The aldotetroses have two chiralatrial centres ("asymmetric carbon atoms") and so 4 different stereoisomers are possible.
The naturally occurring aldotetroses are:
The 2 D-aldotetroses are:
The ketotetroses have 1 chiral centres and therefore 2 possible stereoisomers — Erythrulose (L- and D-form).
The D-ketotetrose is:
CH2OH | C=O | HC-OH | CH2OH D-Erythrulose
The only naturally occurring ketotetrose is D-erythrulose.
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