Without using activating agents, a variety of free aromatic hydroxamic acids could be rearranged to aromatic amines in the presence of base alone.

A redox potential controlled intermolecular [2 + 2] cross-cycloaddition has been developed in the presence of a thioxanthylium photoredox catalyst. Electron-rich styrenes such as β-bromostyrene (E p/2 = +1.61 V vs SCE) were selectively oxidized by a thioxanthylium photoredox catalyst (E 1/2 (C*/C•–) = +1.76 V vs SCE) to styryl radical cations and reacted with styrene (E p/2 = +1.97 V vs SCE) to furnish polysubstituted cyclobutanes in high yields. The present reaction can be successfully applied to intermolecular [2 + 2] cross-cycloaddition of β-halogenostyrenes, which cannot be effectively achieved by the hitherto reported representative organophotoredox catalysts.

Intramolecular [4 + 2] cycloaddition reaction of 6-(4-alkenyloxymethylene)-2,4-cyclohexadien-1-ones generated from the reaction between substituted salicylaldehydes and unsaturated alcohols under mild conditions was investigated. In general, the reaction furnished tricyclic compounds containing the pyranobenzopyran skeleton with trans-fused B/C ring in very good yields (Tables and ). Furthermore, geometry of the olefinic bond of the starting material is retained during the reaction.

A variety of aromatic and aliphatic hydroxamic acids were converted to the corresponding primary amines via base-mediated rearrangement. This rearrangement could proceed with less than 1 equiv. of K2CO3 in polar solvents under thermal conditions with no external reagents. This rearrangement has several features including no external activating agents needed for promoting the rearrangement, less than one equivalent of a base is sufficient for the reaction, and a clean reaction in which only carbon dioxide is produced as a by-product. A self-propagating mechanism via an isocyanate intermediate is proposed and elementary reaction steps, namely, chain propagation reactions are supported by experiments.

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