Torsional effects in glycoside reactivity: saccharide couplings mediated by acetal protecting groups

Fraser‐Reid, Bert; Wu, Zufan; Andrews, C. Webster; Skowronski, Evan W.; Bowen, J. Phillip

doi:10.1021/ja00004a066

Cited by 197 publications

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“…This is similar to the direct influence of the ease of rotation around the C(2)-C(3) bond on the rate of glycoside hydrolysis found by Feather and Harris [32] and consistent with a change of the pyranose-ring conformation during thermolysis. That the torsional strain of the annulated dioxane ring on the hydrolytic stability of glycosides, which has been described by Fraser-Reid and coworkers [25] is not observed in the thermolysis of the benzylidenated diazirines 5,7, and 9 may be explained by the strongly exergonic nature of the thermolysis (release of ring strain and formation of N J and, thus, an early transition state, while a late transition state close to the oxycarbenium intermediate is characteristic for glycoside hydrolysis. A comparison of the steric and electronic factors on the thermal stability of glycosylidene-diazirines, on the one hand, and on the hydrolytic stability of glycosides, on the other hand, may thus contribute to distinguish early and late events on these reaction paths.…”

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confidence: 73%

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Glycosylidene Carbenes. Part 13. Synthesis and thermolysis of representative 1‐azi‐glycoses

Vasella

Witzig

Waldraff

et al. 1993

Helvetica Chimica Acta

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Dedicated to Duilio Arigoni on the occasion of his 65th birthday (1.X.93)In the context of the hypothesis postulating a heterolytic cleavage of a C-N bond during thermolysis of alkoxydiazirines (Scheme I ) , we report the preparation of the diazirines4,5,7, and 8, the kinetic parameters for the thermolysis in MeOH of the diazirines 1 and 4-9, and the products of their thermolysis in an aprotic environment. The diazirines 4,5,7, and 8 (Schemes 2-5) were prepared from the known hemiacetals 10,19,34 (prepared from 31 in an improved way), and 42 according to an established method. The oximes 11,20,35, and 43 were obtained from the corresponding hemiacetals as (E/Z)-mixtures; 43 was formed together wlth the cyclic hydroxylamine 44. Oxidation of 11,35, and 43 (N-chlorosuccinimide/l,8-diazabicyclo[5.4.0]undec-7-ene (NCS/DBU) or NaIO,) gave good yields of the (Z)-hydroximolactones 12, 36, and 45, while the oxime 20 led to a mixture of the (E)-and (2)-hydroximolactones 21 and 22, which adopt different conformations. Their configuration was assigned, inter alia, by a comparison with the enol ethers 28 and 29, which were obtained, together with 30, from the reaction of the diazirine 5 with benzaldehyde and PBu,. Treatment of the hydroximolactone 0-sulfonates 13,23,37, and 46 with NH,/MeOH afforded the diaziridines 15,25,38, and 47 in good yields, while the (E)-sulfonate 24 decomposed readily. Oxidation of the diaziridines gave 4, 5, 7, and 8, respectively. Thermolysis of the diazirines 1 and 4-9 in MeOH yielded the anomeric methyl glycosides 50/51,16/17,26/27,52/53,39/40,48/49, and 54/55, respectively. A comparison of the kinetic data of the thermolysis at four different temperatures shows the importance of conformational and electronic factors and is compatible with the hypothesis of a heterolytic cleavage of a C-N bond. An early transition state is evidenced by the absence of torsional strain by an annulated 1,3-dioxane ring. Thermolysis of 1 in MeCN at 23" led mostly to the diastereoisoineric (Z,Z)-, (E,E)-, and (E,Z)-lactone azines 56, 57, and 58 (Scheme 6), which convert to 56 under mild conditions, and to 59 (3%). The benzyloxyglucal59 was obtained in higher yields (IS%), together with 44% of 5658, by thermolysis of solid 1. Similarly, thermolysis at higher temperatures of4 in toluene, THF, or dioxane and of 9 in CH2C12 or THF yielded the (Z,Z)-lactone azines 60 and 61, respectively, the latter being accompanied by the dihydro-oxazole 62.Introduction. -Diazirines [I] are important precursors of carbenes, and the mechanism of their thermolysis attracted considerable attention, which focused on the concertedness of the cleavage of the two C-N bonds and the homo-or heterolytic nature of the bond breaking [2-71. The mechanism and the kinetics of the thermolysis of (a1koxy)alkyldiazirines (see [8] and earlier papers of the series [9-131) have not been studied, but we hypothesized that thermolysis of 1-azisugars, such as 1, i.e. cyclic (alkoxy)alkyldiazirines, is initiated by heterolysis of one of the C( 1)-N bonds, accord-

