8-O-sialylation of neuraminic acid

Synthesis of Neu5Ac alpha(2-3)Gal beta(1-4)Glc and Neu5Ac alpha(2-8)Neu5Ac derivatives 12 alpha and 14 alpha, From lactose derivative 6 and 2,3-dehydro-Neu5Ac derivative 7, respectively, with anchimerically assisted Neu5Ac donor 3e, 3f, or 3g has been studied. Reaction of halogenose 3e, having a 3-phenoxythiocarbonyloxy moiety as the assisting group, afforded with 6 and 7 in the presence of equimolar amounts of AgOTf as promoter alpha-sialosides 11 alpha and 13 alpha which were readily deoxygenated to afford 12 alpha and 14 alpha, respectively. Reaction of phosphite 3f, having a 3-bromo substituent as the potentially anchimeric assisting group, furnished in the presence of catalytic amounts of TMSOTf with 6 as acceptor alpha-sialoside 15 alpha and with 7 as acceptor beta-sialoside 17 beta, which gave on debromination 12 alpha and 14 beta, respectively. Reaction of readily available phosphite 3g, having a 3-thiobenzoyloxy group as the anchimeric assisting group, afforded with 6 and 7 in the presence of catalytic amounts of TMSOTf in very high yields cl-linked sialosides 21 alpha and 22 alpha, which could be readily deoxygenated to afford 12 alpha and 14 alpha. Thus, 3g is an almost ideal sialyl donor for the high-yielding generation of alpha-glycosidic linkages also to low-reactive acceptors.