Special Considerations with Sugars

Although the chemical modifications to sugars and sugar derivatives described in this chapter generally follow Sn2 mechanisms, special consideration must be given to the cyclic nature of most 

FIGURE 6.2 Types of hydroxyl groups found in sugars. 

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Although aldol condensation is another type of ionic reaction, it deserves special consideration. It can lead to two classes of sugars. Condensation of an aldehyde in an a-position of a free carbonyl group under mildly akaline conditions still seems possible. This very simple reaction can also be quite efficient. Thus diacetal 7.59, quantitatively prepared by treatment of mannose with acetone in an acidic medium, is at equilibrium with the carbonyl tautomer. Condensation with formaldehyde, used in aqueous solution in the presence of potassium carbonate, gives the hydroxymethylated sugar 7.60 in 86% yield (Ho 1979). 

Aminoglycal derivatives are of special interest in connection with the synthesis of sugar constituents of antibiotics [164], and their preparation is covered above inO Sect 6. However, reactions of hex-2-enop)ranosides with chlorosulfonyl isocyanate [214,215], which involve double bond migration, (113 to 114), should also be mentioned since they are one-step procedures of considerable synthetic potential (O Scheme 38). 

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