Synthesis of Unprecedented α/β‐Alternate (1→4)‐Glucans via Stereoselective Iterative Glycosylation

Glucans are major biomaterials on the earth, with α-(1→4)-glucans (i. e., amylose) and β-(1→4)-glucans (i. e., cellulose) being the most abundant ones, which are relevant to energy storage and structural function, respectively. Interestingly, (1→4)-glucans with alternate α/β-linkages, namely herewith amycellulose, have never been disclosed in nature. Here we report a robust glycosylation protocol for the stereoselective construction of the 1,2-cis-α- and 1,2-trans-β-glucosidic linkages, which employs an optimal combination of glycosyl N-phenyltrifluoroacetimidates as donors, TMSNTf2 as promoter, CH2 Cl2 /nitrile or CH2 Cl2 /THF as solvents. A broad substrate scope has been demonstrated by coupling five imidate donors with eight glycosyl acceptors, in which most of the glycosylations lead to high yield and exclusively 1,2-cis-α- or 1,2-trans-β-selectivity. Applying this glycosylation protocol and with an iterative manner, the unprecedented α/β-alternate (1→4)-glucans up to a 16-mer have been synthesized. Differently from amylose, that adopts a compact helicoidal arrangement, the synthetic amycellulose features an extended ribbon-like conformation, comparable to the extended shape of cellulose.