ChemComm
Communication
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4
Scheme 2 Synthetic elaborations on the catalytic products.
were obtained with results comparable to the two-component
procedure (Table 2, entries 2, 8 and 19), and, importantly, the
reaction scope could be expanded to the unstable imines 1i,j
derived from enolisable aldehydes (Table 2, entries 11 and 12).
The relative configuration of adduct 3k was determined as
2,4-cis (and E at the double bond) by NMR spectroscopy,
whereas a comparison of the calculated (TD-DFT) with the
experimental ECD spectrum16 of this compound gave its absolute
configuration as (2S,4R),17 in line with previous phosphoric acid
catalysed Povarov reactions.3f–m,8c
Cycloadducts 3a and 3p were then subjected to a few
transformations at their enecarbamate moieties (Scheme 2).
Hydrolysis in 3a gave aldehyde 5, conveniently isolated after
reduction or Wittig olefination. Reduction of the double bond
in Boc protected 3p furnished the primary amine 8, whereas
addition of indole to the N-Cbz enecarbamate of 3a was efficiently
carried out under phosphoric acid (4c) catalysis.13b,c The stereo-
chemistry of the newly formed chiral centre in 9 could be partially
controlled by the enantiomer of the catalyst 4c employed, and,
remarkably, this transformation could be readily implemented in a
one-pot process, furnishing adduct 9 in 61% yield, 96 : 4 dr and
99% ee directly from imine 1a, dienophile 2a and indole (see ESI†).
In summary, disclosing a new vinylogous reactivity of 1-acyl-
amino dienes 2 towards N-aryl imines 1, we have developed
the first catalytic enantioselective vinylogous Povarov reaction,
promoted by chiral phosphoric acids.
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˜
´
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˜
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unreported. Instead, dienes 2 have been broadly used in Diels–Alder
reactions. For an example in multistep synthesis: (a) L. E. Overman
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10 The terminal carbon atom in 2 should be not only the least sterically
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11 Although in ref. 3k 2-hydroxystyrenes are mentioned as vinylogous
Mannich donors in Povarov reactions, by following the definition in
ref. 6 the whole phenol moiety is the activating EDG. Thus the
reaction with 2-hydroxystyrenes can be better considered as a
Povarov reaction with activated styrenes.
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We acknowledge financial support from the University
of Bologna (RFO). We thank Stefano Pirani for preliminary
experiments.
Notes and references
1 For a recent review describing both biological significance and
preparation of 1,2,3,4-tetrahydroquinolines: V. Sridharan, P. A. 14 Catalyst 4h was prepared by modifying substantially (see ESI†) the
´
Suryavanshi and C. Menendez, Chem. Rev., 2011, 111, 7157.
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model accounting for the observed stereoselectivity, see ESI†.
c
882 Chem. Commun., 2013, 49, 880--882
This journal is The Royal Society of Chemistry 2013