Gold catalysis: 1,3-oxazines by cyclisation of allene amides
A Stephen K Hashmi1, Andreas M Schuster, Sebastian Litters
1Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany. hashmi@hashmi.de
Abstract:
A series of allene amides was prepared and their gold-catalysed cyclisation was investigated. The formation of six-membered rings, 1,3-oxazines, was observed. Dihydropyrroles originating from intramolecular hydroamination of the distal C=C double bonds of the allenes were minor side products. Mechanistic studies by in situ (31)P NMR spectroscopy showed only one additional species during the conversion in each case; a computational study of the different allyl gold(I) species involved allowed this to be assigned as the σ-allyl gold species bearing the gold catalyst at the sterically less hindered methylene end. The regiospecific deuterodeauration of this intermediate confirmed a S(E) '-type mechanism for this last step of the catalytic cycle.
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