Synthesis of (±)‐Azaascorbic Acid

The synthesis of the hitherto unknown aza analog of ascorbic acid (±)‐10a is described. Aldol type reaction of the tetramic acid derivative 4 with tert‐butyldimethylsilyloxy acetaldehyde furnished a mixture of the aldol (±)‐5b and the diastereomeric O‐Boc protected aldols (±)‐6a and (±)‐6b. By X‐ray structural analysis the major reaction product was shown to be the threo isomer 6a. Desilylation of 6a led to the alcohol 7. Removal of the remaining protective groups yielded racemic azaascorbic acid 10a. Similar deprotection of the aldol 5b led only to impure azaisoascorbic acid (±)‐10b which was characterized as the dimethyl ether 12b.