Neutral glycosylation, using microwave irradiation, is novel in the realm of carbohydrate chemistry. Providing mild reaction conditions, decreased reaction times and enabling good yields with high purity are the major advantages of microwave promoted reactions. Microwave assisted glycosylation is a rapidly emerging method in the field of carbohydrate chemistry, however, only very few reports have described microwave assisted glycosylation. We have developed a microwave labile protecting group (MWLPG) which is selectively removed when microwave irradiation is employed and it is orthogonal to other commonly used protecting groups. A dinitrophenyl (DNP) ring, at the anomeric position, gave good yields and high alpha selectivity with simple to complex glycosyl acceptors. The glycoside of DNP is labile to nucleophiles such as ammonia in methanol, but it is not reactive enough to undergo conventional glycosylation. However DNP donor is highly reactive in microwave heating under neutral and slightly basic conditions. In contrast to the conventional glycosylation, this can be achieved without the use of chemical promoters, usually consisting of Lewis or Bronsted acids. Thermal activation of highly electron withdrawing sugar donors in the presence of microwave irradiation and in the absence of any external additives albeit for the partnering acceptor and solvent to dissolve the coupling partners. DMF proved to be the best solvent for the reaction which is polar aprotic solvent with high dielectric constant. DNP donors decorated with electron-donating benzyl ethers showed increased reactivity. We have established a fast, high temperature glycosylation method by employing precise microwave heating.

Microwave-assisted glycosylation reactions in the absence of activators

ACS Fall 2021

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