Five-Atom Rings

There is little doubt therefore that all the true analogs of L-ascorbic acid discussed above contain the unsaturated five atom ring system to which are attached two enolic hydroxyl groups on the carbon atoms of the double bond. 

The chemistrv of sugars rcarbohydiatttsi is a major subject area, covered by many specialist texts. Note monosaccharides can exist as open-chain or as cyclic forms, where the oxygen atom forms part of the ring. Those with six-atom rings are called pyranoses. and those with five-atom rings are described as furenoses. 

From the fact that the Cig-amino acid shows no tendency to lactamize, the conclusion (if erroneous, p. 418) has been drawn that the ring broken by the barium hydroxide-hydrogen peroxide oxidation is a five-atom ring or a much larger ring, and almost certainly not a six-atom ring. 

In contrast to o-glucose and D-galactose, D-fructose is a ketohexose. It forms the five-atom ring when the hydroxyl group on carbon 5 reacts with the carbon of the ketone group. The new hydroxyl group is on carbon 2. 

The smallest ring of a cyclic acetal contains five atoms. Ring-opening polymerization of cyclic monomers with rings composed of more than four atoms is a reversible process due to a relatively low negative AHp (enthalpy of polymerization) value (Table 1) [see Chapter 4.02). Thus, monomer-polymer equilibrium is established, and if the equilibrium monomer concentration [M]e is higher than its starting concentration, polymerization is not possible. This is the case with the six-membered cyclic acetal 1,3-dioxane. 

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