1 and 2, accumulating evidence suggests that such intermediates may not be involved in stereoselective substitution processes. The fact that reactions that would be expected to proceed through episulfonium, [6,7,[13][14][15] episelenonium, [13,16] or iodonium ions [9,10] (9, Figure 1) are not completely selective [17] indicates that these reactions may occur by a different pathway or through multiple pathways. The formation of the product that would not correspond to stereospecific opening of the onium ion 9 (i.e., the 1,2-cis product) is typically explained by invoking oxocarbenium ions 10 as reactive intermediates.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Nucleophilic Substitution Reactions of 2‐Phenylthio‐Substituted Carbohydrate Acetals and Related Systems: Episulfonium Ions vs. Oxocarbenium Ions as Reactive Intermediates

2008

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A powerful approach for controlling the stereochemistry of glycosylation reactions involves using a directing group at an adjacent carbon to influence the stereochemistry of nucleophilic attack. The use of substituents such as I, PhS, and PhSe has proven to be especially valuable because it enables the preparation of 2‐deoxysugars after reductive removal of the directing group. The mechanisms of these reactions are generally considered to involve nucleophilic opening of bridged onium ions and to proceed with inversion. Onium ions, however, equilibrate with open cations, and these open forms can be favored when the cation is highly stabilized. Although the selectivities of reactions of carbohydrate‐derived cations are consistent with the intermediacy of onium ions, the reactions of acyclic cations are not. Instead, consideration of the preferred conformations of the oxocarbenium ions and stereoelectronically favored additions to these intermediates provides a predictive model to explain the diastereoselectivities observed in reactions of oxocarbenium ions bearing proximal heteroatoms.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

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Section: 2-sulfur Migrations and Episulfonium Ionsmentioning

confidence: 99%

Section: 2-sulfur Migrations and Episulfonium Ionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Nucleophilic Substitution Reactions of 2‐Phenylthio‐Substituted Carbohydrate Acetals and Related Systems: Episulfonium Ions vs. Oxocarbenium Ions as Reactive Intermediates

2008

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“…Although three-membered ring onium ions resembling 3 are intermediates in many reactions, , these intermediates may not be involved when an oxygen substituent is attached to the cation (as in oxocarbenium ion 4 ). Glycosylation reactions that should proceed through episulfonium ions − (and their related episelenonium , or iodonium ions 3,4 ) do not always provide trans products exclusively, raising the possibility that oxocarbenium ions such as 4 are involved. This analysis is supported by the observation that benzylic episulfonium ions open rapidly at low temperatures .…”

Section: Introductionmentioning

confidence: 99%

Nucleophilic Substitution Reactions of Sulfur-Substituted Cyclohexanone Acetals: An Analysis of the Factors Controlling Stereoselectivity

Billings

Woerpel

2006

J. Org. Chem.

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The reactions of cyclohexanone acetals substituted with thiophenyl groups (and other heteroatoms) at C-2 demonstrate the powerful influence that these substituents have on the stereoselectivity of nucleophilic substitution reactions. The trans selectivities of these reactions correlate with the behavior of the corresponding ketones. These experiments lend support to the possibility that the reactions of the acetals, which proceed via oxocarbenium ions, are operating under Felkin-Anh control.

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“…5.50 and 5.51, accumulating evidence suggests that such intermediates may not be involved in stereoselective substitution processes. The fact that the reactions that would be expected to proceed through episulfonium [111,112,[118][119][120], episelenonium [118,121], or iodonium ions [114,115] are not completely selective [122] indicates that these reactions may occur by a different pathway or through multiple pathways (Fig. 5.52).…”

Section: Superarmed Glycosyl Donors In Glycosylation Reactionsmentioning

confidence: 99%

Armed-Disarmed Concept in the Synthesis of Glycosidic Bond

Miljković

2013

Electrostatic and Stereoelectronic Effects in Carbohydrate Chemistry

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-Reid et al. reported [1] that n-pentenyl glycosides undergo chemospecific cleavage 1 ! 2 ! 3, with N-bromosuccinimide under conditions that leave a wide variety of other protecting groups unaffected ( Fig. 5.1).According to the proposed mechanism ( Fig. 5.1), the addition of water to the reaction mixture will lead to hydrolysis but the addition of alcohol or another sugar having a free hydroxyl group will lead to the formation of glycoside or a disaccharide. In the course of these studies, Fraser-Reid et al. [1] observed that sugar molecules having an acyloxy group at the C2 carbon (6 in Fig. 5.2) hydrolyzed much more slowly than if they had an alkoxy group (7 in Fig. 5.2). This observation suggested that the n-pentenyl glycoside could be made more or less reactive (it could be armed or disarmed) by the type of protecting group placed on the C2 oxygen.The ability of the C2 acyloxy group to disarm the n-pentenyl glycoside was rationalized as shown in Fig. 5.3. Thus, it can be assumed that the cyclic halonium ions 8 and 9 are formed reversibly [2] and that the electron density on the glycosidic oxygen is depleted in 2-O-acyl derivatives so that nucleophilic attack of this oxygen on the halonium ion is less favored than in the 2-O-alkyl counterpart 9. The reason for this is that the electron-withdrawing group at the C2 carbon makes the C2 carbon slightly positively charged so that the formation of another positive charge at the C1 carbon is not a favorable process [there cannot exist two positive charges on two neighboring carbons (C1 and C2) (Fig. 5.3).].Consequently, the armed glycosyl donors react with electrophiles faster than the disarmed ones, and therefore in a solution containing both an armed and disarmed glycosyl donor molecules, whereby disarmed glycosyl donor molecules have one free hydroxyl group, the reaction with an electrophile will result in crosscoupling and not self-coupling, i.e., an armed glycosyl donor will react with the disarmed one, whereas a disarmed donor will not react with itself [3] as illustrated in Fig. 5.5. The promoters for the activation of n-pentenyl group [4,5] are iodonium dicollidine perchlorate (IDCP) and N-iodosuccinimide/triethylsilyl triflate (NIS/Et 3 SiOTf).M. Miljkovic, Electrostatic and Stereoelectronic Effects in Carbohydrate Chemistry,

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Stereoselective synthesis of nucleosides from 1-thio and 1-seleno glycosides through consecutive 1,2-migration and glycosylation under Mitsunobu conditions

Cited by 18 publications

References 18 publications

Nucleophilic Substitution Reactions of 2‐Phenylthio‐Substituted Carbohydrate Acetals and Related Systems: Episulfonium Ions vs. Oxocarbenium Ions as Reactive Intermediates

Nucleophilic Substitution Reactions of 2‐Phenylthio‐Substituted Carbohydrate Acetals and Related Systems: Episulfonium Ions vs. Oxocarbenium Ions as Reactive Intermediates

Nucleophilic Substitution Reactions of Sulfur-Substituted Cyclohexanone Acetals: An Analysis of the Factors Controlling Stereoselectivity

Armed-Disarmed Concept in the Synthesis of Glycosidic Bond

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