Experimental and computational studies of ⁿJ(⁷⁷Se,¹H) selenium–proton couplings in selenoglycosides

Abstract

Selenoglycosides are important starting materials in synthetic carbohydrate chemistry and play a role in biological interactions as well. Both aspects are influenced by the conformation around the glycosidic bond. Here, we present a combined experimental and computational approach to measure and evaluate (n)J((77)Se, (1)H) coupling constants for their use in conformational analysis. The measurements were carried out using a modified CPMG-HSQMBC pulse scheme which yields pure absorption antiphase multiplets to allow accurate determination of the (n)J(XH) values regardless of the size of the proton-proton couplings. Theoretical calculations were performed at the Second-Order Polarization Propagator Approach (SOPPA) level. Population-averaged values calculated for geminal and vicinal couplings are in a good agreement with experiment indicating an adequate theoretical level of the calculations. Experimental observations and computations alike have indicated that two-bond (77)Se-(1)H couplings, (2)J((77)Se, (1)H), in a H1-C1-Se-X moiety are very sensitive to the torsion angle around the C1-Se-bond and will, therefore, be useful for conformational studies.

Stereochemical behavior of 77Se‐1H spin‐spin coupling constants in pyrazolyl‐1,3‐diselenanes and 1,2‐diselenolane

“…As an example, 2 J (Se,H 4a ) ≈ −1 Hz and 2 J (Se,H 4e ) ≈ +30 Hz in 1 , or 3 J (Se,H 4a ) ≈ 1 Hz, whereas 3 J (Se,H 4e ) ≈ 13 Hz in 6 . With our previous results as basis, we expected that these differences were to be accounted for the selenium lone pair effect for geminal couplings and the Karplus‐type relationship for vicinal coupling constants.…”

J(77Se,1H) and J(77Se,13C) couplings of seleno‐carbohydrates obtained by 77Se satellite 1D 13C spectroscopy and 77Se selective HR‐HMBC spectroscopy

“…There seems to be a tendency for the J value to follow the dihedral angle. It has been reported that 2 J (Se, H) is strongly dependent on the orientation of the selenium lone pair and 3 J (Se, H) follows classical Karplus‐type equation from experimental and theoretical approaches . 2 J (Se, C) have not been studied intensively and little is known about 3 J (Se, C) values .…”

“…There are few applications of 77 Se‐NMR to carbohydrates . Perhaps most noteworthy has been a study by Harmark et al of the interaction between selenium‐labeled carbohydrates and lectins based on differences in 77 Se chemical shifts and line widths .…”

“…Our group is expanding the chemical synthesis of seleno sugars and oligosaccharides for applications in structural biology . Useful information on the conformational properties of seleno‐carbohydrates can be obtained from spin coupling constants such as 2‐3 J ( 77 Se, 1 H) . Extension to 1‐3 J ( 77 Se, 13 C) requires 1 H, 13 C, 77 Se triple resonance experiments, and J (Se, Se) and J (Se, C) in the octahedral clusters are possible .…”

Stereochemical study of the sterically crowded phenylselanylalkenes by means of 77Se1H spin–spin coupling constants

“…Considered herewith are five most typical representatives of this series, compounds 1-5 (Scheme 1), obtained via heterogeneous Ni-based catalytic regioselective hydroselenation of terminal alkynes (to give compounds 1-3) and stereoselective hydroselenation of internal alkynes (to give compounds 4 and 5), as described elsewhere [3b] . Based on our previous results on the marked stereospecificity of geminal and vicinal J(Se,H) couplings in respect with the mutual orientation of the selenium lone pair and the corresponding coupling pathway in divinyl selenide, [4] selenophenes, [5] selenosugars [6] and selenium-containing heterocycles, [7] in this communication we employ vicinal selenium-proton couplings across double bond measured experimentally and calculated theoretically for the stereochemical study of 1-5 dealing with the internal rotation around the Se-C bonds. These results are supplemented with the energy-based theoretical conformational analysis performed at the MP2/6-311G * * level.…”