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

Abstract

AbstractGlucans 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, CH2Cl2/nitrile or CH2Cl2/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.