25.23There are seven possible pentuloses, that is, five-carbon ketoses. The ketone carbonyl can be locatedat either C-2 or C-3. When the carbonyl group is at C-2, there are two stereogenic centers, givingrise to four stereoisomers (two pairs of enantiomers).When the carbonyl group is located at C-3, there are only three stereoisomers, because one of themis 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 CH2OH substituents.(b)The alditol obtained on reduction of D-apiose retains the stereogenic center. It is chiral andoptically active.(c,d)Cyclic hemiacetal formation in D-apiose involves addition of a CH
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