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27.17.2.2.4 Method 4: Ene Reactions of Aldehyde N-tert-Butyl- and N-Phenylhydrazones

DOI: 10.1055/sos-SD-027-00576

Kim, S.; Yoon, J.-Y.Science of Synthesis, (200427690.

Thermally induced reactions of tert-butylhydrazones 81 of aliphatic aldehydes with ethyl propenoate in refluxing toluene or xylene yield tert-butyldiazenyl esters 82.[‌83‌] These products can be isolated, but usually are tautomerized and hydrolyzed with trifluoroacetic acid to give γ-oxo esters 83 in good yields (Scheme 18). Similar, thermal ene reactions can be performed with propenecarbonitrile, but methyl crotonate, propenoic acid, methyl vinyl ketone, and methyl β,β-dimethylpropenoate are unsuitable substrates. The reaction also fails with the tert-butylhydrazones of ketones and benzaldehyde; however, the phenylhydrazones of aliphatic aldehydes can be used in conjunction with ethyl propenoate giving phenyldiazenyl esters 82 (R2=Ph) in moderate yields (5660%). These products can be tautomerized and hydrolyzed to γ-oxo esters 83 (in overall yields of 4152%) under similar conditions to those used for tert-butylhydrazones; however, in general tert-butylhydrazones give better yields than phenylhydrazones.

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