Phosphonium Salt Catalyzed <i>O</i> ‐Glycosylation of Glycosyl Trichloroacetimidates

Comprehensive Summary A novel phosphonium salt catalyst has been developed for the efficient and stereoselective glycosylation of glycosyl trichloroacetimidates. Using perbenzoylated or perbenzylated donors, the method enables the synthesis of glycosides or oligosaccharides in good to excellent yields with high β‐selectivity, demonstrating broad substrate compatibility. The catalyst exhibits exceptional catalytic activity, enabling the glycosylations of low‐reactivity substrates with equivalent yields even at –78 °C. The protocol has been successfully applied to the iterative synthesis of a hexasaccharide, highlighting its utility in oligosaccharide assembly. This work establishes a practical, efficient, and robust organocatalytic platform for carbohydrate synthesis.