Addition and cycloaddition reactions of the electrophilic vinyl nitroso compounds 3-nitrosobut-3-en-2-one, 2-nitrosopropenal, and ethyl 2-nitrosopropenoate

Abstract

1-Chlorobutane-2,3-dione 2-oxime (2a) reacts with sodium carbonate and, in the presence of olefins, gives the oxazines (4) stereoselectively and in good yield. 3-Nitrosobut-3-en-2-one (1a) is postulated to be the intermediate in these reactions. Similar additions occur with 3-chloro-2-hydroxyiminopropanal (2b) and with ethyl bromopyruvate 2-oxime (3), although the oxazines are generally formed in lower yield. Competition experiments using pairs of olefins indicate that the intermediate (1a) adds preferentially to electron-rich olefins.

The vinyl nitroso intermediates (1) act as electrophiles towards indole; the products are the corresponding 3-alkylindoles (11). Analogous reactions are observed with pyrrole, 1-methylpyrrole, 1,3-dimethoxybenzene, and NN-dimethylaniline; with 3-methylindole, addition takes place at the 3-position to give the cycloadducts (12).

3-Nitrosopent-3-en-2-one (19) has been generated from 4-chloropentane-2,3-dione 3-oxime. This vinyl nitroso intermediate also undergoes cycloaddition to olefins, in competition with a hydrogen shift from the terminal methyl group which leads to isomerisation to pent-2-ene-2,3-dione 3-oxime (20).