Photolabile ortho-Nitro-Benzyl Carbonate as a Permanent Hydroxyl Protecting Group for the Synthesis of Digalactosyl Diacylglycerol

Traditional protecting groups are often removed under harsh conditions with potentially hazardous reagents, thereby impeding the convenient synthesis of oligosaccharides and glycosides. Herein, we present to utilize the photolabile ortho-nitro-benzyl carbonate (oNBC) as a permanent hydroxyl protecting group for stereocontrolled synthesis of glycosides. The Ph₃PO-modulated glycosylation with strongly disarmed per-O-oNBC-protected glycosyl ynenoates preferred to afford glycosides with excellent α-selectivities via the β-phosphonium transition state. Based on the oNBC-mediated galactosylation, synthesis of the glycolipid digalactosyl diacylglycerol (DGDG) containing six double bonds and two esters was achieved in a straightforward manner.