Branched Hexasaccharides

2 Branched Hexasaccharides - Compounds of this set with glucose at the reducing end to have been prepared are siaLe ganglioside compounds 14, with sulfate ester groups separately at the 6-positions of residues 3 and 4, 15, which 

Mannose-terminating compounds to have been described ate 17, which is the Le determinant linked to a linear mannotriose, and 18 with two lactosamine 

---

Schmidt and co-workers have also independently developed the self-cleavable diene linker 14 and solid-phase synthesis of the branched hexasaccharide 16 has been achieved (O Scheme 6) [12], The ring-closing metathesis of 15 provided the allyl glycoside 16 from which the allyl protecting group was readily removed by Ir-mediated isomerization followed by hydrolysis. 

Four other oligosaccharides (18-21, Fig. 6) corresponding to fragments of either the linear or the branched hexasaccharide moieties of the arabinan have been synthesized as methyl glycosides [55,74]. These were obtained in a maimer similar to the disaccharide and trisaccharide via glycosylation of methyl glycoside acceptors with thioglycoside donors. 

Very recently the syntheses of the pentasaccharide chain of glycoside (4) and of the doubly branched hexasaccharide chain of pectinioside E (8) have been reported by R. R. Schmidt and coworkers (69, 70). 

With this system and the building blocks 19,28, and 45 shown in the scheme up to a branched hexasaccharide W-glycan was successfully obtained (11 steps on solid phase, 19% overall yield 86% per step) (see 34). This was a major improvement over the results reported in Scheme 4. A careful study of this reaction sequence revealed that the second glycosylation step is incomplete, thus leading also to byproduct formation (48). Therefore, a capping step of unreacted hydroxy groups based on benzoyl isocyanate reaction was introduced (25). As expected and shown in Scheme 11, this way clean product formation could be gained, however the overall yield was not increased. 

In this paper we will focus more specifically on the third one, in which the scaffold unit is originated from monosaccharides. Compare to classical Pamam, Boltorn cores, carbohydrate units add much more 3D information coded in the chirality in almost each carbon. The diversity of such stmcture has been calculated for oligosaccharides and is veiy impressive, Laine found over 1.05 x 10 possible branched hexasaccharides starting from a basis set of 6 different sugars. Compared with a set of 6 different amino acids, there is nearly 10 times less hexapeptides Thereby the possible structures of a carbohydrate-based dendrimers are tremendous and open a large possible choice of scaffolds and thus properties. In addition, various characteristics of such dendrimer can be easily controlled such as their sizes, shapes, topologies, flexibilities and their surface properties. 

Scheme 16.6. Synthesis of a branched hexasaccharide using a self-cleavable diene linker.

Branched Hexasaccharides.—Hasegawa s group have synthesized the tumour-associated ganglioside sialyl Lewis X (16) [cf. 15], - ... 

Alteromonas macleodii svhsp.figiensis isolated from deep sea hydrothermal vent in North Fijian Basin is reported to produce EPS which is sul-fated heteropolysaccharide and has high uronic acids with pyruvate. The repeating unit of the EPS is a branched hexasaccharide containing glucose. 

---