Golden control: The title reaction (see scheme) proceeds with high stereocontrol to generate the heterocyclic products in good yield. Experiments to probe the mechanism were performed.

Metal -catalyzed [2+2+m] cycloadditions (m = 1-3) of 1,nenynes (n = 5-7) with small molecules are powerful tools for accessing complicated hetero-and carbocycles. [1,2] These reactions are appealing because their mechanisms can be altered by using different metal complexes. As shown in Scheme 1, low-valent metal species react with these enynes to form metallocycloalkene intermediates (Ia) initially, [3] whereas gold(I) catalysts generate the cyclopropyl gold carbene Ib which has a resonance structure, that is, the alkenylgold carbocation Ib'.[4] We sought a new catalytic [2+2+m] cycloaddition of 1,n-enynes to generate an additional functionality on the resulting cycloadducts. Although catalytic cycloadditions of nitrosobenzenes have been intensively studied, [5,6] we are aware of no report on the cycloaddition of nitroso species with 1,n-enynes (n = 5-7).[6] We report herein the gold-catalyzed cyclization/oxidative [3+2] cycloadditions of 1,5-enynes with nitrosobenzenes. Notably, nitrosobenzenes not only provide two-atom building units, but also lead to the alkyne oxidation. Such a reaction pattern is unprecedented in metal-catalyzed cycloadditions of 1,n-enynes. This oxidative cycloaddition was realized with the treatment of the 1,5-enyne 1 a (1 equiv) with nitrosobenzene (2 a; 1.5 equiv) and [P(tBu) 2 (o-biphenyl)AuCl]/AgNTf 2 (5 mol %) in 1,2-dichloroethane (DCE; 25 8C, 24 h) to give the oxidative cycloadduct 3 a in 21 % yield, together with the cycloisomerization product 4 a in 70 % yield (Table 1, entry 1).[7] The yield of 3 a was increased to 41 % with [{P(tBu) 2 (o-biphenyl)}-AuCl]/AgSbF 6 (5 mol %, entry 2). For the two above-mentioned entries, we also isolated 1,2-diphenyldiazene oxide (5 a) which was identified upon comparison of its NMR spectra and mass data with those of an authentic sample.[8]

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