Septanose

The compounds usually known as monosaccharide anhydrides or glycose anhydrides (earlier glycosans ), formation of which involves the anomeric hydroxy group, are named by the same procedure. In these cases the order of preference of ring size designators is pyranose > furanose > septanose. However, three- or four-membered rings should normally be cited as anhydro if there is a choice. 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

Qualitative relaxation-studies have also been reported for an extensive series of derivatives of inositols, pentopyranoses, l,6-anhydro-/3-D-hexopyranoses, furanoses, and septanoses. In all instances, the experimentally determined Ri(ns) values reflect the anticipated geometry. For the furanose derivatives especially, they provide a better means for distinguishing between epimeric pairs than the relatively ambiguous interpretation of coupling-constant data. 

CJ14H2XCIO7 5-0-(Chloroacetyl)-l,2 3,4-di-0-isopropylidene-0 -D-gluco- septanose GLUSPT10 31 30 353 465... 

An elegant route to derivatives of septanose from the open-chain hexoses has been reported (Fig. 43).59 The protected septanose 137 was converted in few standard steps into free septanosides. 

The molecular recognition of septanose carbohydrates has been investigated in depth by using concanavalin A68 as a model lectin. Complex formation was analysed by STD experiments and showed the first direct evidence of binding, by a natural protein, for this class of ring-expanded carbohydrate molecules. 

II. Synthesis of Septanose-Containing Mono-, Di-, and Oligo-Saccharides 127... 

IV. Biochemical and Biological Investigations Using Septanose Carbohydrates I63... 

A distinction must be made between septanosides that lack (6-unsubstituted) or contain (6-substituted) an exocyclic hydroxymethyl group at C-6. In the same way that D-glucose can form a septanose ring by connecting the 6-hydroxyl group to the anomeric center as in Fig. 1, all of the common aldohexoses can adopt a septanose... 

Fig. 1. Higher-order sugars L-g/yceroD-manno-Heptose (1, open-chain and 2, a-pyranose) and A -acetylneurami ilic acid (3, a-pyranose form). Structures 4-8 depict D-glucose (4) as its open-chain Fischer projection, and as the a-oxetose (5), a-furanose (6), a-pyranose (7), and a-septanose (8) ring forms.

Even though the yields of the septanose derivatives in these reactions are decidedly modest, the products were isolated in sufficient quantities to permit analysis of their structures and conformations. Importantly, these routes provided enough of the respective septanoses to allow the preparation of alkylated and acylated glycoseptanosyl isomers. 

Scheme 4. Septanose minor products in the acetonation of D-glucose.

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