Glyoxal monohydrazones

Starting from glyoxal monohydrazones and active methylene compounds, N-anilino-pyrroles were prepared under solvent-free conditions by use of piperidine as catalyst [79] (Scheme 8.55). 

Base-catalyzed Reactions of Glyoxal Monohydrazones with Active Methylene Compounds 276... 

Chiral Auxiliary. Chiral 1,2-diamines have often been used as chiral auxiliaries in various carbon-carbon bond-forming reactions. The reaction of a diamine with an aldehyde gives a chiral aminal which can undergo stereoselective reactions. This was applied in the synthesis of enantiomerically pure a-hydrazino aldehydes by stereoselective addition of carbon nucleophiles onto the aminal of glyoxal monohydrazone (eqs 2 and 3). In this reaction, the use of 1,2-diaminocyclohexane gave lower diastereomeric excesses than with the related 1,2-diphenyl ethylenediamine. 

Allylation of acylsilanes and a-ketoesters proceeds normally. Monoallylation of a-diketones is also easily realized, whereas glyoxal A7Ai-monohydrazone gives 1,7-octa-diene-4,5-diol. However, sequential reactions of the glyoxal monohydrazone with RLi and then allylindium reagent lead to unsymmetrical diols. ... 

The monohydrazone of glyoxal is stabilized by resonance. Hence, it is quite difficult to achieve acetalization. 

The reaction of chiral aminals of the monohydrazone of glyoxal 6.49 with organolithium reagents is stereoselective. After Raney nickel treatment, followed by protection of the amine and hydrolysis, a-aminoaldehydes are obtained with an excellent enantiomeric excess [302]. Reaction of organomagnesium reagents with 6.49 in toluene, followed by the same treatment, gives the other enantiomer [1196] (Figure 6.41). 

A related process involves the condensation of ketones with the mono-IV,IV-dimethylhydrazone of glyoxal. Base-catalyzed condensation gives the monohydrazone of a 2-ene-1,4-dione and reductive cleavage of the N—N bond affords 2,3-disubstituted pyrroles (Scheme 8) <7 CB49i>. 

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