Development of a Microwave-assisted Chemoselective Synthesis of Oxime-linked Sugar Linkers and Trivalent Glycoclusters

A rapid, high-yielding microwave-mediated synthetic procedure was developed and optimized using a model system of monovalent sugar linkers, with the ultimate goal of using this method for the synthesis of multivalent glycoclusters. The reaction occurs between the aldehyde/ketone on the sugars and an aminooxy moiety on the linker/trivalent core molecules used in this study, yielding acid-stable oxime linkages in the products and was carried out using equimolar quantities of reactants under mild aqueous conditions. Because the reaction is chemoselective, sugars can be incorporated without the use of protecting groups and the reactions can be completed in as little as 30 min in the microwave. As an added advantage, in the synthesis of the trivalent glycoclusters, the fully substituted trivalent molecules were the major products produced in excellent yields. These results illustrate the potential of this rapid oxime-forming microwave-mediated reaction in the synthesis of larger, more complex glycoconjugates and glycoclusters for use in a wide variety of biomedical applications.


NMR Analysis:
In protic solutions such as D2O, the oxime products reported in this paper would consist of a mixture of not only the ring opened E/Z isomers (major), but also the ring closed -glycosides (minor) as illustrated in Figure 1 of the main body of the paper. [2] Given the persistence of these different isomers, it is important to note some of the features present in the 1 H NMR spectra.
First, in the 1 H spectra of 9-15 and 17, the two downfield signals found between 7.0-8.0 ppm correspond to the open chain E/Z isomers, with the most downfield and dominant signal the E isomer, and the more upfield signal the Z isomer. Table S1 summarizes the E/Z ratios for the products. What can also be noted from the 1 H NMR spectra of 9-15 and 17 is the presence of several small signals integrating for less than 1H each. These should not be interpreted as impurities, but instead as minor isomer species. All of the products correspond to either a single spot by TLC or a single peak eluted by SEC. Additionally, the products all give rise to the expected number of protons, and finally, they have all been confirmed by high-resolution mass spectrometry (HRMS). For example, from the evaluation of the 1 H NMR of Compound 9, it can clearly be seen that the expected ratio of 3:1 for the Boc on the linker (1.40 ppm) to the acetyl protons on the sugar (2.02 ppm) matches that calculated using the peak integrations, despite the sum of the oxime signals at 7.49 ppm (E isomer) and 6.83 ppm (Z isomer) adding to only 0.9 total protons. Based on this information, it is therefore reasonable that the integration of the combined ring opened oxime E and Z signals will not integrate to a full proton, but that some portion of the product is present in the ring-closed form. It would be expected that most of the ring-closed glycosides would predominantly exist in the more favorable -configuration where the linker is equatorial. A determination of the  ratio of these minor products could not be obtained from the current 1D 1 H NMR data given the complexity of the signals overlapping in the region where the anomeric protons would be observed, and the expectation that the -isomer would only be present in trace amounts at the most. However, an estimation of the percentage of product in the ring open oxime could be made from the sum of the E and Z integrations (Table S1) and was found to range from 71-90%, depending on the sugar.
For the ketose-derived products, 20 and 21, there is no oxime proton present, therefore, no signals appear in the 7.0-8.0 ppm range as was seen for the aldose-derived products, 9-15 and 17.
Instead, a strong signal corresponding to the proximal methylene group of the core adjacent to the oxime was used. This group gives rise to two broad apparent triplet signals at 4.28-29 (E isomer) and 4.15 ppm (Z isomer) for 20 and 21. These signals were assigned based on their similarity to the linker molecule used in the NMR study reported by Szabo and coworkers.
[1] The ratio of these two peaks was therefore used to estimate the E/Z ratio of the ring-opened oxime present. Unfortunately, as there is no E/Z proton available, the percentage of ring-opened oxime could not be estimated for 20 and 21.