Stereochemistry of Nucleophilic Substitution Reactions Depending upon Substituent: Evidence for Electrostatic Stabilization of Pseudoaxial Conformers of Oxocarbenium Ions by Heteroatom Substituents.

Abstract

Nucleophilic substitution reactions of C-3 and C-4 alkyl-substituted tetrahydropyran acetates proceed via pseudoequatorially substituted oxocarbenium ions. The corresponding heteroatom-substituted acetates react via pseudoaxially oriented oxocarbenium ions. Title investigation is presented in continuation of previously published studies. -(AYALA, L.; LUCERO, C. G.; ROMERO, J. A. C.; TABACCO, S. A.; WOERPEL*, K. A.; J. Am. Chem. Soc. 125 (2003) 50, 15521-15528; Dep. Chem., Univ. Calif., Irvine, CA 92697, USA; Eng.) -Nuesgen 17-126

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

“…Thus, important issues involve the possible involvement of fused and bridging ions versus that of glycosyl oxocarbenium ions and/or reactive intermediates such as glycosyl triflates in the transition state for glycosidic bond formation, with evidence presented both for, and against, dependent on the system and conditions under study. 177,178,[221][222][223][224][225][226][227][228][229] This delicate balance between stereodirecting participation and alternative mechanisms is a prominent feature of the involvement of esters in glycosylation and apparent from the comparison of a recent study on 1,2-trans-glycosylation directed by the presence of 2-O-alkoxymethyl methyl groups, 230 and one of the many instances of intramolecular aglycone delivery, 231 a common strategy for 1,2-cis glycoside formation (Scheme 31). [232][233][234]…”

Characterization of glycosyl dioxolenium ions and their role in glycosylation reactions

“…The second set ( Fig. 2b) features benzyl ethers and benzoate esters (10)(11)(12)(13)(14)(15)(16)(17)(18) used in a matrix of model glycosylation reactions, as these represent the most commonly used protecting groups in synthetic carbohydrate chemistry. The benzyl ether and benzoate esters are structurally very similar, while differing significantly in electronic properties and their ability to stabilize an oxocarbenium ion 35,36 .…”

“…Importantly, LRP potentially allows for the utilization of the relative stereochemistry of C-3, C-4, or C-6 groups to control the facial selectivity in glycosylation reactions thereby enabling the stereoselective synthesis of C-2-deoxy and 1,2-cis glycosides. However, contradictory results have been reported and there is an ongoing debate as to the role and strength of this stereoelectronic effect [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] . Indirect proof for LRP has been derived from the stereochemical outcome of glycosylation reactions and studies using model systems 24 .…”

A Convenient Approach towards the Synthesis of ADMDP Type Iminosugars and Nojirimycin Derivatives from Sugar-Derived Lactams

“…Addition of TMSCN to gluco-imine 2 b proceeded syn to the BnO substituent at the C-3 position. This is a result of conformational control of the process, according to Woerpel’s model ( Figure 2 a) [ 29 , 30 , 31 , 32 , 33 , 34 ]. In the latter case, the nucleophilic addition of TMSCN to arabino -imine 2a occurs with low stereoselectivity, leading to the separable mixture of isomers 3a and 2- epi -3a in a ratio of 60:40.…”