7352
A. G. Cook et al. / Tetrahedron Letters 42 (2001) 7349–7352
enamine to the aldehyde and secondary amine, the
latter then adding to the maleate ester.
3. Davies, M.; Evans, F. P. Trans. Faraday Soc. 1955, 51,
1506.
4. Gunbaum, Z.; Patai, S.; Rappoport, Z. J. Chem. Soc. (B)
There are two lines of evidence that show that enamines
are produced in these reactions as proposed in the
mechanism shown above. The first is the appearance of
enamine signals in the NMR spectra of the products.
For example the reaction mixture produced by use of
aminal 7a shows 13C NMR signals at 131.7 and 130.0
ppm (CꢁCꢀN) corresponding to the signals of known
enamine 16.26 The second line of evidence is the pro-
duction of cycloaddition product 17 when aminal 5 is
allowed to react with dimethyl maleate. It is identified
by the GC–MS spectrum of the product mixture. This
compound is produced by the cycloaddition of enamine
product 15 with dimethyl maleate through a known
reaction9,10 in a ratio of 3:1 addition product to trapped
enamine product 17. The balance of the enamine
appears as the hydrolyzed product due to trace
amounts of water. The ratio of enamine product to
addition product as determined by a combination of
GC–MS and NMR is about one to one in all these
reactions. This is the expected ratio for the postulated
mechanism. In summary, for this reaction there is a
ratio of isomerization to addition product of 1:1, and a
ratio of addition product to enamine product of 1:1.
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Cook, A. G., Ed., Marcel Dekker: New York, 1988.
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