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this reaction, the sterically favorable product 3m together with its intermediate E or F is totally inhibited, which results in no cyclization
regioisomeric product 3n was obtained in 90% yield as an unsepa- product being obtained.
rated mixture. Furthermore, naphthyl substituted cinnamylalde-
In summary, a convenient and expeditious route to structurally
hydes 1o and 1p can also be applied to this reaction, providing a diverse indenamine derivatives via an in situ formed N-sulfonyl-
useful method for the construction of indene-containing polycarbo- iminum ion initiated cyclization protocol has been developed. This
cycles (Table 2, 3o–p). Interestingly, the crystal growth of 3o in the methodology features using economical and readily available sub-
mixing solution of ether and petroleum ether resulted in its C–C stituted cinnamylaldehydes and sulfonylamines as the starting
double bond regioisomeric single crystal 3o0 exclusively.16 The materials, and generation of water as the only by-product.
structures of compounds 3o0 and 3p were confirmed by X-ray It displays high efficiency in the construction of two bonds
crystallographic analysis.17 Notably, thiophene and furan cannot and one ring in a single-step. Efforts are currently underway in
be used as the heteroaromatic nucleophiles in this reaction, con- our laboratory to further refine this method in organic synthesis
tinued efforts geared towards conversion of 1s or 1t to its cyclization and will be reported in due course.
product resulted in an intractable mixture. In addition, cinnamic
We thank the Natural Science Foundation of China (21162013)
ketone such as (E)-3-methyl-4-phenylbut-3-en-2-one (1u) was also and Beijing National Laboratory for Molecular Sciences (BNLMS) for
found to be an unsuitable substrate for this reaction, which might financial support, and Prof. Zhi-Xiang Yu for helpful discussions.
be attributed to the difficulty in the formation of an iminium ion
intermediate from the ketone.
Notes and references
1 (a) J. Royer and M Bonin, Chem. Rev., 2004, 104, 2311; (b) S. F. Martin,
To gain some understanding of the mechanism, the imine
4a was tested in this reaction (Scheme 3).18 Under the catalysis
Acc. Chem. Res., 2002, 35, 895.
of FeCl3 or TfOH, 4a was smoothly transformed into the
cyclization product 3a in 96% and 97% yields, respectively. In
the absence of the catalyst, 4a could not be converted into 3a.
Therefore, we assumed that the iminium ion might be the
reactive intermediate in this reaction.
Based on the above results, a plausible mechanism for this
reaction is proposed in Scheme 4. The first step is the nucleophilic
addition of a sulfonylamine to the carbonyl group of the a-substituted
aldehyde with the aid of FeCl3 to give the intermediate A, which then
loses a FeꢁCl3(OH) group to afford the electrophilic iminium ion C.
The iminium ion C/E can undergo isomerization to F/G, followed by
an intramolecular Friedel–Crafts alkylation reaction would give rise to
the corresponding indenamine derivatives. In this ring closure reac-
tion, we speculate that the isomerizations of intermediate C to its less
stable resonance structure E and E to its sterically unfavorable
rotamer F are facilitated by the substrates bearing an a-substituent;
upon using aldehydes without an a-substituent, the formation of
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Scheme 4 Plausible mechanism.
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Chem. Commun., 2014, 50, 4119--4122 | 4121