Azide-Alkene Cycloaddition and Visible-Light-Mediated Wolff Rearrangement of Sugar Enones.

The synthesis of branched carbohydrate β-amino esters is described using the Wolff rearrangement of a diazoketone derived from the pyrolysis product (-)-levoglucosenone (LGO). An azide-alkene cycloaddition on LGO catalyzed by methanesulfonic acid directly afforded β-amino diazoketones via the triazoline. A subsequent visible-light-mediated Wolff rearrangement resulted in ring-contraction, giving a series of branched deoxyamino sugars with excellent diastereoselectivity (>20:1). The use of 2,2,2-trifluoroethanol as solvent at subambient temperature improved the yields of the ester relative to reactions performed in methanol or with other alcohol nucleophiles. Amine acylation or sulfonylation prior to the Wolff rearrangement led to the formation of ring-opened chiral 4-aminodihydrofurans in good yields, presumably due to the stabilization of the oxocarbenium ion during ring-opening and elimination.