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12.3.1.4.3.3 Method 3: Cyclizations Involving 1,2-Diimines

DOI: 10.1055/sos-SD-012-00398

Grimmett, M. R.Science of Synthesis, (200212377.

1,2-Diaminoalkanes or -alkenes can be replaced in these reactions by suitable 1,2-diimines. Thus, compounds of type 104, readily available by treatment of glyoxal with a primary amine, can be converted into a 2-substituted 1,3-dialkylimidazolium chloride 105 by treatment with dry hydrogen chloride which can then be hydrolyzed to the 1,3-dialkyl-2-formylimidazolium chloride 106 (Scheme 59).[‌220‌] Similar N-trimethylsilyl-substituted diimines form dihydroimidazol-2-ones when reacted with phosgene.[‌221‌] There are obvious similarities when cyanogen reacts with a variety of aldehydes in the presence of hydrochloric acid to give 2-substituted 4,5-dichloroimidazoles 107. Yields lie in the range 3672%, and the method is potentially useful because 4,5-dichloroimidazoles are not always easy to make. Unfortunately, the process seems to be restricted to aldehydes that cannot enolize. If they can enolize they tend to undergo aldol-type reactions instead. Basic heterocyclic aldehydes merely form salts with hydrochloric acid under the conditions.[‌222‌]

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