CÁLCULO DEL INVERSOR

3.3.1.1 Interrogating Permanent Donor Leaving Group (C1)

Over the past century, a host of different leaving groups and corresponding activators to induce anomeric cleavage have been introduced as part of the quest for mild, selective, and high-yielding glycosylation reactions.61-70For the systematic exploration of the effects different leaving groups have on the stereochemical outcome of glycosylation, a perbenzylatedglucosyl α-trichloroacetimidate (Schmidt donor), glucosyl β-ethanethio ether, or glucosyl α-n-dibutylphosphate were reacted with isopropanol as model acceptor in DCM at temperatures ranging from -50 → 30 °C (Figure 3.3). To minimize differences in the conjugate bases/byproducts present in the solution, the activation conditions for each leaving group were chosen such that triflate anions were present in all cases. All other variables were kept constant (e.g. pyran core: glucose, acceptor: iPrOH, conjugate base: triflate (TfO-), solvent: DCM). The glucosyl donors with the three classes of explored leaving group gave nearly identical stereochemical outcomes under the conditions studied. Differences were in conversion and yield at low temperatures. A rapid drop of glycosylation yields was observed for thioglucosides from 88% at 10 °C to 45% at –10 °C. Glucosyl phosphate donors behaved similarly, exhibiting a 60% yield at -10 °C, which subsequently dropped to 30% at -30 °C. Along with the drop in yield, a slight decrease (< 7%) in α- selectivity is observed as compared to the trichloroacetimidate donor.

38 -60 -40 -20 0 20 40 0 20 40 60 80 100 α -sele ctivi ty (% ) Temperature (°C)

Figure 3.3: Comparison of stereoselectivities for glycosylations for glucose, bearing one of three leaving groups, with iPrOH as acceptor in DCM. For full experimental details, see entries 13-18, 41-44, 313-317 of Table 6.1 in Chapter 6. Figure code: Glucose (▲); Trichloroacetimidate (blue) with TfOH (0.2 equiv.); ethyl thioether (red) with TfOH (0.2 equiv.) and N-iodosuccinamide (1.2 equiv.); n-butylphosphate (green) with TMSOTf (1.2 equiv.).

3.3.1.2 Influence of donor C1 position stereochemistry

The influence of the stereochemistry of the C1 position of the donor was investigated. A model glycosylation reaction involved coupling both α- and β- glucosyl trichloroimidate (TCA) with isopropanol in DCM and studied for the entire temperature range of the solvent (Figure 3.4). It can be seen that stereochemistry of the C1 position of the donor has no influence on the stereoselectivity of the reaction. This, however, is not always the case, as will be discussed in detail in Section 4.1 of Chapter 4.

39 -60 -40 -20 0 20 40 0 20 40 60 80 100 α -sele ctivi ty (% ) Temperature (°C)

Figure 3.4: Comparison of C1 position stereochemistry of donor (perbenzylated glycosyl α- and β- trichloroacetimidates) on the stereochemical outcome of the reaction in DCM. iPrOH was used as acceptor and TfOH as activator. For full experimental details, see entries 13-34 of Table 6.1 in Chapter 6. Figure code: α-trichloroacetimidate glucose; (blue▲); β-trichloroacetimidate glucose (pale blue)

3.3.1.3 Implication of leaving groups

After systematic studies of the effect of glycosyl donor leaving groups at different temperatures, it can be seen that the choice of leaving group does not affect the stereoselectivity of the reaction (Figures 3.3 and 3.4). Hence, for the rest of our systematic study identifying the influence of other factors glycosyl trichloroacetimidates were used, due to their facility of activation and broad reaction range. It was revealed that there is almost a linear relation between stereoselectivity and temperature, when model glycosylations were performed in DCM. For DCM, the lowest temperature studied is -50 °C due to the cooling limitation of the automated platform. The upper temperature range of solvent was dictated by the boiling point, for DCM the upper temperature examined was 30 °C. It was observed that the selectivity of the coupling of glucose and isopropanol in DCM, favors the formation of the β-product at -50 °C, with a selectivity of 73%. The alpha selectivity increases linearly with an increase in temperature, and at 30 °C the α-product was the major product (61% selectivity). This temperature sensitivity (the slope of the plotted data) was calculated at 0.41%/°C. These values serve as a comparison benchmark for all other variables examined herein. While the stereochemistry of some trichloroacetimidate donors, when reacted with TfOH, has been shown to have an influence

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on the stereochemical outcome of glycosylations,83-84 under the standard conditions in DCM, no difference was observed between the α- and β-glucose donor.

3.3.1.4 Donor Stereochemistry (C2 and C4)

To probe the effect of donor stereochemistry at the C2- and C4-positions, model glycosylations were studied using glucose, galactose, and mannose donors. The stability of the intermediate and the activity of the donor can be influenced by through-bond or through- space hyperconjugation of the ether groups of the pyran core. For donors having non- participating groups, the C2 position has a huge influence, generating conformationally locked and unlocked glucose (equatorial C2 ether)/mannose (axial C2 ether) derivatives (Figure 3.5),85 as well as for less common derivatives such as gluco-/mannosamine and the C2 fluorinated derivatives.86

Figure 3.5: Comparison of three different monomers – glucose, galactose and mannose. In the case of mannose, axial C2 ether obstructs β-bond formation.

In order to investigate this aspect further, coupling of isopropanol with the α- glucosyl and mannosyl trichloroacetimidates were compared in DCM. A significant (30%) decrease in temperature sensitivity was observed when the C2 benzyl ether is axial, as is the case for mannose (Figure 3.6). The α-product were favored for mannose, and the selectivity is less sensitive to temperature, with α:β ratios ranging from 48:52 (-50 °C) to 61:39 (30 °C). Monosaccharides differing with respect to the C4 position, galactose (axial C4 ether) and glucose (equatorial C4 ether), exhibit similar temperature sensitivities (Tsens = 0.43%/°C) although galactose is 1.13 times more likely to give the β-product (9% more β-product formed) than glucose, ranging from 81% α-selectivity at -50 °C to 49% at 30 °C (Figure 3.6).

It can be inferred from the results that there are inherent preferences of glycosylating agents concerning mechanistic pathways and stereoselectivity. At low temperatures the β- product is favored by glycosyl donors, exhibiting a moderate degree of temperature

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sensitivity and follows a more SN2-like reaction pathway as temperature decreases.87-88

Galactosyl donors have a higher preference for the formation of the β-product. However, the C2-position is a significantly influential functionality, and the formation of the α-product was favored in case of mannose which proceeded via a more SN1-like pathway (Figure 3.6).

After the mannose donor is activated by TfOH, it forms a solvent separated ion pair mannosyl triflate.87-88 These inherent preferences can be enhanced or overridden by the other reaction variables from -50 °C to 30 °C (vide infra).This temperature depends on the solvent used for the glycosylation reaction.

-60 -40 -20 0 20 40 0 20 40 60 80 100 α -sele ctivi ty (% ) Temperature (°C)

Figure 3.6: Comparison of the stereochemical outcome of three different trichloroacetimidates donors – glucose, galactose and mannose – reacting with isopropanol and TfOH. For full experimental details, see entries 13-18, 149-154, 232-237 of Table 6.1 in Chapter 6. Figure code: Glucose (▲); Galactose (■); Mannose (●); DCM (blue).

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