One-Pot Stereoselective Synthesis of the Naphthofurano-Iminosugars via a [3 + 2] Cycloaddition Reaction

This study presents the synthesis of novel naphthofurano-iminosugars (<b>4</b>) using 2,3-<i>O</i>-isopropylidene D-ribose tosylate (<b>1a</b>), anilines (<b>2</b>), and 1,4-benzoquinone (<b>3a</b>) as starting materials through key iminium ion/enamine intermediates via [3 + 2] cyclization reactions at room temperature. The reaction has unique regioselectivity and stereoselectivity with moderate to excellent yields. The adaptability of this method has been demonstrated using various substituted anilines, on which both electron-donating and electron-withdrawing groups were well employed in the reactions. Notably, the treatment of the fused multicyclic iminosugar <b>4</b> with TFA efficiently leads to an interesting unexpected pyridinium salt (<b>8</b>), possible via four sequential steps: deprotection of the 2,3-<i>O</i>-isopropylidene group, furan ring opening, dehydration condensation of the OH groups, and elimination of water.