Glycoside acetoxy

Scheme 9. Synthesis of /i-acetoxy glycoside 49 and glycosidation of 44 to give 50) .

Although trityl perchlorate is used to accomplish the glycosidation of the C-8 hydroxyl in 44 with acetoxy glycoside 49, control experiments have demonstrated that no reaction takes place in the presence of 4 A molecular sieves or 2,6-di-terf-butylpyridine. This observation suggests that the actual catalyst is not trityl perchlorate, but perchloric acid. Consistent with this conclusion is the observation that catalytic amounts of a strong Brpnsted acid such as triflic or perchloric acid can catalyze the glycosidation of 44 with 49 in the absence of trityl perchlorate. 

Glycosides 5c, 5d and 5g together with glycosides 6, Fig. (6) and 7, Fig. (7) are characterized by additional acetoxy or hydroxy groups in the side chain. 

Fig. (7). Structure of 25-acetoxi-3P,12a-dihydroxyholost-9(l l)-ene aglyoone based glycosides...

Fig. (12). Structure of 16(5-acetoxy-3(J-hydroxyholost-7

Some of the glycosides containing a l6(5-acetoxy group also present an allylic hydroxyl group at C-25, Fig. (13). 

Four glycosides isolated from the sea cucumber Cucumaria lefevrei [50] are the only examples of holothurins with a 16a-acetoxy group in their aglycones, Fig. (14). Lefevreiosides A2 (14b), B (14c) and C (14d) show the same monosuliated tetrasaccharide chain and differ in the degree of unsaturation or the position of the double bond in their side chains. Lefevreioside Ai (14a) is the desulfated analog of glycoside 14b. 

The remarkable stability of the thiazole ring allowed synthetic manipulations of the thiazolyl ketol acetates, which extended considerably the scope of the above C-formylation method of furanoses and pyranoses. Instead of the reductive removal of the acetoxy group, the /V-glycosidation of either a- or p-anomer 73 with TMSN3 afforded stereoselectively the azido galactopyranoside 75 in 88% isolated yield (Scheme 22) [77]. The cleavage of the... 

Glycosides." The construction of p-glycosides has relied heavily on the anomeric effect (equatorial C2-acetoxy substituent). A recent approach relies on generation of a radical at an alkoxy-substituted anomeric position. The precursors (1) can be obtained as shown in equation (I). The same strategy can be applied to construction of (J-linked disaccharides. 

The aromatic phenol was varied to explore the scope of the O-to-C conversion with mannosyl phosphates. Using phosphate 9, the a-C-mannosides of 2-naphthol and 3-benzyloxy phenol (23 and 25, Table 1) were synthesized in excellent yield. O-Mannosides were obtained exclusively with less nucleophilic aromatic systems, such as 3-acetoxy phenol. Several non-phenolic aromatic systems were unsuccessful in the formation of C-aryl or O-aryl glycosides. Reaction of 9 with furan, thiophene, trimethoxybenzene, and indole in the presence of TMSOTf did not result in any product formation. Interestingly, activation of 9 in the absence of any aromatic nucleophiles gave 26 as the major product via an intramolecular C-glycosylation (Figure 1) (79). 

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