could be efficiently converted to THBCs (48–86%) when reacted
with allylmethylcarbonate and Pd(PPh3)4 in refluxing toluene.
In summary, we have shown that metal-catalyzed tandem
isomerization/cyclization of N-allyltryptamines constitutes an
efficient alternative to the Pictet–Spengler reaction for the
synthesis of THBCs. Several metal catalysts were shown to
mediate the transformation, and the strategy thus points to an
underexplored role of metal catalysts in iminium chemistry. At
this stage, it must be emphasized that the proposed mechanism
(Fig. 2) for the reaction process is merely tentative and will be
subject of future research. Notably, when using a Pd catalyst,
THBCs can be synthesized via a novel one-pot tandem process
that involves Pd-catalyzed Tsuji–Trost reaction, isomerization,
and N-alkyliminium cyclization, thus providing a metal-catalyzed
variant of the Pictet–Spengler reaction.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3345–3347 3347