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Konformacje pierścienia monosacharydowego

Stępień Ł., Liberek B.

Wiadomości Chemiczne

2012

[Z] 66, 1-2

67-92

Conformational studies of the pyranose and furanose rings are important and have been carried out for years. Knowledge of the conformation of monosaccharide is essential for understanding its physical, chemical and biological properties. Additionally, conformation of the sugar ring plays a crucial role in the stereochemistry of the reaction in which it participates. This paper provides a basic knowledge concerning the conformational preferences of a monosaccharide ring. The pyranose ring conformations are defined with reference to a cyclohexane ring [1–3]. Factors influencing stability of the pyranose chair conformation are discussed: the 1,3-diaxial interactions [4–6] and anomeric effect [7–10]. It is shown how to estimate the relative stabilities of the two chair conformations on the basis of the Angyal destabilizing factors [11, 12]. It is also demonstrated how to use the NMR spectroscopy for conformational analysis [13–23]. Conformations of the unsaturated pyranose rings are defined and studied mainly on the glycals [24–29], the monosaccharides with a double bond between the C1 and C2 carbon atoms. Factors influencing stability of the unsaturated pyranose ring are disscused: the allylic effect [30] also named the vinylogous anomeric effect [31–34], and quasi 1,3-diaxial interactions [35, 36]. Finally, the furanose ring conformations and pseudorotational itinerary for a d-aldofuranose ring are presented [37]. Two parameters defining the furanose ring conformation are introduced: the amplitude of pseudorotation [38, 39], named also the maximum torsion angle [4] and pseudorotational phase angle [4]. The possible ways to study the conformation of the furanose ring are disscused [40–44].

The Properties of the Dihydrogen-Bonded Dimer (BH3NH3)2

Cybulski H., Sadlej J.

Polish Journal of Chemistry

2007

Vol. 81, nr 5-6

683-697

The energetic and spectroscopic properties of the dihydrogen-bonded dimer (BH3NH3)2 with the B–H bonds as proton acceptors and the N–H bonds as proton donors were calculated by means of the second-orderMller-Plesset perturbation theory and density functional theory methods. The C2h head-to-tail structure with four equivalent dihydrogen bonds was found to be the most stable one. The symmetry-adapted perturbation theory calculations showed that (BH3NH3)2 dimer is bonded by electrostatic-induction-dispersion interactions and the interaction energy decomposition is comparable with this calculated for water dimer. The protons acting as proton donors are deshielded while the protons acting as proton acceptors are shielded upon dimer formation. The one-bond dihydrogen-bond transmitted proton-proton coupling constant has a noticeable value of 1.9 Hz. Among the three-bond dihydrogen-bond transmitted reduced coupling constants the largest one is the coupling between nitrogen and boron nuclei with a value of 5.6.10 19.T2.J -1.