Azasugar Syntheses and Multistep Cascade Rearrangements via Hetero Diels-Alder Cycloadditions with Nitroso Dienophiles

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Hetero Diels-Alder (HDA) reactions with nitroso dienophiles R-N=0 are shown to represent the key step in the stereospecific total synthesis of azasugar derivatives. A series of amino-deoxysugars, both racemic and chiral, have been prepared from the primary cycloadducts of such HDA-reactions. Asymmetric induction during these cycloadditions were achieved either with chiral dienes, or with chiral nitroso dienophiles. In some instances the primary HDA cycloadducts underwent spontaneous cascading rearrangements to thermodynamically more stable entities. These rearrangements were triggered off by the breaking at r.t. of the weak N-O single bond.