DISCUSIÓN DE RESULTADOS

A range of different potential descriptors were obtained using SPARTAN software (Table 4.1) to quantify the molecular property of donor, acceptor, activator and solvent, with density functional theory (DFT) calculations. Descriptors for donor, acceptor, activator and solvent were chosen based on the empirical understanding of the glycosylation mechanism (Figure 3.22 in Chapter 3) gained in this research work (Figure 4.1).

Table 4.1: the potential descriptors.

Figure 4.1: The selected descriptors.

4.2.1 Donor

Starting with the coupling partners, it was critical to identify and quantify their reactivity, capturing steric and electronic effects of both the nucleophile and electrophile. Following the screening of potential descriptors for the steric/electronic properties of the donor, five variables were identified (Figure 4.2). Numerical quantification of donor properties consists of the reactivity of the C1 position, which describes the electrophilicity, and the relative orientations of the substituents of the pyran ring, which describes the stereochemistry and sterics (Figure 4.2). With respect to the anomeric position (C1), it has

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previously been revealed that the reactivity of the donor could be numerically quantified by using the 1H NMR chemical shift.105 Based on this work, we calculated (DFT calculation using basis set B3LYP 6-31G*) the anomeric 13C NMR chemical shift, which is not only correlated to the reactivity of the donor but also allows for differentiation of leaving groups. Previously, three different leaving groups were investigated (trichloroacetimidate (- OC(CCl3)=NH), ethylthioether (-SEt), phosphate (-OP(OnBu)2=O and -OP(OPh)2=O). In

the case of trichloroacetimidate, an oxygen is bonded to anomeric carbon, resulting in C1

13_{C NMR shifts between 98.3–103.0 ppm. The ethylthioether derivatives contain a C-S bond}

at C1 and exhibit an upfield shift in the NMR (81.4–86.1 ppm).

Figure 4.2: a, 13_{Carbon NMR chemical shift (ppm). b, Dihedral angle (°) of X1-C1-C2-O2. c, 3D map of} donor chemical subspace (X: Dihedral angle (°) of O2-C2-C3-O3, Y: Dihedral angle (°) of O3-C3-C4-O4, Z: Dihedral angle (°) of O4-C4-C5-C6). Basis set: B3LYP 6-31G* level of theory.

Donor C1 shift (ppm) X1O2 (°) O2O3 (°) O3O4 (°) O4C6 (°)

Glc1α 98.4 56.8 66.3 -64.5 61.1 Glc1β 103.0 -69.4 71.9 -66.1 58.4 Glc2β 81.4 -50.2 61.2 -70.9 74.2 Gal1α 100.6 54.8 60.8 58.9 -56.7 Gal2β 84.1 -66.4 75.0 31.4 -40.9 Man1α 99.9 171.4 -57.4 -62.5 60.3 Man2α 86.1 147.0 -33.2 -73.5 76.5

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To quantify the donor’s stereochemical properties, the dihedral angles of substituents at positions C1, C2, C3, C4 of the pyranose ring were considered. First, dihedral angle (X1- C1-C2-O2) between oxygen/sulfur at the anomeric position and the C2 oxygen, providing information about the orientation of both the C2 position as well as the leaving group. Clockwise (+) values for this descriptor indicate that the leaving group is α (Figure 4.3a/c) while counterclockwise (-) is the β orientation (Figure 4.3b). The mannose α-donor is differentiated from the respective glucose and galactose donors with angles ranging from +147.0 to +177.5° (Figure 4.3c) as compared to -69.4– +60.7° (Figure 4.3a/b). Dihedral angles with the respective sugars are given in the table of Figure 4.2.

Figure 4.3: Comparison of dihedral angle of O1-C1-C2-O2 for Glc1α, Glc1β and Man1α. For Gal1α, Gal1β, refer tothe table of Figure 4.2.

Similarly, the remaining dihedral angles from the C2 to the C5 positions describe the orientation of the rest of the pyran ring. Mannose only has negative values for the O2- C2-C3-O3 dihedral (-33.2 – -60.5°) due to the axial orientation of C2 position, which results in a counterclockwise rotation to the C3 substituent (Figure 4.4c). However, glucose and galactose have positive values ranging from 61.2° to 75.0° due to the equatorial orientation of the C2 group (Figure 4.4a/b). These dihedral angles with the respective sugars are also given in the table of Figure 4.2.

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Figure 4.4: Comparison of dihedral angle of O2-C2-C3-O3 for Glc1α, Galα and Man1α. For Gal1β, Gal1β, refer tothe table of Figure 4.2.

In the case of galactose, dihedral angle of O4-C4-C5-C6 is and (-) due to the axial O4 substituent on C4 position whereas glucose and mannose donor have (+) value (Figure 4.5). Change of the dihedral angles leads to the changing of the hyperconjugation and “through the space effects”52 _{which can subsequently alter and generate homoconjugation}

and remote double hyperconjugation effects. Quantifying these “through the space effects” can result in quantifying the steric and overall electronics of the molecule, which is an important parameter for the numerical quantification of donor.

Figure 4.5: Comparison of dihedral angle of O4-C5-C5-C6 for Glc1α, Galα and Man1α. For Gal1β, Gal1β, refer tothe table of Figure 4.2.

4.2.2 Acceptor

The most important parameter to be considered in the acceptor (nucleophile) is the nature of the nucleophilic oxygen. In previous research by Codée,74 _{Mayr’s nucleophilicity}

parameters and field inductive parameters were used for correlate stereochemistry outcome of the glycosylation reactions with a set of simple alcohols. However, these parameters are

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an experimentally derived values, this limits the scope of the acceptor. In this study, nucleophilicity is characterized by the 17O NMR chemical shift (B3LYP 6-311G*), which shows the electron distribution of the oxygen according to the local geometry (binding partners, bond lengths, angles between bonds, etc.). As the number of adjacent methyl groups is increased (methanol, ethanol, isopropanol to tert-butanol), the 17_{O NMR shift}

decreases stepwise by about 31.2 ppm, ascribed to hyperconjugative donations of the -CC and -CH orbitals into the LPO* and the *-CO orbitals, respectively. In addition, when

strong electron withdrawing substituents such as fluorine are bonded to the α-carbon on ethanol, the 17_{O NMR chemical shift offsets hyperconjugation donations, as evidenced by}

di-, and trifluoroethanol have similar chemical shifts to methanol. To describe the steric hindrance of the acceptor, the exposed surface area of the oxygen and α-carbon, respectively, was calculated using a space-filling model (Å2) using basis set B3LYP 6- 311G*. As the number of methyl groups increased, oxygen and α-carbon exposed surface areas decreased about 0.27 Å2 and 7.7 Å2, respectively. However, when fluorine is bonded to the α-carbon of ethanol, the oxygen exposed area increased about 0.31Å2 and α-carbon exposed area slightly decreased 0.51 Å2 _{(Figure 4.6).}

Figure 4.6: 3D map of acceptor chemical subspace (X: exposed surface area (Å2_{) of Oxygen in a space-} filling model, Y: exposed surface area (Å2_{) of α-Carbon in a space-filling model, Z:} 17_{Oxygen NMR} chemical shift of hydroxyl group of acceptor). Basis set: B3LYP 6-311G* level of theory.

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