Thermal mutarotation

Among the most interesting examples of varying sugar sweetness is the fact that crystalline or freshly dissolved / -D-fructopyranose is about twice as sweet as sucrose, but after mutarotation or during thermal mutarotation, sweetness diminishes markedly (3). a-D-Glucopyranose is about two-thirds as sweet as sucrose, but the mutarotated solution is even less... 

This is one example of the second criterion mentioned previously. However, the other OH substituents, depending upon the furanose ring conformation, are either eclipsed or in the anti conformation. In the former they are disposed to form a strong intramolecular hydrogen bond in the latter they are incapable of such bonding. Further evidence to support the contention that free / -D-fructofuranose is nearly tasteless is seen in the thermal mutarotation (3) of D-fructose. As the temperature of D-fruc-... 

Figure 1. Change in the sweetness and concentration of o-fructo pyranose during thermal mutarotation...

Mutarotation at S is also reported to be catalyzed by light,1041 possibly via an O-bound sulfenate intermediate (equation 162). Adamson and coworkers report that photodecomposition to Co j is the final outcome.1049 For sulfinate complexes donor S to O isomerization occurs1020,1049 and red O-bound (N,0)[Co(cyst02)(en)2](C104)2-H20 has been isolated.1029 This process (equation 163 hv 350-450 nm) is thermally reversible (r1/2 600h, r.t.) but which O atom is involved, axial or equatorial, is not known. The O-bound form is now asymmetric, and some specificity might be expected. 

---