Chemical synthesis of oligosaccharides requires the selective protection and deprotection of various carbohydrate hydroxyl groups. To achieve selectivity in removal of protecting groups, a wide armamentarium of deprotection conditions has been developed. For example, those groups that may be oxidatively removed, such as p-methoxyphenyl (PMP) [1,2] and p-methoxybenzyl (PMB) [2], are frequently used in carbohydrate synthesis. Generally, chemical oxidation with ceric ammonium nitrate [1-4] is effective for their removal. However, there have been instances in which ceric ammonium nitrate, as well as other chemical oxidation methods, were ineffective in removing the PMP or PMB protecting group [5][6][7]. (Attempted removal of the PMP group from compounds 1, 2, 7 and 8 with CAN or by other chemical oxidations [7] were unsuccessful.) This note describes an electrochemical method [8] for the efficient removal of the PMP group from the anomeric position of various pyranoses. This method has been successful in several cases where chemical procedures have failed [6].
Removal of the PMP group of compounds 1-8 was accomplished by anodic oxidation of the PMP-glycosides at + 1.55 V in aqueous acetonitrile to produce the reducing sugars in > 74% yields (Table 1). Conversion of the resulting reducing sugar to the glycosyl fluoride may be accomplished without purification (Entry C). Thus, the crude product from compound 3 was readily converted, by treatment with DAST, into the corresponding glucosyl fluoride [9]. Entry H shows that this methodology can easily be used on > 0.100 mmol scale reactions.