A New Route to 3-Amino Sugars. A Concise Synthesis of l-Daunosamine and d-Ristosamine Derivatives

An asymmetric aldol strategy has been developed for the synthesis of l-daunosamine and d-ristosamine derivatives starting from noncarbohydrate precursors. Lithium and boron enolate mediated aldol reactions of <b>12</b> with <i>O</i>-TBS lactaldehyde gave non-Evans <i>syn</i> and Evans <i>syn</i> aldol products, respectively, with high selectivity. The chemical efficiency of the lithium enolate reactions were higher than the corresponding reactions with the boron enolates. Curtius rearrangement of lactone acids <b>23</b> and <b>26</b> gave the corresponding <i>N</i>-BOC amino lactones <b>30</b> and <b>32</b> in 64% and 62%, respectively, with complete retention of configuration. Lactone <b>30</b> was converted by a two-step sequence to <i>N</i>-benzoyldaunosamide <b>40</b>. The overall yield for the amino sugar <b>40</b> was 18% over six steps. Similarly, lactone <b>32</b> was converted to <i>N</i>-benzoylristosamide <b>42</b> with an overall yield of 18% starting from <b>12</b>.