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Chart 4. Scope of the Endo-Selective [3 + 2] Cycloaddition
Reaction
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a
After a single recrystallization from a crude mixture of 8f with 80% ee.
nitroalkenes, where respective syn and anti addition products
were obtained with high diastereo- and enantioselectivities. In
addition, the stereoselective formation of exo-5 and endo-8 has
also been achieved from syn-4 and anti-7, respectively, under
base catalysis, clearly demonstrating the stepwise nature of the
[3 + 2] cycloaddition reactions of N-metalated azomethine
ylides. The preparation of a diverse array of chiral compounds
using various chiral catalyst species, particularly derived from a
single chiral source, should enrich our molecular-level under-
standing of asymmetric catalysis. Mechanistic studies and the
extension of the stereodivergent catalysis approach to other
carbon−carbon bond-forming reactions are currently under-
way, and our results will be reported in due course.
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ASSOCIATED CONTENT
■
S
* Supporting Information
(13) For recent examples of Mannich-type reactions, see: (a) Jiang,
J.; Xu, H.-D.; Xi, J.-B.; Ren, B.-Y.; Lv, F.-P.; Guo, X.; Jiang, L.-Q.;
Zhang, Z.-Y.; Hu, W.-H. J. Am. Chem. Soc. 2011, 133, 8428. (b) Lu, G.;
Yoshino, T.; Morimoto, H.; Matsunaga, S.; Shibasaki, M. Angew.
Chem., Int. Ed. 2011, 50, 4382.
(14) Under unoptimized conditions using 10 mol % CuOTf/1/Et3N
in THF, the formation endo-5a was observed in 90% yield with 87% ee
(endo:exo >25:1). Our full account for the endo-selective reactions will
be reported elsewhere.
Experimental procedures and spectral data for all compounds.
This material is available free of charge via the Internet at
AUTHOR INFORMATION
■
Corresponding Author
(15) No syn addition product was identified in thermal and catalyzed
reactions of 6. For the structural misassignment of anti-7 as syn-7 and
endo-8 as anti-7, see: Li, W.; Liu, H.; Du, D.-M. Synlett 2009, 925.
(16) For example, the use of 10 mol % Et3N or DBU in MeOH led to
the exclusive formation of endo-8 in 95% yield within 18 h at 23 °C.
(17) The stepwise nature of the reaction was confirmed by
interrupting an unfinished reaction to identify anti-7 and by
monitoring the conversion of anti-7 to endo-8. For a previous
asymmetric organocatalyst approach to endo-8 with 46−65% ee, see:
Xue, M.-X.; Zhang, X.-M.; Gong, L.-Z. Synlett 2008, 691.
ACKNOWLEDGMENTS
■
This research was supported by IUPUI through a Research
Support Funding Grant (2011 RSFG). S.K. is a 2010 Eli Lilly
Summer Research Fellowship recipient. Authors thank Dr. Karl
Dria for his assistance with mass spectral analysis (CHE-
0821661). The Bruker 500 MHz NMR spectrometer was
purchased via a NSF-MRI Award (CHE-0619254).
REFERENCES
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