Some ketoses may be accessible from similar aldoses by isomerization via an enediol intermediate; for example, on standing in aqueous base, glucose, fructose, and mannose will slowly interconvert since they share an enediol form . ( See mutarotation ).
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The open-chain form is thermodynamically unstable, and it spontaneously isomerizes to the cyclic forms . ( Although the ring closure reaction could in theory create four-or three-atom rings, these would be highly strained, and are not observed in practice . ) In solutions at room temperature, the four cyclic isomers interconvert over a time scale of hours, in a process called mutarotation.
