Derivatives—Natural and Laboratory

Figure 11 Methyl glucopyranosides.

The hydroxyl group formed as a result of ring closure represents a site for attachment of a broad variety of substituents. Compounds that are formed in such reactions are full acetals (ketals) and thus no longer undergo interconversion at the anomeric center. The configuration of such glycosides is therefore either alpha or beta depending on the relationship between the C-1 group and the projection of the ring; if on the same side, designate alpha, otherwise, beta (Fig. 11). A wide variety of natural and man-made derivatives (glycosides) are known with the substituents, aglycones, varying from simple methyl groups to complex organic molecules including other sugars (see below).

In addition to substitution of the anomeric hydroxyl, many modifications of the hydroxyl loci are known in nature. Most prevalent are those in which the hydroxyl group at C-2 is replaced by an amino function, generally acetylated. The sugar 2-deoxy2-acetamido-D-glucose (N-acetylglucosamine) is distributed throughout nature and, in its polymeric form (chitin), forms the organic matrix of insect and arthropod exoskeletons. Hence, it is likely the second most prevalent organic molecule on earth. Other variations include oxidation (C-6 or C-1) to form carboxyl groups and loss of a hydroxyl to form deoxy sugars (Fig. 12).

Figure 12 Sugars related to D-glucose that occur in nature.