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Dolbier HW Solutions 623 - 717 CARBOHYDRATES 25.23 There...

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25.23 There are seven possible pentuloses, that is, fi ve-carbon ketoses. The ketone carbonyl can be located at either C-2 or C-3. When the carbonyl group is at C-2, there are two stereogenic centers, giving rise to four stereoisomers (two pairs of enantiomers). When the carbonyl group is located at C-3, there are only three stereoisomers, because one of them is a meso form and is superposable on its mirror image. 25.24 ( a ) Carbon-2 is the only stereogenic center in D -apiose. Carbon-3 is not a stereogenic center; it bears two identical CH 2 OH substituents. ( b ) The alditol obtained on reduction of D -apiose retains the stereogenic center. It is chiral and optically active. ( c , d ) Cyclic hemiacetal formation in D -apiose involves addition of a CH
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