Of the two chair conformations of -L-ribose, the one with the greater number of equatorialsubstituents is more stable.25.7The equation describing the equilibrium isLet Apercent isomer; 100Apercent isomer. ThenA(29.3°)(100A)(17.0°)100(14.2°)46.3A3120Percent isomer67%Percent isomer(100A)33%25.8Review carbohydrate terminology by referring to text Table 25.1. Aketotetroseis a four-carbon ke-tose. Writing a Fischer projection for a four-carbon ketose reveals that only one stereogenic centeris present, and thus there are only two ketotetroses. They are enantiomers of each other and areknown as D- and L-erythrulose.25.9(b)Because L-fucose is 6-deoxy-L-galactose, first write the Fischer projection formula ofD-galactose, and then transform it to its mirror image,
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