Glycosylation with KDO

FIGURE 8.3 a-Selective glycosylation of KDO with a disaccharide acceptor using the microfluidic method. Alloc, allyloxycarbonyl Cbz, benzyloxycarbonyl, PMB, p-methoxybenzyl. 

FIGURE 8.4 a-Selective glycosylation of monosaccharide acceptors with KDO using the microfluidic method. 

KDO glycosylations using monosaccharide acceptors were also examined [22], Combination of TfOH as catalyst and CH3CN as solvent gave better results than other catalysts (TMSOTf, TBSOTf) and solvents (CPME, CH2C12, toluene). The a selectivity was increased under microflow conditions, though the yields between batch (86%, a/p=85/15) and microflow conditions (83%, a/p=92/8) were comparable (Fig. 8.4). 

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LPS partial structures 70 and 71 with KDO moiety (Scheme 14.9) [94, 95], KDO fluoride (60, 62) and A,-phenyltrifluoroacetimida(c (68) were used to selectively afford the a isomer. For donor 68, the a-glycosylation was effectively achieved by using a microreactor (see Section 8.2.2 for details). 

Catalysis with protic acids has traditionally been used in Fischer glycosylations. As with the recent variations of the above procedure, Lewis acid catalysis is also valuable for dehydrative glycosylations with protected derivatives (Scheme 4.40). Trimethylsilyl triflate has been shown to promote reactions of glucuronic acid hemiacetals [420], and pyridine and TMSOTf was used for the synthesis of cw-glucosides [421]. Pyridinium / -toluenesulfonate (PPTS) was used for the synthesis of glycosides of the biologically important Kdo [422]. 

The glycosyl-residue compositions of the three purified fractions (Table 1) were very similar with a predominance of galacturonic acid, rhamnose and arabinose. The presence in the three purified fractions of the rare monosaccharides characteristic of RG-II (e.g. 2-( -methyl-L-fucose, 2-O-methyl-D-xylose, apiose, Kdo, Dha and aceric acid) was confirmed by GC-CIMS analysis. The molar ratios corresponded approximately to the known structure of the RG-II molecule (Figure 1) and to previously published data for RG-II from sycamore [26], rice [4], arabidospis leaves [8] and Pectinol [12]. 

Anomeric triphenylphosphonium salts have been used as well as phenylsul-fides,but in the latter case extra stabilization is necessary (see below). Anomeric nitrosugars, which have been extensively studied in C-glycosylation reactions by Vasella, will be covered in Sect. 2.2.1 and ester enolates derived from 3-deoxy-2-ketoulosonic acids (sialic acid and KDO derivatives), which bear a structural similarity to 2-deoxy pyranosides, will be covered in Sect. 4.4. Deprotonation of anomeric phenylsulfones has been discussed in Sect. 2.1.1 and additional transformations on closely related compounds are presented in Scheme 14 [20]. Alkylation of phenylsulfone 54 with epoxide 55 provides adduct 56 which eliminates benzenesulfinic acid at room temperature to give the C(l)-alkylated glycal 57 a similar elimination is also observed with adducts derived from... 

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