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confidence: 73%

“…Na,CO, soh. (90 ml), and brine (2 x 90 ml), dried (Na2S04), and evaporated: 1.48 g of crude 34 (1 (4) (25). 1 u n d 4 9 .…”

Section: Experimental Partmentioning

confidence: 99%

Glycosylidene Carbenes. Part 13. Synthesis and thermolysis of representative 1‐azi‐glycoses

Vasella

Witzig

Waldraff

et al. 1993

Helvetica Chimica Acta

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“…[232] Since then, conformation-constraining protecting groups have been used frequently to increase or decrease the reactivity of donors and to enable orthogonal activation in the presence of other conventional armed or disarmed donors. [233] Recently, the cause of the disarming effect of 4,6-Oacetal groups on hexopyranosyl donors was scrutinized and attributed not only to torsional but also to electronic effects.…”

Section: Conformation-constraining Protecting Groupsmentioning

confidence: 99%

New Principles for Glycoside‐Bond Formation

2009

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Increased understanding of the important roles that oligosaccharides and glycoconjugates play in biological processes has led to a demand for significant amounts of these materials for biological, medicinal, and pharmacological studies. Therefore, tremendous effort has been made to develop new procedures for the synthesis of glycosides, whereby the main focus is often the formation of the glycosidic bonds. Accordingly, quite a few review articles have been published over the past few years on glycoside synthesis; however, most are confined to either a specific type of glycoside or a specific strategy for glycoside synthesis. In this Review, new principles for the formation of glycoside bonds are discussed. Developments, mainly in the last ten years, that have led to significant advances in oligosaccharide and glycoconjugate synthesis have been compiled and are evaluated.

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“…They proposed that an S N 2-like mechanism could be favoured by either 1) the use of protecting groups that would destabilise an oxacarbenium ion intermediate or by 2) modifying the auxiliary to increase the stability of the sulfonium ion. [42] If hypothesis (1) is correct, then destabilising the oxacarbenium ion by exploiting the torsional and electronic constraints of a benzylidene acetal protecting group [50] should also favour an S N 2-like reaction. Oxathiane 10 was protected with a benzylidene acetal before acetylation of O3 and oxidation with mCPBA to give sulfoxide 55 (Scheme 7).…”

Section: Wwwchemeurjorgmentioning

confidence: 99%

Do Glycosyl Sulfonium Ions Engage in Neighbouring‐Group Participation? A Study of Oxathiane Glycosyl Donors and the Basis for their Stereoselectivity

Fascione

Kilner

Leach

et al. 2011

Chemistry A European J

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Neighbouring-group participation has long been used to control the synthesis of 1,2-trans-glycosides. More recently there has been a growing interest in the development of similar strategies for the synthesis of 1,2-cis-glycosides, in particular the use of auxiliary groups that generate sulfonium ion intermediates. However, there has been some debate over the role of sulfonium ion intermediates in these reactions: do sulfonium ions actually engage in neighbouring-group participation, or are they a resting state of the system prior to reaction through an oxacarbenium ion intermediate? Herein, we describe the reactivities and stereoselectivities of a family of bicyclic thioglycosides in which an oxathiane ring is fused to the sugar to form a trans-decalin-like structure. A methyl sulfonium ion derived from one such glycosyl donor is so stable that it can be crystallised from ethanol, yet it reacts with complete stereoselectivity at high temperature. The importance of a ketal group in the oxathiane ring for maintaining this high stereoselectivity is investigated using a combination of experiment and ab initio calculations. The data are discussed in terms of S(N)1 and S(N)2 type mechanisms. Trends in stereoselectivity across a series of compounds are more consistent with selective addition to oxacarbenium ions rather than a shift between S(N)1 and S(N)2 mechanisms.

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Torsional effects in glycoside reactivity: saccharide couplings mediated by acetal protecting groups

Cited by 197 publications

References 0 publications

Glycosylidene Carbenes. Part 13. Synthesis and thermolysis of representative 1‐azi‐glycoses

Glycosylidene Carbenes. Part 13. Synthesis and thermolysis of representative 1‐azi‐glycoses

New Principles for Glycoside‐Bond Formation

Do Glycosyl Sulfonium Ions Engage in Neighbouring‐Group Participation? A Study of Oxathiane Glycosyl Donors and the Basis for their Stereoselectivity

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