Chemical Synthesis of β‐(1,4)‐d‐Oligomannurarates with Purported Anti‐Alzheimer Activity

Sodium oligomannate, a sophisticated mixture of acidic linear β‐(1,4)‐d‐oligomannurarate ranging from dimers to decamers with an average molecular weight of 670∼880 Da, has gained conditional approval for the treatment of Alzheimer's disease. Herein, we develop an efficient approach to the chemical synthesis of well‐defined β‐(1,4)‐d‐mannurarate di‐, tetra‐, hexa‐ and octasaccharides (1‐4), utilizing N‐phenyl trifluoroacetimidate donors and thioglycoside acceptors for the stereoselective assembly of the desired β‐(1,4)‐d‐mannuronate linkages. This approach features a straightforward protecting group strategy with only two types of protecting groups: tert‐butyldimethylsilyl (TBS) for the 4‐OH at the non‐reducing end and benzyl (Bn) for free hydroxyl and carboxylic acid moieties, thereby ensuring the stereoselective formation of the β‐mannosyl bonds and facilitating sequential transformation into the target β‐(1,4)‐d‐mannurarate oligosaccharides. The scalability of the present synthesis of these oligosaccharides paves the way for in‐depth structure‐activity relationship studies and pharmacological investigations of sodium oligomannates.