P -Linked disaccharides

Similarly, per-O-trimethylsilylated mono- and P-linked disaccharides (lactose and cellobiose, not melibiose) could be converted into the corresponding a-glycosyl iodides, which upon SN2 displacement with CbT using TBACN mainly afforded P-cyano derivatives in good overall yields [191]. The cyanoglycosides were transformed into aminomethyl glycosides via reduction under mild conditions (Scheme 2.52). 

Widmalm et al.354 reported the measurement of 3J(C, H) couplings through glycosidic linkages in p-linked disaccharides and also for vicinally disubstituted trisaccharides. Widmalm with other co-authors355 reported the measurement of trans-glycosidic 3J(C,H) couplings in the trisaccharide 2 -fucosyllactose. 

Furuhata et al. [266] condensed the chloride (370) with the disaccharide derivative (384) using the mixture of mercury salts in dichloromethane at 20 °C for 60 hours to give 21 % of a- and 8 % of P-linked disaccharide together with 60% of elimination product (376). 

Condensation [276] of the bromide (406) with the unsaturated derivative (377) gave the P-linked disaccharide (408) (58 %) which was brominated at the double bond and the glycosyl bromide produced condensed with the glucoside (409) to give, in... 

With the bromide (412) and the glucoside (409) in the presence of silver triflate in benzene at 20 °C both the a-linked (28 %) and the P-linked (53 %) disaccharides were obtained. In toluene at —10 °C more of the a- (64%) and less of the p- (15%) linked disaccharides were formed. The major product was the P-linked disaccharide (32%) using mercury(II) cyanide and mercury(II) bromide as catalysts. 

An acetal tethered compound can easily be prepared by treatment of equimolar amounts of a 2-propenyl ether derivative of a saccharide with a sugar hydroxyl in the presence of a catalytic amount of acid. Activation of the anomeric thio moiety of the tethered compound with N-iodosuccinimide (NIS) in dichloromethane results in the formation of the p-linked disaccharide. In this reaction, no a-linked disaccharide is usually detected. It is of interest to note that when this reaction was performed in the presence of methanol, no methyl glycosides are obtained. This experiment indicates that the glycosylation proceeds via a concerted reaction and not a free anomeric oxocarbenium ion. 

Marra, A, Mallet, J-M, Amatore, C, Sinay, P, Glycosylation using a one-electron-transfer homogeneous reagent a novel and efficient synthesis of p-linked disaccharides, Synlett, 572-574, 1990. 

A new type of latent-active glycosyl donors, 2 -carboxybenxyl glycosides and their benzyl esters, have been recently introduced [62-64]. The active carboxylic acids are readily available from the latent benzyl esters by catalytic hydrogenation in the presence of ammonium acetate. In an example [62] shown in Scheme 5.24 the latent mannoside (136) was glycosylated with the active donor (135) to provide exclusively the p-linked disaccharide (137) in excellent yield. The latent... 

SCHEME 8.1 Solid-phase glycosylation to form P-linked disaccharides. 

The principle of intramolecular C-mannosylation has been extended to the synthesis of O-linked 3-mannosides [109]. In the first step, a chlorodimethylsilyl moiety is introduced to the primary hydroxyl group of the (9-benzylated aglycone 38 to afford compound 39. Next, chlorosilane 39 is connected to the thiomannoside 40 to give the silaketal intermediate 41. Subsequent conversion to the sulfoxide 42 followed by Kahne-activation [110] affords the p-linked disaccharide 43 stereospecifi-cally (Scheme 13). 

In the first phase the vinyl ether 51 is prepared from the acetate 50 by methylene-transfer from the Tebbe-reagent. Next, the thioglycoside 51 and the aglycone 52 are connected by acid-catalyzed addition of the latter to the vinyl ether moiety to afford the acetal-tethered intermediate 53. In the final phase, NIS/TfOH-induced anomeric activation proceeds in concert with the attack of the tethered oxygen at the anomeric carbon to afford the P-linked disaccharide 54 (Scheme 16) [114]. 

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