Ketoses temperature effects

The results of a detailed study of D-fructose and five O-glucosyl-substi-tuted D-fructoses17 confirmed that increasing the temperature increases the proportion of the furanose forms. This effect is much more noticeable in water and in pyridine than in dimethyl sulfoxide. In other ketoses, the proportion of the keto form also increases with increasing temperature.12... 

The above methods are so designed that the oxidation of ketoses by sodium hypoiodite is restricted to a minimum. Bailey and Hopkins studied the conditions under which ketoses would be oxidized. The reaction rate increased with the temperature, and the extent of the reaction, which was studied over the temperature range of 1 to 35 , went through a minimum at 15°. An excess of alkali in the 17-37 range apparently caused enolization of the D-fructose and increased the extent of the oxidation. When the alkali was added progressively in small amounts, this effect was increased 4-5 times and was independent of the D-fructose concentration. Oxalic acid was isolated and the presence of D-erythronic acid was assumed. 

Cyclic acetals of ketoses are prepared most commonly from acetone or benzaldehyde formaldehyde, acetaldehyde, butanone, and cyclohexanone have been used occasionally. These carbonyl reagents are frequently used directly, although such derivatives as 2,2-di-methoxy- or 2,2-diethoxy -propane (acetone dialkyl acetals), or l,l-dimethoxyethane (acetaldehyde diethyl acetal), are often employed in experiments in which intermediate acetals are of interest,or in which the presence of water in the reaction mixture adversely affects the yield of products. A polymeric form of an aldehyde is the reagent to be preferred whenever the monomer is volatile for example, acetaldehyde is often used in the form of a trimer, paraldehyde, and formaldehyde is employed as formalin solution, as paraformaldehyde, or as polyoxymethylene. An excess of the carbonyl reagent is generally used as the solvent, and the condensation is usually effected at room temperature. 

---