Disaccharides induction

The stereoselectivity in cyclizations of vinyl ether systems to form furanosyl disaccharides is controlled by both the side chain substituent (1,4-asymmetric induction) and the configuration of the existing pyranoside linkage (equation 49).142 Substrates with no side chain substituent show no stereoselectivity. 

Based on the work of Danishefsky [70] on the cycloaddition of aldehydes to dienes bearing various menthyl auxiliaries, Stoodley et al. have used butadienyl glycosides 2-58 with the aim to synthesise (1—>l)-linked disaccharides [103]. The reaction was performed with p-nitrobenzaldehyde 2-57 in the presence of an europium salt since benzaldehyde itself did not react. Reasonable induction was obtained in the presence of the chiral (-)-Eu(hfc)3 as a matched pair to give 2-59 as the major product (Fig. 2-17). 

Diels-Alder methodology is well suited to the preparation of C-monosaccharides and higher carbohydrate analogs. In general, these reactions provide products with high degrees of stereochemical induction as well as exhibiting synthetically useful yields. Further examples of Diels-Alder reactions applied to the preparation of C-disaccharides will be elaborated upon in Chapter 8. 

One of the most stable disaccharides isa,a-trehalose that contains two C-2 hydroxyl groups and is therefore about ten times more stable than maltose or lactose. Increased stability determines, in particular, the negative inductive effect of the hydroxyl groups at C-2. The hydrolysis rate of the galactose-O-glucose bond in raf nose, and also of the glucose-O-fructose bond in the reducing disaccharide turanose (Table 4.14), is comparable to the hydrolysis rate of maltose or lactose. 

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